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<rfc category="std" docName="draft-ietf-nfsv4-rfc5661sesqui-msns-04" obsoletes="5661" ipr="pre5378Trust200902" updates="" submissionType="IETF" xml:lang="en" tocInclude="true" tocDepth="2" symRefs="false" sortRefs="false" version="3">
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  <front>
    <title abbrev="NFSv4.1 with Namespace Update ">
    Network File System (NFS) Version 4 Minor Version 1 Protocol

    </title>
    <seriesInfo name="Internet-Draft" value="draft-ietf-nfsv4-rfc5661sesqui-msns-04"/>
    <author fullname="David Noveck" initials="D." surname="Noveck" role="editor">
      <organization abbrev="NetApp">NetApp</organization>
      <address>
        <postal>
          <street>1601 Trapelo Road, Suite 16</street>
          <city>Waltham</city>
          <region>MA</region>
          <code>02451</code>
          <country>USA</country>
        </postal>
        <phone>+1-781-768-5347</phone>
        <email>dnoveck@netapp.com</email>
      </address>
    </author>
    <author initials="C." surname="Lever" fullname="Charles Lever">
      <organization abbrev="ORACLE">
        Oracle Corporation
      </organization>
      <address>
        <postal>
          <street>1015 Granger Avenue</street>
          <city>Ann Arbor</city>
          <region>MI</region>
          <code>48104</code>
          <country>United States of America</country>
        </postal>
        <phone>+1 248 614 5091</phone>
        <email>chuck.lever@oracle.com</email>
      </address>
    </author>
    <date year="2020"/>
    <area>Transport</area>
    <workgroup>NFSv4</workgroup>
    <abstract>
      <t>
	This document describes the Network File System (NFS) version 4
	minor version 1,
        including features retained from the base protocol (NFS version 4 minor
        version 0, which is specified in RFC 7530) and protocol
        extensions made subsequently.  The later minor version
        has no dependencies on NFS version 4 minor version 0, and
        is considered a separate protocol.
      </t>
      <t>
	This document obsoletes RFC5661.  It substantially revises the treatment
	of features relating to multi-server namespace, superseding the
	description of those features appearing in RFC5661.
      </t>
    </abstract>
  </front>
  <middle>
    <section anchor="intro" numbered="true" toc="default">
      <name>Introduction</name>
      <section anchor="intro_the_document" numbered="true" toc="default">
        <name>Introduction to this Update</name>
        <t>
      Two important features previously defined in minor version 0 but
      never fully addressed in minor version 1 are trunking, the
      simultaneous use of
      multiple connections between a client and server, potentially to
      different network addresses, and transparent state migration, which
      allows a file system to be transferred between servers in a way that
      provides to the client the ability to maintain its existing locking
      state across the transfer.
        </t>
        <t>
      The revised description of the NFS version 4 minor version 1
      (NFSv4.1) protocol presented in this update is necessary to enable
      full use of these features together with other multi-server namespace
      features.  This document is in the form of an updated description of
      the NFSv4.1 protocol previously defined in RFC5661
      <xref target="RFC5661" format="default"/>.
      RFC5661 is obsoleted by  this document. However, the update has a
      limited scope and is focused on enabling full use of trunking and
      transparent state migration. The need for these changes is discussed
      in Appendix A.  Appendix B describes the specific changes made to
      arrive at the current text.
        </t>
        <t>
      This limited-scope update replaces the current NFSv4.1 RFC with the
      intention of providing an authoritative and complete specification, the
      motivation for which is discussed in
      <xref target="I-D.roach-bis-documents" format="default"/>,
      addressing the issues within the scope of the update. However, it will
      not address issues that are known but outside of this limited scope
      as could expected by a full update of the protocol. Below are some
      areas which are known to need addressing in a future update of the
      protocol.
        </t>
        <ul spacing="normal">
          <li>
        Work needs to be done with regard to
        RFC8178 <xref target="RFC8178" format="default"/> which establishes NFSv4-wide
	versioning rules.  As
        RFC5661 is currently inconsistent with
	that document, changes are needed in order
	to arrive at a situation in which there
	would be no need for RFC8178 to update the NFSv4.1 specification.
      </li>
          <li>
        Work needs to be done with regard to
	RFC8434 <xref target="RFC8434" format="default"/>, which establishes the requirements
	for pNFS layout types, which are not clearly defined in
	RFC5661.  When that
	work is done and the resulting documents approved,
	the new NFSv4.1 specification document will provide a clear set
	of requirements for layout types and a description of the file layout
	type that conforms to those requirements.  Other layout types will
	have their own specification documents that conforms to those
	requirements as well.
      </li>
          <li>
            <t>
	Work needs to be done to address many errata reports relevant to
	RFC 5661, other than errata report 2006 <xref target="Err2006" format="default"/>,
	which is addressed in this document.
	Addressing that report was not deferrable because of the
	interaction of the changes suggested there
	and the newly described handling of state and session migration.
            </t>
            <t>
        The errata reports  that have been deferred and that will need to
        be addressed in a later document include reports currently assigned
	a range of statuses in the errata reporting system including reports
	marked Accepted and those marked Hold For Document Update
	because the change was
	too minor to address immediately.
            </t>
            <t>
        In addition, there is a set of other reports, including at least one
        in state Rejected, which will need to be addressed in a later document.
	This will involve making changes to consensus decisions reflected
	in RFC 5661, in situation in which the working group has decided that
	the treatment in RFC 5661 is incorrect, and needs to be revised to
	reflect the working group's new consensus and ensure compatibility
	with existing implementations that do not follow the handling
	described in
	in RFC 5661.
            </t>
            <t>
	Note that it is expected that all such errata reports will remain
	relevant to implementers and the authors of an eventual rfc5661bis,
	despite the fact that this document, when approved,
	will obsolete RFC 5661 <xref target="RFC5661" format="default"/>.
            </t>
          </li>
          <li>
	There is a need for a new approach to the description of
	internationalization since the current internationalization section
	(<xref target="internationalization" format="default"/>) has never been
	implemented and does
	not meet the needs of the NFSv4 protocol.  Possible solutions are
	to create a new internationalization section modeled on that in
	<xref target="RFC7530" format="default"/> or to create a new document describing
	internationalization for all
        NFSv4 minor versions and reference that document in the RFCs
	defining both NFSv4.0 and NFSv4.1.
      </li>
          <li>
	There is a need for a revised treatment of security
        in NFSv4.1.  The issues with the existing treatment are discussed in
        <xref target="SECBAD" format="default"/>.
      </li>
        </ul>
        <t>
      Until the above work is done, there will not be a consistent set of
      documents providing a description of the NFSv4.1 protocol and any
      full description would involve documents updating other documents
      within the specification.  The updates applied by
      RFC8434 <xref target="RFC8434" format="default"/> and RFC8178 <xref target="RFC8178" format="default"/>
      to RFC5661 also apply to this specification, and will apply to
      any subsequent v4.1 specification until that work is done.
        </t>
      </section>
      <section anchor="intro_the_protocol" numbered="true" toc="default">
        <name>The NFS Version 4 Minor Version 1 Protocol</name>
        <t>
      The NFS version 4 minor version 1 (NFSv4.1) protocol
      is the second minor version of the NFS version 4
      (NFSv4) protocol. The first minor version, NFSv4.0, is
      now described in RFC 7530 <xref target="RFC7530" format="default"/>.  It generally
      follows the guidelines for minor versioning that are
      listed in Section 10 of RFC 3530.  However, it
      diverges from guidelines 11 ("a client and server
      that support minor version X must support minor
      versions 0 through X-1") and 12 ("no new features may be
      introduced as mandatory in a minor version"). These
      divergences are due to the introduction of
      the sessions model for managing non-idempotent
      operations and the RECLAIM_COMPLETE operation.
      These two new features are infrastructural in
      nature and simplify implementation of existing and
      other new features.  Making them anything but REQUIRED
      would add undue complexity to protocol definition and
      implementation.  NFSv4.1 accordingly updates the
      <xref target="minor_versioning" format="default">minor versioning
      guidelines</xref>.

        </t>
        <t>
      As a minor version, NFSv4.1 is consistent with the overall
      goals for NFSv4, but extends the protocol so as to
      better meet those goals, based on experiences with NFSv4.0.
      In addition, NFSv4.1 has adopted some additional goals, which
      motivate some of the major extensions in NFSv4.1.
        </t>
      </section>
      <section numbered="true" toc="default">
        <name>Requirements Language</name>
        <t>The key words "MUST", "MUST NOT",
"REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED",
"MAY", and "OPTIONAL" in this document are to be
interpreted as described in <xref target="RFC2119" format="default">RFC 2119</xref>.
</t>
      </section>
      <section anchor="scope_of_doc" numbered="true" toc="default">
        <name>Scope of This Document</name>
        <t>

   This document describes the NFSv4.1 protocol. With
   respect to NFSv4.0, this document does not:

        </t>
        <ul spacing="normal">
          <li>
       describe the NFSv4.0 protocol, except where needed
       to contrast with NFSv4.1.

   </li>
          <li>
       modify the specification of the NFSv4.0 protocol.

   </li>
          <li>
       clarify the NFSv4.0 protocol.

   </li>
        </ul>
      </section>
      <section anchor="version4_goals" numbered="true" toc="default">
        <name>NFSv4 Goals</name>
        <t>
      The NFSv4 protocol is a further revision of the NFS protocol
      defined already by NFSv3
      <xref target="RFC1813" format="default"/>.  It retains
      the essential characteristics of previous versions: easy
      recovery; independence of transport protocols, operating systems, and
      file systems; simplicity; and good performance.  NFSv4 has the following goals:

        </t>
        <ul spacing="normal">
          <li>
            <t>
          Improved access and good performance on the Internet
            </t>
            <t>
          The protocol is designed to transit firewalls easily, perform well
          where latency is high and bandwidth is low, and scale to very
          large numbers of clients per server.
            </t>
          </li>
          <li>
            <t>
          Strong security with negotiation built into the protocol
            </t>
            <t>
          The protocol builds on the work of the ONCRPC working group in
          supporting the RPCSEC_GSS protocol.  Additionally, the
        NFSv4.1 protocol provides a mechanism to allow clients and
        servers the ability to negotiate security and require clients and servers to
          support a minimal set of security schemes.
            </t>
          </li>
          <li>
            <t>
          Good cross-platform interoperability
            </t>
            <t>
          The protocol features a file system model that provides a useful,
          common set of features that does not unduly favor one file system
          or operating system over another.
            </t>
          </li>
          <li>
            <t>
          Designed for protocol extensions
            </t>
            <t>
          The protocol is designed to accept standard extensions within a
          framework that enables and encourages backward compatibility.
            </t>
          </li>
        </ul>
      </section>
      <section anchor="minor_version1_goals" numbered="true" toc="default">
        <name>NFSv4.1 Goals</name>
        <t>
      NFSv4.1 has the following goals, within the framework
      established by the overall NFSv4 goals.
        </t>
        <ul spacing="normal">
          <li>
          To correct significant structural weaknesses and oversights
          discovered in the base protocol.
        </li>
          <li>
          To add clarity and specificity to areas left
          unaddressed or not addressed in sufficient
          detail in the base protocol. However, as stated
          in <xref target="scope_of_doc" format="default"/>, it is not
          a goal to clarify the NFSv4.0 protocol in the
          NFSv4.1 specification.

        </li>
          <li>
          To add specific features based on experience with the existing
          protocol and recent industry developments.
        </li>
          <li>
          To provide protocol support to take advantage of clustered
          server deployments including the ability to provide scalable
          parallel access to files distributed among multiple servers.
        </li>
        </ul>
      </section>
      <section anchor="intro_definitions" numbered="true" toc="default">
        <name>General Definitions</name>
        <t>
      The following definitions provide an appropriate context for the reader.
        </t>
        <dl newline="false" spacing="normal">
          <dt>Byte:</dt>
          <dd anchor="byte">
          In this document, a byte is an octet, i.e., a datum
          exactly 8 bits in length.
        </dd>
          <dt>Client:</dt>
          <dd anchor="client_def">
            <t>
          The client is the entity that accesses the NFS server's
          resources.  The client may be an application that contains
          the logic to access the NFS server directly.  The client
          may also be the traditional operating system client that
	  provides remote file system services for a set of applications.
            </t>
            <t>
          A client is uniquely identified by a client owner.
            </t>
            <t>
          With reference to byte-range locking, the client is also the entity that
          maintains a set of locks on behalf of one or more
          applications.  This client is responsible for crash or
          failure recovery for those locks it manages.
            </t>
            <t>
          Note that multiple clients may share the same transport and
          connection and
          multiple clients may exist on the same network node.
            </t>
          </dd>
          <dt>Client ID:</dt>
          <dd>
          The client ID is a 64-bit quantity used as a unique, short-hand reference to
          a client-supplied verifier and client owner.  The server is
          responsible for supplying the client ID.
        </dd>
          <dt>Client Owner:</dt>
          <dd>
          The client owner is a unique string, opaque to the server,
          that identifies a client. Multiple network connections and source
          network addresses originating from those connections may share
          a client owner. The server is expected to treat requests
          from connections with the same client owner as coming from
          the same client.
        </dd>
          <dt>File System:</dt>
          <dd>
          The file system is the collection of objects on a server (as
          identified by the major identifier of a server
          owner, which is defined later in this section)
          that share the same fsid attribute (see <xref target="attrdef_fsid" format="default"/>).

        </dd>
          <dt>Lease:</dt>
          <dd>
            <t>
          A lease is an interval of time defined by the server for which the
          client is irrevocably granted locks.  At the end of a
          lease period, locks may be revoked if the lease has not
          been extended.  A lock must be revoked if a conflicting
          lock has been granted after the lease interval.
            </t>
            <t>
          A server grants a client a single lease for all state.
            </t>
          </dd>
          <dt>Lock:</dt>
          <dd>
          The term "lock" is used to refer to byte-range (in UNIX environments,
          also known as record)
          locks, share reservations, delegations, or layouts unless
          specifically stated otherwise.
        </dd>
          <dt>Secret State Verifier (SSV):</dt>
          <dd>
          The SSV is a unique secret key shared between a client and
          server.  The SSV serves as the secret key for an internal (that
          is, internal to NFSv4.1) Generic Security Services (GSS)
          mechanism (the SSV GSS mechanism;
          see <xref target="ssv_mech" format="default"/>).  The SSV GSS mechanism uses the
          SSV to compute message integrity code (MIC) and Wrap tokens.
          See <xref target="protect_state_change" format="default"/> for more details on how NFSv4.1 uses
          the SSV and the SSV GSS mechanism.

        </dd>
          <dt>Server:</dt>
          <dd>
          The Server is the entity responsible for coordinating
          client access to a set of file systems and is identified by a server
          owner. A server can span multiple network addresses.
        </dd>
          <dt>Server Owner:</dt>
          <dd>
          The server owner identifies the server to the client.
          The server owner consists of a major identifier and a minor identifier.
          When the client has two connections each to a peer with the
          same major identifier, the client assumes that both peers are
          the same server (the server namespace is the
          same via each connection) and that
          lock state is sharable across both connections. When each peer
          has both the same major and minor identifiers, the client
          assumes that each connection might be associable with the same session.
        </dd>
          <dt>Stable Storage:</dt>
          <dd>
            <t>
          Stable storage is storage from which data stored by
          an NFSv4.1 server can be recovered without data
          loss from multiple power failures (including cascading
          power failures, that is, several power failures in quick
          succession), operating system failures, and/or hardware
          failure of components other than the storage medium itself
          (such as disk, nonvolatile RAM, flash memory, etc.).
            </t>
            <t>
          Some examples of stable storage that are allowable for an
          NFS server include:
            </t>
            <ol spacing="normal" type="1">
              <li>
               Media commit of data; that is, the modified data has
               been successfully written to the disk media, for
               example, the disk platter.
             </li>
              <li>
               An immediate reply disk drive with battery-backed,
               on-drive intermediate storage or uninterruptible power
               system (UPS).
             </li>
              <li>
               Server commit of data with battery-backed intermediate
               storage and recovery software.
             </li>
              <li>
               Cache commit with uninterruptible power system (UPS) and
               recovery software.
             </li>
            </ol>
          </dd>
          <dt>Stateid:</dt>
          <dd>
          A stateid is a 128-bit quantity returned by a server that uniquely
          defines the open and locking states provided by the server
          for a specific open-owner or lock-owner/open-owner pair
          for a specific file and type of lock.
        </dd>
          <dt>Verifier:</dt>
          <dd>
          A verifier is a 64-bit quantity generated by the client that the server
          can use to determine if the client has restarted and lost
           all previous lock state.
        </dd>
        </dl>
      </section>
      <section anchor="feature-overview" numbered="true" toc="default">
        <name>Overview of NFSv4.1 Features</name>
        <t>
      The major features of
      the NFSv4.1 protocol will be reviewed in brief.  This will be done
      to provide an appropriate context for both the reader who is familiar
      with the previous versions of the NFS protocol and the reader
      who is new to the NFS protocols.  For the reader new to the NFS protocols,
      there is still a set of fundamental knowledge that is expected.
      The reader should be familiar with the External Data
      Representation (XDR) and Remote Procedure Call (RPC) protocols
      as described in <xref target="RFC4506" format="default"/> and <xref target="RFC5531" format="default"/>.
      A basic knowledge of file systems and distributed file systems is expected as well.
        </t>
        <t>
      In general, this specification of NFSv4.1 will
      not distinguish those features added in minor version
      1 from those present in the base protocol but
      will treat NFSv4.1 as a unified whole.  See <xref target="intro_differences" format="default"/> for a summary of
      the differences between NFSv4.0 and NFSv4.1.

        </t>
        <section anchor="rpc_and_security" numbered="true" toc="default">
          <name>RPC and Security</name>
          <t>
        As with previous versions of NFS, the External Data Representation
        (XDR) and Remote Procedure Call (RPC) mechanisms used for the NFSv4.1 protocol are those defined in
        <xref target="RFC4506" format="default"/> and <xref target="RFC5531" format="default"/>.  To
        meet end-to-end security requirements, the RPCSEC_GSS framework
        <xref target="RFC2203" format="default"/> is used to extend the basic
        RPC security.  With the
        use of RPCSEC_GSS, various mechanisms can be provided to offer
        authentication, integrity, and privacy to the NFSv4 protocol.
        Kerberos V5 is used as described in
        <xref target="RFC4121" format="default"/> to provide one
        security framework.
        With the use of
        RPCSEC_GSS, other mechanisms may also be specified and used for NFSv4.1 security.
          </t>
          <t>
        To enable in-band security negotiation, the NFSv4.1 protocol
        has operations that provide the client a method of
        querying the server about its policies regarding which security
        mechanisms must be used for access to the server's file system
        resources.  With this, the client can securely match the security
        mechanism that meets the policies specified at both the client and
        server.
          </t>
          <t>
	NFSv4.1 introduces parallel access (see <xref target="parallel_access" format="default"/>), which is
	called pNFS.

The security framework
	described in this section is
	significantly modified by the
	introduction of pNFS (see <xref target="security_considerations_pnfs" format="default"/>),
	because data access is sometimes not over
	RPC.  The level of significance varies
	with the storage protocol (see <xref target="storage_protocol" format="default"/>) and can be as low as zero
        impact (see <xref target="file_security_considerations" format="default"/>).

          </t>
        </section>
        <section anchor="protocol_structure" numbered="true" toc="default">
          <name>Protocol Structure</name>
          <section anchor="core_protocol" numbered="true" toc="default">
            <name>Core Protocol</name>
            <t>
          Unlike NFSv3, which used a series of ancillary
          protocols (e.g., NLM, NSM (Network Status Monitor), MOUNT), within all minor versions
          of NFSv4 a single RPC protocol is used to make requests to
          the server.

Facilities that had been separate protocols, such
          as locking, are now integrated within a single unified
          protocol.
            </t>
          </section>
          <section anchor="parallel_access" numbered="true" toc="default">
            <name>Parallel Access</name>
            <t>
          Minor version 1 supports high-performance data access to a
          clustered server implementation by enabling a separation of
          metadata access and data access, with the latter done to
          multiple servers in parallel.
            </t>
            <t>
          Such parallel data access is controlled by recallable
          objects known as "layouts", which are integrated into the
          protocol locking model.  Clients direct requests for
          data access to a set of data servers specified by the
          layout via a data
          storage protocol which may be NFSv4.1 or may be another
          protocol.
            </t>
            <t>
	  Because the protocols used for parallel
	  data access are not necessarily
	  RPC-based, the RPC-based security model
	  (<xref target="rpc_and_security" format="default"/>) is
	  obviously impacted (see <xref target="security_considerations_pnfs" format="default"/>).
	  The degree of impact varies with the
	  storage protocol (see <xref target="storage_protocol" format="default"/>) used for
	  data access, and can be as low as zero (see
	  <xref target="file_security_considerations" format="default"/>).

            </t>
          </section>
        </section>
        <section anchor="file_system_model" numbered="true" toc="default">
          <name>File System Model</name>
          <t>
        The general file system
        model used for the NFSv4.1 protocol
        is the same as previous versions.  The server file system is
        hierarchical with the regular files contained within being
        treated as opaque byte
        streams.  In a slight departure, file and directory names are encoded
        with UTF-8 to deal with the basics of internationalization.
          </t>
          <t>
        The NFSv4.1 protocol does not require a separate
        protocol to provide for the initial mapping between path
        name and filehandle.  All file systems exported by a server
        are presented as a tree so that all file systems are reachable
        from a special per-server global root filehandle.  This
        allows LOOKUP operations to be used to perform functions
        previously provided by the MOUNT protocol.  The server
        provides any necessary pseudo file systems to bridge any
        gaps that arise due to unexported gaps between exported
        file systems.
          </t>
          <section anchor="intro_filehandles" numbered="true" toc="default">
            <name>Filehandles</name>
            <t>
          As in previous versions of the NFS protocol, opaque
          filehandles are used to identify individual files
          and directories.  Lookup-type and create operations
          translate file and directory names to
          filehandles, which are then used to identify objects
          in subsequent operations.
            </t>
            <t>
          The NFSv4.1 protocol provides support for
          persistent filehandles, guaranteed to be valid
          for the lifetime of the file system object designated.
          In addition, it provides support to servers to provide
          filehandles with more limited validity guarantees,
          called volatile filehandles.
            </t>
          </section>
          <section anchor="intro_attributes" numbered="true" toc="default">
            <name>File Attributes</name>
            <t>
	  The NFSv4.1 protocol has a rich and extensible
	  file object attribute structure, which is divided
	  into REQUIRED, RECOMMENDED, and named attributes
	  (see <xref target="file_attributes" format="default"/>).

            </t>
            <t>
	  Several (but not all) of the REQUIRED attributes
	  are derived from the attributes of NFSv3 (see
	  the definition of the fattr3 data type in <xref target="RFC1813" format="default"/>). An example of a REQUIRED
	  attribute is the file object's type (<xref target="attrdef_type" format="default"/>) so that regular files
	  can be distinguished from directories (also known
	  as folders in some operating environments) and
	  other types of objects. REQUIRED attributes are
	  discussed in <xref target="mandatory_attributes_intro" format="default"/>.

            </t>
            <t>
	  An example of three RECOMMENDED attributes are
	  acl, sacl, and dacl.  These attributes define an
	  Access Control List (ACL) on a file object
	  (<xref target="acl" format="default"/>).  An ACL provides
	  directory and file access control beyond the
	  model used in NFSv3.   The ACL definition allows
	  for specification of specific sets of permissions
	  for individual users and groups.  In addition,
	  ACL inheritance allows propagation of access
	  permissions and restrictions down a directory tree
	  as file system objects are created.  RECOMMENDED
	  attributes are discussed in <xref target="recommended_attributes_intro" format="default"/>.

            </t>
            <t>
          A named attribute is an opaque byte stream that is associated
          with a directory or file and referred to by a string name.
          Named attributes are meant to be used by client applications
          as a method to associate application-specific data with a
          regular file or directory.  NFSv4.1 modifies named attributes
          relative to NFSv4.0 by tightening the allowed operations in
          order to prevent the development of non-interoperable
          implementations.  Named attributes are discussed in <xref target="named_attributes_intro" format="default"/>.

            </t>
          </section>
          <section anchor="PREP-intro" numbered="true" toc="default">
            <name>Multi-Server Namespace</name>
            <t>
      NFSv4.1 contains a number of features to allow
      implementation of namespaces that cross server boundaries
      and that allow and facilitate a non-disruptive transfer of
      support for individual file systems between servers.  They
      are all based upon attributes that allow one file system to
      specify alternate, additional, and new location information
      that specifies how the client may access
      that file system.
            </t>
            <t>
      These attributes can be used to provide for individual active
      file systems:
            </t>
            <ul spacing="normal">
              <li>
        Alternate network addresses to access the
        current file system instance.
      </li>
              <li>
        The locations of alternate file system instances
        or replicas to be used in the event that the current
        file system instance becomes unavailable.
      </li>
            </ul>
            <t>
      These file system location
      attributes may be used together with the concept
      of absent file systems, in which a position in the server
      namespace is associated with locations on other servers without
      there being any corresponding file system instance on the
      current server. For example,
            </t>
            <ul spacing="normal">
              <li>
        These attributes may be used with absent file systems
        to implement referrals whereby one server may direct the
        client to a file system provided by another server.  This
        allows extensive multi-server namespaces to be constructed.
      </li>
              <li>
        These attributes may be provided when a previously
        present file system becomes absent.  This allows
        non-disruptive migration of file systems to alternate
        servers.
      </li>
            </ul>
          </section>
        </section>
        <section anchor="intro_locking" numbered="true" toc="default">
          <name>Locking Facilities</name>
          <t>
        As mentioned previously, NFSv4.1 is a single protocol that
        includes locking facilities.  These locking facilities
        include support for many types of locks including a number
        of sorts of recallable locks.  Recallable locks such as
        delegations allow the client to be assured that certain
        events will not occur so long as that lock is held.  When
        circumstances change, the lock is recalled
        via a callback request.  The assurances provided by
        delegations allow more extensive caching to be done safely
        when circumstances allow it.
          </t>
          <t>
	The types of locks are:
          </t>
          <ul spacing="normal">
            <li>
            Share reservations as established by OPEN operations.
          </li>
            <li>
            Byte-range locks.
          </li>
            <li>
            File delegations, which are recallable locks that assure
            the holder that inconsistent opens and file changes cannot
            occur so long as the delegation is held.
          </li>
            <li>
            Directory delegations, which are recallable locks
            that assure the holder that inconsistent directory
            modifications cannot occur so long as the delegation
            is held.
          </li>
            <li>
            Layouts, which are recallable objects that assure the
            holder that direct access to the file data may be
            performed directly by the client and that no change
            to the data's location that is inconsistent with that access
            may be made so long as the layout is held.
          </li>
          </ul>
          <t>
        All locks for a given client are tied together under a
        single client-wide lease.  All requests made on sessions
        associated with the client renew that lease.  When the client's
        lease
        is not promptly renewed, the client's locks are subject to revocation.
        In the event of server restart, clients have the
        opportunity to safely reclaim their locks within a special
        grace period.
          </t>
        </section>
      </section>
      <section anchor="intro_differences" numbered="true" toc="default">
        <name>Differences from NFSv4.0</name>
        <t>
      The following summarizes the major differences between minor version
      1 and the base protocol:
        </t>
        <ul spacing="normal">
          <li>
          Implementation of the sessions model (<xref target="Session" format="default"/>).
        </li>
          <li>
          Parallel access to data (<xref target="pnfs" format="default"/>).
        </li>
          <li>
          Addition of the RECLAIM_COMPLETE operation to better structure
          the lock reclamation process (<xref target="OP_RECLAIM_COMPLETE" format="default"/>).
        </li>
          <li>
            <t>
         Enhanced delegation support as follows.

            </t>
            <ul spacing="normal">
              <li>
	   Delegations on directories and other
	   file types in addition to regular files (<xref target="OP_GET_DIR_DELEGATION" format="default"/>, <xref target="OP_WANT_DELEGATION" format="default"/>).

	 </li>
              <li>
	   Operations to optimize acquisition of recalled
	   or denied delegations (<xref target="OP_WANT_DELEGATION" format="default"/>, <xref target="OP_CB_PUSH_DELEG" format="default"/>, <xref target="OP_CB_RECALLABLE_OBJ_AVAIL" format="default"/>).

	 </li>
              <li>
	   Notifications of changes to files and directories
	   (<xref target="OP_GET_DIR_DELEGATION" format="default"/>, <xref target="OP_CB_NOTIFY" format="default"/>).

	 </li>
              <li>
	   A method to allow a server to indicate that it is
	   recalling one or more delegations for resource
	   management reasons, and thus a method to allow
	   the client to pick which delegations to return
	   (<xref target="OP_CB_RECALL_ANY" format="default"/>).

        </li>
            </ul>
          </li>
          <li>
	  Attributes can be set atomically
	  during exclusive file create via the OPEN operation
	  (see the new EXCLUSIVE4_1 creation method in
	  <xref target="OP_OPEN" format="default"/>).

        </li>
          <li>
	  Open files can be preserved if removed and the
	  hard link count ("hard link" is defined in
	  an <xref target="hardlink" format="default">Open Group</xref> standard) goes
	  to zero, thus obviating the
	  need for clients to rename deleted files to
	  partially hidden names -- colloquially called
	  "silly rename" (see the new
	  OPEN4_RESULT_PRESERVE_UNLINKED reply flag in
	  <xref target="OP_OPEN" format="default"/>).

        </li>
          <li>
	  Improved compatibility with Microsoft Windows for
	  Access Control Lists (<xref target="attrdef_sacl" format="default"/>, <xref target="attrdef_dacl" format="default"/>, <xref target="auto_inherit" format="default"/>).

        </li>
          <li>
          Data retention (<xref target="retention" format="default"/>).

        </li>
          <li>
          Identification of the implementation of the NFS client
          and server (<xref target="OP_EXCHANGE_ID" format="default"/>).

        </li>
          <li>
	  Support for notification of the availability of
	  byte-range locks (see the new
	  OPEN4_RESULT_MAY_NOTIFY_LOCK reply flag in <xref target="OP_OPEN" format="default"/> and see <xref target="OP_CB_NOTIFY_LOCK" format="default"/>).

        </li>
          <li>
          In NFSv4.1, LIPKEY and SPKM-3 are not required security mechanisms
          <xref target="RFC2847" format="default"/>.
        </li>
        </ul>
      </section>
    </section>
    <section anchor="Core_Infrastructure" numbered="true" toc="default">
      <name>Core Infrastructure</name>
      <section anchor="Introduction" numbered="true" toc="default">
        <name>Introduction</name>
        <t>
  NFSv4.1 relies on core infrastructure common to nearly
  every operation. This core infrastructure is described in the remainder
  of this section.
        </t>
      </section>
      <!-- Introduction -->

 <section anchor="RPC_and_XDR" numbered="true" toc="default">
        <name>RPC and XDR</name>
        <t>
  The NFSv4.1 protocol is a Remote Procedure Call (RPC)
  application that uses RPC version 2 and the corresponding eXternal
  Data Representation (XDR) as defined in
  <xref target="RFC5531" format="default"/> and
  <xref target="RFC4506" format="default"/>.
        </t>
        <section anchor="RPC-based_Security" numbered="true" toc="default">
          <name>RPC-Based Security</name>
          <t>
   Previous NFS versions have been thought of as having a
   host-based authentication model, where the NFS server
   authenticates the NFS client, and trusts the client
   to authenticate all users.
   Actually, NFS has always depended on RPC for
   authentication. One of the first forms of RPC authentication,
   AUTH_SYS, had no strong authentication and
   required a host-based authentication
   approach. NFSv4.1 also depends on RPC for basic security
   services and mandates RPC support for a user-based
   authentication model. The user-based authentication
   model has user principals authenticated by a server, and
   in turn the server authenticated by user principals.
   RPC provides some basic security services that are used
   by NFSv4.1.
          </t>
          <section anchor="RPC_Security_Flavors" numbered="true" toc="default">
            <name>RPC Security Flavors</name>
            <t>
     As described in Section 7.2 ("Authentication") of <xref target="RFC5531" format="default"/>,
     RPC security is encapsulated in the RPC header, via a
     security or authentication flavor, and information
     specific to the specified security flavor.
     Every RPC header conveys information used to identify
     and authenticate a client and server. As discussed in
     <xref target="RPCSEC_GSS_and_Security_Services" format="default"/>,
     some security flavors provide additional security
     services.
            </t>
            <t>
     NFSv4.1 clients and servers MUST implement RPCSEC_GSS.
     (This requirement to implement is not a requirement to
     use.)  Other flavors, such as AUTH_NONE and
     AUTH_SYS, MAY be implemented as well.
            </t>
            <section anchor="RPCSEC_GSS_and_Security_Services" numbered="true" toc="default">
              <name>RPCSEC_GSS and Security Services</name>
              <t>
      RPCSEC_GSS <xref target="RFC2203" format="default"/> uses the
      functionality of GSS-API <xref target="RFC2743" format="default"/>.  This allows for the
      use of various security mechanisms by the RPC layer
      without the additional implementation overhead of
      adding RPC security flavors.
              </t>
              <section anchor="Authentication_Integrity_Privacy" numbered="true" toc="default">
                <name>Identification, Authentication, Integrity, Privacy</name>
                <t>
      Via the GSS-API, RPCSEC_GSS can be used to identify and authenticate
      users on clients to servers, and servers to users. It can also
      perform integrity checking on the entire RPC message, including
      the RPC header, and on the arguments or results. Finally, privacy,
      usually via encryption, is a service available with RPCSEC_GSS.
      Privacy is performed on the arguments and results. Note that
      if privacy is selected, integrity, authentication, and identification
      are enabled.
      If privacy is not selected, but integrity is selected, authentication
      and identification are enabled. If integrity and privacy are not
      selected, but authentication is enabled,
      identification is enabled. RPCSEC_GSS does not provide identification as
      a separate service.
                </t>
                <t>
      Although GSS-API has an authentication service distinct from its
      privacy and integrity services, GSS-API's
      authentication service is not used for RPCSEC_GSS's authentication
      service. Instead, each RPC request and response header is
      integrity protected with the GSS-API integrity service, and
      this allows RPCSEC_GSS to offer per-RPC authentication and
      identity. See <xref target="RFC2203" format="default"/> for more information.
                </t>
                <t>
      NFSv4.1 client and servers MUST support RPCSEC_GSS's integrity and authentication
      service. NFSv4.1 servers MUST support RPCSEC_GSS's privacy service.
      NFSv4.1 clients SHOULD support  RPCSEC_GSS's privacy service.

                </t>
              </section>
              <!-- Identity, Authentication, Integrity, Privacy -->

     <section anchor="security_mechs" numbered="true" toc="default">
                <name>Security Mechanisms for NFSv4.1</name>
                <t>
      RPCSEC_GSS, via GSS-API, normalizes access to mechanisms that
      provide security services. Therefore, NFSv4.1 clients and servers
      MUST support the Kerberos V5 security mechanism.
                </t>
                <t>
      The use of RPCSEC_GSS requires selection of mechanism,
      quality of protection (QOP), and service (authentication,
      integrity, privacy).  For the mandated security mechanisms,
      NFSv4.1 specifies that a QOP of zero is used, leaving it up
      to the mechanism or the mechanism's configuration to map
      QOP zero to
      an appropriate level of protection.
      Each mandated mechanism specifies a minimum set of cryptographic
      algorithms for implementing integrity and privacy. NFSv4.1
      clients and servers MUST be implemented on operating environments
      that comply with the REQUIRED cryptographic algorithms
      of each REQUIRED mechanism.
                </t>
                <section anchor="kerberosv5" numbered="true" toc="default">
                  <name>Kerberos V5</name>
                  <t>
       The Kerberos V5 GSS-API mechanism as described in
       <xref target="RFC4121" format="default"/> MUST be implemented with
       the RPCSEC_GSS services as specified in the following
       table:
                  </t>
                  <artwork name="" type="" align="left" alt=""><![CDATA[
   column descriptions:
   1 == number of pseudo flavor
   2 == name of pseudo flavor
   3 == mechanism's OID
   4 == RPCSEC_GSS service
   5 == NFSv4.1 clients MUST support
   6 == NFSv4.1 servers MUST support

   1      2        3                    4                     5   6
   ------------------------------------------------------------------
   390003 krb5     1.2.840.113554.1.2.2 rpc_gss_svc_none      yes yes
   390004 krb5i    1.2.840.113554.1.2.2 rpc_gss_svc_integrity yes yes
   390005 krb5p    1.2.840.113554.1.2.2 rpc_gss_svc_privacy    no yes
      ]]></artwork>
                  <t>
       Note that the number and name of the pseudo flavor
       are presented here as a mapping aid to the implementor.
       Because the NFSv4.1 protocol includes a method to negotiate
       security and it understands the GSS-API mechanism, the pseudo flavor
       is not needed.  The pseudo flavor is needed for the NFSv3 since the security negotiation is done via
       the MOUNT protocol as described in <xref target="RFC2623" format="default"/>.
                  </t>
                  <t>
       At the time NFSv4.1 was specified, the Advanced Encryption
       Standard (AES) with HMAC-SHA1 was
       a REQUIRED algorithm set for Kerberos V5. In contrast, when
       NFSv4.0 was specified, weaker algorithm sets were REQUIRED for
       Kerberos V5, and were REQUIRED in the NFSv4.0 specification, because
       the Kerberos V5 specification at the time did not specify stronger
       algorithms.
       The NFSv4.1 specification does not specify REQUIRED algorithms
       for Kerberos V5, and instead, the implementor is expected
       to track the evolution of the Kerberos V5 standard if and when
       stronger algorithms are specified.

                  </t>
                  <section anchor="krb5_sec_consider" numbered="true" toc="default">
                    <name>Security Considerations for Cryptographic Algorithms in Kerberos V5</name>
                    <t>
          When deploying NFSv4.1, the strength of the security achieved depends
          on the existing Kerberos V5 infrastructure. The algorithms
          of Kerberos V5 are not directly exposed to or selectable by the
          client or server, so there is some due diligence required by
          the user of NFSv4.1 to ensure that security is acceptable
          where needed.
                    </t>
                  </section>
                </section>
                <!-- Kerberos V5  -->

      </section>
              <!-- Security mechanisms for NFSv4.1  -->

     <section anchor="GSS_Server_Principal" numbered="true" toc="default">
                <name>GSS Server Principal</name>
                <t>
      Regardless of what security mechanism under RPCSEC_GSS
      is being used, the NFS server MUST identify itself
      in GSS-API via a GSS_C_NT_HOSTBASED_SERVICE name type.
      GSS_C_NT_HOSTBASED_SERVICE names are of the form:
                </t>
                <artwork name="" type="" align="left" alt=""><![CDATA[
     service@hostname
     ]]></artwork>
                <t>
      For NFS, the "service" element is
                </t>
                <artwork name="" type="" align="left" alt=""><![CDATA[
     nfs
     ]]></artwork>
                <t>
      Implementations of security mechanisms will convert
      nfs@hostname to various different forms.  For Kerberos
      V5, the following form is RECOMMENDED:
                </t>
                <artwork name="" type="" align="left" alt=""><![CDATA[
     nfs/hostname
     ]]></artwork>
              </section>
              <!-- GSS Server Principal -->
    </section>
            <!-- RPCSEC_GSS and Security Services -->
   </section>
          <!-- RPC Security Flavors -->
  </section>
        <!-- RPC-based Security -->
 </section>
      <!-- RPC and XDR -->

 <section anchor="COMPOUND_and_CB_COMPOUND" numbered="true" toc="default">
        <name>COMPOUND and CB_COMPOUND</name>
        <t>
   A significant departure from the versions of the NFS
   protocol before NFSv4 is the introduction of the
   COMPOUND procedure.  For the NFSv4 protocol,
   in all minor versions, there are exactly two RPC procedures,
   NULL and COMPOUND.  The COMPOUND procedure is defined
   as a series of individual operations and these operations
   perform the sorts of functions performed by traditional
   NFS procedures.
        </t>
        <t>
   The operations combined within a COMPOUND
   request are evaluated in order by the server, without
   any atomicity guarantees.  A limited set of facilities
   exist to pass results from one operation to another.  Once an
   operation returns a failing result, the evaluation ends
   and the results of all
   evaluated operations are returned to the client.
        </t>
        <t>
   With the use of the COMPOUND procedure, the client is able to build
   simple or complex requests.  These COMPOUND requests allow for a
   reduction in the number of RPCs needed for logical file system
   operations.  For example, multi-component look up requests can
   be constructed by combining multiple LOOKUP operations.  Those
   can be further combined with operations such as GETATTR, READDIR,
   or OPEN plus READ to do more complicated sets of operation without
   incurring additional latency.
        </t>
        <t>
   NFSv4.1 also contains a considerable set of
   callback operations in which the server makes an RPC
   directed at the client.  Callback RPCs have a similar
   structure to that of the normal server requests.
   In all minor versions of the NFSv4 protocol,
   there are two callback RPC procedures:
   CB_NULL and CB_COMPOUND.  The CB_COMPOUND procedure is defined
   in an analogous fashion to that of COMPOUND
   with its own set of callback operations.
        </t>
        <t>
   The addition of new server and callback operations within the
   COMPOUND and CB_COMPOUND request
   framework provides a means of extending the protocol in
   subsequent minor versions.
        </t>
        <t>
   Except for a small number of operations needed for session
   creation, server requests and callback requests are performed
   within the context of a session.  Sessions provide a client
   context for every request and support robust replay
   protection for non-idempotent requests.
        </t>
      </section>
      <!-- COMPOUND and CB_COMPOUND -->

 <section anchor="Client_Identifiers" numbered="true" toc="default">
        <name>Client Identifiers and Client Owners</name>
        <t>
    For each operation that obtains or depends on locking state, the
    specific client needs to be identifiable by the server.

        </t>
        <t>
    Each distinct client instance is represented
    by a client ID.  A client ID is a 64-bit identifier
    representing a specific client at a given time.
    The client ID is changed whenever the client re-initializes,
    and may change when the server re-initializes.
    Client IDs are used to support lock identification
    and crash recovery.

        </t>
        <t>
    During steady state operation,
    the client ID associated with each operation
    is derived from the session (see <xref target="Session" format="default"/>) on which the operation is sent. A session is associated with
    a client ID when the session is created.
        </t>
        <t>
    Unlike NFSv4.0, the only NFSv4.1 operations possible before a
    client ID is established are those needed to
    establish the client ID.
        </t>
        <t>
    A sequence of an EXCHANGE_ID operation followed by a
    CREATE_SESSION operation using that client ID
    (eir_clientid as returned from EXCHANGE_ID)
    is required to establish and confirm the
    client ID on the server.  Establishment of identification by a
    new incarnation of the client also has the effect of immediately
    releasing any locking state that a previous incarnation of that
    same client might have had on the server.  Such released state
    would include all byte-range lock, share reservation, layout state, and -- where the server supports neither the CLAIM_DELEGATE_PREV nor CLAIM_DELEG_CUR_FH claim types -- all delegation state associated with the same client with the same
    identity. For discussion of delegation state recovery, see
    <xref target="delegation_recovery" format="default"/>. For discussion of layout state
    recovery, see <xref target="pnfs_client_recovery" format="default"/>.
        </t>
        <t>
    Releasing such state requires that the server be able to determine
    that one client instance is the successor of another.  Where this
    cannot be done, for any of a number of reasons, the locking state
    will remain for a time subject to lease expiration
    (see <xref target="lease_renewal" format="default"/>)
    and the new client will need to wait for
    such state to be removed, if it makes conflicting lock requests.
        </t>
        <t>
    Client identification is encapsulated in the following client owner
    data type:
        </t>
        <artwork name="" type="" align="left" alt=""><![CDATA[
struct client_owner4 {
        verifier4       co_verifier;
        opaque          co_ownerid<NFS4_OPAQUE_LIMIT>;
};
 ]]></artwork>
        <t>
    The first field, co_verifier, is a client incarnation
    verifier, allowing the server to distinguish successive incarnations
    (e.g. reboots) of the same client.  The server will start the process of
    canceling the client's leased state if co_verifier
    is different than what the server has previously
    recorded for the identified client (as specified in
    the co_ownerid field).

        </t>
        <t>
    The second field, co_ownerid, is a variable length string that uniquely defines
    the client so that subsequent instances of the same client bear the
    same co_ownerid with a different verifier.
        </t>
        <t>
    There are several considerations for how the client
    generates the co_ownerid string:
        </t>
        <ul spacing="normal">
          <li>
        The string should be unique so that multiple clients
        do not present the same string. The consequences of
        two clients presenting the same string range from
        one client getting an error to one client having its
        leased state abruptly and unexpectedly cancelled.
      </li>
          <li>
        The string should be selected so that subsequent incarnations
        (e.g., restarts) of the same client cause the client to present
        the same string. The implementor
        is cautioned from an approach that requires the string to
        be recorded in a local file because this precludes the use
        of the implementation in an environment where there is no local
        disk and all file access is from an NFSv4.1 server.
      </li>
          <li>
        The string should be the same for each server network address that
        the client accesses.
        This way, if a server has multiple interfaces, the client
        can trunk traffic over multiple network paths
        as described in <xref target="Trunking" format="default"/>.
        (Note: the precise opposite was advised in the NFSv4.0
        specification <xref target="RFC3530" format="default"/>.)
      </li>
          <li>
        The algorithm for generating the string should not
        assume that the client's network address will not
        change, unless the client implementation knows it
        is using statically assigned network addresses.
        This includes changes between client incarnations
        and even changes while the client is still running
        in its current incarnation.  Thus, with dynamic
        address assignment, if the
        client includes just the client's network address
        in the co_ownerid string, there is a real risk
        that after the
        client gives up the network address, another
        client, using a similar algorithm for generating
        the co_ownerid string, would generate a conflicting
        co_ownerid string.

      </li>
        </ul>
        <t>
    Given the above considerations, an example of a well-generated co_ownerid
    string is one that includes:
        </t>
        <ul spacing="normal">
          <li>
        If applicable, the client's statically assigned network address.
      </li>
          <li>
            <t>
        Additional information that tends to be unique, such as one or more
        of:
            </t>
            <ul spacing="normal">
              <li>
            The client machine's serial number (for privacy reasons, it is best
            to perform some one-way function on the serial number).
          </li>
              <li>
            A Media Access Control (MAC) address (again, a one-way function should be performed).
          </li>
              <li>
            The timestamp of when the NFSv4.1 software was first installed
            on the client (though this is subject to the previously mentioned
            caution about using information that is stored in a file, because the
            file might only be accessible over NFSv4.1).
          </li>
              <li>
            A true random number. However, since this number ought to be the same
            between client incarnations, this shares the same problem as that of
            using the timestamp of the software installation.
          </li>
            </ul>
          </li>
          <li>
        For a user-level NFSv4.1 client, it should contain additional
        information to distinguish the client from other user-level clients
        running on the same host, such as a process identifier or other unique
        sequence.
      </li>
        </ul>
        <t>
    The client ID is assigned by the server (the eir_clientid result from EXCHANGE_ID)
    and should be chosen so that it will not
    conflict with a client ID previously assigned by the
    server.  This applies across server restarts.
        </t>
        <t>
    In the event of a server restart, a client may find
    out that its current client ID is no longer valid when
    it receives an NFS4ERR_STALE_CLIENTID error.  The precise
    circumstances depend on the characteristics of the
    sessions involved, specifically whether the session is
    persistent (see <xref target="Persistence" format="default"/>), but in
    each case the client will receive this error when it attempts
    to establish a new session with the existing client ID and
    receives the error NFS4ERR_STALE_CLIENTID, indicating that a new
    client ID needs to be obtained via EXCHANGE_ID and the new session
    established with that client ID.

        </t>
        <t>
    When a session is not persistent, the client will find out that
    it needs to create a new session as a result of getting an
    NFS4ERR_BADSESSION, since the session in question was lost
    as part of a server restart.  When the existing client ID is
    presented to a server as part of creating a session
    and that client ID is not recognized, as would happen after a server
    restart, the server will reject the request with the error
    NFS4ERR_STALE_CLIENTID.
        </t>
        <t>
    In the case of the session being persistent, the
    client will re-establish communication using the
    existing session after the restart.  This session
    will be associated with the existing client ID but
    may only be used to retransmit operations that the
    client previously transmitted and did not see replies
    to. Replies to operations that the server previously performed
    will come from the reply cache; otherwise,
    NFS4ERR_DEADSESSION will be returned.
    Hence, such a session is referred to as "dead". In this situation,
    in order to perform new operations, the client needs to
    establish a new session.  If an attempt is made to
    establish this new session with the existing client ID,
    the server will reject the request with
    NFS4ERR_STALE_CLIENTID.
        </t>
        <t>
    When NFS4ERR_STALE_CLIENTID is received in either of
    these situations, the client needs to obtain a
    new client ID by use of the EXCHANGE_ID operation, then
    use that client ID as the basis of a new session, and
    then proceed to
    any other necessary recovery for the server restart case (see
    <xref target="server_failure" format="default"/>).
        </t>
        <t>
    See the descriptions of EXCHANGE_ID
    (<xref target="OP_EXCHANGE_ID" format="default"/>) and CREATE_SESSION
    (<xref target="OP_CREATE_SESSION" format="default"/>) for a complete
    specification of these operations.
        </t>
        <section numbered="true" toc="default">
          <name>Upgrade from NFSv4.0 to NFSv4.1</name>
          <t>
    To facilitate upgrade from NFSv4.0 to NFSv4.1, a server
    may compare a value of data type client_owner4 in an EXCHANGE_ID with a
    value of data type nfs_client_id4 that was established using the SETCLIENTID operation of
    NFSv4.0. A server that does so will allow
    an upgraded client to avoid waiting
    until the lease (i.e., the lease established by the NFSv4.0 instance
    client) expires.
    This requires that the value of data type client_owner4 be constructed
    the same way as the value of data type nfs_client_id4.  If the latter's
    contents included the server's network address (per the
    recommendations of the NFSv4.0 specification <xref target="RFC3530" format="default"/>), and
    the NFSv4.1 client does not wish to use a client
    ID that prevents trunking, it should send two
    EXCHANGE_ID operations.  The first EXCHANGE_ID will
    have a client_owner4 equal to the nfs_client_id4.
    This will clear the state created by the NFSv4.0
    client. The second EXCHANGE_ID will not have the
    server's network address. The state created for the
    second EXCHANGE_ID will not have to wait for lease
    expiration, because there will be no state to expire.

          </t>
        </section>
        <section numbered="true" toc="default">
          <name>Server Release of Client ID</name>
          <t>
     NFSv4.1 introduces a new operation called
     DESTROY_CLIENTID (<xref target="OP_DESTROY_CLIENTID" format="default"/>),
     which the client SHOULD use to destroy a client ID it
     no longer needs. This permits graceful, bilateral release of
     a client ID. The operation cannot be used if there are sessions
     associated with the client ID, or state with an unexpired lease.
          </t>
          <t>
     If the server determines that the client holds no associated state
     for its client ID (associated state includes unrevoked sessions,
     opens, locks, delegations, layouts, and wants), the server MAY
     choose to unilaterally release the client ID in order to
     conserve resources.

     If the client
     contacts the server after this release, the server
     MUST ensure that the client receives the appropriate error
     so that it will use the EXCHANGE_ID/CREATE_SESSION
     sequence to establish a new client ID.
     The server ought to be very hesitant to
     release a client ID since the resulting work on the
     client to recover from such an event will be the same
     burden as if the server had failed and restarted.
     Typically, a server would not release a client ID
     unless there had been no activity from that client
     for many minutes.  As long as there are sessions,
     opens, locks, delegations, layouts, or wants, the
     server MUST NOT release the client ID. See <xref target="loss_of_session" format="default"/> for discussion on
     releasing inactive sessions.

          </t>
        </section>
        <!-- Server Release of Client ID -->
   <section anchor="cowner_conflicts" numbered="true" toc="default">
          <name>Resolving Client Owner Conflicts</name>
          <t>
     When the server gets an EXCHANGE_ID for a client owner that
     currently has no state, or that has state but the lease has expired,
     the server MUST allow the
     EXCHANGE_ID and confirm the new client ID if followed by the
     appropriate CREATE_SESSION.
          </t>
          <t>
     When the server gets an EXCHANGE_ID for a
     new incarnation of a client owner that
     currently has an old incarnation with state and an unexpired lease, the
     server is allowed to dispose of the state of the
     previous incarnation of the client owner if
     one of the following is true:

          </t>
          <ul spacing="normal">
            <li>
       The principal that created the client ID for the client owner
       is the same as the principal that is sending the EXCHANGE_ID operation.
       Note that if the client ID was created with
       SP4_MACH_CRED state protection (<xref target="OP_EXCHANGE_ID" format="default"/>),
       the principal MUST be based on RPCSEC_GSS authentication,
       the RPCSEC_GSS service used MUST be integrity or
       privacy, and the
       same GSS mechanism and principal
       MUST be used as that used when the client ID
       was created.
     </li>
            <li>
       The client ID was established with SP4_SSV
       protection (<xref target="OP_EXCHANGE_ID" format="default"/>,

    <xref target="protect_state_change" format="default"/>)

       and the client sends the EXCHANGE_ID with the
       security flavor set to RPCSEC_GSS using the GSS
       SSV mechanism (<xref target="ssv_mech" format="default"/>).

     </li>
            <li>
       The client ID was established with SP4_SSV
       protection, and under the conditions described herein,
       the EXCHANGE_ID was sent with SP4_MACH_CRED state protection.
       Because the SSV might not persist
       across client and server restart, and because
       the first time a client sends EXCHANGE_ID to
       a server it does not have an SSV, the client
       MAY send the subsequent EXCHANGE_ID without
       an SSV RPCSEC_GSS handle.  Instead, as with
       SP4_MACH_CRED protection, the principal MUST be
       based on RPCSEC_GSS authentication, the RPCSEC_GSS
       service used MUST be integrity or privacy, and the
       same GSS mechanism and principal MUST be used as
       that used when the client ID was created.

     </li>
          </ul>
          <t>
     If none of the above situations apply, the server
     MUST return NFS4ERR_CLID_INUSE.

          </t>
          <t>
     If the server accepts the principal and co_ownerid
     as matching that which created the client ID, and
     the co_verifier in the EXCHANGE_ID differs from the
     co_verifier used when the client ID was created,
     then after the server receives a CREATE_SESSION that
     confirms the client ID, the server deletes state.

     If the co_verifier values are the same (e.g., the
     client either is updating properties of the client ID
     (<xref target="OP_EXCHANGE_ID" format="default"/>) or
     is attempting trunking (<xref target="Trunking" format="default"/>),
     the server MUST NOT delete state.

          </t>
        </section>
        <!-- Handling Client Owner Conflicts -->
 </section>
      <!-- Client Identifiers -->
 <section anchor="Server_Owners" numbered="true" toc="default">
        <name>Server Owners</name>
        <t>
  The server owner is similar to a client owner
  (<xref target="Client_Identifiers" format="default"/>), but unlike the
  client owner, there is no shorthand server ID.
  The server owner is defined in the following data type:
        </t>
        <artwork name="" type="" align="left" alt=""><![CDATA[
struct server_owner4 {
 uint64_t       so_minor_id;
 opaque         so_major_id<NFS4_OPAQUE_LIMIT>;
};
 ]]></artwork>
        <t>
   The server owner is returned from
   EXCHANGE_ID. When the so_major_id fields are the same in
   two EXCHANGE_ID results, the connections that each EXCHANGE_ID
   were sent over can be assumed to address the same server
   (as defined in <xref target="intro_definitions" format="default"/>). If
   the so_minor_id fields are also the same, then not only
   do both connections connect to the same server, but the
   session can be shared across both
   connections. The reader is cautioned that multiple
   servers may deliberately or accidentally claim to have
   the same so_major_id or so_major_id/so_minor_id; the
   reader should examine Sections <xref target="Trunking" format="counter"/> and
   <xref target="OP_EXCHANGE_ID" format="counter"/> in order to avoid
   acting on falsely matching server owner values.
        </t>
        <t>
   The considerations for generating an so_major_id are
   similar to that for generating a co_ownerid string (see
   <xref target="Client_Identifiers" format="default"/>). The consequences
   of two servers generating conflicting so_major_id values
   are less dire than they are for co_ownerid conflicts
   because the client can use RPCSEC_GSS to compare the
   authenticity of each server
   (see <xref target="Trunking" format="default"/>).
        </t>
      </section>
      <!-- Server Owners -->

 <section anchor="Security_Service_Negotiation" numbered="true" toc="default">
        <name>Security Service Negotiation</name>
        <t>
    With the NFSv4.1 server potentially offering
    multiple security mechanisms, the client needs a method
    to determine or negotiate which mechanism is to be
    used for its communication with the server.  The NFS
    server may have multiple points within its file system
    namespace that are available for use by NFS clients.
    These points can be considered security policy boundaries,
    and, in some NFS implementations, are tied to NFS export points.
    In turn, the NFS server may be configured such that each
    of these security policy boundaries may have different or multiple
    security mechanisms in use.
        </t>
        <t>
    The security negotiation between client and server
    SHOULD be done with a secure channel to eliminate
    the possibility of a third party intercepting the
    negotiation sequence and forcing the client and server
    to choose a lower level of security than required or
    desired.  See
    <xref target="SECCON" format="default"/> for further discussion.
        </t>
        <section anchor="NFSv4_Security_Tuples" numbered="true" toc="default">
          <name>NFSv4.1 Security Tuples</name>
          <t>
   An NFS server can assign one or more "security tuples" to each
   security policy boundary in its namespace. Each security tuple
   consists of a security flavor
   (see <xref target="RPC_Security_Flavors" format="default"/>) and, if the flavor
   is RPCSEC_GSS, a GSS-API mechanism Object Identifier (OID), a GSS-API quality of
   protection, and an RPCSEC_GSS service.
          </t>
        </section>
        <!-- NFSv4.1 Security Tuples -->

  <section anchor="SECINFO_and_SECINFO_NO_NAME" numbered="true" toc="default">
          <name>SECINFO and SECINFO_NO_NAME</name>
          <t>
   The SECINFO and SECINFO_NO_NAME operations allow the client to
   determine, on a per-filehandle basis, what security tuple is to be
   used for server access.  In general, the client will not have to
   use either operation except during initial communication with the
   server or when the client crosses security policy boundaries at the
   server.  However, the server's policies may also change at any time
   and force the client to negotiate a new security tuple.
          </t>
          <t>
   Where the use of different security tuples would affect the type of
   access that would be allowed if a request was sent over the same
   connection used for the SECINFO or SECINFO_NO_NAME operation
   (e.g., read-only vs. read-write) access, security tuples that allow
   greater access should be presented first.  Where the general level
   of access is the same and different security flavors limit the
   range of principals whose privileges are recognized (e.g., allowing
   or disallowing root access), flavors supporting the greatest range
   of principals should be listed first.
          </t>
        </section>
        <!-- SECINFO and SECINFO_NO_NAME -->

  <section anchor="Security_Error" numbered="true" toc="default">
          <name>Security Error</name>
          <t>
   Based on the assumption that each NFSv4.1 client
   and server MUST support a minimum set of security (i.e.,
   Kerberos V5 under RPCSEC_GSS),
   the NFS client will initiate file access to the server
   with one of the minimal security tuples.  During
   communication with the server, the client may receive an
   NFS error of NFS4ERR_WRONGSEC.  This error allows the
   server to notify the client that the security tuple
   currently being used contravenes the server's
   security policy. The client is then responsible for
   determining (see <xref target="using_secinfo" format="default"/>) what
   security tuples are available at the server and choosing
   one that is appropriate for the client.

          </t>
          <section anchor="using_secinfo" numbered="true" toc="default">
            <name>Using NFS4ERR_WRONGSEC, SECINFO, and SECINFO_NO_NAME</name>
            <t>
   This section explains the mechanics of NFSv4.1 security negotiation.
            </t>
            <section anchor="putfh_series" numbered="true" toc="default">
              <name>Put Filehandle Operations</name>
              <t>

   The term "put filehandle operation" refers to
   PUTROOTFH, PUTPUBFH, PUTFH, and RESTOREFH. Each of the subsections
   herein describes how the server handles a subseries of operations
   that starts with a put filehandle operation.
              </t>
              <section anchor="PUTFHplusSAVEFH" numbered="true" toc="default">
                <name>Put Filehandle Operation + SAVEFH</name>
                <t>
    The client is saving a filehandle for a future
    RESTOREFH, LINK, or RENAME.  SAVEFH MUST NOT
    return NFS4ERR_WRONGSEC. To determine whether or not the put
    filehandle operation returns NFS4ERR_WRONGSEC,
    the server implementation pretends SAVEFH is not in
    the series of operations and examines which of the
    situations described in the other subsections of <xref target="putfh_series" format="default"/> apply.

                </t>
              </section>
              <!-- Put Filehandle Operation + SAVEFH -->
   <section anchor="PUTFHplusPUTFH" numbered="true" toc="default">
                <name>Two or More Put Filehandle Operations</name>
                <t>
    For a series of N put filehandle operations, the server
    MUST NOT return NFS4ERR_WRONGSEC to the first N-1 put
    filehandle operations.

The Nth put filehandle operation
    is handled as if it is the first in a subseries of
    operations.
    For example, if the
    server received a COMPOUND request with this series of
    operations -- PUTFH, PUTROOTFH, LOOKUP -- then the
    PUTFH operation is ignored for NFS4ERR_WRONGSEC purposes, and the
    PUTROOTFH, LOOKUP subseries is processed as according
    to <xref target="PUTFHplusLOOKUP" format="default"/>.
                </t>
              </section>
              <!-- PUTFH + PUTFH -->
   <section anchor="PUTFHplusLOOKUP" numbered="true" toc="default">
                <name>Put Filehandle Operation + LOOKUP (or OPEN of an Existing Name)</name>
                <t>
    This situation also applies to a put filehandle operation followed
    by a LOOKUP or an OPEN operation that specifies an existing component name.
                </t>
                <t>
    In this situation, the client is potentially crossing
    a security policy boundary, and the set of security tuples
    the parent directory supports may differ from those of
    the child.
    The server implementation may decide whether to impose
    any restrictions on security policy administration.
    There are at least three approaches (sec_policy_child is
    the tuple set of the child export, sec_policy_parent is
    that of the parent).
                </t>
                <ol spacing="normal" type="(%c)">
                  <li>
     sec_policy_child &lt;= sec_policy_parent (&lt;= for subset).  This
     means that the set of security tuples specified on the
     security policy of a child directory is always a subset
     of its parent directory.
   </li>
                  <li>
    sec_policy_child ^ sec_policy_parent != {} (^ for intersection, {}
    for the empty set). This means that the set of security tuples specified
    on the security policy of a child directory always has a non-empty intersection
    with that of the parent.
   </li>
                  <li>
    sec_policy_child ^ sec_policy_parent == {}.  This means that the
    set of security tuples specified on the security policy of a child directory
    may not intersect with that of the parent. In other words, there
    are no restrictions on how the system administrator may
    set up these tuples.
   </li>
                </ol>
                <t>
    In order for a server to support approaches (b)
    (for the case when a client chooses a flavor that is
    not a member of sec_policy_parent) and (c), the put
    filehandle operation cannot return NFS4ERR_WRONGSEC
    when there is a security tuple mismatch.  Instead,
    it should be returned from the LOOKUP (or OPEN by
    existing component name) that follows.

                </t>
                <t>
    Since the above guideline does not contradict approach
    (a), it should be followed in general. Even if approach
    (a) is implemented, it is possible for the security
    tuple used to be acceptable for the target of LOOKUP
    but not for the filehandles used in the put filehandle operation. The
    put filehandle operation
    could be a PUTROOTFH or PUTPUBFH, where the
    client cannot know the security tuples for the root
    or public filehandle. Or the security policy for the
    filehandle used by the put filehandle operation
    could have changed since the
    time the filehandle was obtained.
                </t>
                <t>
    Therefore, an NFSv4.1 server MUST NOT return NFS4ERR_WRONGSEC
    in response to the put filehandle operation
    if the operation
    is immediately followed by a LOOKUP or an OPEN by component name.
                </t>
              </section>
              <!-- PUTFH + LOOKUP -->

   <section anchor="PUTFHplusLOOKUPP" numbered="true" toc="default">
                <name>Put Filehandle Operation + LOOKUPP</name>
                <t>
    Since SECINFO only works its way down, there is no way LOOKUPP can
    return NFS4ERR_WRONGSEC without SECINFO_NO_NAME. SECINFO_NO_NAME
    solves this issue via style
    SECINFO_STYLE4_PARENT, which works in the opposite direction as SECINFO.
    As with <xref target="PUTFHplusLOOKUP" format="default"/>, a put filehandle operation
    that is followed by a LOOKUPP
    MUST NOT return NFS4ERR_WRONGSEC.
    If the server does not support SECINFO_NO_NAME, the client's
    only recourse is to send the put filehandle operation,
    LOOKUPP, GETFH sequence
    of operations with every security tuple it supports.
                </t>
                <t>
    Regardless of whether SECINFO_NO_NAME is supported, an
    NFSv4.1 server  MUST NOT return NFS4ERR_WRONGSEC in
    response to a put filehandle operation if the
    operation is immediately followed by a LOOKUPP.
                </t>
              </section>
              <!-- PUTFH + LOOKUPP -->

   <section anchor="PUTFHplusSECINFO" numbered="true" toc="default">
                <name>Put Filehandle Operation + SECINFO/SECINFO_NO_NAME</name>
                <t>
    A security-sensitive client is allowed to choose
    a strong security tuple when querying a server to
    determine a file object's permitted security tuples.
    The security tuple chosen by the client does not have
    to be included in the tuple list of the security policy
    of either the parent directory indicated in the put filehandle
    operation or the child file object indicated in SECINFO (or any parent directory
    indicated in SECINFO_NO_NAME). Of course, the server has to be
    configured for whatever security
    tuple the client selects; otherwise, the request will
    fail at the RPC layer with an appropriate authentication error.
                </t>
                <t>
    In theory, there is no connection between the security
    flavor used by SECINFO or SECINFO_NO_NAME and those
    supported by the security policy.  But in practice, the
    client may start looking for strong flavors from those
    supported by the security policy, followed by those in
    the REQUIRED set.
                </t>
                <t>
    The NFSv4.1 server MUST NOT return NFS4ERR_WRONGSEC to a
    put filehandle operation that
    is immediately followed by SECINFO or SECINFO_NO_NAME.
    The NFSv4.1 server MUST NOT return NFS4ERR_WRONGSEC from SECINFO or
    SECINFO_NO_NAME.
                </t>
              </section>
              <!-- PUTFH + SECINFO -->

   <section anchor="PUTFHplusNothing" numbered="true" toc="default">
                <name>Put Filehandle Operation + Nothing</name>
                <t>
    The NFSv4.1 server MUST NOT return NFS4ERR_WRONGSEC.
                </t>
              </section>
              <!-- PUTFH + Nothing -->

   <section anchor="PUTFHplusAnythingElse" numbered="true" toc="default">
                <name>Put Filehandle Operation + Anything Else</name>
                <t>
    "Anything Else" includes OPEN by filehandle.
                </t>
                <t>
    The security policy enforcement applies to the
    filehandle specified in the put filehandle operation. Therefore, the
    put filehandle operation MUST
    return NFS4ERR_WRONGSEC when there is a security tuple
    mismatch. This avoids the complexity of
    adding NFS4ERR_WRONGSEC as an allowable error to every
    other operation.
                </t>
                <t>
    A COMPOUND containing the series put filehandle
    operation + SECINFO_NO_NAME (style SECINFO_STYLE4_CURRENT_FH) is an
    efficient way for the client to recover from
    NFS4ERR_WRONGSEC.
                </t>
                <t>
    The NFSv4.1 server MUST NOT return NFS4ERR_WRONGSEC to
    any operation other than a put filehandle operation,
    LOOKUP, LOOKUPP, and OPEN (by component name).
                </t>
              </section>
              <!-- PUTFH + Anything Else -->

   <section anchor="aftersecinfo" numbered="true" toc="default">
                <name>Operations after SECINFO and SECINFO_NO_NAME</name>
                <t>

     Suppose a client sends a COMPOUND procedure
     containing the series SEQUENCE, PUTFH,
     SECINFO_NONAME, READ, and suppose the security tuple
     used does not match that required for the target
     file. By rule (see <xref target="PUTFHplusSECINFO" format="default"/>),
     neither PUTFH nor SECINFO_NO_NAME can
     return NFS4ERR_WRONGSEC. By rule (see <xref target="PUTFHplusAnythingElse" format="default"/>), READ cannot return
     NFS4ERR_WRONGSEC. The issue is resolved by the fact
     that SECINFO and SECINFO_NO_NAME consume the current
     filehandle (note that this is a change from NFSv4.0). This leaves no current filehandle for
     READ to use, and READ returns NFS4ERR_NOFILEHANDLE.

                </t>
              </section>
              <!-- Operations after SECINFO and SECINFO_NO_NAME" -->

  </section>
            <section anchor="link_rename" numbered="true" toc="default">
              <name>LINK and RENAME</name>
              <t>
   The LINK and RENAME operations use both the current
   and saved filehandles.
   Technically, the server MAY return NFS4ERR_WRONGSEC from
   LINK or RENAME
   if the security policy of the
   saved filehandle rejects the security flavor used in the
   COMPOUND request's credentials.  If the server does so,
   then if there is no intersection between the security
   policies of saved and current filehandles, this means that it
   will be impossible for the client to perform the intended
   LINK or RENAME operation.

              </t>
              <t>
   For example, suppose the client sends this COMPOUND
   request: SEQUENCE, PUTFH bFH, SAVEFH, PUTFH aFH,
   RENAME "c" "d", where filehandles bFH and aFH refer
   to different directories.  Suppose no common security
   tuple exists between the security policies of aFH and
   bFH. If the client sends the request using credentials
   acceptable to bFH's security policy but not aFH's
   policy, then the PUTFH aFH operation will fail with
   NFS4ERR_WRONGSEC. After a SECINFO_NO_NAME request,
   the client sends SEQUENCE, PUTFH bFH, SAVEFH, PUTFH
   aFH, RENAME "c" "d", using credentials acceptable to
   aFH's security policy but not bFH's policy. The server
   returns NFS4ERR_WRONGSEC on the RENAME operation.

              </t>
              <t>
   To prevent a client from an endless sequence of a
   request containing LINK or RENAME, followed by a request
   containing SECINFO_NO_NAME or SECINFO, the server MUST detect
   when the security policies of the current and saved
   filehandles have no mutually acceptable security tuple,
   and MUST NOT return NFS4ERR_WRONGSEC from LINK or RENAME
   in that situation. Instead
   the server MUST do one of two things:
              </t>
              <ul spacing="normal">
                <li>
    The server can return NFS4ERR_XDEV.
   </li>
                <li>
    The server can
    allow the security policy of the current filehandle to
    override that of the saved filehandle, and so return NFS4_OK.
   </li>
              </ul>
            </section>
          </section>
          <!-- Using NFS4ERR_WRONGSEC, SECINFO, and SECINFO_NO_NAME -->
 </section>
        <!-- Security Error -->
 </section>
      <!-- Security Service Negotiation -->

 <section anchor="minor_versioning" numbered="true" toc="default">
        <name>Minor Versioning</name>
        <t>
  To address the requirement of an NFS protocol that can evolve as the
  need arises, the NFSv4.1 protocol contains the rules and
  framework to allow for future minor changes or versioning.
        </t>
        <t>
  The base assumption with respect to minor versioning is that any
  future accepted minor version will be
  documented in one or more Standards Track RFCs.
  Minor version 0 of the NFSv4 protocol is represented by
  <xref target="RFC3530" format="default"/>, and minor version 1 is represented by
  this RFC.
  The COMPOUND and CB_COMPOUND
  procedures support the encoding of the minor version
  being requested by the client.
        </t>
        <t>
  The following items represent the basic rules for the development of
  minor versions.  Note that a future minor version may modify
  or add to the following rules as part of the minor version definition.
        </t>
        <ol spacing="normal" type="1">
          <li>
            <t>
  Procedures are not added or deleted.
            </t>
            <t>
  To maintain the general RPC model, NFSv4 minor versions will
  not add to or delete procedures from the NFS program.
            </t>
          </li>
          <li>
            <t>
  Minor versions may add operations to the COMPOUND and CB_COMPOUND
  procedures.
            </t>
            <t>
  The addition of operations to the COMPOUND and CB_COMPOUND procedures
  does not affect the RPC model.

            </t>
            <ul spacing="normal">
              <li>
                <t>
  Minor versions may append attributes to the bitmap4 that represents
  sets of attributes and to the fattr4 that represents sets of attribute
  values.
                </t>
                <t>
  This allows for the expansion of the attribute model to allow for
  future growth or adaptation.
                </t>
              </li>
              <li>
                <t>
  Minor version X must append any new attributes after the last
  documented attribute.
                </t>
                <t>
  Since attribute results are specified as an opaque array of
  per-attribute, XDR-encoded results, the complexity of adding new
  attributes in the midst of the current definitions would be too
  burdensome.
                </t>
              </li>
            </ul>
          </li>
          <li>
            <t>
 Minor versions must not modify the structure of an existing
 operation's arguments or results.
            </t>
            <t>
 Again, the complexity of handling multiple structure definitions for a
 single operation is too burdensome.  New operations should be added
 instead of modifying existing structures for a minor version.
            </t>
            <t>
 This rule does not preclude the following adaptations in a minor version:
            </t>
            <ul spacing="normal">
              <li>
 adding bits to flag fields, such as new attributes to GETATTR's bitmap4
 data type, and providing corresponding variants of opaque arrays,
 such as a notify4 used together with such bitmaps
 </li>
              <li>
 adding bits to existing attributes like ACLs that have flag words
 </li>
              <li>
 extending enumerated types (including NFS4ERR_*) with new values
 </li>
              <li>
 adding cases to a switched union
 </li>
            </ul>
          </li>
          <li>
 Minor versions must not modify the structure of existing attributes.
 </li>
          <li>
            <t>
 Minor versions must not delete operations.
            </t>
            <t>
 This prevents the potential reuse of a particular operation "slot" in
 a future minor version.
            </t>
          </li>
          <li>
 Minor versions must not delete attributes.
 </li>
          <li>
 Minor versions must not delete flag bits or enumeration values.
 </li>
          <li>
            <t>
 Minor versions may declare an operation MUST NOT be implemented.
            </t>
            <t>
 Specifying that an operation MUST NOT be implemented is equivalent
 to obsoleting an operation.  For the client, it means that the
 operation MUST NOT be sent to the server.  For the server, an NFS
 error can be returned as opposed to "dropping" the request as an XDR
 decode error.  This approach allows for the obsolescence of an
 operation while maintaining its structure so that a future minor version can reintroduce the operation.
            </t>
            <ol spacing="normal" type="1">
              <li>
 Minor versions may declare that an attribute MUST NOT be implemented.
 </li>
              <li>
 Minor versions may declare that a flag bit or enumeration value MUST NOT
 be implemented.
 </li>
            </ol>
          </li>
          <li>
 Minor versions may downgrade features from REQUIRED to RECOMMENDED,
 or RECOMMENDED to OPTIONAL.
 </li>
          <li>
 Minor versions may upgrade features from OPTIONAL to RECOMMENDED, or
 RECOMMENDED to REQUIRED.
 </li>
          <li>
 A client and server that support minor version X SHOULD support minor
 versions zero through X-1 as well.

 </li>
          <li>
            <t>
 Except for infrastructural changes, a minor version must not
 introduce REQUIRED new features.
            </t>
            <t>
 This rule allows for the introduction of new functionality and forces
 the use of implementation experience before designating a feature as
 REQUIRED. On the other hand, some classes of features are
 infrastructural and have broad effects. Allowing infrastructural features
 to be RECOMMENDED or OPTIONAL complicates implementation of the minor version.
            </t>
          </li>
          <li>
 A client MUST NOT attempt to use a stateid, filehandle, or similar
 returned object from the COMPOUND procedure with minor version X for
 another COMPOUND procedure with minor version Y, where X != Y.
 </li>
        </ol>
      </section>
      <!-- Minor Versioning -->

 <section anchor="Non-RPC-based_Security_Services" numbered="true" toc="default">
        <name>Non-RPC-Based Security Services</name>
        <t>
  As described in
  <xref target="Authentication_Integrity_Privacy" format="default"/>,
  NFSv4.1 relies on RPC for identification,
  authentication, integrity, and privacy. NFSv4.1 itself
  provides or enables additional security services as described in the
  next several subsections.
        </t>
        <section anchor="Authorization" numbered="true" toc="default">
          <name>Authorization</name>
          <t>
   Authorization to access a file object via an NFSv4.1
   operation is ultimately determined by the NFSv4.1
   server. A client can predetermine its access to a file
   object via the OPEN (<xref target="OP_OPEN" format="default"/>)
   and the ACCESS (<xref target="OP_ACCESS" format="default"/>)
   operations.
          </t>
          <t>
   Principals with appropriate access rights can modify the
   authorization on a file object via the SETATTR
   (<xref target="OP_SETATTR" format="default"/>) operation.  Attributes that affect
   access rights include mode, owner, owner_group, acl, dacl, and
   sacl. See <xref target="file_attributes" format="default"/>.
          </t>
        </section>
        <!-- Authorization -->

  <section anchor="Auditing" numbered="true" toc="default">
          <name>Auditing</name>
          <t>
   NFSv4.1 provides auditing on a per-file object basis, via the acl
   and sacl attributes as described in <xref target="acl" format="default"/>.  It is
   outside the scope of this specification to specify audit log
   formats or management policies.
          </t>
        </section>
        <!-- Auditing -->

  <section anchor="Intrusion_Detection" numbered="true" toc="default">
          <name>Intrusion Detection</name>
          <t>
   NFSv4.1 provides alarm control on a per-file object basis, via the
   acl and sacl attributes as described in <xref target="acl" format="default"/>.
   Alarms may serve as the basis for intrusion detection.  It is
   outside the scope of this specification to specify heuristics for
   detecting intrusion via alarms.
          </t>
        </section>
        <!-- Intrusion Detection -->
 </section>
      <!-- Non-RPC-based Security Services -->

 <section anchor="Transport_Layers" numbered="true" toc="default">
        <name>Transport Layers</name>
        <section anchor="Required_and_Recommended_Transport_Attributes" numbered="true" toc="default">
          <name>REQUIRED and RECOMMENDED Properties of Transports</name>
          <t>
   NFSv4.1 works over Remote Direct Memory Access (RDMA) and non-RDMA-based transports with
   the following attributes:
          </t>
          <ul spacing="normal">
            <li>
    The transport supports reliable delivery of data, which
    NFSv4.1 requires but neither NFSv4.1 nor RPC has facilities
    for ensuring  <xref target="Chet" format="default"/>.
   </li>
            <li>
    The transport delivers data in the order it was sent.
    Ordered delivery simplifies detection of transmit
    errors, and simplifies the sending of arbitrary sized
    requests and responses via the record marking
    protocol <xref target="RFC5531" format="default"/>.
   </li>
          </ul>
          <t>
   Where an NFSv4.1 implementation supports operation
   over the IP network protocol, any transport used between
   NFS and IP MUST be among the IETF-approved congestion
   control transport protocols.  At the time this document
   was written, the only two transports that had the above
   attributes were TCP and the Stream
   Control Transmission Protocol (SCTP).  To enhance the
   possibilities for interoperability, an NFSv4.1
   implementation MUST support operation over the TCP
   transport protocol.
          </t>
          <t>
   Even if NFSv4.1 is used over a non-IP network
   protocol, it is RECOMMENDED that the transport support
   congestion control.
          </t>
          <t>
   It is permissible for a connectionless transport to
   be used under NFSv4.1; however, reliable and in-order
   delivery of data combined with congestion control
   by the connectionless transport is
   REQUIRED.  As a consequence, UDP by itself MUST NOT be used
   as an NFSv4.1 transport. NFSv4.1 assumes that a client transport
   address and server transport address used to send data
   over a transport together constitute a connection,
   even if the underlying transport eschews the concept
   of a connection.

          </t>
        </section>
        <!-- Required and Recommended Transport Attributes -->

  <section anchor="Client_and_Server_Transport_Behavior" numbered="true" toc="default">
          <name>Client and Server Transport Behavior</name>
          <t>
   If a connection-oriented transport (e.g., TCP) is used,
   the client and server SHOULD use long-lived connections
   for at least three reasons:
          </t>
          <ol spacing="normal" type="1">
            <li>
    This will prevent the weakening of the transport's
    congestion control mechanisms via short-lived
    connections.
   </li>
            <li>
    This will improve performance for the WAN environment
    by eliminating the need for connection setup
    handshakes.
   </li>
            <li>
    The NFSv4.1 callback model differs from NFSv4.0, and
    requires the client and server to maintain a
    client-created backchannel (see <xref target="conn_chann_assoc" format="default"/>) for the server to use.
   </li>
          </ol>
          <t>
   In order to reduce congestion, if a connection-oriented
   transport is used, and the request is not the NULL
   procedure:
          </t>
          <ul spacing="normal">
            <li>
    A requester MUST
    NOT retry a request unless the connection the request
    was sent over was lost before the reply was
    received.
   </li>
            <li>
    A replier MUST
    NOT silently drop a request, even if the request is a
    retry.  (The silent drop behavior of RPCSEC_GSS
    <xref target="RFC2203" format="default"/> does not apply
    because this behavior happens at the RPCSEC_GSS layer,
    a lower layer in the request processing.)  Instead, the
    replier SHOULD return an appropriate error (see
    <xref target="Slot_Identifiers_and_Server_Reply_Cache" format="default"/>),
    or it MAY disconnect the connection.
   </li>
          </ul>
          <t>

    When sending a reply, the replier MUST send the reply
    to the same full network address (e.g., if using an
    IP-based transport, the source port of the requester
    is part of the full network address) from which the requester
    sent the request. If using a connection-oriented
    transport, replies MUST be sent on the same connection from which
    the request was received.

          </t>
          <t>

    If a connection is dropped after the replier receives
    the request but before the replier sends the reply, the
    replier might have a pending reply.
    If a connection is established with the same
    source and destination full network address as the
    dropped connection, then the replier MUST NOT send
    the reply until the requester retries the request. The
    reason for this prohibition is that the requester MAY
    retry a request over a different connection (provided that connection
    is associated with the original request's session).

          </t>
          <t>
   When using RDMA transports, there are other reasons for not
   tolerating retries over the same connection:
          </t>
          <ul spacing="normal">
            <li>
     RDMA transports use "credits" to enforce flow control, where
     a credit is a right to a peer to transmit a message.
     If one peer were to retransmit a request (or reply), it would
     consume an additional credit.
     If the replier
     retransmitted a reply, it would certainly result in an RDMA
     connection loss, since the requester would typically only post a
     single receive buffer for each request.  If the requester
     retransmitted a request, the additional credit consumed on the
     server might lead to RDMA connection failure unless the client
     accounted for it and decreased its available credit, leading to
     wasted resources.
    </li>
            <li>
     RDMA credits present a new issue to the reply cache in
     NFSv4.1.  The reply cache may be used when a connection within a
     session is lost, such as after the client reconnects.  Credit
     information is a dynamic property of the RDMA connection, and stale
     values must not be replayed from the cache.  This implies that the
     reply cache contents must not be blindly used when replies are
     sent from it, and credit information appropriate to the channel
     must be refreshed by the RPC layer.
    </li>
          </ul>
          <t>
   In addition, as described in
   <xref target="Retry_and_Replay" format="default"/>, while a session is active,
   the NFSv4.1 requester MUST NOT stop waiting for a reply.
          </t>
        </section>
        <!-- Client and Server Transport Behavior -->

  <section anchor="Ports" numbered="true" toc="default">
          <name>Ports</name>
          <t>
   Historically, NFSv3 servers have listened over
   TCP port 2049.  The registered port 2049 <xref target="RFC3232" format="default"/>
   for the NFS protocol should be the default configuration.  NFSv4.1
   clients SHOULD NOT use the RPC binding protocols as described in
   <xref target="RFC1833" format="default"/>.
          </t>
        </section>
        <!-- Ports -->

 </section>
      <!-- Transport Layers -->

 <section anchor="Session" numbered="true" toc="default">
        <name>Session</name>
        <t>
   NFSv4.1 clients and servers MUST support and MUST use the session
   feature as described in this section.

        </t>
        <section anchor="Motivation_and_Overview" numbered="true" toc="default">
          <name>Motivation and Overview</name>
          <t>
   Previous versions and minor versions of NFS have suffered from
   the following:
          </t>
          <ul spacing="normal">
            <li>
    Lack of support for Exactly Once Semantics (EOS). This includes
    lack of support for EOS through server failure and recovery.
   </li>
            <li>
    Limited callback support, including no support for sending callbacks
    through firewalls, and races between replies to normal requests
    and callbacks.
   </li>
            <li>
    Limited trunking over multiple network paths.
   </li>
            <li>
    Requiring machine credentials for fully secure operation.
   </li>
          </ul>
          <t>
   Through the introduction of a session, NFSv4.1 addresses the
   above shortfalls with practical solutions:
          </t>
          <ul spacing="normal">
            <li>
    EOS is enabled by a reply cache with a bounded size,
    making it feasible to keep the cache in persistent storage and enable
    EOS through server failure and recovery. One reason that
    previous revisions of NFS did not support EOS was
    because some EOS approaches often limited parallelism.
    As will be explained in
    <xref target="Exactly_Once_Semantics" format="default"/>,
    NFSv4.1 supports both EOS and unlimited parallelism.
   </li>
            <li>
    The NFSv4.1 client (defined in <xref target="intro_definitions" format="default"/>,
    <xref target="client_def" format="default"/>) creates transport
    connections and provides them to the server to use for sending
    callback requests, thus solving the firewall issue
    (<xref target="OP_BIND_CONN_TO_SESSION" format="default"/>). Races between
    responses from client requests and callbacks caused by
    the requests are detected via the session's sequencing
    properties that are a consequence of EOS
    (<xref target="sessions_callback_races" format="default"/>).
   </li>
            <li>
    The NFSv4.1 client can associate an arbitrary number of connections with
    the session, and thus provide trunking (<xref target="Trunking" format="default"/>).
   </li>
            <li>
    The NFSv4.1 client and server produce a session key independent of client
    and server machine credentials which can be
    used to compute a digest for protecting critical session management operations
    (<xref target="protect_state_change" format="default"/>).
   </li>
            <li>
    The NFSv4.1 client can also create secure RPCSEC_GSS contexts
    for use by the session's backchannel that do not require
    the server to authenticate to a client machine principal
    (<xref target="Backchannel_RPC_Security" format="default"/>).
   </li>
          </ul>
          <t>
   A session is a dynamically created, long-lived server object
   created by a client and used over time from one or more transport
   connections.  Its function is to maintain the server's state
   relative to the connection(s) belonging to a client instance.  This
   state is entirely independent of the connection itself, and indeed
   the state exists whether or not the connection exists. A client may
   have one or more sessions associated with it so that
   client-associated state may be accessed using any of the sessions
   associated with that client's client ID, when connections are
   associated with those sessions. When no connections are associated
   with any of a client ID's sessions for an extended time, such
   objects as locks, opens, delegations, layouts, etc. are subject to
   expiration.  The session serves as an object representing a means
   of access by a client to the associated client state on the server,
   independent of the physical means of access to that state.
          </t>
          <t>
   A single client may create multiple sessions. A single session MUST
   NOT serve multiple clients.
          </t>
        </section>
        <!-- Motivation and Overview -->

  <section anchor="NFSv4_Integration" numbered="true" toc="default">
          <name>NFSv4 Integration</name>
          <t>
   Sessions are part of NFSv4.1 and not NFSv4.0. Normally, a major
   infrastructure change such as sessions would require a new major
   version number to an Open Network Computing (ONC) RPC program like
   NFS. However, because NFSv4 encapsulates its functionality in a single procedure, COMPOUND,
   and because COMPOUND can support an arbitrary number of
   operations, sessions have been added to NFSv4.1 with little difficulty. COMPOUND includes
   a minor version number field, and for NFSv4.1 this minor version
   is set to 1. When the NFSv4 server processes a COMPOUND with
   the minor version set to 1, it expects a different set of
   operations than it does for NFSv4.0. NFSv4.1 defines the
   SEQUENCE operation, which is required for every
   COMPOUND that operates over an established session, with the
   exception of some session administration operations, such
   as DESTROY_SESSION (<xref target="OP_DESTROY_SESSION" format="default"/>).
          </t>
          <section anchor="SEQUENCE_and_CB_SEQUENCE" numbered="true" toc="default">
            <name>SEQUENCE and CB_SEQUENCE</name>
            <t>
     In NFSv4.1, when the SEQUENCE operation is present, it MUST be
     the first operation in the COMPOUND procedure. The primary purpose
     of SEQUENCE is to carry the session identifier. The session identifier
     associates all other operations in the COMPOUND procedure with
     a particular session. SEQUENCE also contains required information
     for maintaining EOS (see <xref target="Exactly_Once_Semantics" format="default"/>).
     Session-enabled NFSv4.1 COMPOUND requests thus have the form:
            </t>
            <artwork name="" type="" align="left" alt=""><![CDATA[
    +-----+--------------+-----------+------------+-----------+----
    | tag | minorversion | numops    |SEQUENCE op | op + args | ...
    |     |   (== 1)     | (limited) |  + args    |           |
    +-----+--------------+-----------+------------+-----------+----
    ]]></artwork>
            <t>
    and the replies have the form:
            </t>
            <artwork name="" type="" align="left" alt=""><![CDATA[
    +------------+-----+--------+-------------------------------+--//
    |last status | tag | numres |status + SEQUENCE op + results |  //
    +------------+-----+--------+-------------------------------+--//
            //-----------------------+----
            // status + op + results | ...
            //-----------------------+----
    ]]></artwork>
            <t>
     A CB_COMPOUND procedure request and reply has a similar form to
     COMPOUND, but
     instead of a SEQUENCE operation, there is a CB_SEQUENCE operation.
     CB_COMPOUND also has an additional field called "callback_ident", which
     is superfluous in NFSv4.1 and MUST be ignored by
     the client. CB_SEQUENCE has the same information
     as SEQUENCE, and also includes other information needed to resolve
     callback races
    (<xref target="sessions_callback_races" format="default"/>).
            </t>
          </section>
          <!-- SEQUENCE and CB_SEQUENCE -->

   <section anchor="Client_ID_and_Session_Association" numbered="true" toc="default">
            <name>Client ID and Session Association</name>
            <t>
    Each client ID (<xref target="Client_Identifiers" format="default"/>) can have
    zero or more active sessions. A client ID and associated
    session are required to perform file access in
    NFSv4.1. Each time a session is used (whether by a client sending
    a request to the server or the client replying to a callback
    request from the server), the state leased to its associated
    client ID is automatically renewed.

            </t>
            <t>
    State (which can consist of share reservations, locks, delegations,
    and layouts (<xref target="intro_locking" format="default"/>)) is tied to
    the client ID. Client state is not tied to any individual session.
    Successive state changing operations from a given state
    owner MAY go over different sessions, provided the
    session is associated with the same client ID. A callback
    MAY arrive over a different session than that of the request
    that originally acquired the state pertaining to the
    callback. For example, if session A is used to
    acquire a delegation, a request to recall the
    delegation MAY arrive over session B if both sessions are
    associated with the same client ID. Sections
    <xref target="Session_Callback_Security" format="counter"/> and
    <xref target="Backchannel_RPC_Security" format="counter"/> discuss
    the security considerations around callbacks.
            </t>
          </section>
          <!-- Client ID and Session Association -->
  </section>
        <!-- NFSv4 Integration -->

  <section anchor="Channels" numbered="true" toc="default">
          <name>Channels</name>
          <t>
   A channel is not a connection. A channel represents the
   direction ONC RPC requests are sent.
          </t>
          <t>
   Each session has one or two channels: the fore channel and the backchannel.
   Because there are at most two channels per session, and because each
   channel has a distinct purpose, channels are not assigned
   identifiers.
          </t>
          <t>
   The fore channel is
   used for ordinary requests from the client to the server, and
   carries COMPOUND requests and responses.
   A session always has a fore channel.
          </t>
          <t>
   The backchannel is used for callback requests from server
   to client, and carries CB_COMPOUND requests and responses.
   Whether or not there is a backchannel is decided by the
   client; however, many features of NFSv4.1 require a backchannel.
   NFSv4.1 servers MUST support backchannels.
          </t>
          <t>
   Each session has resources for each channel,
   including separate reply caches (see
   <xref target="Slot_Identifiers_and_Server_Reply_Cache" format="default"/>).

   Note that even the backchannel requires a reply cache (or, at least,
   a slot table in order to detect retries) because
   some callback operations are nonidempotent.
          </t>
          <section anchor="conn_chann_assoc" numbered="true" toc="default">
            <name>Association of Connections, Channels, and Sessions</name>
            <t>
    Each channel is associated with zero or more transport
    connections (whether of the same transport protocol or different
    transport protocols).  A connection can be associated with
    one channel or both channels of a session; the client
    and server negotiate whether a connection will carry
    traffic for one channel or both channels via the
    CREATE_SESSION (<xref target="OP_CREATE_SESSION" format="default"/>) and the BIND_CONN_TO_SESSION (<xref target="OP_BIND_CONN_TO_SESSION" format="default"/>) operations. When a
    session is created via CREATE_SESSION, the connection
    that transported the CREATE_SESSION request is
    automatically associated with the fore channel, and
    optionally the backchannel. If the client specifies no
    state protection (<xref target="OP_EXCHANGE_ID" format="default"/>)
    when the session is created, then when SEQUENCE is
    transmitted on a different connection, the connection
    is automatically associated with the fore channel of
    the session specified in the SEQUENCE operation.

            </t>
            <t>
    A connection's association with a session is
    not exclusive.  A connection associated with the channel(s)
    of one session may be simultaneously
    associated with the channel(s) of other sessions including
    sessions associated with other client IDs.

            </t>
            <t>
    It is permissible for connections of multiple transport
    types to be associated with the same channel. For
    example, both TCP and RDMA connections can be
    associated with the fore channel.  In the event an
    RDMA and non-RDMA connection are associated with the
    same channel, the maximum number of slots SHOULD be
    at least one more than the total number of RDMA credits
    (<xref target="Slot_Identifiers_and_Server_Reply_Cache" format="default"/>).
   This way, if all RDMA credits are used, the non-RDMA
   connection can have at least one outstanding request.
   If a server supports multiple transport types, it MUST
   allow a client to associate connections from each transport
   to a channel.

            </t>
            <t>
    It is permissible for a connection of one type of
    transport to be associated with the fore channel,
    and a connection of a different type to be associated
    with the backchannel.

            </t>
          </section>
        </section>
        <!-- Channels -->
  <section anchor="Server_Scope" numbered="true" toc="default">
          <name>Server Scope</name>
          <t>
      Servers each specify a server scope value in the form
      of an opaque string eir_server_scope returned as part of
      the results of an EXCHANGE_ID operation.  The purpose of
      the server scope is to allow a group of servers to
      indicate to clients that a set of servers sharing the
      same server scope value has arranged to use distinct
      values of opaque identifiers so that the two servers never
      assign the same value to two distinct objects. Thus, the identifiers
      generated by two servers within that set can be assumed compatible
      so that, in certain important cases,
      identifiers generated by one server in that set may be
      presented to
      another server of the same scope.
          </t>
          <t>
      The use of such compatible values does not imply that
      a value generated by one server will always be accepted
      by another.  In most cases, it will not.  However, a
      server will not inadvertently accept a value generated by another
      server.  When it does accept it, it will be because
      it is recognized as valid and carrying the same meaning
      as on another server of the same scope.
          </t>
          <t>
      When servers are of the same server scope, this compatibility
      of values applies to the following identifiers:
          </t>
          <ul spacing="normal">
            <li>
        Filehandle values.  A filehandle value accepted by two
        servers of the same server scope denotes the same object.
        A WRITE operation sent to one server is reflected immediately
        in a READ sent to the other.
      </li>
            <li>
        Server owner values.  When the server scope values are
        the same, server owner value may be validly compared.
        In cases where the server scope values are different, server
        owner values are treated as different even if they
        contain identical strings of bytes.
      </li>
          </ul>
          <t>
      The coordination among servers required to provide such
      compatibility can be quite minimal, and limited to a simple
      partition of the ID space.  The recognition of common values
      requires additional implementation, but this can be tailored
      to the specific situations in which that recognition is
      desired.
          </t>
          <t>
      Clients will have occasion to compare the server scope values
      of multiple servers under a number of circumstances, each of
      which will be discussed under the appropriate functional
      section:
          </t>
          <ul spacing="normal">
            <li>
        When server owner values received in response to
        EXCHANGE_ID operations sent to multiple network
        addresses are compared for the purpose of determining
        the validity of various forms of trunking, as described
        in <xref target="SEC11-USES-trunk" format="default"/>.                       .
      </li>
            <li>
        When network or server reconfiguration causes the same
        network address to possibly be directed to different
        servers, with the necessity for the client to determine
        when lock reclaim should be attempted, as described
        in <xref target="reclaim_locks" format="default"/>.
      </li>
          </ul>
          <t>
      When two replies from EXCHANGE_ID, each from two different
      server network addresses, have the same server scope, there
      are a number of ways a client can validate that the common
      server scope is due to two servers cooperating in a group.
          </t>
          <ul spacing="normal">
            <li>
        If both EXCHANGE_ID requests were sent with RPCSEC_GSS
	(<xref target="RFC2203" format="default"/>, <xref target="RFC5403" format="default"/>,
	<xref target="RFC7861" format="default"/>)
        authentication and the server principal is the same for
        both targets, the equality of server scope is validated.
        It is RECOMMENDED that two servers intending to share the
        same server scope and server_owner major_id also share the
	same principal name.  In some cases, this
	simplifies the client's task of validating server scope.
      </li>
            <li>
        The client may accept the appearance of the second
        server in the fs_locations or fs_locations_info attribute
        for a relevant file system.  For example, if there is
        a migration event for a particular file system
        or there are locks to be reclaimed on a particular file
        system, the attributes for that particular file system
        may be used.  The client sends the GETATTR request to
        the first server for the fs_locations or
        fs_locations_info attribute with RPCSEC_GSS
        authentication.  It may need to do this in advance
        of the need to verify the common server scope.
        If the client successfully authenticates the reply
        to GETATTR, and the GETATTR request and reply containing
        the fs_locations or fs_locations_info attribute refers
        to the second server, then the equality of server scope
        is supported.  A client may choose to limit the use of
        this form of support to information relevant to the
        specific file system involved (e.g. a file system
        being migrated).
      </li>
          </ul>
        </section>
        <section anchor="Trunking" numbered="true" toc="default">
          <name>Trunking</name>
          <t>
      Trunking is the use of multiple connections between a
      client and server in order to increase the speed of data
      transfer. NFSv4.1  supports two types of trunking:
      session trunking and client ID trunking.
          </t>
          <t>
      In the context of a single server network address, it
      can be assumed that all connections are accessing the
      same server and NFSv4.1
      servers MUST support both forms of trunking.  When
      multiple connections use a set of network addresses
      accessing the same server, the server
      MUST support both forms of trunking.
      NFSv4.1 servers in a clustered configuration MAY allow
      network addresses for different servers to use client ID
      trunking.
          </t>
          <t>
     Clients may use either form of trunking as long as they
     do not, when trunking between different server network
     addresses, violate the servers' mandates as to the
     kinds of trunking to be allowed (see below).  With regard
     to callback channels, the client MUST allow the server to
     choose among all callback channels valid for a given
     client ID and MUST support trunking when the connections
     supporting the backchannel allow session or client ID
     trunking to be used for callbacks.
          </t>
          <t>
     Session trunking is essentially the association of multiple
     connections, each with potentially different target and/or source
     network addresses, to the same session.  When the target network
     addresses (server addresses) of the two connections are the same,
     the server MUST
     support such session trunking.  When the target network addresses
     are different, the server MAY indicate such support using the
     data returned by the EXCHANGE_ID operation (see below).
          </t>
          <t>
     Client ID trunking is the association of multiple
     sessions to the same client ID.  Servers MUST support client ID
     trunking for two target network addresses whenever they allow
     session trunking for those same two network addresses.
     In addition, a server MAY, by presenting the same
     major server owner ID
     (<xref target="Server_Owners" format="default"/>) and server scope
     (<xref target="Server_Scope" format="default"/>), allow an additional
     case of client ID trunking.  When two
     servers return the same major server owner and server
     scope, it means that the two servers are cooperating on
     locking state management, which is a prerequisite
     for client ID trunking.

          </t>
          <t>
     Distinguishing when the client is allowed to use session and
     client ID trunking requires understanding how the results of the
     EXCHANGE_ID (<xref target="OP_EXCHANGE_ID" format="default"/>)
     operation identify a server.
     Suppose a client sends EXCHANGE_IDs over two different
     connections, each with a possibly different target
     network address, but each EXCHANGE_ID operation has the same
     value in the eia_clientowner field.  If the same
     NFSv4.1 server is listening over each connection,
     then each EXCHANGE_ID result MUST return the same
     values of eir_clientid, eir_server_owner.so_major_id,
     and eir_server_scope. The client can then treat each
     connection as referring to the same server (subject
     to verification; see
     <xref target="PREP-trunk-verify" format="default"/> below),
     and it can use each connection to trunk requests and
     replies.

     The client's choice is whether session trunking
     or client ID trunking applies.

          </t>
          <dl newline="false" spacing="normal">
            <dt>Session Trunking.</dt>
            <dd>
              <t>

       If the eia_clientowner argument is the same in
       two different EXCHANGE_ID requests, and
       the eir_clientid, eir_server_owner.so_major_id,
       eir_server_owner.so_minor_id, and eir_server_scope
       results match in both EXCHANGE_ID results, then
       the client is permitted to perform session trunking.
       If the client has no session mapping to the tuple of
       eir_clientid, eir_server_owner.so_major_id, eir_server_scope, and
       eir_server_owner.so_minor_id, then it creates
       the session via a CREATE_SESSION operation over one
       of the connections, which associates the connection
       to the session. If there is a session for the tuple,
       the client can send BIND_CONN_TO_SESSION to associate
       the connection to the session.
              </t>
              <t>
       Of course, if the client
       does not desire to use session trunking, it is not
       required to do so.  It can invoke
       CREATE_SESSION on the connection. This will result
       in client ID trunking as described below.  It can also
       decide to drop the connection if it does not choose to
       use trunking.
              </t>
              <t/>
            </dd>
            <dt>Client ID Trunking.</dt>
            <dd>
              <t>

       If the eia_clientowner argument is the same in
       two different EXCHANGE_ID requests, and
       the eir_clientid, eir_server_owner.so_major_id,
       and eir_server_scope
       results match in both EXCHANGE_ID results, then
       the client is permitted to perform client ID trunking
       (regardless of whether the eir_server_owner.so_minor_id results match).
       The client can associate
       each connection with different sessions, where
       each session is associated with the same server.

              </t>
              <t>

       The client completes the act of client ID trunking by invoking
       CREATE_SESSION on each connection, using the same
       client ID that was returned in eir_clientid. These
       invocations create two sessions and also associate
       each connection with its respective session.  The client
       is free to decline to use client ID trunking by simply
       dropping the connection at this point.

              </t>
              <t>

       When doing client ID trunking, locking state
       is shared across sessions associated with that same
       client ID. This requires the server to coordinate
       state across sessions and the client to be able to
       associate the same locking state with multiple sessions.

              </t>
            </dd>
          </dl>
          <t>
      It is always possible that, as a result of various sorts
      of reconfiguration events, eir_server_scope and
      eir_server_owner values may be different on subsequent
      EXCHANGE_ID requests made to the same network address.
          </t>
          <t>
      In most cases such reconfiguration events will be
      disruptive and indicate that an IP address formerly connected
      to one server is now connected to an entirely different one.
          </t>
          <t>
      Some guidelines on client handling of such situations follow:
          </t>
          <ul spacing="normal">
            <li>
        When eir_server_scope changes, the client has no assurance
        that any id's it obtained previously (e.g. file handles) can
        be validly used on the new server, and, even if the new
        server accepts them, there is no assurance that this is not
        due to accident.  Thus, it is best to treat all such state
        as lost/stale although a client may assume that the
        probability  of inadvertent acceptance is low and treat
        this situation as within the next case.
      </li>
            <li>
        When eir_server_scope remains the same and
        eir_server_owner.so_major_id changes, the client can use
        the filehandles it has, consider its locking state lost,
	and attempt
	to reclaim or otherwise re-obtain its locks.  It might find
	that
        its file handle is now stale.  However, if NFS4ERR_STALE is not
	returned, it can proceed to reclaim or otherwise re-obtain its
	open locking state.
      </li>
            <li>
        When eir_server_scope and
        eir_server_owner.so_major_id remain the same,
        the client has to use the now-current values
        of eir_server_owner.so_minor_id in deciding on appropriate
        forms of trunking.  This may result in connections being
	dropped or new sessions being created.
      </li>
          </ul>
          <section anchor="PREP-trunk-verify" numbered="true" toc="default">
            <name>Verifying Claims of Matching Server Identity</name>
            <t>
     When the server responds using two different connections claiming
     matching or partially matching eir_server_owner,
     eir_server_scope, and eir_clientid values, the client
     does not have to trust the servers' claims. The client
     may verify these claims before trunking traffic in
     the following ways:

            </t>
            <ul spacing="normal">
              <li>
                <t>
      For session trunking,
      clients SHOULD
      reliably verify if connections between different
      network paths are in fact associated with the same NFSv4.1
      server and usable on the same session, and servers
      MUST allow clients to perform reliable verification.
      When a client ID is created, the client SHOULD specify that
      BIND_CONN_TO_SESSION is to be verified according to the
      SP4_SSV or SP4_MACH_CRED (<xref target="OP_EXCHANGE_ID" format="default"/>)
      state protection options.  For SP4_SSV, reliable
      verification depends on a shared secret (the
      SSV) that is established via the SET_SSV (see
      <xref target="OP_SET_SSV" format="default"/>) operation.

                </t>
                <t>

      When a new connection is associated with the
      session (via the BIND_CONN_TO_SESSION operation,
      see <xref target="OP_BIND_CONN_TO_SESSION" format="default"/>), if
      the client specified SP4_SSV state protection for the
      BIND_CONN_TO_SESSION operation, the client MUST send
      the BIND_CONN_TO_SESSION with RPCSEC_GSS protection,
      using integrity or privacy, and an RPCSEC_GSS handle created
      with the GSS SSV mechanism (see <xref target="ssv_mech" format="default"/>).

                </t>
                <t>

      If the client mistakenly tries to associate a
      connection to a session of a wrong server, the
      server will either reject the attempt because
      it is not aware of the session identifier of the
      BIND_CONN_TO_SESSION arguments, or it will reject
      the attempt because the RPCSEC_GSS authentication
      fails.  Even if the server mistakenly or maliciously
      accepts the connection association attempt, the
      RPCSEC_GSS verifier it computes in the response
      will not be verified by the client, so the client will
      know it cannot use the connection for trunking the
      specified session.  </t>
                <t> If the
      client specified SP4_MACH_CRED state protection, the
      BIND_CONN_TO_SESSION operation will use RPCSEC_GSS
      integrity or privacy, using the same credential that
      was used when the client ID was created. Mutual
      authentication via RPCSEC_GSS assures the client
      that the connection is associated with the correct
      session of the correct server.

                </t>
                <t/>
              </li>
              <li>
                <t>
      For client ID trunking, the client has at least two
      options for verifying that the same client ID
      obtained from two different EXCHANGE_ID operations
      came from the same server.  The first option is
      to use RPCSEC_GSS authentication when sending each
      EXCHANGE_ID operation. Each time an EXCHANGE_ID is sent with
      RPCSEC_GSS authentication, the client notes the
      principal name of the GSS target.  If the EXCHANGE_ID
      results indicate that client ID trunking is possible,
      and the GSS targets' principal names are the same,
      the servers are the same and client ID trunking is
      allowed.

                </t>
                <t>

      The second option for verification is to
      use SP4_SSV protection.  When the client sends
      EXCHANGE_ID, it specifies SP4_SSV protection. The
      first EXCHANGE_ID the client sends always has to
      be confirmed by a CREATE_SESSION call. The client
      then sends SET_SSV. Later, the client
      sends EXCHANGE_ID to a second destination
      network address different from the one the first
      EXCHANGE_ID was sent to.
      The client checks that each EXCHANGE_ID reply has the
      same eir_clientid, eir_server_owner.so_major_id, and
      eir_server_scope. If so, the client verifies the
      claim by sending a CREATE_SESSION operation to the second
      destination address, protected with RPCSEC_GSS integrity
      using an RPCSEC_GSS handle returned by the second
      EXCHANGE_ID. If the server accepts the CREATE_SESSION
      request, and if the client verifies the RPCSEC_GSS
      verifier and integrity codes, then the client has
      proof the second server knows the SSV, and thus
      the two servers are cooperating for the purposes of
      specifying server scope and client ID trunking.

                </t>
              </li>
            </ul>
          </section>
        </section>
        <section anchor="Exactly_Once_Semantics" numbered="true" toc="default">
          <name>Exactly Once Semantics</name>
          <t>
   Via the session, NFSv4.1 offers exactly once semantics (EOS)
   for requests sent over a channel. EOS is supported on both the
   fore channel and backchannel.
          </t>
          <t>
   Each COMPOUND or CB_COMPOUND request that is sent
   with a leading SEQUENCE or CB_SEQUENCE operation MUST
   be executed by the receiver exactly once. This requirement
   holds regardless of whether the request is sent with reply
   caching specified (see <xref target="optional_reply_caching" format="default"/>).
   The requirement holds even if the requester is sending the
   request over a session created between a pNFS data client
   and pNFS data server. To understand the rationale for this requirement,
   divide the requests into three
   classifications:
          </t>
          <ul spacing="normal">
            <li>
    Non-idempotent requests.
   </li>
            <li>
    Idempotent modifying requests.
   </li>
            <li>
    Idempotent non-modifying requests.
   </li>
          </ul>
          <t>
    An example of a non-idempotent request is
    RENAME. Obviously, if a replier executes the
    same RENAME request twice, and the first execution succeeds,
    the re-execution will fail. If the replier returns the
    result from the re-execution, this result is incorrect.
    Therefore, EOS is required for non-idempotent requests.
          </t>
          <t>
    An example of an idempotent modifying request is
    a COMPOUND request containing a WRITE operation.
    Repeated execution of the same WRITE
    has the same effect as execution of that WRITE a single time.
    Nevertheless, enforcing EOS for WRITEs and other idempotent
    modifying requests is necessary
    to avoid data corruption.
          </t>
          <t>
    Suppose a client sends WRITE A to a
    noncompliant server that does not enforce EOS, and
    receives no response, perhaps due to a network
    partition.  The client reconnects to the server and
    re-sends WRITE A. Now, the server has
    outstanding two instances of A.  The
    server can be in a situation in which it executes and
    replies to the retry of A, while the first
    A is still waiting in the server's internal I/O system for some
    resource.  Upon receiving the
    reply to the second attempt of WRITE A,
    the client believes its WRITE is done so it is free
    to send WRITE B, which overlaps the byte-range of
    A.  When the original A is dispatched from the server's
    I/O system and
    executed (thus the second time A will have
    been written), then what has been
    written by B can be overwritten and thus corrupted.
          </t>
          <t>
    An example of an idempotent non-modifying request
    is a COMPOUND containing SEQUENCE, PUTFH, READLINK,
    and nothing else. The re-execution of such a
    request will not cause data corruption or
    produce an incorrect result. Nonetheless,
    to keep the implementation simple,
    the replier MUST enforce EOS for all requests, whether or not
    idempotent and non-modifying.
          </t>
          <t>
    Note that true and complete EOS is not possible unless the
    server persists the reply cache in stable storage, and unless the
    server is somehow implemented to never require a restart
    (indeed, if such a server exists, the distinction between a
    reply cache kept in stable storage versus one that is not is
    one without meaning). See <xref target="Persistence" format="default"/> for
    a discussion of persistence in the reply cache.
    Regardless, even if the server does not persist the reply cache,
    EOS improves robustness and correctness over previous versions
    of NFS because the legacy duplicate request/reply caches were
    based on the ONC RPC transaction identifier (XID).
    <xref target="Slot_Identifiers_and_Server_Reply_Cache" format="default"/>
    explains the shortcomings of the XID as a basis for
   a reply cache and describes how NFSv4.1 sessions improve
   upon the XID.
          </t>
          <section anchor="Slot_Identifiers_and_Server_Reply_Cache" numbered="true" toc="default">
            <name>Slot Identifiers and Reply Cache</name>
            <t>
     The RPC layer provides a transaction ID (XID), which,
     while required to be unique, is not
     convenient for tracking requests for two reasons.
     First, the XID is only
     meaningful to the requester; it cannot be interpreted
     by the replier except to test for equality with
     previously sent requests. When consulting an RPC-based
     duplicate request cache, the opaqueness of the XID requires
     a computationally expensive look up (often via a hash that
     includes XID and source address). NFSv4.1 requests use
     a non-opaque slot ID, which is an index into a slot table,
     which is far more efficient. Second, because RPC requests
     can be executed by the replier in any order, there is
     no bound on the number of requests that may be outstanding
     at any time. To achieve perfect EOS, using ONC RPC
     would require storing all replies in the reply cache.
     XIDs are 32 bits; storing over four billion (2^32) replies
     in the reply cache is not practical. In practice, previous versions
     of NFS have chosen to store a fixed number of replies in
     the cache, and to use a least recently used (LRU) approach to
     replacing cache entries with new entries when the cache
     is full. In NFSv4.1, the number of outstanding requests is
     bounded by the size of the slot table, and a sequence ID
     per slot is used to tell the replier when it is safe to
     delete a cached reply.
            </t>
            <t>
     In the NFSv4.1 reply cache, when the requester sends a new request,
     it selects a slot ID in the
     range 0..N, where N is the replier's current maximum slot ID
     granted to the requester on the session over which the request is to be
     sent. The value of N starts out as equal to
     ca_maxrequests - 1 (<xref target="OP_CREATE_SESSION" format="default"/>), but
     can be adjusted by the response to SEQUENCE or CB_SEQUENCE as described
     later in this section.
     The slot ID must be unused by any of the requests that the
     requester has already active on the session.  "Unused" here means the
     requester has no outstanding request for that slot ID.
            </t>
            <t>
     A slot contains a sequence ID and the cached reply corresponding to
     the request sent with that sequence ID. The sequence ID is a
     32-bit unsigned value, and is therefore in the range 0..0xFFFFFFFF (2^32 - 1).
     The first time a slot is used, the requester MUST specify
     a sequence ID of one (<xref target="OP_CREATE_SESSION" format="default"/>).
     Each time a slot is reused, the request MUST specify a sequence ID
     that is one greater than that of the previous request on the
     slot. If the previous sequence ID was 0xFFFFFFFF, then the next
     request for the slot MUST have the sequence ID set to zero (i.e.,
     (2^32 - 1) + 1 mod 2^32).
            </t>
            <t>
     The sequence ID accompanies the slot ID in each request. It is
     for the critical check at the replier: it used to efficiently
     determine whether a request using a certain
     slot ID is a retransmit or a new, never-before-seen request.  It is
     not feasible for the requester to assert that it is retransmitting to
     implement this, because for any given request the requester cannot
     know whether the replier has seen it unless the replier actually replies.  Of
     course, if the requester has seen the reply, the requester would
     not retransmit.
            </t>
            <t>
     The replier compares each received request's
     sequence ID with the last one previously received for that slot ID,
     to see if the new request is:
            </t>
            <ul spacing="normal">
              <li>
      A new request, in which the sequence ID is one greater
      than that previously seen in the slot (accounting for sequence
      wraparound).  The replier proceeds to execute the new request,
      and the replier
      MUST increase the slot's sequence ID by one.
     </li>
              <li>
      A retransmitted request, in which the sequence ID is equal to
      that currently recorded in the slot.
      If the original request has
      executed to completion, the replier returns the cached
      reply. See <xref target="Retry_and_Replay" format="default"/> for direction on how the replier
      deals with retries of requests that are still in progress.
     </li>
              <li>
      A misordered retry, in which the sequence ID
      is less than (accounting for sequence wraparound)
      that previously seen in the slot.  The
      replier MUST return NFS4ERR_SEQ_MISORDERED (as the
      result from SEQUENCE or CB_SEQUENCE).
     </li>
              <li>
      A misordered new request, in which the sequence ID
      is two or more than (accounting for sequence
      wraparound) that previously seen in the
      slot. Note that because the sequence ID MUST
      wrap around to zero once it reaches 0xFFFFFFFF, a
      misordered new request and a misordered retry
      cannot be distinguished. Thus, the replier MUST
      return NFS4ERR_SEQ_MISORDERED (as the result from
      SEQUENCE or CB_SEQUENCE).
     </li>
            </ul>
            <t>
     Unlike the XID, the slot ID is always within a specific
     range; this has two implications.  The first
     implication is that for a given session, the replier
     need only cache the results of a limited number of
     COMPOUND requests.
     The second implication derives
     from the first, which is that unlike XID-indexed reply
     caches (also known as duplicate request caches - DRCs),
     the slot ID-based reply cache cannot be overflowed.
     Through use of the sequence ID to identify
     retransmitted requests, the replier does not need to
     actually cache the request itself, reducing the
     storage requirements of the reply cache further.  These
     facilities make it practical to maintain all the
     required entries for an effective reply cache.

            </t>
            <t>
     The slot ID, sequence ID, and session ID therefore take over the traditional role
     of the XID and source network address in the replier's
     reply cache implementation.
     This approach is considerably
     more portable and completely robust -- it is not subject to the
     reassignment of ports as clients reconnect over IP
     networks.  In addition, the RPC XID is not used in the reply cache,
     enhancing robustness of the cache in the face of any rapid reuse of
     XIDs by the requester. While the replier does not care
     about the XID for the purposes of reply cache management
     (but the replier MUST return the same XID that was in the request),
     nonetheless there are considerations for the XID in NFSv4.1
     that are the same as all other previous versions of NFS.
     The RPC XID remains in each message and needs to be formulated
     in NFSv4.1 requests as in any other ONC RPC request. The reasons
     include:
            </t>
            <ul spacing="normal">
              <li>
     The RPC layer retains its existing semantics and implementation.
    </li>
              <li>
     The requester and replier must be able to interoperate at the
     RPC layer, prior to the NFSv4.1 decoding of the SEQUENCE or CB_SEQUENCE
     operation.
    </li>
              <li>
     If an operation is being used that does not start with
     SEQUENCE or CB_SEQUENCE (e.g., BIND_CONN_TO_SESSION),
     then the RPC XID is needed for correct operation to
     match the reply to the request.

    </li>
              <li>
     The SEQUENCE or CB_SEQUENCE operation may generate an error.
     If so, the embedded slot ID, sequence ID, and session ID (if
     present) in the request will not be in the reply, and the
     requester has only the XID to match the reply to the request.
    </li>
            </ul>
            <t>
     Given that well-formulated XIDs continue to be required,
     this raises the question: why do SEQUENCE and CB_SEQUENCE replies
     have a session ID, slot ID, and sequence ID? Having the session ID
     in the reply means that the requester does not have to use the
     XID to look up
     the session ID, which would be necessary if the connection were
     associated with multiple sessions. Having the slot ID and sequence ID
     in the reply means that the requester does not have to use the XID to
     look up the slot ID and sequence ID.
     Furthermore, since the XID is only 32 bits, it is too small to
     guarantee the re-association of a reply with its request
     <xref target="rpc_xid_issues" format="default"/>; having
     session ID, slot ID, and sequence ID in the reply allows the
     client to validate that the reply in fact belongs to the matched request.
            </t>
            <t>
     The SEQUENCE (and CB_SEQUENCE) operation also carries
     a "highest_slotid" value, which carries additional
     requester slot usage information.  The requester MUST
     always indicate the slot ID representing the outstanding request with the
     highest-numbered slot
     value.
     The requester should in all cases provide the most
     conservative value possible, although it can be increased somewhat
     above the actual instantaneous usage to maintain some minimum or
     optimal level.  This provides a way for the requester to yield unused
     request slots back to the replier, which in turn can use the
     information to reallocate resources.
            </t>
            <t>
     The replier
     responds with both a new target highest_slotid and an
     enforced highest_slotid, described as follows:
            </t>
            <ul spacing="normal">
              <li>
                <t>
      The target highest_slotid is
      an indication to the requester of the highest_slotid the replier
      wishes the requester to be using.  This permits the replier to withdraw
      (or add) resources from a requester that has been found to not be
      using them, in order to more fairly share resources among a varying
      level of demand from other requesters.  The requester must always comply
      with the replier's value updates, since they indicate newly
      established hard limits on the requester's access to session
      resources.  However, because of request pipelining, the requester may
      have active requests in flight reflecting prior values; therefore,
      the replier must not immediately require the requester to comply.
                </t>
                <t/>
              </li>
              <li>
                <t>
      The enforced highest_slotid indicates the highest slot ID
      the requester is permitted to use on a subsequent SEQUENCE or
      CB_SEQUENCE operation. The replier's enforced highest_slotid SHOULD
      be no less than the highest_slotid the requester indicated
      in the SEQUENCE or CB_SEQUENCE arguments.

                </t>
                <t>

      A requester can be intransigent with respect to lowering its
      highest_slotid argument to a Sequence operation, i.e. the requester
      continues to ignore the target highest_slotid in the response to
      a Sequence operation, and continues to set its highest_slotid
      argument to be higher than the target highest_slotid. This can
      be considered particularly egregious behavior when the replier
      knows there are no outstanding requests with slot IDs higher than
      its target highest_slotid.  When faced with such intransigence,
      the replier is free to take more forceful action, and MAY reply with
      a new enforced highest_slotid that is less than its previous
      enforced highest_slotid.  Thereafter, if the requester continues
      to send requests with a highest_slotid that is greater than
      the replier's new enforced highest_slotid, the server MAY return
      NFS4ERR_BAD_HIGH_SLOT, unless the slot ID in the request is greater
      than the new enforced highest_slotid and the request is a retry.

                </t>
                <t>

      The replier SHOULD retain the slots it wants to retire
      until
      the requester sends a request with a highest_slotid less than
      or equal to the replier's new enforced highest_slotid.

                </t>
                <t>

      The requester can also be intransigent with
      respect to sending non-retry requests that have a slot ID that
      exceeds the replier's highest_slotid.
      Once the replier has forcibly lowered the enforced
      highest_slotid, the requester is only allowed to
      send retries on slots that exceed the replier's highest_slotid.
      If a request is received with a slot ID that is higher than
      the new enforced highest_slotid, and the sequence ID
      is one higher than what is in the slot's reply cache, then
      the server can both retire the slot and return NFS4ERR_BADSLOT
      (however, the server MUST NOT do one and not the other).
      The reason it is safe to retire the slot
      is because by using the next sequence ID, the requester
      is indicating it has received the previous reply for the
      slot.
                </t>
                <t/>
              </li>
              <li>
     The requester SHOULD use the lowest available
     slot when sending a new request.  This way, the
     replier may be able to retire slot entries faster.
     However, where the replier is actively adjusting
     its granted highest_slotid,
     it will not be able
     to use only the receipt of the slot ID and highest_slotid
     in the request.  Neither the slot ID nor the
     highest_slotid used in a request may reflect the
     replier's current idea of the requester's session
     limit, because the request may have been sent from the
     requester before the update was received.  Therefore,
     in the downward adjustment case, the replier may have
     to retain a number of reply cache entries at least as
     large as the old value of maximum requests
     outstanding, until it can infer that the requester
     has seen a reply containing the new granted highest_slotid.
     The replier can infer that the requester has seen such a
     reply when it receives a new request with the same
     slot ID as the request replied to and the next higher
     sequence ID.
    </li>
            </ul>
            <section anchor="cacheseq" numbered="true" toc="default">
              <name>Caching of SEQUENCE and CB_SEQUENCE Replies</name>
              <t>
      When a SEQUENCE or CB_SEQUENCE operation is
      successfully executed, its reply MUST always be
      cached. Specifically, session ID, sequence ID,
      and slot ID MUST be cached in the reply cache.
      The reply from SEQUENCE also includes the highest
      slot ID, target highest slot ID, and status flags. Instead
      of caching these values, the server MAY
      re-compute the values from the current
      state of the fore channel, session, and/or client
      ID as appropriate.  Similarly, the reply from
      CB_SEQUENCE includes a highest slot ID and target
      highest slot ID. The client
      MAY re-compute the values from the
      current state of the session as appropriate.

              </t>
              <t>

       Regardless of whether or not a replier is re-computing highest slot ID,
       target slot ID, and status on replies to retries, the requester
       MUST NOT assume that the values are being re-computed whenever it
       receives a reply after a retry is sent, since it has no way
       of knowing whether the reply it has received was sent by the
       replier in response to the retry or is a delayed response to
       the original request.  Therefore, it may be the case that
       highest slot ID, target slot ID, or status bits may reflect
       the state of affairs when the request was first executed.
       Although acting based on such delayed information is valid,
       it may cause the receiver of the reply to do unneeded work.  Requesters
       MAY choose to send additional requests to get the current
       state of affairs or use the state of affairs reported by
       subsequent requests, in preference to acting immediately
       on data that might be out of date.

              </t>
            </section>
            <section anchor="err_sequence" numbered="true" toc="default">
              <name>Errors from SEQUENCE and CB_SEQUENCE</name>
              <t>
      Any time SEQUENCE or CB_SEQUENCE returns an error, the
      sequence ID of the slot MUST NOT change. The replier MUST NOT
      modify the reply cache entry for the slot whenever an error
      is returned from SEQUENCE or CB_SEQUENCE.
              </t>
            </section>
            <!-- Errors from SEQUENCE and CB_SEQUENCE -->
     <section anchor="optional_reply_caching" numbered="true" toc="default">
              <name>Optional Reply Caching</name>
              <t>
       On a per-request basis, the requester can choose to
       direct the replier to cache the reply to all operations
       after the first operation (SEQUENCE or CB_SEQUENCE) via
       the sa_cachethis or csa_cachethis fields of the arguments
       to SEQUENCE or CB_SEQUENCE.
       The reason it would not direct the replier to cache
       the entire reply is that the request is composed of all
       idempotent operations <xref target="Chet" format="default"/>.
       Caching the reply may offer little benefit. If
       the reply is too large (see

       <xref target="COMPOUND_Sizing_Issues" format="default"/>),

       it may not be cacheable anyway. Even if the reply to
       idempotent request is small enough to cache, unnecessarily
       caching the reply slows down the server and increases
       RPC latency.
              </t>
              <t>
       Whether or not the requester requests the reply to be cached
       has no effect on the slot processing. If the
       result of SEQUENCE or CB_SEQUENCE is NFS4_OK, then
       the slot's sequence ID MUST be incremented by one.
       If a requester does not direct the replier to cache
       the reply, the replier MUST do one of following:
              </t>
              <ul spacing="normal">
                <li>
        The replier can cache the entire original reply.
        Even though sa_cachethis or csa_cachethis is FALSE,
        the replier is always free to cache. It may choose
        this approach in order to simplify implementation.
       </li>
                <li>
                  <t>
        The replier enters into its reply cache a reply consisting
        of the original results to the SEQUENCE or CB_SEQUENCE
        operation, and with the next operation in
        COMPOUND or CB_COMPOUND having the error NFS4ERR_RETRY_UNCACHED_REP.
        Thus, if the requester later retries the request, it will
        get NFS4ERR_RETRY_UNCACHED_REP.

        If a replier receives a retried Sequence operation where the reply
        to the COMPOUND or CB_COMPOUND was not cached, then the replier,

                  </t>
                  <ul spacing="normal">
                    <li>
	  MAY return NFS4ERR_RETRY_UNCACHED_REP
	  in reply to a Sequence operation if the
	  Sequence operation is not the first
	  operation (granted, a requester that
	  does so is in violation of the NFSv4.1
	  protocol).

        </li>
                    <li>
	  MUST NOT return
	  NFS4ERR_RETRY_UNCACHED_REP in reply to
	  a Sequence operation if the Sequence
	  operation is the first operation.

        </li>
                  </ul>
                </li>
                <li>
        If the second operation is an illegal operation, or an
        operation that was legal in a previous minor version of
        NFSv4 and MUST NOT
        be supported in the current minor version (e.g., SETCLIENTID), the
        replier MUST NOT ever return NFS4ERR_RETRY_UNCACHED_REP.
        Instead the replier MUST return NFS4ERR_OP_ILLEGAL or
        NFS4ERR_BADXDR or NFS4ERR_NOTSUPP as appropriate.
       </li>
                <li>
        If the second operation can result in another error status,
        the replier MAY return a status other than NFS4ERR_RETRY_UNCACHED_REP,
        provided the operation is not executed in such a way that the state
        of the replier is changed. Examples of such
        an error status include: NFS4ERR_NOTSUPP returned for an
        operation that is legal but not REQUIRED in the current
        minor versions, and thus not supported by the replier;
        NFS4ERR_SEQUENCE_POS; and NFS4ERR_REQ_TOO_BIG.
       </li>
              </ul>
              <t>
	The discussion above assumes that the
	retried request matches the original
	one.  <xref target="false_retry" format="default"/>
	discusses what the replier might do, and
	MUST do when original and retried requests do not match.
        Since the replier may
	only cache a small amount of the
	information that would be required to
	determine whether this is a case of a
	false retry, the replier may send to the
	client any of the following responses:

              </t>
              <ul spacing="normal">
                <li>
         The cached reply to the original request (if the replier has cached
         it in its entirety and the users of the original request and retry match).
        </li>
                <li>
          A reply that consists only of the Sequence operation with the error
	  NFS4ERR_FALSE_RETRY.
        </li>
                <li>
	A reply consisting of the response to Sequence  with the status
	NFS4_OK, together with the second operation as it appeared in the retried
	request with an error of NFS4ERR_RETRY_UNCACHED_REP or other error as
	described above.
        </li>
                <li>
          A reply that consists of the response to Sequence with the status
	NFS4_OK, together with the second operation as it appeared in the original
	request with an error of NFS4ERR_RETRY_UNCACHED_REP or other error as
	described above.
        </li>
              </ul>
              <section anchor="false_retry" numbered="true" toc="default">
                <name>False Retry</name>
                <t>
	If a requester sent a Sequence operation
	with a slot ID and sequence ID that are
	in the reply cache but the replier
	detected that the retried request is not
	the same as the original request,
	including a retry that has different
	operations or different arguments in the
	operations from the original and a retry
	that uses a different principal in the
	RPC request's credential field that
	translates to a different user, then this
	is a false retry. When the replier
	detects a false retry, it is permitted
	(but not always obligated) to return
	NFS4ERR_FALSE_RETRY in response to the
	Sequence operation when it detects a
	false retry.

                </t>
                <t>
	Translations of particularly privileged
	user values to other users due to the
	lack of appropriately secure credentials,
	as configured on the replier, should be
	applied before determining whether the
	users are the same or different. If the
	replier determines the users are
	different between the original request
	and a retry, then the replier MUST return
	NFS4ERR_FALSE_RETRY.

                </t>
                <t>
	If an operation of the retry is an
	illegal operation, or an operation that
	was legal in a previous minor version of
	NFSv4 and MUST NOT be supported in the
	current minor version (e.g., SETCLIENTID),
	the replier MAY return
	NFS4ERR_FALSE_RETRY (and MUST do so if
	the users of the original request and
	retry differ). Otherwise, the replier MAY return
	NFS4ERR_OP_ILLEGAL or NFS4ERR_BADXDR or
	NFS4ERR_NOTSUPP as appropriate.  Note
	that the handling is in contrast for how the
	replier deals with retries requests with
	no cached reply. The difference is due to
	NFS4ERR_FALSE_RETRY being a valid error
	for only Sequence operations, whereas
	NFS4ERR_RETRY_UNCACHED_REP is a valid
	error for all operations except illegal
	operations and operations that MUST NOT be
	supported in the current minor version of
	NFSv4.

                </t>
              </section>
            </section>
            <!-- Optional Reply Caching -->
    </section>
          <!-- Slot Identifiers and Server Reply Cache -->

    <section anchor="Retry_and_Replay" numbered="true" toc="default">
            <name>Retry and Replay of Reply</name>
            <t>
     A requester MUST NOT retry a request, unless
     the connection it used to send the request
     disconnects. The requester can then reconnect
     and re-send the request, or it can re-send the
     request over a different connection that is
     associated with the same session.
            </t>
            <t>
     If the requester is a server wanting to re-send a callback
     operation over the backchannel of a session, the requester
     of course cannot reconnect because only the client can
     associate connections with the backchannel. The
     server can re-send the request over another connection that
     is bound to the same session's backchannel. If there is no
     such connection, the server
     MUST indicate that the session has no backchannel by setting
     the SEQ4_STATUS_CB_PATH_DOWN_SESSION flag bit in the response
     to the next SEQUENCE operation from the client. The client MUST
     then associate a connection with the session (or destroy
     the session).
            </t>
            <t>
     Note that it is not fatal for a requester to retry
     without a disconnect between the request and retry.
     However, the retry does consume resources, especially
     with RDMA, where each request, retry or not, consumes
     a credit. Retries for no reason, especially retries
     sent shortly after the previous attempt, are a poor
     use of network bandwidth and defeat the purpose of a
     transport's inherent congestion control system.
            </t>
            <t>
     A requester MUST wait for a reply to a request before using
     the slot for another request. If it does not wait for
     a reply, then the requester does not know what
     sequence ID to use for the slot on its next request.
     For example, suppose a requester sends a request with sequence ID
     1, and does not wait for the response. The next time it uses
     the slot, it sends the new request with sequence ID 2.
     If the replier has not seen the request with sequence ID 1, then
     the replier is not expecting sequence ID 2, and rejects the
     requester's new request with NFS4ERR_SEQ_MISORDERED (as the
     result from SEQUENCE or CB_SEQUENCE).
            </t>
            <t>
     RDMA fabrics do not guarantee that the memory handles
     (Steering Tags) within each RPC/RDMA "chunk" <xref target="RFC8166" format="default"/>
     are valid on a scope
     outside that of a single connection.  Therefore, handles used by
     the direct operations become invalid after connection loss.  The
     server must ensure that any RDMA operations that must be replayed
     from the reply cache use the newly provided handle(s) from the
     most recent request.
            </t>
            <t>
     A retry might be sent while the original request is still in
     progress on the replier. The replier SHOULD deal with the issue
     by returning NFS4ERR_DELAY as the reply to SEQUENCE or CB_SEQUENCE
     operation, but implementations MAY return NFS4ERR_MISORDERED.
     Since errors from SEQUENCE and CB_SEQUENCE are
     never recorded in the reply cache, this approach allows the
     results of the execution of the original request to be
     properly recorded in the reply cache (assuming that the requester
     specified the reply to be cached).
            </t>
          </section>
          <!-- Retry and Replay -->

    <section anchor="sessions_callback_races" numbered="true" toc="default">
            <name>Resolving Server Callback Races</name>
            <t>
     It is possible for server callbacks to arrive at the
     client before the reply from related fore channel
     operations. For example, a client may have been
     granted a delegation to a file it has opened, but the
     reply to the OPEN (informing the client of the
     granting of the delegation) may be delayed in the
     network. If a conflicting operation arrives at the
     server, it will recall the delegation using the
     backchannel, which may be on a different
     transport connection, perhaps even a different
     network, or even a different session associated with
     the same client ID.
            </t>
            <t>
     The presence of a session between the client and server
     alleviates this issue. When a session is in place,
     each client request is uniquely identified by its {
     session ID, slot ID, sequence ID } triple. By the rules under which
     slot entries (reply cache entries) are
     retired, the server has knowledge whether the client
     has "seen" each of the server's replies. The server
     can therefore provide sufficient information to the
     client to allow it to disambiguate between an
     erroneous or conflicting callback race
     condition.
            </t>
            <t>
     For each client operation that might result in some
     sort of server callback, the server SHOULD "remember"
     the { session ID, slot ID, sequence ID } triple of the client request
     until the slot ID retirement rules allow the server to
     determine that the client has, in fact, seen the
     server's reply. Until the time the { session ID, slot ID,
     sequence ID } request triple can be retired, any recalls
     of the associated object MUST carry an array of these
     referring identifiers (in the CB_SEQUENCE operation's
     arguments), for the benefit of the client.  After this
     time, it is not necessary for the server to provide
     this information in related callbacks, since it is
     certain that a race condition can no longer occur.
            </t>
            <t>
     The CB_SEQUENCE operation that begins each server
     callback carries a list of "referring" { session ID, slot ID,
     sequence ID } triples.  If the client finds the request
     corresponding to the referring session ID, slot ID, and sequence ID
     to be currently outstanding (i.e., the server's reply has
     not been seen by the client), it can determine that
     the callback has raced the reply, and act
     accordingly. If the client does not find the request
     corresponding to the referring triple to be outstanding (including
     the case of a session ID referring to a destroyed session),
     then there is no race with respect to this triple.
     The server SHOULD limit the referring triples
     to requests that refer to just those that apply to the objects
     referred to in
     the CB_COMPOUND procedure.
            </t>
            <t>
     The client must not simply wait forever for the
     expected server reply to arrive before responding to the
     CB_COMPOUND that won the race,
     because it is possible
     that it will be delayed indefinitely. The client should
     assume the likely case that the reply will arrive within
     the average round-trip time for COMPOUND requests to the
     server, and wait that period of time. If
     that period of time
     expires, it can respond to the CB_COMPOUND with
     NFS4ERR_DELAY.  There are other scenarios under which callbacks
     may race replies.
     Among them are pNFS layout recalls as described in
     <xref target="pnfs_operation_sequencing" format="default"/>.
            </t>
          </section>
          <!-- Resolving server callback races with sessions -->
   <section anchor="COMPOUND_Sizing_Issues" numbered="true" toc="default">
            <name>COMPOUND and CB_COMPOUND Construction Issues</name>
            <t>
    Very large requests and replies may pose both buffer
    management issues (especially with RDMA) and reply
    cache issues. When the session is created
    (<xref target="OP_CREATE_SESSION" format="default"/>), for each channel (fore and
    back), the client and server
    negotiate the maximum-sized request they will
    send or process (ca_maxrequestsize), the maximum-sized reply
    they will return or process (ca_maxresponsesize), and the
    maximum-sized reply they will store in the reply cache
    (ca_maxresponsesize_cached).
            </t>
            <t>
    If a request exceeds ca_maxrequestsize, the reply will
    have the status NFS4ERR_REQ_TOO_BIG. A replier MAY
    return NFS4ERR_REQ_TOO_BIG as the status for the first operation
    (SEQUENCE or CB_SEQUENCE) in the request (which means that
    no operations in the request executed and that the
    state of the slot in the reply cache is unchanged), or it MAY
    opt to return it on a subsequent operation in the same
    COMPOUND or CB_COMPOUND request (which means that at least one
    operation did execute and that the state of the slot in the reply cache does
    change). The replier SHOULD set NFS4ERR_REQ_TOO_BIG on the
    operation that exceeds ca_maxrequestsize.
            </t>
            <t>
    If a reply exceeds ca_maxresponsesize, the reply will
    have the status NFS4ERR_REP_TOO_BIG. A replier MAY
    return NFS4ERR_REP_TOO_BIG as the status for the first operation
    (SEQUENCE or CB_SEQUENCE) in the request, or it MAY
    opt to return it on a subsequent operation (in the same
    COMPOUND or CB_COMPOUND reply). A replier MAY return NFS4ERR_REP_TOO_BIG
    in the reply to SEQUENCE or CB_SEQUENCE, even if the response
    would still exceed ca_maxresponsesize.
            </t>
            <t>
    If sa_cachethis or csa_cachethis is TRUE, then the
    replier MUST cache a reply except if an error is
    returned by the SEQUENCE or CB_SEQUENCE operation (see
    <xref target="err_sequence" format="default"/>). If the reply exceeds
    ca_maxresponsesize_cached (and sa_cachethis or
    csa_cachethis is TRUE), then the server MUST return
    NFS4ERR_REP_TOO_BIG_TO_CACHE. Even if
    NFS4ERR_REP_TOO_BIG_TO_CACHE (or any other error for
    that matter) is returned on an operation other than the
    first operation (SEQUENCE or CB_SEQUENCE), then
    the reply MUST be cached if sa_cachethis or
    csa_cachethis is TRUE.
    For example, if a COMPOUND has eleven
    operations, including SEQUENCE, the fifth operation is
    a RENAME, and the tenth operation is a READ for one
    million bytes, the server may return
    NFS4ERR_REP_TOO_BIG_TO_CACHE on the tenth operation.
    Since the server executed several operations, especially
    the non-idempotent RENAME, the client's request to
    cache the reply needs to be honored in order for the
    correct operation of exactly once semantics. If the
    client retries the request, the server will have cached
    a reply that contains results for ten of the eleven requested
    operations, with
    the tenth operation having a status of NFS4ERR_REP_TOO_BIG_TO_CACHE.
            </t>
            <t>
    A client needs to take care that, when sending
    operations that change the current filehandle (except for
    PUTFH, PUTPUBFH, PUTROOTFH, and RESTOREFH), it
    does not exceed the maximum reply buffer before the GETFH
    operation. Otherwise, the client will have to retry
    the operation that changed the current filehandle, in order
    to obtain the desired filehandle.
    For the OPEN operation (see <xref target="OP_OPEN" format="default"/>),
    retry is not always available as an option.
    The following guidelines for the handling of
    filehandle-changing operations are advised:
            </t>
            <ul spacing="normal">
              <li>
     Within the same COMPOUND procedure, a client
     SHOULD send GETFH immediately after a current
     filehandle-changing operation. A client
     MUST send GETFH after a current filehandle-changing operation
     that is also non-idempotent (e.g., the OPEN operation), unless
     the operation is RESTOREFH. RESTOREFH is
     an exception, because even though it is
     non-idempotent, the filehandle RESTOREFH
     produced originated from an operation that
     is either idempotent (e.g., PUTFH, LOOKUP),
     or non-idempotent (e.g., OPEN, CREATE). If the
     origin is non-idempotent, then because the client
     MUST send GETFH after the origin operation, the
     client can recover if RESTOREFH returns an error.

    </li>
              <li>
     A server MAY return NFS4ERR_REP_TOO_BIG or
     NFS4ERR_REP_TOO_BIG_TO_CACHE (if sa_cachethis is TRUE)
     on a filehandle-changing operation if the reply would
     be too large on the next operation.
    </li>
              <li>
     A server SHOULD return NFS4ERR_REP_TOO_BIG or
     NFS4ERR_REP_TOO_BIG_TO_CACHE (if sa_cachethis is TRUE)
     on a filehandle-changing, non-idempotent operation if the reply would
     be too large on the next operation, especially if the operation
     is OPEN.
    </li>
              <li>
     A server MAY return NFS4ERR_UNSAFE_COMPOUND to a non-idempotent
     current filehandle-changing operation, if
     it looks at the next operation (in the same COMPOUND procedure)
     and finds it is
     not GETFH. The server SHOULD do this if it is unable to
     determine in advance whether the total response size
     would exceed ca_maxresponsesize_cached or ca_maxresponsesize.
    </li>
            </ul>
          </section>
          <!-- COMPOUND and CB_COMPOUND Construction Issues -->
   <section anchor="Persistence" numbered="true" toc="default">
            <name>Persistence</name>
            <t>
    Since the reply cache is bounded, it is practical for
    the reply cache to persist across server restarts.
    The replier MUST persist the following information
    if it agreed to persist the session (when the session
    was created; see <xref target="OP_CREATE_SESSION" format="default"/>):

            </t>
            <ul spacing="normal">
              <li>
     The session ID.
    </li>
              <li>
     The slot table including the sequence ID and cached reply for
     each slot.
    </li>
            </ul>
            <t>
    The above are sufficient for a replier to provide EOS semantics
    for any requests that were sent and executed before the server
    restarted.
    If the replier is a client, then there is no need for
    it to persist any more information, unless the client will
    be persisting all other state across client restart, in which case,
    the server will never see any NFSv4.1-level protocol manifestation
    of a client restart.
    If the replier is a server, with just the
    slot table and session ID persisting,
    any requests the client retries after the server restart will
    return the results that are cached in the reply cache,
    and any new requests (i.e., the sequence ID is one greater than the
    slot's sequence ID) MUST be rejected with NFS4ERR_DEADSESSION
    (returned by SEQUENCE). Such a session is considered dead.
    A server MAY re-animate a session
    after a server restart so that the session will accept new
    requests as well as retries. To re-animate a session,
    the server needs to persist additional information
    through server restart:
            </t>
            <ul spacing="normal">
              <li>
     The client ID. This is a prerequisite to let the client
     create more sessions associated with the same client ID
     as the re-animated session.
    </li>
              <li>
     The client ID's sequence ID that is used for creating
     sessions (see Sections <xref target="OP_EXCHANGE_ID" format="counter"/> and
     <xref target="OP_CREATE_SESSION" format="counter"/>). This is a
     prerequisite to let the client create more sessions.
    </li>
              <li>
     The principal that created the client ID. This
     allows the server to authenticate the client when
     it sends EXCHANGE_ID.
    </li>
              <li>
     The SSV, if SP4_SSV state protection was
     specified when the client ID was created (see <xref target="OP_EXCHANGE_ID" format="default"/>). This lets the
     client create new sessions, and associate connections
     with the new and existing sessions.
    </li>
              <li>
     The properties of the client ID as defined in
     <xref target="OP_EXCHANGE_ID" format="default"/>.
    </li>
            </ul>
            <t>
    A persistent reply cache places certain demands on the server.
    The execution of the sequence of operations (starting with SEQUENCE)
    and placement of its results in the persistent cache MUST be atomic. If
    a client retries a sequence of operations that was previously
    executed on the server, the only acceptable outcomes are either
    the original cached reply or an indication that the client ID
    or session has been lost (indicating a catastrophic loss
    of the reply cache or a session that has been deleted because
    the client failed to use the session for an extended period
    of time).
            </t>
            <t>
    A server could fail and restart in the middle of a
    COMPOUND procedure that contains one or more non-idempotent
    or idempotent-but-modifying operations. This creates
    an even higher challenge for atomic execution and
    placement of results in the reply cache. One way
    to view the problem is as a single transaction consisting of
    each operation in the COMPOUND followed by storing
    the result in persistent storage, then finally a transaction
    commit. If there is a failure before the transaction
    is committed, then the server rolls back the transaction.
    If the server itself fails, then when it restarts, its
    recovery logic could roll back the transaction
    before starting the NFSv4.1 server.
            </t>
            <t>
    While the description of the
    implementation for atomic execution of the request
    and caching of the reply
    is beyond the scope of this document, an example implementation
    for NFSv2 <xref target="RFC1094" format="default"/> is described in <xref target="ha_nfs_ibm" format="default"/>.
            </t>
          </section>
          <!-- Persistence -->
  </section>
        <!-- Exactly Once Semantics -->
  <section anchor="RDMA_Considerations" numbered="true" toc="default">
          <name>RDMA Considerations</name>
          <t>
   A complete discussion of the operation of RPC-based
   protocols over RDMA transports is in <xref target="RFC8166" format="default"/>. A
   discussion of the operation of NFSv4, including NFSv4.1,
   over RDMA is in <xref target="RFC8267" format="default"/>.  Where RDMA is considered,
   this specification assumes the use of such a layering;
   it addresses only the upper-layer issues relevant to
   making best use of RPC/RDMA.

          </t>
          <section anchor="RDMA_Connection_Resources" numbered="true" toc="default">
            <name>RDMA Connection Resources</name>
            <t>
    RDMA requires its consumers to register memory and post
    buffers of a specific size and number for receive
    operations.

            </t>
            <t>
    Registration of memory can be a relatively high-overhead operation,
    since it requires pinning of buffers, assignment of attributes
    (e.g., readable/writable), and initialization of hardware
    translation.  Preregistration is desirable to reduce overhead.
    These registrations are specific to hardware interfaces and even to
    RDMA connection endpoints; therefore, negotiation of their limits is
    desirable to manage resources effectively.
            </t>
            <t>
    Following basic registration, these buffers must be posted by
    the RPC layer to handle receives.  These buffers remain in use by
    the RPC/NFSv4.1 implementation; the size and number of them must be
    known to the remote peer in order to avoid RDMA errors that would
    cause a fatal error on the RDMA connection.
            </t>
            <t>
    NFSv4.1 manages slots as resources on a per-session
    basis (see <xref target="Session" format="default"/>), while RDMA
    connections manage credits on a per-connection basis.
    This means that in order for a peer to send data over
    RDMA to a remote buffer, it has to have both an NFSv4.1
    slot and an RDMA credit.  If multiple RDMA connections
    are associated with a session, then if the total number
    of credits across all RDMA connections associated with
    the session is X, and the number of slots in the session
    is Y, then the maximum number of outstanding requests
    is the lesser of X and Y.

            </t>
          </section>
          <!-- RDMA Connection Resources -->
   <section anchor="Flow_Control" numbered="true" toc="default">
            <name>Flow Control</name>
            <t>
    Previous versions of NFS do not provide flow control;
    instead, they rely on the windowing provided by
    transports like TCP to throttle requests.  This does
    not work with RDMA, which provides no operation flow
    control and will terminate a connection in error when
    limits are exceeded.

    Limits such as maximum number of requests
    outstanding are therefore negotiated when a session
    is created (see the ca_maxrequests field in <xref target="OP_CREATE_SESSION" format="default"/>).  These limits then
    provide the maxima within which each connection associated
    with the session's channel(s) must remain.
    RDMA connections are managed within these limits as
    described in Section 3.3 of <xref target="RFC8166" format="default"/>; if there are multiple
    RDMA connections, then the maximum number of requests
    for a channel will be divided among the RDMA
    connections.  Put a different way, the onus is on the
    replier to ensure that the total number of RDMA credits
    across all connections associated with the replier's
    channel does exceed the channel's maximum number of
    outstanding requests.

            </t>
            <t>
    The limits may also be modified
    dynamically at the replier's choosing by manipulating
    certain parameters present in each NFSv4.1 reply. In
    addition, the CB_RECALL_SLOT callback operation (see
    <xref target="OP_CB_RECALL_SLOT" format="default"/>) can be sent by
    a server to a client to return RDMA credits to the
    server, thereby lowering the maximum number of requests
    a client can have outstanding to the server.

            </t>
          </section>
          <!-- Flow Control -->

   <section anchor="Padding" numbered="true" toc="default">
            <name>Padding</name>
            <t>
        Header padding is requested by each peer at session initiation
        (see the ca_headerpadsize argument to CREATE_SESSION in
        <xref target="OP_CREATE_SESSION" format="default"/>), and
        subsequently used by the RPC RDMA layer, as described in <xref target="RFC8166" format="default"/>.
        Zero padding is permitted.
            </t>
            <t>
        Padding leverages the useful property
        that RDMA preserve alignment of data, even when they are
        placed into anonymous (untagged) buffers.  If requested, client
        inline writes will insert appropriate pad bytes within the request
        header to align the data payload on the specified boundary.  The
        client is encouraged to add sufficient padding (up to the
        negotiated size) so that
        the "data" field of the WRITE operation
        is aligned.
        Most servers can make good use of such padding,
        which allows them to chain receive buffers in such a way that any
        data carried by client requests will be placed into appropriate
        buffers at the server, ready for file system processing.  The
        receiver's RPC layer encounters no overhead from skipping over pad
        bytes, and the RDMA layer's high performance makes the insertion
        and transmission of padding on the sender a significant
        optimization.  In this way, the need for servers to perform RDMA
        Read to satisfy all but the largest client writes is obviated.  An
        added benefit is the reduction of message round trips on the network
        -- a potentially good trade, where latency is present.
            </t>
            <t>
        The value to choose for padding is subject to a number of criteria.
        A primary source of variable-length data in the RPC header is the
        authentication information, the form of which is client-determined,
        possibly in response to server specification.  The contents of
        COMPOUNDs, sizes of strings such as those passed to RENAME, etc. all
        go into the determination of a maximal NFSv4.1 request size and
        therefore minimal buffer size.  The client must select its offered
        value carefully, so as to avoid overburdening the server, and vice
        versa.  The benefit of an appropriate padding value is higher
        performance.
            </t>
            <artwork name="" type="" align="left" alt=""><![CDATA[
                 Sender gather:
     |RPC Request|Pad  bytes|Length| -> |User data...|
     \------+----------------------/      \
             \                             \
              \    Receiver scatter:        \-----------+- ...
         /-----+----------------\            \           \
         |RPC Request|Pad|Length|   ->  |FS buffer|->|FS buffer|->...
   ]]></artwork>
            <t>
        In the above case, the server may recycle unused buffers to the
        next posted receive if unused by the actual received request, or
        may pass the now-complete buffers by reference for normal write
        processing.  For a server that can make use of it, this removes
        any need for data copies of incoming data, without resorting to
        complicated end-to-end buffer advertisement and management.  This
        includes most kernel-based and integrated server designs, among
        many others.  The client may perform similar optimizations, if
        desired.
            </t>
          </section>
          <!-- Padding -->
   <section anchor="dual" numbered="true" toc="default">
            <name>Dual RDMA and Non-RDMA Transports</name>
            <t>
    Some RDMA transports (e.g., RFC 5040 <xref target="RFC5040" format="default"/>)
    permit a "streaming" (non-RDMA) phase,
    where ordinary traffic might flow before "stepping up"
    to RDMA mode, commencing RDMA traffic.  Some RDMA
    transports start connections always in RDMA mode.
    NFSv4.1 allows, but does not assume, a streaming phase
    before RDMA mode.  When a connection
    is associated with a session, the client and server negotiate whether the
    connection is used in RDMA or non-RDMA mode  (see Sections
    <xref target="OP_CREATE_SESSION" format="counter"/> and
    <xref target="OP_BIND_CONN_TO_SESSION" format="counter"/>).
            </t>
          </section>
          <!-- RDMA Transports -->

  </section>
        <!-- RDMA Considerations -->

  <section anchor="Sessions_Security" numbered="true" toc="default">
          <name>Session Security</name>
          <section anchor="Session_Callback_Security" numbered="true" toc="default">
            <name>Session Callback Security</name>
            <t>
    Via session/connection association, NFSv4.1 improves security over
    that provided by NFSv4.0 for the backchannel.  The
    connection is client-initiated (see
    <xref target="OP_BIND_CONN_TO_SESSION" format="default"/>) and subject to the same
    firewall and routing checks as the fore channel.
    At the client's option (see <xref target="OP_EXCHANGE_ID" format="default"/>),
    connection association is fully authenticated before being
    activated (see <xref target="OP_BIND_CONN_TO_SESSION" format="default"/>).
    Traffic from the server over the
    backchannel is authenticated exactly as the client specifies
    (see <xref target="Backchannel_RPC_Security" format="default"/>).
            </t>
          </section>
          <!-- Session Callback Security -->
    <section anchor="Backchannel_RPC_Security" numbered="true" toc="default">
            <name>Backchannel RPC Security</name>
            <t>
     When the NFSv4.1 client establishes the backchannel, it
     informs the  server of the security flavors and principals
     to use when sending requests. If the security flavor is
     RPCSEC_GSS, the client expresses the principal in the form
     of an established RPCSEC_GSS context.  The server is free
     to use any of the flavor/principal combinations the client
     offers, but it MUST NOT use unoffered combinations.

     This way, the client need not provide a target
     GSS principal for the backchannel as it did with
     NFSv4.0, nor does the server have to implement an
     RPCSEC_GSS initiator as it did with NFSv4.0 <xref target="RFC3530" format="default"/>.

            </t>
            <t>
     The CREATE_SESSION (<xref target="OP_CREATE_SESSION" format="default"/>)
     and BACKCHANNEL_CTL (<xref target="OP_BACKCHANNEL_CTL" format="default"/>)
     operations allow the client to specify flavor/principal combinations.
            </t>
            <t>
     Also note that the SP4_SSV state protection mode
     (see Sections <xref target="OP_EXCHANGE_ID" format="counter"/> and <xref target="protect_state_change" format="counter"/>) has the side
     benefit of providing SSV-derived RPCSEC_GSS contexts (<xref target="ssv_mech" format="default"/>).
            </t>
          </section>
          <!-- Backchannel RPC Security -->

    <section anchor="protect_state_change" numbered="true" toc="default">
            <name>Protection from Unauthorized State Changes</name>
            <t>
     As described to this point in the specification, the state model
     of NFSv4.1 is vulnerable to an attacker that
     sends a SEQUENCE operation with a forged session ID and with a slot ID that
     it expects the legitimate client to use next. When the legitimate client
     uses the slot ID with the same sequence number, the server
     returns the attacker's result from the reply cache, which
     disrupts the legitimate client and thus denies service to it.
     Similarly, an attacker could send a CREATE_SESSION with a forged
     client ID to create a new session associated with the client ID.
     The attacker could send requests using the new session that
     change locking state, such as LOCKU operations to release locks
     the legitimate client has acquired. Setting a security
     policy on the file that requires RPCSEC_GSS credentials when
     manipulating the file's state is one potential work around,
     but has the disadvantage of preventing a legitimate client from
     releasing state when RPCSEC_GSS is required to do so, but
     a GSS context cannot be obtained (possibly because the user
     has logged off the client).
            </t>
            <t>
     NFSv4.1 provides three options to a client for state protection,
     which are specified when a client creates
     a client ID via EXCHANGE_ID (<xref target="OP_EXCHANGE_ID" format="default"/>).
            </t>
            <t>
     The first (SP4_NONE) is to simply waive state protection.
            </t>
            <t>
     The other two options (SP4_MACH_CRED and SP4_SSV)
     share several traits:
            </t>
            <ul spacing="normal">
              <li>
      An RPCSEC_GSS-based credential is used to authenticate
      client ID and session maintenance operations,
      including creating and destroying a session,
      associating a connection with the session, and
      destroying the client ID.
     </li>
              <li>
      Because RPCSEC_GSS is used to authenticate
      client ID and session maintenance, the attacker cannot
      associate a rogue connection with a legitimate session, or
      associate a rogue session with a legitimate client ID in
      order to maliciously alter the client ID's lock state
      via CLOSE, LOCKU, DELEGRETURN, LAYOUTRETURN, etc.
     </li>
              <li>
      In cases where the server's security policies on a
      portion of its namespace require RPCSEC_GSS authentication,
      a client may have to use an RPCSEC_GSS credential
      to remove per-file state (e.g., LOCKU, CLOSE, etc.).
      The server may require that the principal that removes
      the state match certain criteria (e.g.,
      the principal might have to be the same as the one
      that acquired the state). However, the client might
      not have an RPCSEC_GSS context for such a principal,
      and might not be able to create such a context (perhaps
      because the user has logged off). When the client
      establishes SP4_MACH_CRED or SP4_SSV protection,
      it can specify a list of operations that the server MUST
      allow using the machine credential (if SP4_MACH_CRED
      is used) or the SSV credential (if SP4_SSV is used).
     </li>
            </ul>
            <t>
     The SP4_MACH_CRED  state protection option uses a machine
     credential where the principal that
     creates the client ID MUST also be the principal
     that performs client ID and session maintenance
    operations.
     The security of the machine credential state protection approach
     depends entirely on safeguarding the per-machine credential.
     Assuming a proper safeguard using the per-machine credential
     for operations like CREATE_SESSION, BIND_CONN_TO_SESSION,
     DESTROY_SESSION, and DESTROY_CLIENTID will prevent an attacker
     from associating a rogue connection with a session, or
     associating a rogue session with a client ID.
            </t>
            <t>
     There are at least three scenarios for the SP4_MACH_CRED
     option:
            </t>
            <ol spacing="normal" type="1">
              <li>
      The system administrator configures a unique,
      permanent per-machine credential for one of the
      mandated GSS mechanisms (e.g., if Kerberos
      V5 is used, a "keytab" containing a principal derived from a
      client host name could be used).

     </li>
              <li>
      The client is used by a single user, and so the
      client ID and its sessions are used by just that
      user. If the user's credential expires, then session
      and client ID maintenance cannot occur, but since
      the client has a single user, only that user is
      inconvenienced.

     </li>
              <li>
      The physical client has multiple users, but the
      client implementation has a unique client ID for
      each user. This is effectively the same as the
      second scenario, but a disadvantage is that each
      user needs to be allocated at least one session each,
      so the approach suffers from lack of economy.

     </li>
            </ol>
            <t>
     The SP4_SSV protection option uses the SSV (<xref target="intro_definitions" format="default"/>), via RPCSEC_GSS and the SSV GSS
     mechanism (<xref target="ssv_mech" format="default"/>), to protect state from attack.
     The SP4_SSV protection option is intended for the situation
     comprised of a client that has multiple active users and a system
     administrator who wants to avoid the burden of installing a permanent
     machine credential on each client.  The SSV is
     established and updated on the server via SET_SSV (see <xref target="OP_SET_SSV" format="default"/>). To prevent eavesdropping,
     a client SHOULD send SET_SSV via RPCSEC_GSS with
     the privacy service.  Several aspects of the SSV
     make it intractable for an attacker to guess the SSV,
     and thus associate rogue connections with a session,
     and rogue sessions with a client ID:

            </t>
            <ul spacing="normal">
              <li>
      The arguments to and results of SET_SSV include digests of the old and
      new SSV, respectively.
    </li>
              <li>
      Because the initial value of the SSV is zero,
      therefore known, the client that opts for SP4_SSV
      protection and opts to apply SP4_SSV protection to
      BIND_CONN_TO_SESSION and CREATE_SESSION MUST send
      at least one SET_SSV operation before the first
      BIND_CONN_TO_SESSION operation or before the second
      CREATE_SESSION operation on a client ID. If it does
      not, the SSV mechanism will not generate tokens
      (<xref target="ssv_mech" format="default"/>).

      A client SHOULD send SET_SSV as soon as a session
      is created.

    </li>
              <li>
      A SET_SSV request does not replace the SSV with the argument to
      SET_SSV. Instead, the current SSV on the server is logically
      exclusive ORed (XORed) with the argument to SET_SSV.
      Each time a new principal uses a client ID for the first
      time, the client
      SHOULD send a SET_SSV with that principal's RPCSEC_GSS
      credentials, with RPCSEC_GSS service set to RPC_GSS_SVC_PRIVACY.
    </li>
            </ul>
            <t>
     Here are the types of attacks that can be attempted by an attacker named
     Eve on a victim named Bob, and how SP4_SSV protection foils
     each attack:
            </t>
            <ul spacing="normal">
              <li>
                <t>
       Suppose Eve is the first user to log into a
       legitimate client.  Eve's use of an NFSv4.1
       file system will cause the legitimate client to
       create a client ID
       with SP4_SSV protection, specifying that the BIND_CONN_TO_SESSION
       operation MUST use the SSV credential. Eve's use of
       the file system also causes an SSV to be created.  The
       SET_SSV operation that creates the SSV will be protected by
       the RPCSEC_GSS context created by the legitimate
       client, which uses Eve's GSS principal and
       credentials. Eve can eavesdrop on the network while
       her RPCSEC_GSS context is created and the SET_SSV
       using her context is sent. Even if the legitimate
       client sends the SET_SSV with RPC_GSS_SVC_PRIVACY,
       because Eve knows her own credentials, she can
       decrypt the SSV.  Eve can compute an RPCSEC_GSS
       credential that BIND_CONN_TO_SESSION will accept,
       and so associate a new connection with the
       legitimate session. Eve can change the slot ID and
       sequence state of a legitimate session, and/or the
       SSV state, in such a way that when Bob accesses
       the server via the same legitimate client, the
       legitimate client will be unable to use the session.

                </t>
                <t>

       The client's only recourse is to create a new client
       ID for Bob to use, and establish a new SSV for the
       client ID.  The client will be unable to delete
       the old client ID, and will let the lease on the old
       client ID expire.

                </t>
                <t>

       Once the legitimate client establishes an SSV over
       the new session using Bob's RPCSEC_GSS context,
       Eve can use the new session via the legitimate
       client, but she cannot disrupt Bob.  Moreover,
       because the client SHOULD have modified the SSV
       due to Eve using the new session, Bob cannot get
       revenge on Eve by associating a rogue connection
       with the session.

                </t>
                <t>

       The question is how did the legitimate client detect
       that Eve has hijacked the old session?  When the
       client detects that a new principal, Bob, wants to
       use the session, it SHOULD have sent a SET_SSV,
       which leads to the following sub-scenarios:

                </t>
                <ul spacing="normal">
                  <li>
                    <t>
         Let us suppose that from the rogue connection, Eve
         sent a SET_SSV with the same slot ID and sequence ID that
         the legitimate client later uses. The server will
         assume the SET_SSV sent with Bob's credentials is a retry,
         and return to the legitimate
         client the reply it sent Eve.  However, unless Eve can
         correctly guess the SSV the legitimate client will use,
         the digest verification checks in the SET_SSV response
         will fail.  That is an indication to the client that the
         session has apparently been hijacked.
                    </t>
                    <t/>
                  </li>
                  <li>
                    <t>
         Alternatively, Eve sent a SET_SSV with a different slot ID than
         the legitimate client uses for its SET_SSV. Then the digest
         verification of the SET_SSV sent with Bob's credentials fails
         on the server, and the error returned to the client makes it
         apparent that the session has been hijacked.
                    </t>
                    <t/>
                  </li>
                  <li>
                    <t>
         Alternatively, Eve sent an operation other than SET_SSV,
         but with the same slot ID and sequence that the legitimate client
         uses for its SET_SSV. The server returns to the legitimate
         client the response it sent Eve.  The client sees that the
         response is not at all what it expects. The client
         assumes either session hijacking or a server bug, and either way
         destroys the old session.
                    </t>
                    <t/>
                  </li>
                </ul>
              </li>
              <li>
                <t>
       Eve associates a rogue connection with the session
       as above, and then destroys the session. Again, Bob
       goes to use the server from the legitimate client,
       which sends a SET_SSV using Bob's credentials. The client receives an error
       that indicates that the session does not exist. When
       the client tries to create a new session, this
       will fail because the SSV it has does not match that which the
       server has, and now the client knows the session
       was hijacked. The legitimate client establishes a
       new client ID.

                </t>
                <t/>
              </li>
              <li>
                <t>
       If Eve creates a connection before the legitimate
       client establishes an SSV, because the initial
       value of the SSV is zero and therefore known,
       Eve can send a SET_SSV that will pass the digest
       verification check.  However, because the new
       connection has not been associated with the session,
       the SET_SSV is rejected for that reason.

                </t>
                <t/>
              </li>
            </ul>
            <t>
     In summary, an attacker's disruption of state when
     SP4_SSV protection is in use is limited to the
     formative period of a client ID, its first session,
     and the establishment of the SSV. Once a non-malicious
     user uses the client ID, the client quickly detects
     any hijack and rectifies the situation. Once a
     non-malicious user successfully modifies the SSV,
     the attacker cannot use NFSv4.1 operations to disrupt
     the non-malicious user.

            </t>
            <t>
     Note that neither the SP4_MACH_CRED nor
     SP4_SSV protection approaches prevent hijacking
     of a transport connection that has previously been
     associated with a session. If the goal of a counter-threat
     strategy is to prevent connection hijacking, the use of IPsec is RECOMMENDED.
            </t>
            <t>
     If a connection hijack occurs, the hijacker could in
     theory change locking state and negatively impact the
     service to legitimate clients.  However, if the server
     is configured to require the use of RPCSEC_GSS with
     integrity or privacy on the affected file objects, and
     if EXCHGID4_FLAG_BIND_PRINC_STATEID capability (<xref target="OP_EXCHANGE_ID" format="default"/>) is in force, this will
     thwart unauthorized attempts to change locking state.
            </t>
          </section>
          <!-- Protection from Unauthorized State Changes -->
  </section>
        <!-- Sessions Security -->
  <section anchor="ssv_mech" numbered="true" toc="default">
          <name>The Secret State Verifier (SSV) GSS Mechanism</name>
          <t>
   The SSV provides the secret key for a GSS mechanism internal to NFSv4.1
   that NFSv4.1 uses for state protection. Contexts for this
   mechanism are not established via the RPCSEC_GSS
   protocol.  Instead, the contexts are automatically
   created when EXCHANGE_ID specifies
   SP4_SSV protection.  The only tokens
   defined are the PerMsgToken (emitted by GSS_GetMIC)
   and the SealedMessage token (emitted by GSS_Wrap).
          </t>
          <t>
   The mechanism OID for the SSV mechanism is
   iso.org.dod.internet.private.enterprise.Michael
   Eisler.nfs.ssv_mech (1.3.6.1.4.1.28882.1.1).  While the
   SSV mechanism does not define any initial context
   tokens, the OID can be used to let servers indicate
   that the SSV mechanism is acceptable whenever the
   client sends a SECINFO or SECINFO_NO_NAME operation
   (see

   <xref target="Security_Service_Negotiation" format="default"/>).

          </t>
          <t>
   The SSV mechanism defines four subkeys derived from
   the SSV value. Each time SET_SSV is invoked, the subkeys
   are recalculated by the client and server. The
   calculation of each of the four subkeys depends on each
   of the four respective ssv_subkey4 enumerated values. The calculation
   uses the HMAC
   <xref target="RFC2104" format="default"/> algorithm, using the current SSV as the key, the one-way hash
   algorithm as negotiated by EXCHANGE_ID,
   and the input text as represented by the XDR encoded
   enumeration value for that subkey of data type ssv_subkey4.
   If the length of the output of the HMAC algorithm exceeds the length of
   key of the encryption algorithm (which is also negotiated by EXCHANGE_ID),
   then the subkey MUST be truncated from the HMAC output, i.e., if the
   subkey is of N bytes long, then the first N bytes of the HMAC output
   MUST be used for the subkey. The specification of EXCHANGE_ID
   states that the length of the output of the HMAC algorithm MUST NOT
   be less than the length of subkey needed for the encryption algorithm
   (see <xref target="OP_EXCHANGE_ID" format="default"/>).
          </t>
          <artwork name="" type="" align="left" alt=""><![CDATA[

/* Input for computing subkeys */
enum ssv_subkey4 {
        SSV4_SUBKEY_MIC_I2T     = 1,
        SSV4_SUBKEY_MIC_T2I     = 2,
        SSV4_SUBKEY_SEAL_I2T    = 3,
        SSV4_SUBKEY_SEAL_T2I    = 4
};

 ]]></artwork>
          <t>
   The subkey derived from SSV4_SUBKEY_MIC_I2T
   is used for calculating message integrity codes (MICs)
   that originate from the NFSv4.1 client, whether as part
   of a request over the fore channel or a response
   over the backchannel. The subkey derived from
   SSV4_SUBKEY_MIC_T2I is used for MICs originating from the
   NFSv4.1 server. The subkey derived from SSV4_SUBKEY_SEAL_I2T
   is used for encryption text originating from the NFSv4.1
   client, and the subkey derived from SSV4_SUBKEY_SEAL_T2I
   is used for encryption text originating from the
   NFSv4.1 server.
          </t>
          <t>
   The PerMsgToken description is based on an XDR definition:
          </t>
          <artwork name="" type="" align="left" alt=""><![CDATA[

/* Input for computing smt_hmac */
struct ssv_mic_plain_tkn4 {
  uint32_t        smpt_ssv_seq;
  opaque          smpt_orig_plain<>;
};

 ]]></artwork>
          <artwork name="" type="" align="left" alt=""><![CDATA[

/* SSV GSS PerMsgToken token */
struct ssv_mic_tkn4 {
  uint32_t        smt_ssv_seq;
  opaque          smt_hmac<>;
};

 ]]></artwork>
          <t>

   The field smt_hmac is an HMAC calculated by using the
   subkey derived from SSV4_SUBKEY_MIC_I2T or
   SSV4_SUBKEY_MIC_T2I  as the key, the one-way hash algorithm
   as negotiated by EXCHANGE_ID, and the input text
   as represented by data of type ssv_mic_plain_tkn4.
   The field smpt_ssv_seq is the same as smt_ssv_seq.
   The field smpt_orig_plain is the "message" input passed
   to GSS_GetMIC() (see Section 2.3.1 of <xref target="RFC2743" format="default"/>).
   The caller of GSS_GetMIC() provides a pointer to a buffer
   containing the plain text. The SSV mechanism's entry point for
   GSS_GetMIC() encodes this into an opaque array, and the encoding
   will include an initial four-byte length, plus any necessary padding.
   Prepended to this will be the XDR encoded value of smpt_ssv_seq,
   thus making up an XDR encoding of a value of data type
   ssv_mic_plain_tkn4, which in turn is the input into the HMAC.
          </t>
          <t>
   The token emitted by GSS_GetMIC() is XDR encoded and
   of XDR data type ssv_mic_tkn4.  The field smt_ssv_seq
   comes from the SSV sequence number, which is equal to
   one after SET_SSV (<xref target="OP_SET_SSV" format="default"/>)
   is called the first time on a client
   ID.
   Thereafter, the SSV sequence number is incremented on each SET_SSV.
   Thus, smt_ssv_seq represents the version of the SSV at
   the time GSS_GetMIC() was called.  As noted in <xref target="OP_EXCHANGE_ID" format="default"/>, the client and server
   can maintain multiple concurrent versions of the SSV.
   This allows the SSV to be changed without serializing
   all RPC calls that use the SSV mechanism with SET_SSV
   operations.
   Once the HMAC is calculated, it is XDR encoded into
   smt_hmac, which will include an initial four-byte length,
   and any necessary padding. Prepended to this will be
   the XDR encoded value of smt_ssv_seq.

          </t>
          <t>
   The SealedMessage description is based on an XDR definition:
          </t>
          <artwork name="" type="" align="left" alt=""><![CDATA[

/* Input for computing ssct_encr_data and ssct_hmac */
struct ssv_seal_plain_tkn4 {
  opaque          sspt_confounder<>;
  uint32_t        sspt_ssv_seq;
  opaque          sspt_orig_plain<>;
  opaque          sspt_pad<>;
};

 ]]></artwork>
          <artwork name="" type="" align="left" alt=""><![CDATA[

/* SSV GSS SealedMessage token */
struct ssv_seal_cipher_tkn4 {
  uint32_t      ssct_ssv_seq;
  opaque        ssct_iv<>;
  opaque        ssct_encr_data<>;
  opaque        ssct_hmac<>;
};

 ]]></artwork>
          <t>
   The token emitted by GSS_Wrap() is XDR encoded and
   of XDR data type ssv_seal_cipher_tkn4.

          </t>
          <t>
   The ssct_ssv_seq field has the same meaning as smt_ssv_seq.

          </t>
          <t>
   The ssct_encr_data field is the result of encrypting a
   value of the XDR encoded data type ssv_seal_plain_tkn4.
   The encryption key is the subkey derived from SSV4_SUBKEY_SEAL_I2T
   or SSV4_SUBKEY_SEAL_T2I, and the encryption
   algorithm is that negotiated by EXCHANGE_ID.
          </t>
          <t>
   The ssct_iv field is the initialization vector (IV)
   for the encryption algorithm (if applicable) and is
   sent in clear text. The content and size of the IV MUST
   comply with the specification of the encryption algorithm.
   For example, the id-aes256-CBC algorithm MUST use
   a 16-byte initialization vector (IV), which MUST be
   unpredictable for each instance of a value of data type
   ssv_seal_plain_tkn4 that is encrypted with a particular
   SSV key.

          </t>
          <t>
   The ssct_hmac field is the result of computing an HMAC using the value
   of the XDR encoded data type ssv_seal_plain_tkn4 as the input
   text. The key is the subkey derived from SSV4_SUBKEY_MIC_I2T or
   SSV4_SUBKEY_MIC_T2I, and the one-way hash algorithm is that
   negotiated by EXCHANGE_ID.

          </t>
          <t>
   The sspt_confounder field is a random value.

          </t>
          <t>
   The sspt_ssv_seq field is the same as ssvt_ssv_seq.

          </t>
          <t>
   The field sspt_orig_plain field is the original plaintext
   and is the "input_message" input passed to
   GSS_Wrap() (see Section 2.3.3 of <xref target="RFC2743" format="default"/>).
   As with the handling of the plaintext by the SSV mechanism's
   GSS_GetMIC() entry point, the entry point for GSS_Wrap()
   expects a pointer to the plaintext, and will XDR encode
   an opaque array into sspt_orig_plain
   representing the plain text, along with
   the other fields of an instance of data type ssv_seal_plain_tkn4.

          </t>
          <t>
   The sspt_pad field is present to support encryption
   algorithms that require inputs to be in fixed-sized
   blocks.  The content of sspt_pad is zero filled
   except for the length.  Beware that the XDR encoding
   of ssv_seal_plain_tkn4 contains three variable-length
   arrays, and so each array consumes four bytes for an
   array length, and each array that follows the length
   is always padded to a multiple of four bytes per the
   XDR standard.

          </t>
          <t>
   For example, suppose the encryption algorithm uses 16-byte blocks, and
   the sspt_confounder is three bytes long, and
   the sspt_orig_plain field is 15 bytes long.

   The XDR encoding of sspt_confounder uses eight bytes
   (4 + 3 + 1-byte pad),

   the XDR encoding of sspt_ssv_seq uses four bytes,

   the XDR encoding of sspt_orig_plain uses 20 bytes
   (4 + 15 + 1-byte pad),

   and the smallest XDR encoding of the sspt_pad field
   is four bytes.

   This totals 36 bytes. The next multiple of 16 is 48;
   thus, the length field of sspt_pad needs to be set to
   12 bytes, or a total encoding of 16 bytes.

   The total number of XDR encoded bytes is thus 8 +
   4 + 20 + 16 = 48.

          </t>
          <t>
   GSS_Wrap() emits a token that is an XDR
   encoding of a value of data type ssv_seal_cipher_tkn4.

   Note that regardless of whether or not the caller of GSS_Wrap()
   requests confidentiality, the token always has
   confidentiality. This is because the SSV mechanism
   is for RPCSEC_GSS, and RPCSEC_GSS never produces
   GSS_wrap() tokens without confidentiality.

          </t>
          <t>
   There is one SSV per client ID.
   There is a single GSS context for
   a client ID / SSV pair.
   All SSV mechanism RPCSEC_GSS handles of a client ID / SSV pair
   share the same GSS context.
   SSV GSS contexts do not expire except when the SSV
   is destroyed (causes would include the client ID
   being destroyed or a server restart).
   Since one
   purpose of context expiration is to replace keys that
   have been in use for "too long", hence vulnerable to
   compromise by brute force or accident, the client can
   replace the SSV key by
   sending periodic SET_SSV operations, which is done by cycling through
   different users' RPCSEC_GSS credentials. This way, the SSV is
   replaced without destroying the SSV's GSS contexts.
          </t>
          <t>
   SSV RPCSEC_GSS handles can be expired or deleted by the server
   at any time, and the EXCHANGE_ID operation can be used to create
   more SSV RPCSEC_GSS handles. Expiration of SSV RPCSEC_GSS handles
   does not imply that the SSV or its GSS context has expired.
          </t>
          <t>
   The client MUST establish an SSV via SET_SSV before the
   SSV GSS context can be used to emit tokens from GSS_Wrap()
   and GSS_GetMIC(). If SET_SSV has not been successfully
   called, attempts to emit tokens MUST fail.

          </t>
          <t>
   The SSV mechanism does not support replay detection and sequencing
   in its tokens because RPCSEC_GSS does not use those features (See
   Section 5.2.2, "Context Creation Requests", in <xref target="RFC2203" format="default"/>). However, <xref target="rpcsec_ssv_consider" format="default"/> discusses special
   considerations for the SSV mechanism when used with RPCSEC_GSS.

          </t>
        </section>
        <!-- The SSV GSS Mechanism -->

  <section anchor="rpcsec_ssv_consider" numbered="true" toc="default">
          <name>Security Considerations for RPCSEC_GSS When Using the SSV Mechanism</name>
          <t>
    When a client ID is created with SP4_SSV state protection (see <xref target="OP_EXCHANGE_ID" format="default"/>), the client is permitted to associate
    multiple RPCSEC_GSS handles with the single SSV GSS context
    (see <xref target="ssv_mech" format="default"/>). Because of the way RPCSEC_GSS
    (both version 1 and version 2, see <xref target="RFC2203" format="default"/> and
    <xref target="RFC5403" format="default"/>) calculate the verifier of the reply,
    special care must be taken by the implementation of the NFSv4.1
    client to prevent attacks by a man-in-the-middle.  The verifier
    of an RPCSEC_GSS reply is the output of GSS_GetMIC() applied to
    the input value of the seq_num field of the RPCSEC_GSS credential
    (data type rpc_gss_cred_ver_1_t) (see Section 5.3.3.2 of <xref target="RFC2203" format="default"/>). If multiple RPCSEC_GSS handles share the same
    GSS context, then if one handle is used to send a request with the
    same seq_num value as another handle, an attacker could block the
    reply, and replace it with the verifier used for the other handle.

          </t>
          <t>
   There are multiple ways to prevent the attack on the SSV RPCSEC_GSS
   verifier in the reply. The simplest is believed to be as follows.

          </t>
          <ul spacing="normal">
            <li>
   Each time one or more new SSV RPCSEC_GSS handles are created via
   EXCHANGE_ID, the client SHOULD send a SET_SSV operation to modify
   the SSV. By changing the SSV, the new handles will not result in the
   re-use of an SSV RPCSEC_GSS verifier in a reply.

  </li>
            <li>
   When a requester decides to use N SSV RPCSEC_GSS handles, it SHOULD
   assign a unique and non-overlapping range of seq_nums to each SSV
   RPCSEC_GSS handle. The size of each range SHOULD be equal to MAXSEQ
   / N (see Section 5 of <xref target="RFC2203" format="default"/> for the definition
   of MAXSEQ). When an SSV RPCSEC_GSS handle reaches its maximum, it
   SHOULD force the replier to destroy the handle by sending a NULL
   RPC request with seq_num set to MAXSEQ + 1 (see Section 5.3.3.3 of
   <xref target="RFC2203" format="default"/>).

  </li>
            <li>
   When the requester wants to increase or decrease N, it SHOULD force
   the replier to destroy all N handles by sending a NULL RPC request on
   each handle with seq_num set to MAXSEQ + 1. If the requester is the
   client, it SHOULD send a SET_SSV operation before using new handles.
   If the requester is the server, then the client SHOULD send a SET_SSV
   operation when it detects that the server has forced it to destroy a
   backchannel's SSV RPCSEC_GSS handle. By sending a SET_SSV operation,
   the SSV will change, and so the attacker will be unavailable to
   successfully replay a previous verifier in a reply to the requester.

  </li>
          </ul>
          <t>
    Note that if the replier carefully creates the SSV RPCSEC_GSS
    handles, the related risk of a man-in-the-middle splicing a forged
    SSV RPCSEC_GSS credential with a verifier for another handle does
    not exist. This is because the verifier in an RPCSEC_GSS request
    is computed from input that includes both the RPCSEC_GSS handle and
    seq_num (see Section 5.3.1 of <xref target="RFC2203" format="default"/>). Provided the
    replier takes care to avoid re-using the value of an RPCSEC_GSS
    handle that it creates, such as by including a generation number in the
    handle, the man-in-the-middle will not be able to successfully replay
    a previous verifier in the request to a replier.

          </t>
        </section>
        <section anchor="Session_Mechanics_Steady_State" numbered="true" toc="default">
          <name>Session Mechanics - Steady State</name>
          <section anchor="Obligations_of_the_Server" numbered="true" toc="default">
            <name>Obligations of the Server</name>
            <t>
    The server has the primary obligation to monitor the
    state of backchannel resources that the client has
    created for the server (RPCSEC_GSS contexts and backchannel
    connections). If these resources vanish, the
    server takes action as specified in <xref target="Events_Requiring_Server_Action" format="default"/>.
            </t>
          </section>
          <!-- Obligations of the Server -->

   <section anchor="Obligations_of_the_Client" numbered="true" toc="default">
            <name>Obligations of the Client</name>
            <t>
   The client SHOULD honor the following obligations in order to
   utilize the session:
            </t>
            <ul spacing="normal">
              <li>
     Keep a necessary session from going idle on the server. A client
     that requires a session but nonetheless is not
     sending operations risks having the session be destroyed
     by the server. This is because sessions consume
     resources, and resource limitations may force the
     server to cull an inactive session. A server MAY consider
     a session to be inactive if the client has not used
     the session before the session inactivity timer (<xref target="session_inactive" format="default"/>) has expired.

   </li>
              <li>
     Destroy the session when not needed. If a client has
     multiple sessions, one of which has no
     requests waiting for replies, and has been idle for
     some period of time, it SHOULD destroy the session.
   </li>
              <li>
     Maintain GSS contexts and RPCSEC_GSS handles
     for the backchannel. If the client
     requires the server to use the RPCSEC_GSS security
     flavor for callbacks, then it needs to be sure the
     RPCSEC_GSS handles and/or their GSS
     contexts that are handed to the server via BACKCHANNEL_CTL or
     CREATE_SESSION are unexpired.
   </li>
              <li>
     Preserve a connection for a backchannel. The server
     requires a backchannel in order to gracefully recall
     recallable state or notify the client of certain
     events. Note that if the connection is not being used
     for the fore channel, there is no way for the client to tell
     if the connection is still alive (e.g., the server
     restarted without sending a disconnect). The onus is
     on the server, not the client, to determine if the
     backchannel's connection is alive, and to indicate in
     the response to a SEQUENCE operation when the last
     connection associated with a session's backchannel
     has disconnected.

   </li>
            </ul>
          </section>
          <!-- Obligations of the Client -->

   <section anchor="Steps_the_Client_Takes_To_Establish_a_Session" numbered="true" toc="default">
            <name>Steps the Client Takes to Establish a Session</name>
            <t>
     If the client does not have a client ID, the client
     sends EXCHANGE_ID to establish a client ID.  If it
     opts for SP4_MACH_CRED or SP4_SSV protection, in the
     spo_must_enforce list of operations, it SHOULD at
     minimum specify CREATE_SESSION, DESTROY_SESSION,
     BIND_CONN_TO_SESSION, BACKCHANNEL_CTL, and DESTROY_CLIENTID.
     If it opts for SP4_SSV protection, the client needs to
     ask for SSV-based RPCSEC_GSS handles.

            </t>
            <t>
     The client uses the client ID to send a
     CREATE_SESSION on a connection to the server.
     The results of CREATE_SESSION indicate whether or not the
     server will persist the session reply cache through
     a server that has restarted, and the client notes this
     for future reference.

            </t>
            <t>
     If the client specified SP4_SSV state protection
     when the client ID was created, then it SHOULD send
     SET_SSV in the first COMPOUND after the session is
     created. Each time a new principal goes to use the
     client ID, it SHOULD send a SET_SSV again.

            </t>
            <t>
     If the client wants to use delegations, layouts,
     directory notifications, or any other state that
     requires a backchannel, then it needs to add a connection
     to the backchannel if CREATE_SESSION did not already
     do so.  The client creates a connection, and calls
     BIND_CONN_TO_SESSION to associate the connection
     with the session and the session's backchannel. If
     CREATE_SESSION did not already do so, the client MUST
     tell the server what security is required in order
     for the client to accept callbacks. The client does
     this via BACKCHANNEL_CTL. If the client selected
     SP4_MACH_CRED or SP4_SSV protection when it called
     EXCHANGE_ID, then the client SHOULD specify that the
     backchannel use RPCSEC_GSS contexts for security.

            </t>
            <t>
     If the client wants to use additional
     connections for the backchannel, then it needs to call
     BIND_CONN_TO_SESSION on each connection it wants to
     use with the session. If the client wants to use
     additional connections for the fore channel, then
     it needs to call BIND_CONN_TO_SESSION if it specified
     SP4_SSV or SP4_MACH_CRED state protection when the
     client ID was created.

            </t>
            <t>
     At this point, the session has reached steady state.
            </t>
          </section>
          <!-- Steps the Client Takes To Establish a Session -->
  </section>
        <!-- Session Mechanics - Steady State -->

  <section anchor="session_inactive" numbered="true" toc="default">
          <name>Session Inactivity Timer</name>
          <t>
   The server MAY maintain a session inactivity timer for
   each session.  If the session inactivity timer expires,
   then the server MAY destroy the session. To avoid losing
   a session due to inactivity, the client MUST renew
   the session inactivity timer. The length of session
   inactivity timer MUST NOT be less than the lease_time
   attribute (<xref target="attrdef_lease_time" format="default"/>).
   As with lease renewal (<xref target="lease_renewal" format="default"/>),
   when the server receives a SEQUENCE operation,
   it resets the session inactivity timer, and MUST NOT allow the
   timer to expire while the rest of the operations in the
   COMPOUND procedure's request are still executing. Once the
   last operation has finished, the server MUST set the session
   inactivity timer to expire no sooner than the sum of the
   current time and the value of the lease_time attribute.
          </t>
        </section>
        <section anchor="Session_Mechanics_Recovery" numbered="true" toc="default">
          <name>Session Mechanics - Recovery</name>
          <section anchor="Events_Requiring_Client_Action" numbered="true" toc="default">
            <name>Events Requiring Client Action</name>
            <t>
   The following events require client action to recover.
            </t>
            <section numbered="true" toc="default">
              <name>RPCSEC_GSS Context Loss by Callback Path</name>
              <t>
    If all RPCSEC_GSS handles
    granted by the client to the server for callback use have
    expired, the client MUST
    establish a new handle via BACKCHANNEL_CTL. The
    sr_status_flags field of the SEQUENCE results indicates when callback handles
    are nearly expired, or fully expired (see <xref target="OP_SEQUENCE_DESCRIPTION" format="default"/>).
              </t>
            </section>
            <!-- RPCSEC_GSS Context Loss by Callback_Path -->
   <section numbered="true" toc="default">
              <name>Connection Loss</name>
              <t>
    If the client loses the last connection of the session
    and wants to retain the session, then it needs to
    create a new connection, and if, when the client
    ID was created, BIND_CONN_TO_SESSION was specified
    in the spo_must_enforce list, the client MUST use
    BIND_CONN_TO_SESSION to associate the connection with
    the session.

              </t>
              <t>
    If there was a request outstanding at the time
    of connection loss, then if the client wants to continue
    to use the session, it MUST retry the request, as
    described in
    <xref target="Retry_and_Replay" format="default"/>. Note that it
    is not necessary to retry requests over a connection
    with the same source network address or the same
    destination network address as the lost connection. As
    long as the session ID, slot ID, and sequence ID in the
    retry match that of the original request, the server
    will recognize the request as a retry if it executed
    the request prior to disconnect.

              </t>
              <t>
    If the connection that was lost was the last one associated with
    the backchannel, and the client wants to retain the backchannel and/or
    prevent revocation of recallable state, the client needs to
    reconnect, and if it does, it
    MUST associate the connection to the session and backchannel via
    BIND_CONN_TO_SESSION.
    The server SHOULD indicate when it has no callback connection
    via the sr_status_flags result from SEQUENCE.
              </t>
            </section>
            <!-- Connection Disconnect -->
   <section numbered="true" toc="default">
              <name>Backchannel GSS Context Loss</name>
              <t>
    Via the sr_status_flags result of the SEQUENCE operation or
    other means, the client will learn if some or all of
    the RPCSEC_GSS contexts it assigned to the backchannel have
    been lost. If the client wants to retain the backchannel and/or
    not put recallable state subject to revocation,
    the client needs to use BACKCHANNEL_CTL to
    assign new contexts.
              </t>
            </section>
            <!-- Backchannel GSS Context Loss -->

    <section anchor="loss_of_session" numbered="true" toc="default">
              <name>Loss of Session</name>
              <t>
     The replier might lose a record of the session. Causes include:
              </t>
              <ul spacing="normal">
                <li>
        Replier failure and restart.
      </li>
                <li>
        A catastrophe that causes the reply cache to be corrupted or
        lost on the media on which it was stored. This applies
        even if the replier indicated in the CREATE_SESSION results
        that it would persist the cache.
      </li>
                <li>
        The server purges the session of a client that has been
        inactive for a very extended period of time.
      </li>
                <li>
        As a result of configuration changes among a set of clustered
        servers, a network address previously connected to one
        server becomes connected to a different server that has
        no knowledge of the session in question.  Such a configuration
        change will generally only happen when the original server
        ceases to function for a time.
      </li>
              </ul>
              <t>
     Loss of reply cache is equivalent to loss of session.
     The replier indicates loss of session to the requester
     by returning NFS4ERR_BADSESSION on the next operation
     that uses the session ID that refers to the lost
     session.
              </t>
              <t>
     After an event like a server restart, the client may have
     lost its connections. The client assumes for the moment
     that the session has not been lost. It reconnects, and
     if it specified connection association enforcement when
     the session was created, it
     invokes BIND_CONN_TO_SESSION using the session ID. Otherwise,
     it invokes SEQUENCE. If
     BIND_CONN_TO_SESSION or SEQUENCE returns NFS4ERR_BADSESSION, the
     client knows the session is not available to it when communicating
     with that network address. If the connection survives
     session loss, then the next SEQUENCE operation the client
     sends over the connection will get back NFS4ERR_BADSESSION.
     The client again knows the session was lost.
              </t>
              <t>
     Here is one suggested algorithm for the client when it gets
     NFS4ERR_BADSESSION.  It is not obligatory in that, if a
     client does not want to take advantage of such features as
     trunking, it may omit parts of it.  However, it is a useful
     example that draws attention to various possible recovery
     issues:
              </t>
              <ol spacing="normal" type="1">
                <li>
         If the client has other connections to
         other server network addresses
         associated with the same session, attempt
         a COMPOUND with a single operation, SEQUENCE,
         on each of the other connections.
       </li>
                <li>
         If the attempts succeed, the session is still alive,
         and this is a strong indicator that the server's
         network address has moved.
         The client might send an EXCHANGE_ID on the
         connection that returned NFS4ERR_BADSESSION
         to see if there are opportunities for client ID
         trunking (i.e., the same client ID and so_major value
	 are
         returned). The client might use DNS to see if
         the moved network address was replaced with another,
         so that the performance and availability benefits of
         session trunking can continue.
       </li>
                <li>
         If the SEQUENCE requests fail with NFS4ERR_BADSESSION,
         then the session no longer exists on any of the
         server network addresses for which the client has connections
         associated with that session ID. It is possible the
         session is still alive and available on other
         network addresses. The client sends an EXCHANGE_ID
         on all the connections to see if the server owner
         is still listening on those network addresses.
         If the same server owner is returned but a new
         client ID is returned, this is a strong
         indicator of a server restart. If both the same
         server owner and same client ID are
         returned, then this is a strong indication
         that the server did delete the session, and the
         client will need to send a CREATE_SESSION if it
         has no other sessions for that client ID.
         If a different server owner is returned,
         the client can use DNS to find
         other network addresses. If it does not, or if
         DNS does not find any other addresses for the server,
         then the client will be unable to provide NFSv4.1
         service, and fatal errors should be returned
         to processes that were using the server. If the
         client is using a "mount" paradigm, unmounting
         the server is advised.
       </li>
                <li>
         If the client knows of no other connections associated
         with the session ID and server network addresses that
         are, or have been, associated with the session ID,
         then the client can use DNS to find
         other network addresses. If it does not, or if
         DNS does not find any other addresses for the server,
         then the client will be unable to provide NFSv4.1
         service, and fatal errors should be returned
         to processes that were using the server. If the
         client is using a "mount" paradigm, unmounting
         the server is advised.
       </li>
              </ol>
              <t>
      If there is a reconfiguration event that results in the
      same network address being assigned to servers where the
      eir_server_scope value is different, it cannot be guaranteed
      that a session ID generated by the first will be recognized
      as invalid by the first.  Therefore, in managing server
      reconfigurations among servers with different server scope
      values, it is necessary to make sure that all clients have
      disconnected from the first server before effecting
      the reconfiguration.  Nonetheless, clients should not
      assume that servers will always adhere to this requirement;
      clients MUST be prepared to deal with unexpected
      effects of server reconfigurations.
      Even where a session ID is inappropriately
      recognized as valid, it is likely either that the connection
      will not be recognized as valid or that a sequence value
      for a slot will not be correct.  Therefore, when a client
      receives results indicating such unexpected errors, the use of
      EXCHANGE_ID to determine the current server configuration
      is RECOMMENDED.
              </t>
              <t>
      A variation on the above is that after a server's network
      address moves, there is no NFSv4.1 server listening, e.g., no
      listener on port 2049. In this example, one of the following occur: the NFSv4 server returns
      NFS4ERR_MINOR_VERS_MISMATCH, the NFS server returns a
      PROG_MISMATCH error, the RPC listener on 2049 returns
      PROG_UNVAIL, or attempts to reconnect to the network address
      timeout. These SHOULD be treated as equivalent to SEQUENCE
      returning NFS4ERR_BADSESSION for these purposes.
              </t>
              <t>
     When the client detects session loss, it needs to call CREATE_SESSION
     to recover.  Any non-idempotent operations that were in progress
     might have been performed on the server at the time of
     session loss. The client has no general way to recover from this.
              </t>
              <t>
     Note that loss of session does not imply loss of byte-range lock, open, delegation,
     or layout state because locks, opens, delegations, and layouts
     are tied to the client ID and depend on the client ID, not the session.
     Nor does loss of byte-range lock, open, delegation,
     or layout state imply loss of session state, because the session depends
     on the client ID; loss of client ID however does imply loss of
     session, byte-range lock, open, delegation, and layout state.
     See <xref target="server_failure" format="default"/>.
     A session can survive a server restart,
     but lock recovery may still be needed.
              </t>
              <t>
     It is possible that CREATE_SESSION will fail with NFS4ERR_STALE_CLIENTID
     (e.g., the server restarts and does not preserve client ID
     state).
     If so, the client needs to call EXCHANGE_ID, followed by
     CREATE_SESSION.
              </t>
            </section>
            <!-- Loss of Session -->
   </section>
          <!-- Events Requiring Client Action -->

   <section anchor="Events_Requiring_Server_Action" numbered="true" toc="default">
            <name>Events Requiring Server Action</name>
            <t>
     The following events require server action to recover.
            </t>
            <section numbered="true" toc="default">
              <name>Client Crash and Restart</name>
              <t>
    As described in <xref target="OP_EXCHANGE_ID" format="default"/>,
    a restarted client sends EXCHANGE_ID in such a way that it
    causes the server to delete any sessions it had.
              </t>
            </section>
            <!-- Client Crash and Restart -->
    <section anchor="client_crash_no_restart" numbered="true" toc="default">
              <name>Client Crash with No Restart</name>
              <t>
    If a client crashes and never comes back, it will never send
    EXCHANGE_ID with its old client owner. Thus, the server has session
    state that will never be used again. After an extended period of time,
    and if the server has resource constraints, it MAY destroy the old
    session as well as locking state.
              </t>
            </section>
            <!-- Client Crash with No Restart -->
    <section numbered="true" toc="default">
              <name>Extended Network Partition</name>
              <t>
     To the server, the extended network partition may be no
     different from a
     client crash with no
     restart (see
     <xref target="client_crash_no_restart" format="default"/>).
     Unless the server can discern that there is
     a network partition, it is free to treat the
     situation as if the client has crashed permanently.
              </t>
            </section>
            <!-- "Extended Network Partition" -->
    <section numbered="true" toc="default">
              <name>Backchannel Connection Loss</name>
              <t>
     If there were callback requests outstanding at the time
     of a connection loss, then the server
     MUST retry the requests, as described in
     <xref target="Retry_and_Replay" format="default"/>. Note that it
     is not necessary to retry requests over a connection
     with the same source network address or the same destination
     network address as the lost connection. As long as
     the session ID, slot ID, and sequence ID in the retry
     match that of the original request, the callback target will
     recognize the request as a retry even if it did see the request
     prior to disconnect.
              </t>
              <t>
     If the connection lost is the last one associated with the backchannel,
     then the server MUST indicate that in the sr_status_flags field of
     every SEQUENCE reply until the backchannel is re-established.
     There are two situations, each of which uses different
     status flags: no connectivity for the session's backchannel
     and no connectivity for any session backchannel of the client.
     See <xref target="OP_SEQUENCE" format="default"/> for a description of
     the appropriate flags in sr_status_flags.
              </t>
            </section>
            <!-- Backchannel Connection Loss -->
    <section numbered="true" toc="default">
              <name>GSS Context Loss</name>
              <t>
     The server SHOULD monitor when the number of RPCSEC_GSS
     handles assigned to the backchannel reaches one, and when that
     one handle is near expiry (i.e., between
     one and two periods of lease time), and
     indicate so in the sr_status_flags field of all SEQUENCE replies.
     The server MUST indicate when all of the
     backchannel's assigned RPCSEC_GSS handles
     have expired via the sr_status_flags field of all SEQUENCE replies.
              </t>
            </section>
            <!-- GSS Context Loss -->
   </section>
          <!-- Events Requiring Server Action -->
  </section>
        <!-- Session Mechanics - Recovery -->
  <section anchor="pnfs_and_sessions" numbered="true" toc="default">
          <name>Parallel NFS and Sessions</name>
          <t>
   A client and server can potentially be a non-pNFS implementation,
   a metadata server implementation, a data server implementation, or two or
   three types of implementations. The EXCHGID4_FLAG_USE_NON_PNFS,
   EXCHGID4_FLAG_USE_PNFS_MDS, and EXCHGID4_FLAG_USE_PNFS_DS flags
   (not mutually exclusive) are passed in the EXCHANGE_ID arguments
   and results to allow the client to indicate how it wants to use sessions created
   under the client ID, and to allow the server to indicate how it
   will allow the sessions to be used.
   See <xref target="pnfs_session_stuff" format="default"/> for pNFS sessions considerations.
          </t>
        </section>
        <!-- Parallel NFS and Sessions -->
 </section>
      <!-- Session -->
</section>
    <!-- Core Infrastructure -->
<!-- 	$Id: 2009-12-20-TO-rfc5661.xml,v 1.2 2009/12/21 05:59:32 shepler.mre Exp $	 -->
<!-- Copyright (C) The IETF Trust (2007-2008) -->
<!-- Copyright (C) The Internet Society (2006) -->
<section numbered="true" toc="default">
      <name>Protocol Constants and Data Types</name>
      <t>
    The syntax and semantics to describe the data types of the NFSv4.1
    protocol are defined in the XDR <xref target="RFC4506" format="default">RFC 4506</xref> and RPC
    <xref target="RFC5531" format="default">RFC 5531</xref> documents.  The next sections
    build upon the XDR data types to define constants, types, and structures
    specific to this protocol. The full list of XDR data types is in <xref target="RFC5662" format="default"/>.
      </t>
      <section numbered="true" toc="default">
        <name>Basic Constants</name>
        <artwork name="" type="" align="left" alt=""><![CDATA[
const NFS4_FHSIZE               = 128;
const NFS4_VERIFIER_SIZE        = 8;
const NFS4_OPAQUE_LIMIT         = 1024;
const NFS4_SESSIONID_SIZE       = 16;

const NFS4_INT64_MAX            = 0x7fffffffffffffff;
const NFS4_UINT64_MAX           = 0xffffffffffffffff;
const NFS4_INT32_MAX            = 0x7fffffff;
const NFS4_UINT32_MAX           = 0xffffffff;

const NFS4_MAXFILELEN           = 0xffffffffffffffff;
const NFS4_MAXFILEOFF           = 0xfffffffffffffffe;
 ]]></artwork>
        <t>
    Except where noted, all these constants are defined in bytes.
        </t>
        <ul spacing="normal">
          <li>
       NFS4_FHSIZE is the maximum size of a filehandle.
    </li>
          <li>
       NFS4_VERIFIER_SIZE is the fixed size of a verifier.
    </li>
          <li>
       NFS4_OPAQUE_LIMIT is the maximum size of certain
       opaque information.
    </li>
          <li>
       NFS4_SESSIONID_SIZE is the fixed size of a session identifier.
    </li>
          <li>
       NFS4_INT64_MAX is the maximum value of a signed 64-bit integer.
    </li>
          <li>
       NFS4_UINT64_MAX is the maximum value of an unsigned 64-bit integer.
    </li>
          <li>
       NFS4_INT32_MAX is the maximum value of a signed 32-bit integer.
    </li>
          <li>
       NFS4_UINT32_MAX is the maximum value of an unsigned 32-bit integer.
    </li>
          <li>
       NFS4_MAXFILELEN is the maximum length of a regular file.
    </li>
          <li>
       NFS4_MAXFILEOFF is the maximum offset into a regular file.
    </li>
        </ul>
      </section>
      <section numbered="true" toc="default">
        <name>Basic Data Types</name>
        <t>
	These are the base NFSv4.1 data types.
        </t>
        <table anchor="basic_data_types" align="center">
          <thead>
            <tr>
              <th align="left">Data Type</th>
              <th align="left">Definition</th>
            </tr>
          </thead>
          <tbody>
            <tr>
              <td align="left">int32_t</td>
              <td align="left">typedef int int32_t;</td>
            </tr>
            <tr>
              <td align="left">uint32_t</td>
              <td align="left">typedef unsigned int uint32_t;</td>
            </tr>
            <tr>
              <td align="left">int64_t</td>
              <td align="left">typedef hyper int64_t;</td>
            </tr>
            <tr>
              <td align="left">uint64_t</td>
              <td align="left">typedef unsigned hyper uint64_t;</td>
            </tr>
            <tr>
              <td align="left">attrlist4</td>
              <td align="left">typedef opaque attrlist4&lt;&gt;;</td>
            </tr>
            <tr>
              <td align="left"/>
              <td align="left">Used for file/directory attributes.</td>
            </tr>
            <tr>
              <td align="left">bitmap4</td>
              <td align="left">typedef uint32_t bitmap4&lt;&gt;;</td>
            </tr>
            <tr>
              <td align="left"/>
              <td align="left">Used in attribute array encoding.</td>
            </tr>
            <tr>
              <td align="left">changeid4</td>
              <td align="left">typedef uint64_t changeid4;</td>
            </tr>
            <tr>
              <td align="left"/>
              <td align="left">Used in the definition of change_info4.</td>
            </tr>
            <tr>
              <td align="left">clientid4</td>
              <td align="left">typedef uint64_t clientid4;</td>
            </tr>
            <tr>
              <td align="left"/>
              <td align="left">Shorthand reference to client identification.</td>
            </tr>
            <tr>
              <td align="left">count4</td>
              <td align="left">typedef uint32_t count4;</td>
            </tr>
            <tr>
              <td align="left"/>
              <td align="left">Various count parameters (READ, WRITE, COMMIT).</td>
            </tr>
            <tr>
              <td align="left">length4</td>
              <td align="left">typedef uint64_t length4;</td>
            </tr>
            <tr>
              <td align="left"/>
              <td align="left">The length of a byte-range within a file.</td>
            </tr>
            <tr>
              <td align="left">mode4</td>
              <td align="left">typedef uint32_t mode4;</td>
            </tr>
            <tr>
              <td align="left"/>
              <td align="left">Mode attribute data type.</td>
            </tr>
            <tr>
              <td align="left">nfs_cookie4</td>
              <td align="left">typedef uint64_t nfs_cookie4;</td>
            </tr>
            <tr>
              <td align="left"/>
              <td align="left">Opaque cookie value for READDIR.</td>
            </tr>
            <tr>
              <td align="left">nfs_fh4</td>
              <td align="left">typedef opaque nfs_fh4&lt;NFS4_FHSIZE&gt;;</td>
            </tr>
            <tr>
              <td align="left"/>
              <td align="left">Filehandle definition.</td>
            </tr>
            <tr>
              <td align="left">nfs_ftype4</td>
              <td align="left">enum nfs_ftype4;</td>
            </tr>
            <tr>
              <td align="left"/>
              <td align="left">Various defined file types.</td>
            </tr>
            <tr>
              <td align="left">nfsstat4</td>
              <td align="left">enum nfsstat4;</td>
            </tr>
            <tr>
              <td align="left"/>
              <td align="left">Return value for operations.</td>
            </tr>
            <tr>
              <td align="left">offset4</td>
              <td align="left">typedef uint64_t offset4;</td>
            </tr>
            <tr>
              <td align="left"/>
              <td align="left">Various offset designations (READ, WRITE, LOCK, COMMIT).</td>
            </tr>
            <tr>
              <td align="left">qop4</td>
              <td align="left">typedef uint32_t qop4;</td>
            </tr>
            <tr>
              <td align="left"/>
              <td align="left">Quality of protection designation in SECINFO.</td>
            </tr>
            <tr>
              <td align="left">sec_oid4</td>
              <td align="left">typedef opaque sec_oid4&lt;&gt;;</td>
            </tr>
            <tr>
              <td align="left"/>
              <td align="left">Security Object Identifier.  The sec_oid4 data type is not really opaque.  Instead, it contains an ASN.1 OBJECT IDENTIFIER as used by GSS-API in the mech_type argument to GSS_Init_sec_context. See <xref target="RFC2743" format="default"/> for details.</td>
            </tr>
            <tr>
              <td align="left">sequenceid4</td>
              <td align="left">typedef uint32_t sequenceid4;</td>
            </tr>
            <tr>
              <td align="left"/>
              <td align="left">Sequence number used for various session operations (EXCHANGE_ID, CREATE_SESSION, SEQUENCE, CB_SEQUENCE).</td>
            </tr>
            <tr>
              <td align="left">seqid4</td>
              <td align="left">typedef uint32_t seqid4;</td>
            </tr>
            <tr>
              <td align="left"/>
              <td align="left">Sequence identifier used for locking.</td>
            </tr>
            <tr>
              <td align="left">sessionid4</td>
              <td align="left">typedef opaque sessionid4[NFS4_SESSIONID_SIZE];</td>
            </tr>
            <tr>
              <td align="left"/>
              <td align="left">Session identifier.</td>
            </tr>
            <tr>
              <td align="left">slotid4</td>
              <td align="left">typedef uint32_t slotid4;</td>
            </tr>
            <tr>
              <td align="left"/>
              <td align="left">Sequencing artifact for various session operations (SEQUENCE, CB_SEQUENCE).</td>
            </tr>
            <tr>
              <td align="left">utf8string</td>
              <td align="left">typedef opaque utf8string&lt;&gt;;</td>
            </tr>
            <tr>
              <td align="left"/>
              <td align="left">UTF-8 encoding for strings.</td>
            </tr>
            <tr>
              <td align="left">utf8str_cis</td>
              <td align="left">typedef utf8string utf8str_cis;</td>
            </tr>
            <tr>
              <td align="left"/>
              <td align="left">Case-insensitive UTF-8 string.</td>
            </tr>
            <tr>
              <td align="left">utf8str_cs</td>
              <td align="left">typedef utf8string utf8str_cs;</td>
            </tr>
            <tr>
              <td align="left"/>
              <td align="left">Case-sensitive UTF-8 string.</td>
            </tr>
            <tr>
              <td align="left">utf8str_mixed</td>
              <td align="left">typedef utf8string utf8str_mixed;</td>
            </tr>
            <tr>
              <td align="left"/>
              <td align="left">UTF-8 strings with a case-sensitive prefix and a
	case-insensitive suffix.</td>
            </tr>
            <tr>
              <td align="left">component4</td>
              <td align="left">typedef utf8str_cs component4;</td>
            </tr>
            <tr>
              <td align="left"/>
              <td align="left">Represents pathname components.</td>
            </tr>
            <tr>
              <td align="left">linktext4</td>
              <td align="left">typedef utf8str_cs linktext4;</td>
            </tr>
            <tr>
              <td align="left"/>
              <td align="left">Symbolic link contents ("symbolic link" is defined in an <xref target="symlink" format="default">Open Group</xref> standard).</td>
            </tr>
            <tr>
              <td align="left">pathname4</td>
              <td align="left">typedef component4 pathname4&lt;&gt;;</td>
            </tr>
            <tr>
              <td align="left"/>
              <td align="left">Represents pathname for fs_locations.</td>
            </tr>
            <tr>
              <td align="left">verifier4</td>
              <td align="left">typedef opaque verifier4[NFS4_VERIFIER_SIZE];</td>
            </tr>
            <tr>
              <td align="left"/>
              <td align="left">Verifier used for various operations (COMMIT, CREATE, EXCHANGE_ID, OPEN, READDIR, WRITE) NFS4_VERIFIER_SIZE is defined as 8.</td>
            </tr>
          </tbody>
        </table>
        <t>End of Base Data Types</t>
      </section>
      <!-- start here for the structured data types -->

  <section numbered="true" toc="default">
        <name>Structured Data Types</name>
        <section toc="exclude" anchor="nfstime4" numbered="true">
          <name>nfstime4</name>
          <artwork name="" type="" align="left" alt=""><![CDATA[
struct nfstime4 {
        int64_t         seconds;
        uint32_t        nseconds;
};
 ]]></artwork>
          <t>
	The nfstime4 data type gives the number of seconds and
	nanoseconds since midnight or zero hour January 1, 1970
	Coordinated Universal Time (UTC).  Values greater than zero
	for the seconds field denote dates after the zero hour January 1,
	1970.  Values less than zero for the seconds field denote
	dates before the zero hour January 1, 1970.  In both cases, the
	nseconds field is to be added to the seconds field for the
	final time representation.  For example, if the time to be
	represented is one-half second before zero hour January 1, 1970,
	the seconds field would have a value of negative one (-1) and
	the nseconds field would have a value of one-half second
	(500000000).  Values greater than 999,999,999 for nseconds are
	invalid.
          </t>
          <t>
	This data type is used to pass time and date information.  A
	server converts to and from its local representation of time
	when processing time values, preserving as much accuracy as
	possible. If the precision of timestamps stored for a
	file system object is less than defined, loss of precision can
	occur.  An adjunct time maintenance protocol is RECOMMENDED to
	reduce client and server time skew.
          </t>
        </section>
        <section toc="exclude" anchor="time_how4" numbered="true">
          <name>time_how4</name>
          <artwork name="" type="" align="left" alt=""><![CDATA[
enum time_how4 {
        SET_TO_SERVER_TIME4 = 0,
        SET_TO_CLIENT_TIME4 = 1
};
 ]]></artwork>
        </section>
        <section toc="exclude" anchor="settime4" numbered="true">
          <name>settime4</name>
          <artwork name="" type="" align="left" alt=""><![CDATA[
union settime4 switch (time_how4 set_it) {
 case SET_TO_CLIENT_TIME4:
         nfstime4       time;
 default:
         void;
};
 ]]></artwork>
          <t>
	The time_how4 and settime4 data types are used
	for setting timestamps in file object attributes.  If set_it is SET_TO_SERVER_TIME4, then the server
	uses its local representation of time for the time value.
          </t>
        </section>
        <section toc="exclude" anchor="specdata4" numbered="true">
          <name>specdata4</name>
          <artwork name="" type="" align="left" alt=""><![CDATA[
struct specdata4 {
 uint32_t specdata1; /* major device number */
 uint32_t specdata2; /* minor device number */
};
 ]]></artwork>
          <t>
	This data type represents the device numbers for the device file
	types NF4CHR and NF4BLK.
          </t>
        </section>
        <section toc="exclude" anchor="fsid4" numbered="true">
          <name>fsid4</name>
          <artwork name="" type="" align="left" alt=""><![CDATA[
struct fsid4 {
        uint64_t        major;
        uint64_t        minor;
};
 ]]></artwork>
        </section>
        <section toc="exclude" anchor="chg_policy4" numbered="true">
          <name>change_policy4</name>
          <artwork name="" type="" align="left" alt=""><![CDATA[
struct change_policy4 {
        uint64_t        cp_major;
        uint64_t        cp_minor;
};
 ]]></artwork>
          <t>
         The change_policy4 data type is used for the change_policy
         RECOMMENDED attribute.  It provides change sequencing indication
         analogous to the change attribute.  To enable the server to
         present a value valid across server re-initialization without
         requiring persistent storage, two 64-bit quantities are used,
         allowing one to be a server instance ID and the second to be
         incremented non-persistently, within a given server instance.
          </t>
        </section>
        <section toc="exclude" anchor="fattr4" numbered="true">
          <name>fattr4</name>
          <artwork name="" type="" align="left" alt=""><![CDATA[
struct fattr4 {
        bitmap4         attrmask;
        attrlist4       attr_vals;
};
 ]]></artwork>
          <t>
	The fattr4 data type is used to represent file and directory attributes.
          </t>
          <t>
	The bitmap is a counted array of 32-bit integers used to contain bit
	values.  The position of the integer in the array that contains bit n
	can be computed from the expression (n / 32), and its bit within that
	integer is (n mod 32).
          </t>
          <artwork name="" type="" align="left" alt=""><![CDATA[
0            1
+-----------+-----------+-----------+--
|  count    | 31  ..  0 | 63  .. 32 |
+-----------+-----------+-----------+--
	  ]]></artwork>
        </section>
        <section toc="exclude" anchor="change_info4" numbered="true">
          <name>change_info4</name>
          <artwork name="" type="" align="left" alt=""><![CDATA[
struct change_info4 {
        bool            atomic;
        changeid4       before;
        changeid4       after;
};
 ]]></artwork>
          <t>
	This data type is used with the CREATE, LINK, OPEN, REMOVE, and RENAME
	operations to let the client know the value of the change attribute
	for the directory in which the target file system object resides.
          </t>
        </section>
        <section toc="exclude" anchor="netaddr4" numbered="true">
          <name>netaddr4</name>
          <artwork name="" type="" align="left" alt=""><![CDATA[
struct netaddr4 {
        /* see struct rpcb in RFC 1833 */
        string na_r_netid<>; /* network id */
        string na_r_addr<>;  /* universal address */
};
 ]]></artwork>
          <t>
	The netaddr4 data type is used to identify network transport endpoints.
	The na_r_netid and na_r_addr fields respectively contain a netid
        and uaddr. The netid and uaddr concepts are defined in
	<xref target="RFC5665" format="default"/>. The netid and uaddr formats for
        TCP over IPv4 and TCP over IPv6 are defined in <xref target="RFC5665" format="default"/>,
        specifically Tables 2 and 3 and Sections 5.2.3.3 and 5.2.3.4.
          </t>
        </section>
        <section toc="exclude" anchor="state_owner4" numbered="true">
          <name>state_owner4</name>
          <artwork name="" type="" align="left" alt=""><![CDATA[
struct state_owner4 {
        clientid4       clientid;
        opaque          owner<NFS4_OPAQUE_LIMIT>;
};

typedef state_owner4 open_owner4;
typedef state_owner4 lock_owner4;
 ]]></artwork>
          <t>
     The state_owner4 data type is the base type for the
     open_owner4 (<xref target="open_owner4" format="default"/>) and
     lock_owner4 (<xref target="lock_owner4" format="default"/>).
          </t>
          <section toc="exclude" anchor="open_owner4" numbered="true">
            <name>open_owner4</name>
            <t>
	 This data type is used to identify the owner of OPEN state.
            </t>
          </section>
          <section toc="exclude" anchor="lock_owner4" numbered="true">
            <name>lock_owner4</name>
            <t>
	 This structure is used to identify the owner of byte-range
         locking state.
            </t>
          </section>
        </section>
        <section toc="exclude" anchor="open_to_lock_owner4" numbered="true">
          <name>open_to_lock_owner4</name>
          <artwork name="" type="" align="left" alt=""><![CDATA[
struct open_to_lock_owner4 {
        seqid4          open_seqid;
        stateid4        open_stateid;
        seqid4          lock_seqid;
        lock_owner4     lock_owner;
};
 ]]></artwork>
          <t>
	This data type is used for the first LOCK operation done for
	an open_owner4.  It provides both the open_stateid and
	lock_owner, such that the transition is made from a valid
	open_stateid sequence to that of the new lock_stateid
	sequence.  Using this mechanism avoids the confirmation of the
	lock_owner/lock_seqid pair since it is tied to established
	state in the form of the open_stateid/open_seqid.
          </t>
        </section>
        <section toc="exclude" anchor="stateid4" numbered="true">
          <name>stateid4</name>
          <artwork name="" type="" align="left" alt=""><![CDATA[
struct stateid4 {
        uint32_t        seqid;
        opaque          other[12];
};
 ]]></artwork>
          <t>
	This data type is used for the various state sharing
	mechanisms between the client and server.  The client
	never modifies a value of data type stateid.
        The starting value of the
	"seqid" field is undefined.  The server is required to
	increment the "seqid" field by one at each transition
	of the stateid.  This is important since the client will
	inspect the seqid in OPEN stateids to determine the order of
	OPEN processing done by the server.
          </t>
        </section>
        <section toc="exclude" anchor="layouttype4" numbered="true">
          <name>layouttype4</name>
          <artwork name="" type="" align="left" alt=""><![CDATA[
enum layouttype4 {
        LAYOUT4_NFSV4_1_FILES   = 0x1,
        LAYOUT4_OSD2_OBJECTS    = 0x2,
        LAYOUT4_BLOCK_VOLUME    = 0x3
};
 ]]></artwork>
          <t>
	This data type indicates what type of layout is being used.
	The file server advertises the
	layout types it supports through the fs_layout_type file
	system attribute (<xref target="attrdef_fs_layout_type" format="default"/>).
	A client asks for layouts of a particular type in LAYOUTGET,
	and processes those layouts in its layout-type-specific logic.
          </t>
          <t>
	The layouttype4 data type is 32 bits in length.  The range
	represented by the layout type is split into three parts.  Type
        0x0 is reserved. Types
	within the range 0x00000001-0x7FFFFFFF are globally unique and
	are assigned according to the description in <xref target="pnfsiana" format="default"/>; they are maintained by IANA.  Types
	within the range 0x80000000-0xFFFFFFFF are site specific and
	for private use only.
          </t>
          <t>
	The LAYOUT4_NFSV4_1_FILES enumeration specifies that the NFSv4.1
	file layout type, as defined in <xref target="file_layout_type" format="default"/>, is to be used.  The LAYOUT4_OSD2_OBJECTS
	enumeration specifies that the object layout, as defined in
	<xref target="RFC5664" format="default"/>, is to be used.  Similarly,
	the LAYOUT4_BLOCK_VOLUME enumeration specifies that the block/volume
	layout, as defined in <xref target="RFC5663" format="default"/>, is to be
	used.
          </t>
        </section>
        <section toc="exclude" anchor="deviceid4" numbered="true">
          <name>deviceid4</name>
          <artwork name="" type="" align="left" alt=""><![CDATA[
const NFS4_DEVICEID4_SIZE = 16;

typedef opaque  deviceid4[NFS4_DEVICEID4_SIZE];
 ]]></artwork>
          <t>
	Layout information includes device IDs that
	specify a storage device through a compact handle.
	Addressing and type information is obtained
	with the GETDEVICEINFO operation.  Device IDs
	are not guaranteed to be valid across metadata
	server restarts.  A device ID is unique per client
	ID and layout type.  See <xref target="device_ids" format="default"/> for more details.

          </t>
        </section>
        <section toc="exclude" anchor="device_addr4" numbered="true">
          <name>device_addr4</name>
          <artwork name="" type="" align="left" alt=""><![CDATA[
struct device_addr4 {
        layouttype4             da_layout_type;
        opaque                  da_addr_body<>;
};
 ]]></artwork>
          <t>
        The device address is used to set up a communication channel
        with the storage device.  Different layout types will require
        different data types to define how they communicate
        with storage devices.  The opaque da_addr_body field is
        interpreted based on the specified da_layout_type field.
          </t>
          <t>
        This document defines the device address for the NFSv4.1 file
        layout (see <xref target="file_data_types" format="default"/>), which
        identifies a storage device by network IP address and port
        number.  This is sufficient for the clients to communicate
        with the NFSv4.1 storage devices, and may be sufficient for
        other layout types as well.  Device types for object-based storage
        devices and block storage devices (e.g., Small Computer System
         Interface (SCSI) volume labels)
        are defined by their respective layout specifications.
          </t>
        </section>
        <section toc="exclude" anchor="layout_content4" numbered="true">
          <name>layout_content4</name>
          <artwork name="" type="" align="left" alt=""><![CDATA[
struct layout_content4 {
        layouttype4 loc_type;
        opaque      loc_body<>;
};
 ]]></artwork>
          <t>
        The loc_body field is interpreted based on the layout type (loc_type).
        This document defines the loc_body for the NFSv4.1
	file layout type; see <xref target="file_data_types" format="default"/> for its definition.
          </t>
        </section>
        <section toc="exclude" anchor="layout4" numbered="true">
          <name>layout4</name>
          <artwork name="" type="" align="left" alt=""><![CDATA[
struct layout4 {
        offset4                 lo_offset;
        length4                 lo_length;
        layoutiomode4           lo_iomode;
        layout_content4         lo_content;
};
 ]]></artwork>
          <t>
	The layout4 data type defines a layout for a file.  The layout
	type specific data is opaque within lo_content.
        Since layouts are sub-dividable, the offset
	and length together with the file's filehandle, the client ID,
	iomode, and layout type identify the layout.
          </t>
        </section>
        <section toc="exclude" anchor="layoutupdate4" numbered="true">
          <name>layoutupdate4</name>
          <artwork name="" type="" align="left" alt=""><![CDATA[
struct layoutupdate4 {
        layouttype4             lou_type;
        opaque                  lou_body<>;
};
 ]]></artwork>
          <t>
	The layoutupdate4 data type is used by the client to return
	updated layout information to the metadata server via the
	LAYOUTCOMMIT (<xref target="OP_LAYOUTCOMMIT" format="default"/>) operation.
	This data type provides a channel to pass
	layout type specific information (in field lou_body)
        back to the metadata server.
	For example, for the block/volume layout type, this could include the
	list of reserved blocks that were written.  The contents of
	the opaque lou_body argument are determined by the layout type.
	The NFSv4.1 file-based layout
	does not use this data type; if lou_type is LAYOUT4_NFSV4_1_FILES,
        the lou_body field MUST
	have a zero length.
          </t>
        </section>
        <section toc="exclude" anchor="layouthint4" numbered="true">
          <name>layouthint4</name>
          <artwork name="" type="" align="left" alt=""><![CDATA[
struct layouthint4 {
        layouttype4             loh_type;
        opaque                  loh_body<>;
};
 ]]></artwork>
          <t>
	The layouthint4 data type is used by the client to pass in a
	hint about the type of layout it would like created for a particular
	file.  It is the data type specified by the layout_hint
	attribute described in <xref target="attrdef_layout_hint" format="default"/>.
	The metadata server may ignore the hint
	or may selectively ignore fields within the hint.  This hint should
	be provided at create time as part of the initial attributes within
	OPEN.  The loh_body field is specific to the type of layout (loh_type).
        The NFSv4.1 file-based layout uses the nfsv4_1_file_layouthint4
	data type as defined in <xref target="file_data_types" format="default"/>.
          </t>
        </section>
        <section toc="exclude" anchor="layoutiomode4" numbered="true">
          <name>layoutiomode4</name>
          <artwork name="" type="" align="left" alt=""><![CDATA[
enum layoutiomode4 {
        LAYOUTIOMODE4_READ      = 1,
        LAYOUTIOMODE4_RW        = 2,
        LAYOUTIOMODE4_ANY       = 3
};
 ]]></artwork>
          <t>
	The iomode specifies whether the client intends to just read or both
        read and write the data represented by the
	layout.  While the LAYOUTIOMODE4_ANY iomode MUST NOT be used in
        the arguments to the LAYOUTGET operation, it MAY
	be used in the arguments to the LAYOUTRETURN and CB_LAYOUTRECALL
        operations.  The LAYOUTIOMODE4_ANY iomode
	specifies that layouts pertaining to both LAYOUTIOMODE4_READ
        and LAYOUTIOMODE4_RW iomodes are being returned or recalled,
        respectively.  The metadata server's use of the iomode may
        depend on the layout type being used.  The storage devices MAY
        validate I/O accesses against the iomode and reject invalid accesses.
          </t>
        </section>
        <section toc="exclude" anchor="nfs_impl_id4" numbered="true">
          <name>nfs_impl_id4</name>
          <artwork name="" type="" align="left" alt=""><![CDATA[
struct nfs_impl_id4 {
        utf8str_cis   nii_domain;
        utf8str_cs    nii_name;
        nfstime4      nii_date;
};
 ]]></artwork>
          <t>
	This data type is used to identify client and server
	implementation details.  The nii_domain field is the DNS domain
	name with which the implementor is associated.  The nii_name
	field is the product name of the implementation and is
	completely free form.  It is RECOMMENDED that the nii_name be
	used to distinguish machine architecture, machine platforms,
	revisions, versions, and patch levels.  The nii_date field is
	the timestamp of when the software instance was published or
	built.
          </t>
        </section>
        <section toc="exclude" anchor="threshold_item4" numbered="true">
          <name>threshold_item4</name>
          <artwork name="" type="" align="left" alt=""><![CDATA[
struct threshold_item4 {
        layouttype4     thi_layout_type;
        bitmap4         thi_hintset;
        opaque          thi_hintlist<>;
};
 ]]></artwork>
          <t>
	This data type contains a list of hints specific to
	a layout type for helping the client determine when
	it should send I/O directly through the metadata
	server versus the storage devices.  The data type
	consists of the layout type (thi_layout_type),
	a bitmap (thi_hintset) describing the set of
	hints supported by the server (they may differ
	based on the layout type), and a list of hints
	(thi_hintlist) whose content is determined by
	the hintset bitmap.  See the mdsthreshold attribute
	for more details.

          </t>
          <t>
        The thi_hintset field is a bitmap of the following values:
          </t>
          <table align="center">
            <thead>
              <tr>
                <th align="left">name</th>
                <th align="left">#</th>
                <th align="left">Data Type</th>
                <th align="left">Description</th>
              </tr>
            </thead>
            <tbody>
              <tr>
                <td align="left">threshold4_read_size</td>
                <td align="left">0</td>
                <td align="left">length4</td>
                <td align="left">
           If a file's length is less than the value of threshold4_read_size,
           then it is RECOMMENDED that the client read from the file via the MDS and not
           a storage device.

        </td>
              </tr>
              <tr>
                <td align="left">threshold4_write_size</td>
                <td align="left">1</td>
                <td align="left">length4</td>
                <td align="left">
           If a file's length is less than the value of threshold4_write_size,
           then it is RECOMMENDED that the client write to the file via the MDS and not
           a storage device.
        </td>
              </tr>
              <tr>
                <td align="left">threshold4_read_iosize</td>
                <td align="left">2</td>
                <td align="left">length4</td>
                <td align="left">
          For read I/O sizes below this threshold, it is RECOMMENDED to
  	read data through the MDS.
        </td>
              </tr>
              <tr>
                <td align="left">threshold4_write_iosize</td>
                <td align="left">3</td>
                <td align="left">length4</td>
                <td align="left">
          For write I/O sizes below this threshold, it is RECOMMENDED to
  	write data through the MDS.
        </td>
              </tr>
            </tbody>
          </table>
        </section>
        <section toc="exclude" anchor="mdsthreshold4" numbered="true">
          <name>mdsthreshold4</name>
          <artwork name="" type="" align="left" alt=""><![CDATA[
struct mdsthreshold4 {
        threshold_item4 mth_hints<>;
};
 ]]></artwork>
          <t>
        This data type holds an array of elements of data type
        threshold_item4,
	each of which is valid for a particular layout type.  An array
	is necessary because a server can support multiple layout types
	for a single file.
          </t>
        </section>
      </section>
    </section>
    <!-- End of Data Types -->

<!-- 	$Id: 2009-12-20-TO-rfc5661.xml,v 1.2 2009/12/21 05:59:32 shepler.mre Exp $	 -->
<!-- Copyright (C) The IETF Trust (2007-2008) -->
<!-- Copyright (C) The Internet Society (2006) -->
<section anchor="Filehandles" numbered="true" toc="default">
      <name>Filehandles</name>
      <t>
    The filehandle in the NFS protocol is a per-server unique identifier
    for a file system object.  The contents of the filehandle are opaque
    to the client.  Therefore, the server is responsible for translating
    the filehandle to an internal representation of the file system
    object.
      </t>
      <section numbered="true" toc="default">
        <name>Obtaining the First Filehandle</name>
        <t>
      The operations of the NFS protocol are defined in terms of one
      or more filehandles.  Therefore, the client needs a filehandle
      to initiate communication with the server.  With the NFSv3
      protocol (<xref target="RFC1813" format="default">RFC 1813</xref>), there
      exists an ancillary protocol to obtain this first filehandle.
      The MOUNT protocol, RPC program number 100005, provides the
      mechanism of translating a string-based file system pathname to
      a filehandle, which can then be used by the NFS protocols.
        </t>
        <t>
      The MOUNT protocol has deficiencies in the area of security and
      use via firewalls.  This is one reason that the use of the
      public filehandle was introduced in <xref target="RFC2054" format="default">RFC 2054</xref> and <xref target="RFC2055" format="default">RFC 2055</xref>.  With the use of the public
      filehandle in combination with the LOOKUP operation in the NFSv3
      protocol, it has been demonstrated that the
      MOUNT protocol is unnecessary for viable interaction between NFS
      client and server.
        </t>
        <t>
      Therefore, the NFSv4.1 protocol will not use an ancillary
      protocol for translation from string-based pathnames to a filehandle.
      Two special filehandles will be used as starting points for the NFS
      client.
        </t>
        <section numbered="true" toc="default">
          <name>Root Filehandle</name>
          <t>
        The first of the special filehandles is the ROOT filehandle.  The ROOT
        filehandle is the "conceptual" root of the file system namespace at
        the NFS server.  The client uses or starts with the ROOT filehandle
        by employing the PUTROOTFH operation.  The PUTROOTFH operation
        instructs the server to set the "current" filehandle to the ROOT of
        the server's file tree.  Once this PUTROOTFH operation is used, the
        client can then traverse the entirety of the server's file tree with
        the LOOKUP operation.  A complete discussion of the server namespace
        is in <xref target="single_server_namespace" format="default"/>.
          </t>
        </section>
        <section numbered="true" toc="default">
          <name>Public Filehandle</name>
          <t>
        The second special filehandle is the PUBLIC filehandle.  Unlike the
        ROOT filehandle, the PUBLIC filehandle may be bound or represent an
        arbitrary file system object at the server.  The server is responsible
        for this binding.  It may be that the PUBLIC filehandle and the ROOT
        filehandle refer to the same file system object.  However, it is up to
        the administrative software at the server and the policies of the
        server administrator to define the binding of the PUBLIC filehandle
        and server file system object.  The client may not make any
        assumptions about this binding. The client uses the PUBLIC filehandle
        via the PUTPUBFH operation.
          </t>
        </section>
      </section>
      <section numbered="true" toc="default">
        <name>Filehandle Types</name>
        <t>
      In the NFSv3 protocol, there was one type of filehandle
      with a single set of semantics.  This type of filehandle is termed
      "persistent" in NFSv4.1.  The semantics of a persistent
      filehandle remain the same as before.  A new type of filehandle
      introduced in NFSv4.1 is the "volatile" filehandle, which
      attempts to accommodate certain server environments.
        </t>
        <t>
      The volatile filehandle type was introduced to address server
      functionality or implementation issues that make correct
      implementation of a persistent filehandle infeasible.  Some server
      environments do not provide a file-system-level invariant that can be
      used to construct a persistent filehandle.  The underlying server
      file system may not provide the invariant or the server's file system
      programming interfaces may not provide access to the needed invariant.
      Volatile filehandles may ease the implementation of server
      functionality such as hierarchical storage management or file system
      reorganization or migration.  However, the volatile filehandle
      increases the implementation burden for the client.
        </t>
        <t>
      Since the client will need to handle persistent and volatile
      filehandles differently, a file attribute is defined that may be used
      by the client to determine the filehandle types being returned by the
      server.
        </t>
        <section numbered="true" toc="default">
          <name>General Properties of a Filehandle</name>
          <t>
        The filehandle contains all the information the
        server needs to distinguish an individual file.
        To the client, the filehandle is opaque. The
        client stores filehandles for use in a later
        request and can compare two filehandles from the
        same server for equality by doing a byte-by-byte
        comparison.  However, the client MUST NOT otherwise
        interpret the contents of filehandles.  If two
        filehandles from the same server are equal, they
        MUST refer to the same file.  Servers SHOULD try
        to maintain a one-to-one correspondence between
        filehandles and files, but this is not required.
        Clients MUST use filehandle comparisons only to
        improve performance, not for correct behavior.
        All clients need to be prepared for situations
        in which it cannot be determined whether two
        filehandles denote the same object and in such
        cases, avoid making invalid assumptions that might
        cause incorrect behavior.  Further discussion
        of filehandle and attribute comparison in the
        context of data caching is presented in <xref target="data_caching_and_file_identity" format="default"/>.

          </t>
          <t>
        As an example, in the case that two different pathnames when
        traversed at the server terminate at the same file system object, the
        server SHOULD return the same filehandle for each path.  This can
        occur if a hard link (see <xref target="hardlink" format="default"/>) is used
        to create two file names that refer to the same underlying
        file object and associated data.  For example, if paths /a/b/c
        and /a/d/c refer to the same file, the server SHOULD return
        the same filehandle for both pathnames' traversals.
          </t>
        </section>
        <section numbered="true" toc="default">
          <name>Persistent Filehandle</name>
          <t>
        A persistent filehandle is defined as having a fixed value for the
        lifetime of the file system object to which it refers.  Once the
        server creates the filehandle for a file system object, the server
        MUST accept the same filehandle for the object for the lifetime of the
        object.  If the server restarts, the NFS server MUST honor
        the same filehandle value as it did in the server's previous
        instantiation.  Similarly, if the file system is migrated, the new NFS
        server MUST honor the same filehandle as the old NFS server.
          </t>
          <t>
        The persistent filehandle will be become stale or invalid when the
        file system object is removed.  When the server is presented with a
        persistent filehandle that refers to a deleted object, it MUST return
        an error of NFS4ERR_STALE.  A filehandle may become stale when the
        file system containing the object is no longer available.  The file
        system may become unavailable if it exists on removable media and the
        media is no longer available at the server or the file system in whole
        has been destroyed or the file system has simply been removed from the
        server's namespace (i.e., unmounted in a UNIX environment).
          </t>
        </section>
        <section numbered="true" toc="default">
          <name>Volatile Filehandle</name>
          <t>
        A volatile filehandle does not share the same longevity
        characteristics of a persistent filehandle.  The server may
        determine that a volatile filehandle is no longer valid at many
        different points in time.  If the server can definitively determine
        that a volatile filehandle refers to an object that has been removed,
        the server should return NFS4ERR_STALE to the client (as is the case
        for persistent filehandles).  In all other cases where the server
        determines that a volatile filehandle can no longer be used, it should
        return an error of NFS4ERR_FHEXPIRED.
          </t>
          <t>
        The REQUIRED attribute "fh_expire_type" is used by the client to
        determine what type of filehandle the server is providing for a
        particular file system.  This attribute is a bitmask with the
        following values:
          </t>
          <dl newline="false" spacing="normal">
            <dt>FH4_PERSISTENT</dt>
            <dd>
            The value of FH4_PERSISTENT is used to indicate a persistent
            filehandle, which is valid until the object is removed from the
            file system.  The server will not return NFS4ERR_FHEXPIRED for this
            filehandle.  FH4_PERSISTENT is defined as a value in which none of the
            bits specified below are set.
          </dd>
            <dt>FH4_VOLATILE_ANY</dt>
            <dd>
            The filehandle may expire at any time, except as specifically
            excluded (i.e., FH4_NO_EXPIRE_WITH_OPEN).
          </dd>
            <dt>FH4_NOEXPIRE_WITH_OPEN</dt>
            <dd>
            May only be set when FH4_VOLATILE_ANY is set.  If this bit is set,
            then the meaning of FH4_VOLATILE_ANY is qualified to exclude any
            expiration of the filehandle when it is open.
          </dd>
            <dt>FH4_VOL_MIGRATION</dt>
            <dd>
	    The filehandle will expire as a result of a file system
	    transition (migration or replication), in those cases in
	    which the continuity of filehandle use is not specified by
	    handle class information
	    within the fs_locations_info attribute.  When this bit is
	    set, clients without access to fs_locations_info
	    information should assume that filehandles will expire on file
	    system transitions.
          </dd>
            <dt>FH4_VOL_RENAME</dt>
            <dd>
            The filehandle will expire during rename.  This includes a rename by
            the requesting client or a rename by any other client.  If FH4_VOL_ANY
            is set, FH4_VOL_RENAME is redundant.
          </dd>
          </dl>
          <t>
        Servers that provide volatile filehandles that can expire
        while open require special care as regards handling of RENAMEs
        and REMOVEs.  This situation can arise if FH4_VOL_MIGRATION or
        FH4_VOL_RENAME is set, if FH4_VOLATILE_ANY is set and
        FH4_NOEXPIRE_WITH_OPEN is not set, or if a non-read-only file system
        has a transition target in a different handle
         class.  In these cases, the server should deny a RENAME
        or REMOVE that would affect an OPEN file of any of the
        components leading to the OPEN file.  In addition, the server
        should deny all RENAME or REMOVE requests during the grace period,
        in order to make sure that reclaims of files where filehandles
        may have expired do not do a reclaim for the wrong file.
          </t>
          <t>
        Volatile filehandles are especially suitable for implementation
        of the pseudo file systems used to bridge exports.  See
        <xref target="pseudo_fs_volatility" format="default"/> for a discussion of this.
          </t>
        </section>
      </section>
      <section numbered="true" toc="default">
        <name>One Method of Constructing a Volatile Filehandle</name>
        <t>
      A volatile filehandle, while opaque to the client, could contain:
        </t>
        <artwork name="" type="" align="left" alt=""><![CDATA[
[volatile bit = 1 | server boot time | slot | generation number]
o  slot is an index in the server volatile filehandle table

o  generation number is the generation number for the table entry/
   slot
      ]]></artwork>
        <t>
      When the client presents a volatile filehandle, the server makes the
      following checks, which assume that the check for the volatile bit has
      passed.  If the server boot time is less than the current server boot
      time, return NFS4ERR_FHEXPIRED.  If slot is out of range, return
      NFS4ERR_BADHANDLE.  If the generation number does not match, return
      NFS4ERR_FHEXPIRED.
        </t>
        <t>
      When the server restarts, the table is gone (it is volatile).
        </t>
        <t>
      If the volatile bit is 0, then it is a persistent filehandle with a
      different structure following it.

        </t>
      </section>
      <section numbered="true" toc="default">
        <name>Client Recovery from Filehandle Expiration</name>
        <t>
      If possible, the client SHOULD recover from the receipt of an
      NFS4ERR_FHEXPIRED error.  The client must take on additional
      responsibility so that it may prepare itself to recover from the
      expiration of a volatile filehandle.  If the server returns persistent
      filehandles, the client does not need these additional steps.
        </t>
        <t>
      For volatile filehandles, most commonly the client will need to store
      the component names leading up to and including the file system object
      in question.  With these names, the client should be able to recover
      by finding a filehandle in the namespace that is still available or
      by starting at the root of the server's file system namespace.
        </t>
        <t>
      If the expired filehandle refers to an object that has been removed
      from the file system, obviously the client will not be able to recover
      from the expired filehandle.
        </t>
        <t>
      It is also possible that the expired filehandle refers to a file that
      has been renamed.  If the file was renamed by another client, again it
      is possible that the original client will not be able to recover.
      However, in the case that the client itself is renaming the file and
      the file is open, it is possible that the client may be able to
      recover.  The client can determine the new pathname based on the
      processing of the rename request.  The client can then regenerate the
      new filehandle based on the new pathname.  The client could also use
      the COMPOUND procedure to construct a series of operations
      like:
        </t>
        <artwork name="" type="" align="left" alt=""><![CDATA[
          RENAME A B
          LOOKUP B
          GETFH
        ]]></artwork>
        <t>

      Note that the COMPOUND procedure does not provide atomicity.  This
      example only reduces the overhead of recovering from an expired
      filehandle.
        </t>
      </section>
    </section>
    <!-- 	$Id: 2009-12-20-TO-rfc5661.xml,v 1.2 2009/12/21 05:59:32 shepler.mre Exp $	 -->
<!-- Copyright (C) The IETF Trust (2007-2008) -->
<!-- Copyright (C) The Internet Society (2006) -->
<section anchor="file_attributes" numbered="true" toc="default">
      <name>File Attributes</name>
      <t>
    To meet the requirements of extensibility and increased
    interoperability with non-UNIX platforms, attributes need to be handled
    in a flexible manner.  The NFSv3 fattr3 structure contains a
    fixed list of attributes that not all clients and servers are able to
    support or care about.  The fattr3 structure cannot be extended as
    new needs arise and it provides no way to indicate non-support.  With
    the NFSv4.1 protocol, the client is able to query what attributes
    the server supports and construct requests with only those supported
    attributes (or a subset thereof).
      </t>
      <t>
    To this end, attributes are divided into three groups: REQUIRED,
    RECOMMENDED, and named.  Both REQUIRED and RECOMMENDED attributes are
    supported in the NFSv4.1 protocol by a specific and well-defined
    encoding and are identified by number.  They are requested by setting
    a bit in the bit vector sent in the GETATTR request; the server
    response includes a bit vector to list what attributes were returned
    in the response.  New REQUIRED or RECOMMENDED attributes may be added
    to the NFSv4 protocol as part of a new minor version
    by publishing a
    Standards Track RFC that allocates a new attribute number value and
    defines the encoding for the attribute.  See
    <xref target="minor_versioning" format="default"/> for further
    discussion.
      </t>
      <t>
    Named attributes are accessed by the new OPENATTR operation, which
    accesses a hidden directory of attributes associated with a file
    system object.  OPENATTR takes a filehandle for the object and returns
    the filehandle for the attribute hierarchy.  The filehandle for the
    named attributes is a directory object accessible by LOOKUP or READDIR
    and contains files whose names represent the named attributes and
    whose data bytes are the value of the attribute.  For example:
      </t>
      <table align="center">
        <thead>
          <tr>
            <th align="left"/>
            <th align="left"/>
            <th align="left"/>
          </tr>
        </thead>
        <tbody>
          <tr>
            <td align="left">LOOKUP</td>
            <td align="left">"foo"</td>
            <td align="left">; look up file</td>
          </tr>
          <tr>
            <td align="left">GETATTR</td>
            <td align="left">attrbits</td>
            <td align="left"/>
          </tr>
          <tr>
            <td align="left">OPENATTR</td>
            <td align="left"/>
            <td align="left">; access foo's named attributes</td>
          </tr>
          <tr>
            <td align="left">LOOKUP</td>
            <td align="left">"x11icon"</td>
            <td align="left">; look up specific attribute</td>
          </tr>
          <tr>
            <td align="left">READ</td>
            <td align="left">0,4096</td>
            <td align="left">; read stream of bytes</td>
          </tr>
        </tbody>
      </table>
      <t>
    Named attributes are intended for data needed by applications rather
    than by an NFS client implementation.  NFS implementors are strongly
    encouraged to define their new attributes as RECOMMENDED attributes by
    bringing them to the IETF Standards Track process.
      </t>
      <t>
    The set of attributes that are classified as REQUIRED is
    deliberately small since servers need to do whatever it takes to support
    them.  A server should support as many of the RECOMMENDED attributes
    as possible but, by their definition, the server is not required to
    support all of them.  Attributes are deemed REQUIRED if the data is
    both needed by a large number of clients and is not otherwise
    reasonably computable by the client when support is not provided on
    the server.
      </t>
      <t>
    Note that the hidden directory returned by OPENATTR is a convenience
    for protocol processing.  The client should not make any assumptions
    about the server's implementation of named attributes and whether
    or not the underlying file system at the server has a named
    attribute directory.  Therefore, operations such as SETATTR and
    GETATTR on the named attribute directory are undefined.
      </t>
      <section anchor="mandatory_attributes_intro" numbered="true" toc="default">
        <name>REQUIRED Attributes</name>
        <t>
      These MUST be supported by every NFSv4.1 client and server in
      order to ensure a minimum level of interoperability.  The server MUST
      store and return these attributes, and the client MUST be able to
      function with an attribute set limited to these attributes.  With just
      the REQUIRED attributes some client functionality may be impaired or
      limited in some ways.  A client may ask for any of these attributes to
      be returned by setting a bit in the GETATTR request, and the server
      MUST return their value.
        </t>
      </section>
      <section anchor="recommended_attributes_intro" numbered="true" toc="default">
        <name>RECOMMENDED Attributes</name>
        <t>
      These attributes are understood well enough to warrant support in the
      NFSv4.1 protocol.  However, they may not be supported on all
      clients and servers.  A client may ask for any of these attributes to
      be returned by setting a bit in the GETATTR request but must handle
      the case where the server does not return them.  A client MAY ask for
      the set of attributes the server supports and SHOULD NOT request
      attributes the server does not support.  A server should be tolerant
      of requests for unsupported attributes and simply not return them
      rather than considering the request an error.  It is expected that
      servers will support all attributes they comfortably can and only fail
      to support attributes that are difficult to support in their
      operating environments.  A server should provide attributes whenever
      they don't have to "tell lies" to the client.  For example, a file
      modification time should be either an accurate time or should not be
      supported by the server.  At times this will be difficult for
      clients, but a client is better positioned to decide whether and how to
      fabricate or construct an attribute or whether to do without the
      attribute.
        </t>
      </section>
      <section anchor="named_attributes_intro" numbered="true" toc="default">
        <name>Named Attributes</name>
        <t>
      These attributes are not supported by direct encoding in the NFSv4
      protocol but are accessed by string names rather than
      numbers and correspond to an uninterpreted stream of bytes that are
      stored with the file system object.  The namespace for these
      attributes may be accessed by using the OPENATTR operation.  The
      OPENATTR operation returns a filehandle for a virtual "named attribute
      directory", and further perusal and modification of the namespace may
      be done using operations that work on more typical directories.  In
      particular, READDIR may be used to get a list of such named attributes,
      and LOOKUP and OPEN may select a particular attribute.  Creation of
      a new named attribute may be the result of an OPEN specifying file
      creation.
        </t>
        <t>
      Once an OPEN is done, named attributes may be examined and changed
      by normal READ and WRITE operations using the filehandles and stateids
      returned by OPEN.
        </t>
        <t>
      Named attributes and the named attribute directory may have
      their own (non-named) attributes.  Each of these objects MUST have all
      of the REQUIRED attributes and may have additional RECOMMENDED
      attributes.  However, the set of attributes for named attributes
      and the named attribute directory need not be, and
      typically will not be, as large as that for other objects in that
      file system.
        </t>
        <t>
      Named attributes and the named attribute directory might be the
      target of delegations (in the case of the named attribute directory,
      these will be directory delegations).  However, since granting of
      delegations is at the server's discretion, a server
      need not support delegations on named attributes or the named
      attribute directory.
        </t>
        <t>
      It is RECOMMENDED that servers support arbitrary named attributes.  A
      client should not depend on the ability to store any named attributes
      in the server's file system.  If a server does support named
      attributes, a client that is also able to handle them should be able
      to copy a file's data and metadata with complete transparency from
      one location to another; this would imply that names allowed for
      regular directory entries are valid for named attribute names as well.
        </t>
        <t>
      In NFSv4.1, the structure of named attribute directories is
      restricted in a number of ways, in order to prevent the development
      of non-interoperable implementations in which some servers support
      a fully general hierarchical directory structure for named attributes
      while others support a limited but adequate structure for named attributes.
      In such an environment, clients or applications might come to
      depend on non-portable extensions.  The restrictions are:
        </t>
        <ul spacing="normal">
          <li>
          CREATE is not allowed in a named attribute directory.  Thus, such
          objects as symbolic links and special files are not allowed to
          be named attributes.   Further, directories may not be created
          in a named attribute directory, so no hierarchical structure of
          named attributes for a single object is allowed.
        </li>
          <li>
          If OPENATTR is done on a named attribute directory or on
          a named attribute, the server MUST return NFS4ERR_WRONG_TYPE.
        </li>
          <li>
          Doing a RENAME of a named attribute to a different named
          attribute directory or to an ordinary (i.e., non-named-attribute)
          directory is not allowed.
        </li>
          <li>
          Creating hard links between named attribute directories or
          between named attribute directories and ordinary directories
          is not allowed.
        </li>
        </ul>
        <t>
      Names of attributes will not be controlled by this document or other
      IETF Standards Track documents.  See
      <xref target="namedattributesiana" format="default"/>
      for further discussion.
        </t>
      </section>
      <section numbered="true" toc="default">
        <name>Classification of Attributes</name>
        <t>
      Each of the REQUIRED and RECOMMENDED attributes can be classified in
      one of three categories: per server (i.e., the value of the attribute will
      be the same for all file objects that share the same
      server owner; see <xref target="Server_Owners" format="default"/> for a definition of server
      owner), per file system (i.e., the value of the attribute will
      be the same for some or all file objects that share the
      same <xref target="attrdef_fsid" format="default">fsid attribute</xref> and
      server owner), or per file system
      object.  Note that it is possible that some per file system attributes
      may vary within the file system, depending on the value of
      the <xref target="attrdef_homogeneous" format="default">"homogeneous"</xref>
      attribute. Note that the attributes time_access_set and
      time_modify_set are not listed in this section because they are
      write-only attributes corresponding to time_access and time_modify,
      and are used in a special instance of SETATTR.
        </t>
        <ul spacing="normal">
          <li>
            <t>
	  The per-server attribute is:
            </t>
            <ul empty="true" spacing="normal">
              <li>
	      lease_time
	    </li>
            </ul>
          </li>
          <li>
            <t>
	  The per-file system attributes are:
            </t>
            <ul empty="true" spacing="normal">
              <li>
	      supported_attrs, suppattr_exclcreat, fh_expire_type, link_support,
	      symlink_support, unique_handles, aclsupport,
	      cansettime, case_insensitive, case_preserving,
	      chown_restricted, files_avail, files_free,
	      files_total, fs_locations, homogeneous, maxfilesize,
	      maxname, maxread, maxwrite, no_trunc, space_avail,
	      space_free, space_total, time_delta,
              change_policy, fs_status,
	      fs_layout_type, fs_locations_info, fs_charset_cap
	    </li>
            </ul>
          </li>
          <li>
            <t>
	  The per-file system object attributes are:
            </t>
            <ul empty="true" spacing="normal">
              <li>
	      type, change, size, named_attr, fsid, rdattr_error,
	      filehandle, acl, archive, fileid, hidden, maxlink,
	      mimetype, mode, numlinks, owner, owner_group, rawdev,
	      space_used, system, time_access, time_backup,
	      time_create, time_metadata, time_modify,
	      mounted_on_fileid, dir_notif_delay, dirent_notif_delay,
              dacl, sacl,
	      layout_type, layout_hint, layout_blksize, layout_alignment,
              mdsthreshold, retention_get, retention_set, retentevt_get,
              retentevt_set, retention_hold, mode_set_masked
	    </li>
            </ul>
          </li>
        </ul>
        <t>
      For quota_avail_hard, quota_avail_soft, and quota_used, see their
      definitions below for the appropriate classification.
        </t>
      </section>
      <section anchor="rw_attr" numbered="true" toc="default">
        <name>Set-Only and Get-Only Attributes</name>
        <t>
     Some REQUIRED and RECOMMENDED attributes are set-only; i.e., they
     can be set via SETATTR but not retrieved via GETATTR. Similarly, some
     REQUIRED and RECOMMENDED attributes are get-only; i.e., they
     can be retrieved via GETATTR but not set via SETATTR. If a client attempts
     to set a get-only attribute or get a set-only attributes, the server
     MUST return NFS4ERR_INVAL.
        </t>
      </section>
      <section anchor="mandatory_attributes" numbered="true" toc="default">
        <name>REQUIRED Attributes - List and Definition References</name>
        <t>
     The list of REQUIRED attributes appears in <xref target="req_attr_table" format="default"/>.
     The meaning of the columns of the table are:
        </t>
        <ul spacing="normal">
          <li>Name: The name of the attribute.</li>
          <li>Id: The number assigned to the attribute. In
        the event of conflicts between the assigned number and <xref target="RFC5662" format="default"/>, the latter is
        likely authoritative, but should be resolved with Errata to
        this document and/or
        <xref target="RFC5662" format="default"/>. See <xref target="errata" format="default"/> for the Errata process.

</li>
          <li>Data Type: The XDR data type of the attribute.</li>
          <li>
        Acc: Access allowed to the attribute. R means
        read-only (GETATTR may retrieve, SETATTR may not
        set). W means write-only (SETATTR may set, GETATTR
        may not retrieve).  R W means read/write (GETATTR
        may retrieve, SETATTR may set).

     </li>
          <li>Defined in: The section of this specification that describes the
        attribute.</li>
        </ul>
        <table anchor="req_attr_table" align="center">
          <thead>
            <tr>
              <th align="left">Name</th>
              <th align="left">Id</th>
              <th align="left">Data Type</th>
              <th align="left">Acc</th>
              <th align="left">Defined in:</th>
            </tr>
          </thead>
          <tbody>
            <tr>
              <td align="left">supported_attrs</td>
              <td align="left">0</td>
              <td align="left">bitmap4</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_supp_attr" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">type</td>
              <td align="left">1</td>
              <td align="left">nfs_ftype4</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_type" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">fh_expire_type</td>
              <td align="left">2</td>
              <td align="left">uint32_t</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_fh_expire_type" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">change</td>
              <td align="left">3</td>
              <td align="left">uint64_t</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_change" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">size</td>
              <td align="left">4</td>
              <td align="left">uint64_t</td>
              <td align="left">R W</td>
              <td align="left">
                <xref target="attrdef_size" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">link_support</td>
              <td align="left">5</td>
              <td align="left">bool</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_link_support" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">symlink_support</td>
              <td align="left">6</td>
              <td align="left">bool</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_symlink_support" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">named_attr</td>
              <td align="left">7</td>
              <td align="left">bool</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_named_attr" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">fsid</td>
              <td align="left">8</td>
              <td align="left">fsid4</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_fsid" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">unique_handles</td>
              <td align="left">9</td>
              <td align="left">bool</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_unique_handles" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">lease_time</td>
              <td align="left">10</td>
              <td align="left">nfs_lease4</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_lease_time" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">rdattr_error</td>
              <td align="left">11</td>
              <td align="left">enum</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_rdattr_error" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">filehandle</td>
              <td align="left">19</td>
              <td align="left">nfs_fh4</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_filehandle" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">suppattr_exclcreat</td>
              <td align="left">75</td>
              <td align="left">bitmap4</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_suppattr_exclcreat" format="default"/>
              </td>
            </tr>
          </tbody>
        </table>
      </section>
      <section anchor="recommended_attributes" numbered="true" toc="default">
        <name>RECOMMENDED Attributes - List and Definition References</name>
        <t>
     The RECOMMENDED attributes are defined in
     <xref target="rec_attr_tbl" format="default"/>.  The meanings
     of the column headers are the same as
     <xref target="req_attr_table" format="default"/>; see <xref target="mandatory_attributes" format="default"/> for the meanings.

        </t>
        <table anchor="rec_attr_tbl" align="center">
          <thead>
            <tr>
              <th align="left">Name</th>
              <th align="left">Id</th>
              <th align="left">Data Type</th>
              <th align="left">Acc</th>
              <th align="left">Defined in:</th>
            </tr>
          </thead>
          <tbody>
            <tr>
              <td align="left">acl</td>
              <td align="left">12</td>
              <td align="left">nfsace4&lt;&gt;</td>
              <td align="left">R W</td>
              <td align="left">
                <xref target="attrdef_acl" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">aclsupport</td>
              <td align="left">13</td>
              <td align="left">uint32_t</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_aclsupport" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">archive</td>
              <td align="left">14</td>
              <td align="left">bool</td>
              <td align="left">R W</td>
              <td align="left">
                <xref target="attrdef_archive" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">cansettime</td>
              <td align="left">15</td>
              <td align="left">bool</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_cansettime" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">case_insensitive</td>
              <td align="left">16</td>
              <td align="left">bool</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_case_insensitive" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">case_preserving</td>
              <td align="left">17</td>
              <td align="left">bool</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_case_preserving" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">change_policy</td>
              <td align="left">60</td>
              <td align="left">chg_policy4</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_change_policy" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">chown_restricted</td>
              <td align="left">18</td>
              <td align="left">bool</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_chown_restricted" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">dacl</td>
              <td align="left">58</td>
              <td align="left">nfsacl41</td>
              <td align="left">R W</td>
              <td align="left">
                <xref target="attrdef_dacl" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">dir_notif_delay</td>
              <td align="left">56</td>
              <td align="left">nfstime4</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_dir_notif_delay" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">dirent_notif_delay</td>
              <td align="left">57</td>
              <td align="left">nfstime4</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_dirent_notif_delay" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">fileid</td>
              <td align="left">20</td>
              <td align="left">uint64_t</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_fileid" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">files_avail</td>
              <td align="left">21</td>
              <td align="left">uint64_t</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_files_avail" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">files_free</td>
              <td align="left">22</td>
              <td align="left">uint64_t</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_files_free" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">files_total</td>
              <td align="left">23</td>
              <td align="left">uint64_t</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_files_total" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">fs_charset_cap</td>
              <td align="left">76</td>
              <td align="left">uint32_t</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_fs_charset_cap" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">fs_layout_type</td>
              <td align="left">62</td>
              <td align="left">layouttype4&lt;&gt;</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_fs_layout_type" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">fs_locations</td>
              <td align="left">24</td>
              <td align="left">fs_locations</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_fs_locations" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">fs_locations_info</td>
              <td align="left">67</td>
              <td align="left">*</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_fs_locations_info" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">fs_status</td>
              <td align="left">61</td>
              <td align="left">fs4_status</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_fs_status" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">hidden</td>
              <td align="left">25</td>
              <td align="left">bool</td>
              <td align="left">R W</td>
              <td align="left">
                <xref target="attrdef_hidden" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">homogeneous</td>
              <td align="left">26</td>
              <td align="left">bool</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_homogeneous" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">layout_alignment</td>
              <td align="left">66</td>
              <td align="left">uint32_t</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_layout_alignment" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">layout_blksize</td>
              <td align="left">65</td>
              <td align="left">uint32_t</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_layout_blksize" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">layout_hint</td>
              <td align="left">63</td>
              <td align="left">layouthint4</td>
              <td align="left">&nbsp;&nbsp;W</td>
              <td align="left">
                <xref target="attrdef_layout_hint" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">layout_type</td>
              <td align="left">64</td>
              <td align="left">layouttype4&lt;&gt;</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_layout_type" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">maxfilesize</td>
              <td align="left">27</td>
              <td align="left">uint64_t</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_maxfilesize" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">maxlink</td>
              <td align="left">28</td>
              <td align="left">uint32_t</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_maxlink" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">maxname</td>
              <td align="left">29</td>
              <td align="left">uint32_t</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_maxname" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">maxread</td>
              <td align="left">30</td>
              <td align="left">uint64_t</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_maxread" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">maxwrite</td>
              <td align="left">31</td>
              <td align="left">uint64_t</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_maxwrite" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">mdsthreshold</td>
              <td align="left">68</td>
              <td align="left">mdsthreshold4</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_mdsthreshold" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">mimetype</td>
              <td align="left">32</td>
              <td align="left">utf8str_cs</td>
              <td align="left">R W</td>
              <td align="left">
                <xref target="attrdef_mimetype" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">mode</td>
              <td align="left">33</td>
              <td align="left">mode4</td>
              <td align="left">R W</td>
              <td align="left">
                <xref target="attrdef_mode" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">mode_set_masked</td>
              <td align="left">74</td>
              <td align="left">mode_masked4</td>
              <td align="left">&nbsp;&nbsp;W</td>
              <td align="left">
                <xref target="attrdef_mode_set_masked" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">mounted_on_fileid</td>
              <td align="left">55</td>
              <td align="left">uint64_t</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_mounted_on_fileid" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">no_trunc</td>
              <td align="left">34</td>
              <td align="left">bool</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_no_trunc" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">numlinks</td>
              <td align="left">35</td>
              <td align="left">uint32_t</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_numlinks" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">owner</td>
              <td align="left">36</td>
              <td align="left">utf8str_mixed</td>
              <td align="left">R W</td>
              <td align="left">
                <xref target="attrdef_owner" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">owner_group</td>
              <td align="left">37</td>
              <td align="left">utf8str_mixed</td>
              <td align="left">R W</td>
              <td align="left">
                <xref target="attrdef_owner_group" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">quota_avail_hard</td>
              <td align="left">38</td>
              <td align="left">uint64_t</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_quota_avail_hard" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">quota_avail_soft</td>
              <td align="left">39</td>
              <td align="left">uint64_t</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_quota_avail_soft" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">quota_used</td>
              <td align="left">40</td>
              <td align="left">uint64_t</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_quota_used" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">rawdev</td>
              <td align="left">41</td>
              <td align="left">specdata4</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_rawdev" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">retentevt_get</td>
              <td align="left">71</td>
              <td align="left">retention_get4</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_retentevt_get" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">retentevt_set</td>
              <td align="left">72</td>
              <td align="left">retention_set4</td>
              <td align="left">&nbsp;&nbsp;W</td>
              <td align="left">
                <xref target="attrdef_retentevt_set" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">retention_get</td>
              <td align="left">69</td>
              <td align="left">retention_get4</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_retention_get" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">retention_hold</td>
              <td align="left">73</td>
              <td align="left">uint64_t</td>
              <td align="left">R W</td>
              <td align="left">
                <xref target="attrdef_retention_hold" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">retention_set</td>
              <td align="left">70</td>
              <td align="left">retention_set4</td>
              <td align="left">&nbsp;&nbsp;W</td>
              <td align="left">
                <xref target="attrdef_retention_set" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">sacl</td>
              <td align="left">59</td>
              <td align="left">nfsacl41</td>
              <td align="left">R W</td>
              <td align="left">
                <xref target="attrdef_sacl" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">space_avail</td>
              <td align="left">42</td>
              <td align="left">uint64_t</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_space_avail" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">space_free</td>
              <td align="left">43</td>
              <td align="left">uint64_t</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_space_free" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">space_total</td>
              <td align="left">44</td>
              <td align="left">uint64_t</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_space_total" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">space_used</td>
              <td align="left">45</td>
              <td align="left">uint64_t</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_space_used" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">system</td>
              <td align="left">46</td>
              <td align="left">bool</td>
              <td align="left">R W</td>
              <td align="left">
                <xref target="attrdef_system" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">time_access</td>
              <td align="left">47</td>
              <td align="left">nfstime4</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_time_access" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">time_access_set</td>
              <td align="left">48</td>
              <td align="left">settime4</td>
              <td align="left">&nbsp;&nbsp;W</td>
              <td align="left">
                <xref target="attrdef_time_access_set" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">time_backup</td>
              <td align="left">49</td>
              <td align="left">nfstime4</td>
              <td align="left">R W</td>
              <td align="left">
                <xref target="attrdef_time_backup" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">time_create</td>
              <td align="left">50</td>
              <td align="left">nfstime4</td>
              <td align="left">R W</td>
              <td align="left">
                <xref target="attrdef_time_create" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">time_delta</td>
              <td align="left">51</td>
              <td align="left">nfstime4</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_time_delta" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">time_metadata</td>
              <td align="left">52</td>
              <td align="left">nfstime4</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_time_metadata" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">time_modify</td>
              <td align="left">53</td>
              <td align="left">nfstime4</td>
              <td align="left">R</td>
              <td align="left">
                <xref target="attrdef_time_modify" format="default"/>
              </td>
            </tr>
            <tr>
              <td align="left">time_modify_set</td>
              <td align="left">54</td>
              <td align="left">settime4</td>
              <td align="left">&nbsp;&nbsp;W</td>
              <td align="left">
                <xref target="attrdef_time_modify_set" format="default"/>
              </td>
            </tr>
          </tbody>
        </table>
        <t>* fs_locations_info4</t>
      </section>
      <section anchor="attribute_definitions" numbered="true" toc="default">
        <name>Attribute        Definitions</name>
        <section anchor="required_attr" numbered="true" toc="default">
          <name>Definitions of REQUIRED Attributes</name>
          <section toc="exclude" anchor="attrdef_supp_attr" numbered="true">
            <name>Attribute 0: supported_attrs</name>
            <t>
	The bit vector that would retrieve all REQUIRED and
	RECOMMENDED attributes that are supported for this object.
	The scope of this attribute applies to all objects with a
	matching fsid.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_type" numbered="true">
            <name>Attribute 1: type</name>
            <t>
	  Designates the type of an object in terms of one of a number
          of special constants:
            </t>
            <ul spacing="normal">
              <li>
              NF4REG designates a regular file.
            </li>
              <li>
              NF4DIR designates a directory.
            </li>
              <li>
              NF4BLK designates a block device special file.
            </li>
              <li>
              NF4CHR designates a character device special file.
            </li>
              <li>
              NF4LNK designates a symbolic link.
            </li>
              <li>
              NF4SOCK designates a named socket special file.
            </li>
              <li>
              NF4FIFO designates a fifo special file.
            </li>
              <li>
              NF4ATTRDIR designates a named attribute directory.
            </li>
              <li>
              NF4NAMEDATTR designates a named attribute.
            </li>
            </ul>
            <t>
          Within the explanatory text and operation descriptions, the
          following phrases will be used with the meanings given below:
            </t>
            <ul spacing="normal">
              <li>
              The phrase "is a directory" means that the object's
              type attribute is NF4DIR or NF4ATTRDIR.
            </li>
              <li>
              The phrase "is a special file" means that the object's type
              attribute is NF4BLK, NF4CHR, NF4SOCK, or NF4FIFO.
            </li>
              <li>
              The phrases "is an ordinary file" and
              "is a regular file" mean that the object's
              type attribute is NF4REG or NF4NAMEDATTR.
            </li>
            </ul>
          </section>
          <section toc="exclude" anchor="attrdef_fh_expire_type" numbered="true">
            <name>Attribute 2: fh_expire_type</name>
            <t>
	  Server uses this to specify filehandle expiration behavior
	  to the client.  See <xref target="Filehandles" format="default"/> for additional
	  description.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_change" numbered="true">
            <name>Attribute 3: change</name>
            <t>
	  A value created by the server that the client can use to
	  determine if file data, directory contents, or attributes of
	  the object have been modified.  The server may return the
	  object's time_metadata attribute for this attribute's value,
	  but only if the file system object cannot be updated more
	  frequently than the resolution of time_metadata.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_size" numbered="true">
            <name>Attribute 4: size</name>
            <t>
	  The size of the object in bytes.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_link_support" numbered="true">
            <name>Attribute 5: link_support</name>
            <t>
	  TRUE, if the object's file system supports hard links.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_symlink_support" numbered="true">
            <name>Attribute 6: symlink_support</name>
            <t>
	  TRUE, if the object's file system supports symbolic links.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_named_attr" numbered="true">
            <name>Attribute 7: named_attr</name>
            <t>
	  TRUE, if this object has named attributes. In other words,
	  object has a non-empty named attribute directory.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_fsid" numbered="true">
            <name>Attribute 8: fsid</name>
            <t>
	  Unique file system identifier for the file system holding this
	  object.  The fsid attribute has major and minor components, each of
	  which are of data type uint64_t.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_unique_handles" numbered="true">
            <name>Attribute 9: unique_handles</name>
            <t>
	  TRUE, if two distinct filehandles are guaranteed to refer to two
	  different file system objects.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_lease_time" numbered="true">
            <name>Attribute 10: lease_time</name>
            <t>
	  Duration of the lease at server in seconds.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_rdattr_error" numbered="true">
            <name>Attribute 11: rdattr_error</name>
            <t>
	  Error returned from an attempt to retrieve attributes during a READDIR operation.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_filehandle" numbered="true">
            <name>Attribute 19: filehandle</name>
            <t>
	  The filehandle of this object (primarily for READDIR requests).
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_suppattr_exclcreat" numbered="true">
            <name>Attribute 75: suppattr_exclcreat</name>
            <t>
	The bit vector that would set all REQUIRED and
	RECOMMENDED attributes that are supported by the EXCLUSIVE4_1
        method of file creation via the OPEN operation.
	The scope of this attribute applies to all objects with a
	matching fsid.
            </t>
          </section>
        </section>
        <section anchor="recommended_attr" numbered="true" toc="default">
          <name>Definitions of Uncategorized RECOMMENDED Attributes</name>
          <t>
     The definitions of most of the RECOMMENDED attributes follow. Collections
     that share a common category are defined in other sections.
          </t>
          <section toc="exclude" anchor="attrdef_archive" numbered="true">
            <name>Attribute 14: archive</name>
            <t>
	TRUE, if this file has been archived since the time of last
	modification (deprecated in favor of time_backup).
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_cansettime" numbered="true">
            <name>Attribute 15: cansettime</name>
            <t>
	TRUE, if the server is able to change the times for a
	file system object as specified in a SETATTR operation.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_case_insensitive" numbered="true">
            <name>Attribute 16: case_insensitive</name>
            <t>
	TRUE, if file name comparisons on this file system are case
	insensitive.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_case_preserving" numbered="true">
            <name>Attribute 17: case_preserving</name>
            <t>
	TRUE, if file name case on this file system is preserved.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_change_policy" numbered="true">
            <name>Attribute 60: change_policy</name>
            <t>
	A value created by the server that the client can use to
	determine if some server policy related to the current
        file system has been subject to change.  If the value
        remains the same, then the client can be sure that the
        values of the attributes related to fs location
        and the fss_type field of the fs_status attribute have
        not changed.  On the other hand, a change in this value does
        necessarily imply a change in policy.  It is up to the client
        to interrogate the server to determine if some policy relevant to
        it has changed.  See <xref target="chg_policy4" format="default"/> for
        details.
            </t>
            <t>
        This attribute MUST change when the value returned by
        the fs_locations or fs_locations_info attribute changes, when
        a file system goes from read-only to writable or vice versa,
        or when the allowable set of security flavors for the file system
        or any part thereof is changed.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_chown_restricted" numbered="true">
            <name>Attribute 18: chown_restricted</name>
            <t>
	If TRUE, the server will reject any request to change either
	the owner or the group associated with a file if the caller
	is not a privileged user (for example, "root" in UNIX
	operating environments or, in Windows 2000, the "Take
	Ownership" privilege).
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_fileid" numbered="true">
            <name>Attribute 20: fileid</name>
            <t>
	A number uniquely identifying the file within the file system.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_files_avail" numbered="true">
            <name>Attribute 21: files_avail</name>
            <t>
	File slots available to this user on the file system
	containing this object -- this should be the smallest
	relevant limit.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_files_free" numbered="true">
            <name>Attribute 22: files_free</name>
            <t>
	Free file slots on the file system containing this object --
	this should be the smallest relevant limit.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_files_total" numbered="true">
            <name>Attribute 23: files_total</name>
            <t>
	Total file slots on the file system containing this object.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_fs_charset_cap" numbered="true">
            <name>Attribute 76: fs_charset_cap</name>
            <t>
        Character set capabilities for this file system. See
        <xref target="utf8_caps" format="default"/>.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_fs_locations" numbered="true">
            <name>Attribute 24: fs_locations</name>
            <t>
       Locations where this file system may be found.  If the server
       returns NFS4ERR_MOVED as an error, this attribute MUST be
       supported.
       See <xref target="fs_locations" format="default"/> for more details.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_fs_locations_info" numbered="true">
            <name>Attribute 67: fs_locations_info</name>
            <t>
	Full function file system location.
       See <xref target="SEC11-fsli-info" format="default"/> for more details.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_fs_status" numbered="true">
            <name>Attribute 61: fs_status</name>
            <t>
	Generic file system type information.
       See <xref target="fs_status" format="default"/> for more details.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_hidden" numbered="true">
            <name>Attribute 25: hidden</name>
            <t>
	TRUE, if the file is considered hidden with respect to
	the Windows API.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_homogeneous" numbered="true">
            <name>Attribute 26: homogeneous</name>
            <t>
	TRUE, if this object's file system is homogeneous; i.e., all
	objects in the file system (all objects on the server with the
	same fsid) have common values for all per-file-system attributes.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_maxfilesize" numbered="true">
            <name>Attribute 27: maxfilesize</name>
            <t>
	Maximum supported file size for the file system of this object.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_maxlink" numbered="true">
            <name>Attribute 28: maxlink</name>
            <t>
	Maximum number of links for this object.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_maxname" numbered="true">
            <name>Attribute 29: maxname</name>
            <t>
	Maximum file name size supported for this object.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_maxread" numbered="true">
            <name>Attribute 30: maxread</name>
            <t>
	Maximum amount of data the READ operation will return for this object.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_maxwrite" numbered="true">
            <name>Attribute 31: maxwrite</name>
            <t>
	Maximum amount of data the WRITE operation will accept for this object.
	This
	attribute SHOULD be supported if the file is writable.  Lack
	of this attribute can lead to the client either wasting
	bandwidth or not receiving the best performance.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_mimetype" numbered="true">
            <name>Attribute 32: mimetype</name>
            <t>
	MIME body type/subtype of this object.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_mounted_on_fileid" numbered="true">
            <name>Attribute 55: mounted_on_fileid</name>
            <t>
	Like fileid, but if the target filehandle is the root of a
	file system, this attribute represents the fileid of the
	underlying directory.
            </t>
            <t>
	UNIX-based operating environments connect a file system into
	the namespace by connecting (mounting) the file system onto
	the existing file object (the mount point, usually a
	directory) of an existing file system. When the mount point's
	parent directory is read via an API like readdir(), the return
	results are directory entries, each with a component name and
	a fileid. The fileid of the mount point's directory entry will
	be different from the fileid that the stat() system call
	returns. The stat() system call is returning the fileid of the
	root of the mounted file system, whereas readdir() is
	returning the fileid that stat() would have returned before any
	file systems were mounted on the mount point.
            </t>
            <t>
	Unlike NFSv3, NFSv4.1 allows a client's LOOKUP
	request to cross other file systems. The client detects the
	file system crossing whenever the filehandle argument of
	LOOKUP has an fsid attribute different from that of the
	filehandle returned by LOOKUP. A UNIX-based client will
	consider this a "mount point crossing".  UNIX has a legacy
	scheme for allowing a process to determine its current working
	directory. This relies on readdir() of a mount point's parent
	and stat() of the mount point returning fileids as previously
	described.  The mounted_on_fileid attribute corresponds to the
	fileid that readdir() would have returned as described
	previously.
            </t>
            <t>
	While the NFSv4.1 client could simply fabricate a fileid
	corresponding to what mounted_on_fileid provides (and if the
	server does not support mounted_on_fileid, the client has no
	choice), there is a risk that the client will generate a
	fileid that conflicts with one that is already assigned to
	another object in the file system. Instead, if the server can
	provide the mounted_on_fileid, the potential for client
	operational problems in this area is eliminated.
            </t>
            <t>
	If the server detects that there is no mounted point at the
	target file object, then the value for mounted_on_fileid that
	it returns is the same as that of the fileid attribute.
            </t>
            <t>
	The mounted_on_fileid attribute is RECOMMENDED, so the server
	SHOULD provide it if possible, and for a UNIX-based server,
	this is straightforward. Usually, mounted_on_fileid will be
	requested during a READDIR operation, in which case it is
	trivial (at least for UNIX-based servers) to return
	mounted_on_fileid since it is equal to the fileid of a
	directory entry returned by readdir().  If mounted_on_fileid
	is requested in a GETATTR operation, the server should obey an
	invariant that has it returning a value that is equal to the
	file object's entry in the object's parent directory,
	i.e., what readdir() would have returned.  Some operating
	environments allow a series of two or more file systems to be
	mounted onto a single mount point. In this case, for the
	server to obey the aforementioned invariant, it will need to
	find the base mount point, and not the intermediate mount
	points.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_no_trunc" numbered="true">
            <name>Attribute 34: no_trunc</name>
            <t>
	If this attribute is TRUE, then if the client uses a file
        name longer than name_max, an error will be
	returned instead of the name being truncated.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_numlinks" numbered="true">
            <name>Attribute 35: numlinks</name>
            <t>
	Number of hard links to this object.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_owner" numbered="true">
            <name>Attribute 36: owner</name>
            <t>
	The string name of the owner of this object.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_owner_group" numbered="true">
            <name>Attribute 37: owner_group</name>
            <t>
	The string name of the group ownership of this object.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_quota_avail_hard" numbered="true">
            <name>Attribute 38: quota_avail_hard</name>
            <t anchor="quota_avail_hard">
	The value in bytes that represents the amount of additional
	disk space beyond the current allocation that can be allocated
	to this file or directory before further allocations will be
	refused.  It is understood that this space may be consumed by
	allocations to other files or directories.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_quota_avail_soft" numbered="true">
            <name>Attribute 39: quota_avail_soft</name>
            <t anchor="quota_avail_soft">
	The value in bytes that represents the amount of additional
	disk space that can be allocated to this file or directory
	before the user may reasonably be warned.  It is understood
	that this space may be consumed by allocations to other files
	or directories though there is a rule as to which other files
	or directories.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_quota_used" numbered="true">
            <name>Attribute 40: quota_used</name>
            <t anchor="quota_used">
	The value in bytes that represents the amount of disk
	space used by this file or directory and possibly a
	number of other similar files or directories, where the
	set of "similar" meets at least the criterion that
	allocating space to any file or directory in the set
	will reduce the "quota_avail_hard" of every other file
	or directory in the set.

	Note that there may be a number of distinct but
	overlapping sets of files or directories for which a
	quota_used value is maintained, e.g., "all files with a
	given owner", "all files with a given group owner", etc.
	The server is at liberty to choose any of those sets when
        providing the content of the quota_used attribute, but
	should do so in a repeatable way.  The rule may be
	configured per file system or may be "choose the set with
	the smallest quota".
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_rawdev" numbered="true">
            <name>Attribute 41: rawdev</name>
            <t>
	Raw device number of file of type NF4BLK or NF4CHR. The device
        number is split into major and minor numbers.
	If the file's type attribute is not NF4BLK or NF4CHR,
	the value returned SHOULD NOT be considered useful.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_space_avail" numbered="true">
            <name>Attribute 42: space_avail</name>
            <t>
	Disk space in bytes available to this user on the file system
	containing this object -- this should be the smallest
	relevant limit.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_space_free" numbered="true">
            <name>Attribute 43: space_free</name>
            <t>
	Free disk space in bytes on the file system containing this
	object -- this should be the smallest relevant limit.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_space_total" numbered="true">
            <name>Attribute 44: space_total</name>
            <t>
	Total disk space in bytes on the file system containing this object.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_space_used" numbered="true">
            <name>Attribute 45: space_used</name>
            <t>
	Number of file system bytes allocated to this object.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_system" numbered="true">
            <name>Attribute 46: system</name>
            <t>
	This attribute is TRUE if this file is a "system" file with
	respect to the Windows operating environment.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_time_access" numbered="true">
            <name>Attribute 47: time_access</name>
            <t>
	The time_access attribute represents the time of last access to
	the object by a READ operation sent to the server. The notion
	of what is an "access" depends on the server's operating environment
	and/or the server's file system semantics.  For example, for
	servers obeying Portable Operating System Interface (POSIX) semantics, time_access would be updated only
	by the READ and READDIR operations and not any of the operations
	that modify the content of the object <xref target="read_atime" format="default"/>,
	<xref target="readdir_atime" format="default"/>, <xref target="write_atime" format="default"/>. Of
	course, setting the corresponding time_access_set attribute is
	another way to modify the time_access attribute.

            </t>
            <t>
	Whenever the file object resides on a writable file system,
	the server should make its best efforts to record time_access into
	stable storage.  However, to mitigate the performance effects
	of doing so, and most especially whenever the server is
	satisfying the read of the object's content from its cache,
	the server MAY cache access time updates and lazily write them
	to stable storage.  It is also acceptable to give
	administrators of the server the option to disable time_access
	updates.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_time_access_set" numbered="true">
            <name>Attribute 48: time_access_set</name>
            <t>
	Sets the time of last access to the object.  SETATTR use only.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_time_backup" numbered="true">
            <name>Attribute 49: time_backup</name>
            <t>
	The time of last backup of the object.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_time_create" numbered="true">
            <name>Attribute 50: time_create</name>
            <t>
	The time of creation of the object. This attribute does not
	have any relation to the traditional UNIX file attribute
	"ctime" or "change time".
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_time_delta" numbered="true">
            <name>Attribute 51: time_delta</name>
            <t>
	Smallest useful server time granularity.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_time_metadata" numbered="true">
            <name>Attribute 52: time_metadata</name>
            <t>
	The time of last metadata modification of the object.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_time_modify" numbered="true">
            <name>Attribute 53: time_modify</name>
            <t>
	The time of last modification to the object.
            </t>
          </section>
          <section toc="exclude" anchor="attrdef_time_modify_set" numbered="true">
            <name>Attribute 54: time_modify_set</name>
            <t>
	Sets the time of last modification to the object.  SETATTR use only.
            </t>
          </section>
        </section>
      </section>
      <section anchor="owner_owner_group" numbered="true" toc="default">
        <name>Interpreting owner and owner_group</name>
        <t>
      The RECOMMENDED attributes "owner" and "owner_group" (and also
      users and groups within the "acl" attribute) are represented in
      terms of a UTF-8 string.  To avoid a representation that is tied
      to a particular underlying implementation at the client or
      server, the use of the UTF-8 string has been chosen.  Note that
      Section 6.1 of <xref target="RFC2624" format="default">RFC 2624</xref> provides
      additional rationale.  It is expected that the client and server
      will have their own local representation of owner and
      owner_group that is used for local storage or presentation to
      the end user.  Therefore, it is expected that when these
      attributes are transferred between the client and server,
      the local representation is translated to a syntax of the form
      "user@dns_domain".  This will allow for a client and server that
      do not use the same local representation the ability to
      translate to a common syntax that can be interpreted by both.
        </t>
        <t>
      Similarly, security principals may be represented in different
      ways by different security mechanisms.  Servers normally
      translate these representations into a common format,
      generally that used by local storage, to serve as a means of
      identifying the users corresponding to these security
      principals.  When these local identifiers are translated to
      the form of the owner attribute, associated with files created
      by such principals, they identify, in a common format, the
      users associated with each corresponding set of security
      principals.
        </t>
        <t>
      The translation used to interpret owner and group strings is
      not specified as part of the protocol.  This allows various
      solutions to be employed.  For example, a local translation
      table may be consulted that maps a numeric identifier to the
      user@dns_domain syntax.  A name service may also be used to
      accomplish the translation.  A server may provide a more
      general service, not limited by any particular translation
      (which would only translate a limited set of possible strings)
      by storing the owner and owner_group attributes in local
      storage without any translation or it may augment a
      translation method by storing the entire string for attributes
      for which no translation is available while using the local
      representation for those cases in which a translation is
      available.
        </t>
        <t>
      Servers that do not provide support for all possible values of
      the owner and owner_group attributes SHOULD return an error
      (NFS4ERR_BADOWNER) when a string is presented that has no
      translation, as the value to be set for a SETATTR of the
      owner, owner_group, or acl attributes.  When a server does
      accept an owner or owner_group value as valid on a SETATTR
      (and similarly for the owner and group strings in an acl), it
      is promising to return that same string when a corresponding
      GETATTR is done.  Configuration changes (including
      changes from the mapping of the string to the local representation)
      and ill-constructed
      name translations (those that contain aliasing) may make that
      promise impossible to honor.  Servers should make appropriate
      efforts to avoid a situation in which these attributes have
      their values changed when no real change to ownership has
      occurred.
        </t>
        <t>
      The "dns_domain" portion of the owner string is meant to be a
      DNS domain name, for example, user@example.org.  Servers should
      accept as valid a set of users for at least one domain.  A
      server may treat other domains as having no valid
      translations.  A more general service is provided when a
      server is capable of accepting users for multiple domains, or
      for all domains, subject to security constraints.
        </t>
        <t>
      In the case where there is no translation available to the
      client or server, the attribute value will be constructed
      without the "@".  Therefore, the absence of the @ from the
      owner or owner_group attribute signifies that no translation
      was available at the sender and that the receiver of the
      attribute should not use that string as a basis for
      translation into its own internal format.  Even though the
      attribute value cannot be translated, it may still be useful.
      In the case of a client, the attribute string may be used for
      local display of ownership.
        </t>
        <t>
      To provide a greater degree of compatibility with NFSv3,
      which identified users and groups by 32-bit unsigned user
      identifiers and group identifiers, owner and group strings that
      consist of decimal numeric values with no leading zeros can be
      given a special interpretation by clients and servers that
      choose to provide such support.  The receiver may treat such a
      user or group string as representing the same user as would be
      represented by an NFSv3 uid or gid having the corresponding
      numeric value.  A server is not obligated to accept such a
      string, but may return an NFS4ERR_BADOWNER instead.  To avoid
      this mechanism being used to subvert user and group translation,
      so that a client might pass all of the owners and groups in
      numeric form, a server SHOULD return an NFS4ERR_BADOWNER error
      when there is a valid translation for the user or owner
      designated in this way.  In that case, the client must use the
      appropriate name@domain string and not the special form for compatibility.
        </t>
        <t>
      The owner string "nobody" may be used to designate an
      anonymous user, which will be associated with a file created
      by a security principal that cannot be mapped through normal
      means to the owner attribute. Users and implementations
      of NFSv4.1 SHOULD NOT use "nobody" to designate a real user whose access is not anonymous.
        </t>
      </section>
      <section anchor="character_case_attributes" numbered="true" toc="default">
        <name>Character Case Attributes</name>
        <t>
      With respect to the case_insensitive and case_preserving
      attributes, each UCS-4 character (which UTF-8 encodes) can be
      mapped according to Appendix B.2 of
      <xref target="RFC3454" format="default">RFC 3454</xref>.
      For general character handling and internationalization issues,
      see <xref target="internationalization" format="default"/>.
        </t>
      </section>
      <section anchor="dir_not_attrs" numbered="true" toc="default">
        <name>Directory Notification Attributes</name>
        <t>
      As described in <xref target="OP_GET_DIR_DELEGATION" format="default"/>, the
      client can request a minimum delay for notifications of changes
      to attributes, but the server is free to ignore what the client
      requests. The client can determine in advance what notification
      delays the server will accept by sending a GETATTR operation for either or
      both of two directory notification attributes.  When the client
      calls the GET_DIR_DELEGATION operation and asks for attribute
      change notifications, it should request notification delays that
      are no less than the values in the server-provided attributes.
        </t>
        <section toc="exclude" anchor="attrdef_dir_notif_delay" numbered="true">
          <name>Attribute 56: dir_notif_delay</name>
          <t>
	The dir_notif_delay attribute is the minimum number of seconds
	the server will delay before notifying the client of a change
	to the directory's attributes.
          </t>
        </section>
        <section toc="exclude" anchor="attrdef_dirent_notif_delay" numbered="true">
          <name>Attribute 57: dirent_notif_delay</name>
          <t>
	The dirent_notif_delay attribute is the minimum number of seconds
	the server will delay before notifying the client of a change
	to a file object that has an entry in the directory.
          </t>
        </section>
      </section>
      <section anchor="pnfs_attr_full" numbered="true" toc="default">
        <name>pNFS Attribute Definitions</name>
        <section toc="exclude" anchor="attrdef_fs_layout_type" numbered="true">
          <name>Attribute 62: fs_layout_type</name>
          <t>
	The fs_layout_type attribute (see
	<xref target="layouttype4" format="default"/>) applies to a
	file system and indicates what layout types are supported by
	the file system.  When the client encounters a new fsid, the
	client SHOULD obtain the value for the fs_layout_type
	attribute associated with the new file system.  This attribute
	is used by the client to determine if the layout types
	supported by the server match any of the client's supported
	layout types.
          </t>
        </section>
        <section toc="exclude" anchor="attrdef_layout_alignment" numbered="true">
          <name>Attribute 66: layout_alignment</name>
          <t>
	When a client holds layouts on files of a file system, the
        layout_alignment attribute indicates the preferred alignment
        for I/O to files on that file system.  Where possible, the
        client should send READ and WRITE operations with offsets
        that are whole multiples of the layout_alignment attribute.
          </t>
        </section>
        <section toc="exclude" anchor="attrdef_layout_blksize" numbered="true">
          <name>Attribute 65: layout_blksize</name>
          <t>
	When a client holds layouts on files of a file system, the
	layout_blksize attribute indicates the preferred block size
	for I/O to files on that file system.  Where possible, the
	client should send READ operations with a count argument that
	is a whole multiple of layout_blksize, and WRITE operations
	with a data argument of size that is a whole multiple of
	layout_blksize.
          </t>
        </section>
        <section toc="exclude" anchor="attrdef_layout_hint" numbered="true">
          <name>Attribute 63: layout_hint</name>
          <t>
	The layout_hint attribute (see
	<xref target="layouthint4" format="default"/>) may be set on
	newly created files to influence the metadata server's choice
	for the file's layout.  If possible, this attribute is one of
	those set in the initial attributes within the OPEN operation.
	The metadata server may choose to ignore this attribute.  The
	layout_hint attribute is a subset of the layout structure
	returned by LAYOUTGET.  For example, instead of specifying
	particular devices, this would be used to suggest the stripe
	width of a file.  The server implementation determines which
	fields within the layout will be used.
          </t>
        </section>
        <section toc="exclude" anchor="attrdef_layout_type" numbered="true">
          <name>Attribute 64: layout_type</name>
          <t>
	This attribute lists the layout type(s) available for a file.
	The value returned by the server is for informational purposes
	only.  The client will use the LAYOUTGET operation to obtain
	the information needed in order to perform I/O, for example,
	the specific device information for the file and its layout.
          </t>
        </section>
        <section toc="exclude" anchor="attrdef_mdsthreshold" numbered="true">
          <name>Attribute 68: mdsthreshold</name>
          <t>
	This attribute is a server-provided hint used to communicate
	to the client when it is more efficient to send READ and
	WRITE operations to the metadata server or the data server.
	The two types of thresholds described are file size thresholds
	and I/O size thresholds.  If a file's size is smaller than the
	file size threshold, data accesses SHOULD be sent to the
	metadata server.  If an I/O request has a length
        that is below the I/O size threshold,
	the I/O SHOULD be sent to the metadata server.
	Each threshold type is specified separately for read and
	write.
          </t>
          <t>
	The server MAY provide both types of thresholds for a file.
	If both file size and I/O size are provided, the client SHOULD
	reach or exceed both thresholds before sending its read or write
	requests to the data server.  Alternatively, if only one of
	the specified thresholds is reached or exceeded, the I/O requests are
	sent to the metadata server.
          </t>
          <t>
	For each threshold type, a value of zero indicates no READ or WRITE
	should be sent to the metadata server, while a value of all ones
	indicates that all READs or WRITEs should be sent to the metadata
	server.
          </t>
          <t>
	The attribute is available on a per-filehandle basis.  If the
	current filehandle refers to a non-pNFS file or directory, the
	metadata server should return an attribute that is
	representative of the filehandle's file system.  It is suggested
	that this attribute is queried as part of the OPEN operation.
	Due to dynamic system changes, the client should not assume that
	the attribute will remain constant for any specific time period;
	thus, it should be periodically refreshed.
          </t>
        </section>
      </section>
      <!-- "PNFS Attributes" -->

  <section anchor="retention" numbered="true" toc="default">
        <name>Retention Attributes</name>
        <t>
      Retention is a concept whereby a file object can be placed in an
      immutable, undeletable, unrenamable state for a fixed or
      infinite duration of time. Once in this "retained" state, the
      file cannot be moved out of the state until the duration of
      retention has been reached.
        </t>
        <t>
      When retention is enabled, retention MUST extend to the data of
      the file, and the name of file. The server MAY extend retention
      to any other property of the file, including any subset of
      REQUIRED, RECOMMENDED, and named attributes, with the
      exceptions noted in this section.
        </t>
        <t>
      Servers MAY support or not support retention on
      any file object type.
        </t>
        <t>
      The five retention attributes are explained in the next subsections.
        </t>
        <section toc="exclude" anchor="attrdef_retention_get" numbered="true">
          <name>Attribute 69: retention_get</name>
          <t>
      If retention is enabled for the associated file,
      this attribute's value represents the retention
      begin time of the file object.   This attribute's
      value is only readable with the GETATTR operation
      and MUST NOT be modified by the SETATTR operation
      (<xref target="rw_attr" format="default"/>).  The value of the
      attribute consists of:

</t>
          <artwork name="" type="" align="left" alt=""><![CDATA[
const RET4_DURATION_INFINITE    = 0xffffffffffffffff;
struct retention_get4 {
        uint64_t        rg_duration;
        nfstime4        rg_begin_time<1>;
};
 ]]></artwork>
          <t>

      The field rg_duration is the duration in seconds indicating how
      long the file will be retained once retention is enabled. The
      field rg_begin_time is an array of up to one absolute time
      value. If the array is zero length, no beginning retention time
      has been established, and retention is not enabled.
      If rg_duration is equal to RET4_DURATION_INFINITE, the file, once
      retention is enabled, will be retained for an infinite duration.
          </t>
          <t>
      If (as soon as) rg_duration is zero, then rg_begin_time will be
      of zero length, and again, retention is not (no longer) enabled.

          </t>
        </section>
        <section toc="exclude" anchor="attrdef_retention_set" numbered="true">
          <name>Attribute 70: retention_set</name>
          <t>
	This attribute is used to set the retention
	duration and optionally enable retention for
	the associated file object.  This attribute is
	only modifiable via the SETATTR operation and
        MUST NOT be retrieved by the GETATTR operation
        (<xref target="rw_attr" format="default"/>).
	This attribute corresponds to retention_get.
	The value of the attribute consists of:

</t>
          <artwork name="" type="" align="left" alt=""><![CDATA[
struct retention_set4 {
        bool            rs_enable;
        uint64_t        rs_duration<1>;
};
 ]]></artwork>
          <t>

        If the client sets rs_enable to TRUE, then it is enabling
        retention on the file object with the begin time of retention
        starting from the server's current time and date. The
        duration of the retention can also be provided if the
        rs_duration array is of length one.  The duration is the time in
        seconds from the begin time of retention, and if set to
        RET4_DURATION_INFINITE, the file is to be retained forever. If
        retention is enabled, with no duration specified in either
        this SETATTR or a previous SETATTR, the duration defaults to
        zero seconds.  The server MAY restrict the enabling of
        retention or the duration of retention on the basis of the
        ACE4_WRITE_RETENTION ACL permission.  The enabling of
        retention MUST NOT prevent the enabling of event-based
        retention or the modification of the retention_hold
        attribute.
          </t>
          <t>
       The following rules apply to both the retention_set and
       retentevt_set attributes.

          </t>
          <ul spacing="normal">
            <li>
	 As long as retention is not enabled, the client
	 is permitted to decrease the duration.

       </li>
            <li>
	 The duration can always be set to an
	 equal or higher value, even if retention is
	 enabled. Note that once retention is enabled,
	 the actual duration (as returned by the
	 retention_get or retentevt_get attributes;
	 see <xref target="attrdef_retention_get" format="default"/>
	 or <xref target="attrdef_retentevt_get" format="default"/>)
	 is constantly counting down to zero (one unit
	 per second), unless the duration was set to
	 RET4_DURATION_INFINITE.  Thus, it will not be
	 possible for the client to precisely extend the
	 duration on a file that has retention enabled.

       </li>
            <li>
	 While retention is enabled, attempts to disable
	 retention or decrease the retention's duration
	 MUST fail with the error NFS4ERR_INVAL.

       </li>
            <li>
         If the principal attempting to change
         retention_set or retentevt_set does not have
         ACE4_WRITE_RETENTION permissions, the attempt
         MUST fail with NFS4ERR_ACCESS.

       </li>
          </ul>
        </section>
        <section toc="exclude" anchor="attrdef_retentevt_get" numbered="true">
          <name>Attribute 71: retentevt_get</name>
          <t>
	Gets the event-based retention duration, and if enabled, the
        event-based retention begin time of the file object.  This
        attribute is like retention_get, but refers to event-based
        retention.  The event that triggers event-based retention is
        not defined by the NFSv4.1 specification.
          </t>
        </section>
        <section toc="exclude" anchor="attrdef_retentevt_set" numbered="true">
          <name>Attribute 72: retentevt_set</name>
          <t>
	Sets the event-based retention duration, and optionally enables
	event-based retention on the file object.  This attribute
	corresponds to retentevt_get and is like retention_set, but
	refers to event-based retention.  When event-based retention
	is set, the file MUST be retained even if non-event-based
	retention has been set, and the duration of non-event-based
	retention has been reached. Conversely, when non-event-based
	retention has been set, the file MUST be retained even if
	event-based retention has been set, and the duration of
	event-based retention has been reached.  The server MAY
	restrict the enabling of event-based retention or the duration
	of event-based retention on the basis of the
	ACE4_WRITE_RETENTION ACL permission.  The enabling of
	event-based retention MUST NOT prevent the enabling of
	non-event-based retention or the modification of the
	retention_hold attribute.
          </t>
        </section>
        <section toc="exclude" anchor="attrdef_retention_hold" numbered="true">
          <name>Attribute 73: retention_hold</name>
          <t>
	Gets or sets administrative retention holds, one hold per bit
        position.
          </t>
          <t>
	This attribute allows one to 64 administrative holds, one hold
	per bit on the attribute. If retention_hold is not zero, then
	the file MUST NOT be deleted, renamed, or modified, even if
	the duration on enabled event or non-event-based retention has
	been reached.  The server MAY restrict the modification of
	retention_hold on the basis of the ACE4_WRITE_RETENTION_HOLD
	ACL permission.  The enabling of administration retention
	holds does not prevent the enabling of event-based or
	non-event-based retention.
          </t>
          <t>
	If the principal attempting to change retention_hold does
	not have ACE4_WRITE_RETENTION_HOLD permissions,
	the attempt MUST fail with NFS4ERR_ACCESS.
          </t>
        </section>
      </section>
    </section>
    <!--    $Id: 2009-12-20-TO-rfc5661.xml,v 1.2 2009/12/21 05:59:32 shepler.mre Exp $  -->
<!-- Copyright (C) The IETF Trust (2007-2008) -->
<!-- Copyright (C) The Internet Society (2006) -->
<section anchor="acl" numbered="true" toc="default">
      <name>Access Control Attributes</name>
      <t>
        Access Control Lists (ACLs) are file attributes that specify
        fine-grained access control. This section covers the
        "acl", "dacl", "sacl",
        "aclsupport", "mode", and
        "mode_set_masked" file attributes and their
        interactions.  Note that file attributes may apply to any file
        system object.
      </t>
      <section numbered="true" toc="default">
        <name>Goals</name>
        <t>
        ACLs and modes represent two well-established models for
        specifying permissions. This section specifies requirements
        that attempt to meet the following goals:

        </t>
        <ul spacing="normal">
          <li>
            If a server supports the mode attribute, it should provide
            reasonable semantics to clients that only set and retrieve
            the mode attribute.
          </li>
          <li>
            If a server supports ACL attributes, it should provide
            reasonable semantics to clients that only set and retrieve
            those attributes.
          </li>
          <li>
            On servers that support the mode attribute, if ACL
            attributes have never been set on an object, via
            inheritance or explicitly, the behavior should be
            traditional UNIX-like behavior.
          </li>
          <li>
            <t>
            On servers that support the mode attribute, if the ACL
            attributes have been previously set on an object, either
            explicitly or via inheritance:
            </t>
            <ul spacing="normal">
              <li>
                Setting only the mode attribute should effectively
                control the traditional UNIX-like permissions of read,
                write, and execute on owner, owner_group, and other.
              </li>
              <li>
                Setting only the mode attribute should provide
                reasonable security. For example, setting a mode of
                000 should be enough to ensure that future OPEN operations for
                OPEN4_SHARE_ACCESS_READ or OPEN4_SHARE_ACCESS_WRITE by any principal fail, regardless of a
                previously existing or inherited ACL.
              </li>
            </ul>
          </li>
          <li>
            NFSv4.1 may introduce different
            semantics relating to the mode and ACL attributes,
            but it does not render invalid any previously
            existing implementations. Additionally, this
            section provides clarifications based on previous
            implementations and discussions around them.
          </li>
          <li>
            On servers that support both the mode and the acl or
            dacl attributes, the server must keep the two consistent
            with each other.  The value of the mode attribute (with
            the exception of the three high-order bits described in
            <xref target="attrdef_mode" format="default"/>) must be determined entirely
            by the value of the ACL, so that use of the mode is
            never required for anything other than setting the
            three high-order bits.  See <xref target="setattr" format="default"/>
            for exact requirements.
          </li>
          <li>
            When a mode attribute is set on an object, the ACL
            attributes may need to be modified in order to not conflict
            with the new mode. In such cases, it is desirable that the
            ACL keep as much information as possible. This includes
            information about inheritance, AUDIT and ALARM ACEs, and
            permissions granted and denied that do not conflict with
            the new mode.
          </li>
        </ul>
      </section>
      <section numbered="true" toc="default">
        <name>File Attributes Discussion</name>
        <section anchor="attrdef_acl" numbered="true" toc="default">
          <name>Attribute 12: acl</name>
          <t>
          The NFSv4.1 ACL attribute contains an array of Access
          Control Entries (ACEs) that are associated with the file
          system object.  Although the client can set and
          get the acl attribute, the server is responsible for using
          the ACL to perform access control. The client can use the
          OPEN or ACCESS operations to check access without modifying
          or reading data or metadata.
          </t>
          <t>
          The NFS ACE structure is defined as follows:
          </t>
          <artwork name="" type="" align="left" alt=""><![CDATA[
typedef uint32_t        acetype4;
 ]]></artwork>
          <artwork name="" type="" align="left" alt=""><![CDATA[
typedef uint32_t aceflag4;
 ]]></artwork>
          <artwork name="" type="" align="left" alt=""><![CDATA[
typedef uint32_t        acemask4;
 ]]></artwork>
          <artwork name="" type="" align="left" alt=""><![CDATA[
struct nfsace4 {
        acetype4        type;
        aceflag4        flag;
        acemask4        access_mask;
        utf8str_mixed   who;
};
 ]]></artwork>
          <t>
          To determine if a request succeeds, the server processes
          each nfsace4 entry in order.  Only ACEs that have a "who"
          that matches the requester are considered.  Each ACE is
          processed until all of the bits of the requester's access
          have been ALLOWED.  Once a bit (see below) has been ALLOWED
          by an ACCESS_ALLOWED_ACE, it is no longer considered in the
          processing of later ACEs.  If an ACCESS_DENIED_ACE is
          encountered where the requester's access still has unALLOWED
          bits in common with the "access_mask" of the ACE, the
          request is denied.  When the ACL is fully processed, if
          there are bits in the requester's mask that have not been
          ALLOWED or DENIED, access is denied.
          </t>
          <t>
          Unlike the ALLOW and DENY ACE types, the ALARM and AUDIT ACE
          types do not affect a requester's access, and instead are
          for triggering events as a result of a requester's access
          attempt.  Therefore, AUDIT and ALARM ACEs are processed only
          after processing ALLOW and DENY ACEs.
          </t>
          <t>
          The NFSv4.1 ACL model is quite rich. Some server
          platforms may provide access-control functionality that goes
          beyond the UNIX-style mode attribute, but that is not as
          rich as the NFS ACL model.  So that users can take advantage
          of this more limited functionality, the server may support
          the acl attributes by mapping between its ACL model and the
          NFSv4.1 ACL model.  Servers must ensure that the ACL
          they actually store or enforce is at least as strict as the
          NFSv4 ACL that was set.  It is tempting to accomplish this
          by rejecting any ACL that falls outside the small set that
          can be represented accurately.  However, such an approach
          can render ACLs unusable without special client-side
          knowledge of the server's mapping, which defeats the purpose
          of having a common NFSv4 ACL protocol.  Therefore, servers
          should accept every ACL that they can without compromising
          security.  To help accomplish this, servers may make a
          special exception, in the case of unsupported permission
          bits, to the rule that bits not ALLOWED or DENIED by an ACL
          must be denied.  For example, a UNIX-style server might
          choose to silently allow read attribute permissions even
          though an ACL does not explicitly allow those permissions.
          (An ACL that explicitly denies permission to read attributes
          should still be rejected.)
          </t>
          <t>
          The situation is complicated by the fact that a server may
          have multiple modules that enforce ACLs. For example, the
          enforcement for NFSv4.1 access may be different from,
          but not weaker than, the enforcement for local access, and
          both may be different from the enforcement for access
          through other protocols such as SMB (Server Message Block). So it may be useful for
          a server to accept an ACL even if not all of its modules are
          able to support it.
          </t>
          <t>
          The guiding principle with regard to NFSv4 access is
          that the server must not accept ACLs that appear to
          make access to the file more restrictive than it really is.
          </t>
          <section numbered="true" toc="default">
            <name>ACE Type</name>
            <t>
            The constants used for the type field (acetype4) are as
            follows:
            </t>
            <artwork name="" type="" align="left" alt=""><![CDATA[
const ACE4_ACCESS_ALLOWED_ACE_TYPE      = 0x00000000;
const ACE4_ACCESS_DENIED_ACE_TYPE       = 0x00000001;
const ACE4_SYSTEM_AUDIT_ACE_TYPE        = 0x00000002;
const ACE4_SYSTEM_ALARM_ACE_TYPE        = 0x00000003;
 ]]></artwork>
            <t>
            Only the ALLOWED and DENIED bits may be used in the
            dacl attribute, and only the AUDIT and ALARM bits may be
            used in the sacl attribute.  All four are permitted in the
            acl attribute.
            </t>
            <table align="center">
              <thead>
                <tr>
                  <th align="left">Value</th>
                  <th align="left">Abbreviation</th>
                  <th align="left">Description</th>
                </tr>
              </thead>
              <tbody>
                <tr>
                  <td align="left">ACE4_ACCESS_ALLOWED_ACE_TYPE</td>
                  <td align="left">ALLOW</td>
                  <td align="left">
              Explicitly grants the access defined in acemask4 to
              the file or directory.
            </td>
                </tr>
                <tr>
                  <td align="left">ACE4_ACCESS_DENIED_ACE_TYPE</td>
                  <td align="left">DENY</td>
                  <td align="left">
              Explicitly denies the access defined in acemask4 to
              the file or directory.
            </td>
                </tr>
                <tr>
                  <td align="left">ACE4_SYSTEM_AUDIT_ACE_TYPE</td>
                  <td align="left">AUDIT</td>
                  <td align="left">
              Log (in a system-dependent way) any access attempt to
              a file or directory that uses any of the access
              methods specified in acemask4.
            </td>
                </tr>
                <tr>
                  <td align="left">ACE4_SYSTEM_ALARM_ACE_TYPE</td>
                  <td align="left">ALARM</td>
                  <td align="left">
              Generate an alarm (in a system-dependent way) when any
              access attempt is made to a file or directory for the
              access methods specified in acemask4.
            </td>
                </tr>
              </tbody>
            </table>
            <t>
              The "Abbreviation" column denotes how the
              types will be referred to throughout the rest of this
              section.
            </t>
          </section>
          <section anchor="attrdef_aclsupport" numbered="true" toc="default">
            <name>Attribute 13: aclsupport</name>
            <t>
            A server need not support all of the above ACE types.
	    This attribute indicates which ACE types are supported for
	    the current file system.  The bitmask constants used to
	    represent the above definitions within the aclsupport
	    attribute are as follows:
            </t>
            <artwork name="" type="" align="left" alt=""><![CDATA[
const ACL4_SUPPORT_ALLOW_ACL    = 0x00000001;
const ACL4_SUPPORT_DENY_ACL     = 0x00000002;
const ACL4_SUPPORT_AUDIT_ACL    = 0x00000004;
const ACL4_SUPPORT_ALARM_ACL    = 0x00000008;
 ]]></artwork>
            <t>
            Servers that support either the ALLOW or DENY ACE type
            SHOULD support both ALLOW and DENY ACE types.
            </t>
            <t>
            Clients should not attempt to set an ACE unless the server
            claims support for that ACE type. If the server receives a
            request to set an ACE that it cannot store, it MUST reject
            the request with NFS4ERR_ATTRNOTSUPP. If the server
            receives a request to set an ACE that it can store but
            cannot enforce, the server SHOULD reject the request with
            NFS4ERR_ATTRNOTSUPP.
            </t>
            <t>
            Support for any of the ACL attributes is
            optional (albeit RECOMMENDED).
            However, a server that supports either of the new ACL
            attributes (dacl or sacl) MUST allow use of the new ACL
            attributes to access all of the ACE types that it
            supports.  In other words, if such a server supports ALLOW
            or DENY ACEs, then it MUST support the dacl attribute, and
            if it supports AUDIT or ALARM ACEs, then it MUST support
            the sacl attribute.
            </t>
          </section>
          <section anchor="acemask" numbered="true" toc="default">
            <name>ACE Access Mask</name>
            <t>
            The bitmask constants used for the access mask field
            are as follows:
            </t>
            <artwork name="" type="" align="left" alt=""><![CDATA[
const ACE4_READ_DATA            = 0x00000001;
const ACE4_LIST_DIRECTORY       = 0x00000001;
const ACE4_WRITE_DATA           = 0x00000002;
const ACE4_ADD_FILE             = 0x00000002;
const ACE4_APPEND_DATA          = 0x00000004;
const ACE4_ADD_SUBDIRECTORY     = 0x00000004;
const ACE4_READ_NAMED_ATTRS     = 0x00000008;
const ACE4_WRITE_NAMED_ATTRS    = 0x00000010;
const ACE4_EXECUTE              = 0x00000020;
const ACE4_DELETE_CHILD         = 0x00000040;
const ACE4_READ_ATTRIBUTES      = 0x00000080;
const ACE4_WRITE_ATTRIBUTES     = 0x00000100;
const ACE4_WRITE_RETENTION      = 0x00000200;
const ACE4_WRITE_RETENTION_HOLD = 0x00000400;

const ACE4_DELETE               = 0x00010000;
const ACE4_READ_ACL             = 0x00020000;
const ACE4_WRITE_ACL            = 0x00040000;
const ACE4_WRITE_OWNER          = 0x00080000;
const ACE4_SYNCHRONIZE          = 0x00100000;
 ]]></artwork>
            <t>

	   Note that some masks have coincident values, for
	   example, ACE4_READ_DATA and ACE4_LIST_DIRECTORY.
	   The mask entries ACE4_LIST_DIRECTORY,
	   ACE4_ADD_FILE, and ACE4_ADD_SUBDIRECTORY are
	   intended to be used with directory objects,
	   while ACE4_READ_DATA, ACE4_WRITE_DATA, and
	   ACE4_APPEND_DATA are intended to be used with
	   non-directory objects.

            </t>
            <section numbered="true" toc="default">
              <name>Discussion of Mask Attributes</name>
              <dl newline="false" spacing="normal">
                <dt>ACE4_READ_DATA</dt>
                <dd>
                  <dl newline="true" spacing="normal">
                    <dt>Operation(s) affected:</dt>
                    <dd>
                      <dl newline="false" spacing="normal">
                        <dt>READ</dt>
                        <dd/>
                        <dt>OPEN</dt>
                        <dd/>
                      </dl>
                    </dd>
                    <dt>Discussion:</dt>
                    <dd>
                      <t>
		      Permission to read the data of the file.
                      </t>
                      <t>
		      Servers SHOULD allow a user the ability to read the data
		      of the file when only the ACE4_EXECUTE access mask bit is
		      allowed.
                      </t>
                    </dd>
                  </dl>
                </dd>
                <dt>ACE4_LIST_DIRECTORY</dt>
                <dd>
                  <dl newline="true" spacing="normal">
                    <dt>Operation(s) affected:</dt>
                    <dd>
                      <dl newline="false" spacing="normal">
                        <dt>READDIR</dt>
                        <dd/>
                      </dl>
                    </dd>
                    <dt>Discussion:</dt>
                    <dd>
		      Permission to list the contents of a directory.
		    </dd>
                  </dl>
                </dd>
                <dt>ACE4_WRITE_DATA</dt>
                <dd>
                  <dl newline="true" spacing="normal">
                    <dt>Operation(s) affected:</dt>
                    <dd>
                      <dl newline="false" spacing="normal">
                        <dt>WRITE</dt>
                        <dd/>
                        <dt>OPEN</dt>
                        <dd/>
                        <dt>SETATTR of size</dt>
                        <dd/>
                      </dl>
                    </dd>
                    <dt>Discussion:</dt>
                    <dd>
		      Permission to modify a file's data.
		    </dd>
                  </dl>
                </dd>
                <dt>ACE4_ADD_FILE</dt>
                <dd>
                  <dl newline="true" spacing="normal">
                    <dt>Operation(s) affected:</dt>
                    <dd>
                      <dl newline="false" spacing="normal">
                        <dt>CREATE</dt>
                        <dd/>
                        <dt>LINK</dt>
                        <dd/>
                        <dt>OPEN</dt>
                        <dd/>
                        <dt>RENAME</dt>
                        <dd/>
                      </dl>
                    </dd>
                    <dt>Discussion:</dt>
                    <dd>
		      Permission to add a new file in a directory.
		      The CREATE operation is affected when nfs_ftype4
		      is NF4LNK, NF4BLK, NF4CHR, NF4SOCK, or
		      NF4FIFO. (NF4DIR is not listed because it is
		      covered by ACE4_ADD_SUBDIRECTORY.) OPEN is
		      affected when used to create a regular file.
		      LINK and RENAME are always affected.
		    </dd>
                  </dl>
                </dd>
                <dt>ACE4_APPEND_DATA</dt>
                <dd>
                  <dl newline="true" spacing="normal">
                    <dt>Operation(s) affected:</dt>
                    <dd>
                      <dl newline="false" spacing="normal">
                        <dt>WRITE</dt>
                        <dd/>
                        <dt>OPEN</dt>
                        <dd/>
                        <dt>SETATTR of size</dt>
                        <dd/>
                      </dl>
                    </dd>
                    <dt>Discussion:</dt>
                    <dd>
		      The ability to modify a file's data, but only
		      starting at EOF.  This allows for the notion of
		      append-only files, by allowing ACE4_APPEND_DATA
		      and denying ACE4_WRITE_DATA to the same user or
		      group.  If a file has an ACL such as the one
		      described above and a WRITE request is made for
		      somewhere other than EOF, the server SHOULD
		      return NFS4ERR_ACCESS.
		    </dd>
                  </dl>
                </dd>
                <dt>ACE4_ADD_SUBDIRECTORY</dt>
                <dd>
                  <dl newline="true" spacing="normal">
                    <dt>Operation(s) affected:</dt>
                    <dd>
                      <dl newline="false" spacing="normal">
                        <dt>CREATE</dt>
                        <dd/>
                        <dt>RENAME</dt>
                        <dd/>
                      </dl>
                    </dd>
                    <dt>Discussion:</dt>
                    <dd>
		      Permission to create a subdirectory in a
		      directory.  The CREATE operation is affected
		      when nfs_ftype4 is NF4DIR.  The RENAME operation
		      is always affected.
		    </dd>
                  </dl>
                </dd>
                <dt>ACE4_READ_NAMED_ATTRS</dt>
                <dd>
                  <dl newline="true" spacing="normal">
                    <dt>Operation(s) affected:</dt>
                    <dd>
                      <dl newline="false" spacing="normal">
                        <dt>OPENATTR</dt>
                        <dd/>
                      </dl>
                    </dd>
                    <dt>Discussion:</dt>
                    <dd>
		      Permission to read the named attributes of a
		      file or to look up the named attribute
		      directory.  OPENATTR is affected when it is not
		      used to create a named attribute directory.
		      This is when 1) createdir is TRUE, but a named
		      attribute directory already exists, or 2)
		      createdir is FALSE.
		    </dd>
                  </dl>
                </dd>
                <dt>ACE4_WRITE_NAMED_ATTRS</dt>
                <dd>
                  <dl newline="true" spacing="normal">
                    <dt>Operation(s) affected:</dt>
                    <dd>
                      <dl newline="false" spacing="normal">
                        <dt>OPENATTR</dt>
                        <dd/>
                        <dt></dt>
                        <dd/>
                      </dl>
                    </dd>
                    <dt>Discussion:</dt>
                    <dd>
		      Permission to write the named attributes of a
		      file or to create a named attribute directory.
		      OPENATTR is affected when it is used to create a
		      named attribute directory.  This is when
		      createdir is TRUE and no named attribute
		      directory exists.  The ability to check whether
		      or not a named attribute directory exists
		      depends on the ability to look it up; therefore,
		      users also need the ACE4_READ_NAMED_ATTRS
		      permission in order to create a named attribute
		      directory.
		    </dd>
                  </dl>
                </dd>
                <dt>ACE4_EXECUTE</dt>
                <dd>
                  <dl newline="true" spacing="normal">
                    <dt>Operation(s) affected:</dt>
                    <dd>
                      <dl newline="false" spacing="normal">
                        <dt>READ</dt>
                        <dd/>
                        <dt>OPEN</dt>
                        <dd/>
                        <dt>REMOVE</dt>
                        <dd/>
                        <dt>RENAME</dt>
                        <dd/>
                        <dt>LINK</dt>
                        <dd/>
                        <dt>CREATE</dt>
                        <dd/>
                      </dl>
                    </dd>
                    <dt>Discussion:</dt>
                    <dd>
                      <t>
		      Permission to execute a file.
                      </t>
                      <t>
		      Servers SHOULD allow a
		      user the ability to read the data of the file
		      when only the ACE4_EXECUTE access mask bit is
		      allowed.  This is because there is no way to
		      execute a file without reading the contents.
		      Though a server may treat ACE4_EXECUTE and
		      ACE4_READ_DATA bits identically when deciding to
		      permit a READ operation, it SHOULD still allow
		      the two bits to be set independently in ACLs,
		      and MUST distinguish between them when replying
		      to ACCESS operations.  In particular, servers
		      SHOULD NOT silently turn on one of the two bits
		      when the other is set, as that would make it
		      impossible for the client to correctly enforce
		      the distinction between read and execute
		      permissions.
                      </t>
                      <t>
		      As an example, following a SETATTR of the following ACL:
                      </t>
                      <t>
                      nfsuser:ACE4_EXECUTE:ALLOW
                      </t>
                      <t>
		      A subsequent GETATTR of ACL for that file SHOULD return:
                      </t>
                      <t>
                      nfsuser:ACE4_EXECUTE:ALLOW
                      </t>
                      <t>
		      Rather than:
                      </t>
                      <t>
                      nfsuser:ACE4_EXECUTE/ACE4_READ_DATA:ALLOW
                      </t>
                    </dd>
                  </dl>
                </dd>
                <dt>ACE4_EXECUTE</dt>
                <dd>
                  <dl newline="true" spacing="normal">
                    <dt>Operation(s) affected:</dt>
                    <dd>
                      <dl newline="false" spacing="normal">
                        <dt>LOOKUP</dt>
                        <dd/>
                      </dl>
                    </dd>
                    <dt>Discussion:</dt>
                    <dd>
		      Permission to traverse/search a directory.
		    </dd>
                  </dl>
                </dd>
                <dt>ACE4_DELETE_CHILD</dt>
                <dd>
                  <dl newline="true" spacing="normal">
                    <dt>Operation(s) affected:</dt>
                    <dd>
                      <dl newline="false" spacing="normal">
                        <dt>REMOVE</dt>
                        <dd/>
                        <dt>RENAME</dt>
                        <dd/>
                      </dl>
                    </dd>
                    <dt>Discussion:</dt>
                    <dd>
		      Permission to delete a file or directory within
		      a directory.

		      See <xref target="delete-delete_child" format="default"/>
		      for information on ACE4_DELETE and
		      ACE4_DELETE_CHILD interact.

		    </dd>
                  </dl>
                </dd>
                <dt>ACE4_READ_ATTRIBUTES</dt>
                <dd>
                  <dl newline="true" spacing="normal">
                    <dt>Operation(s) affected:</dt>
                    <dd>
                      <dl newline="false" spacing="normal">
                        <dt>GETATTR of file system object attributes</dt>
                        <dd/>
                        <dt>VERIFY</dt>
                        <dd/>
                        <dt>NVERIFY</dt>
                        <dd/>
                        <dt>READDIR</dt>
                        <dd/>
                      </dl>
                    </dd>
                    <dt>Discussion:</dt>
                    <dd>
		      The ability to read basic attributes (non-ACLs)
		      of a file.  On a UNIX system, basic attributes
		      can be thought of as the stat-level attributes.
		      Allowing this access mask bit would mean that the
		      entity can execute "ls -l" and stat.  If a
		      READDIR operation requests attributes, this mask
		      must be allowed for the READDIR to succeed.
		    </dd>
                  </dl>
                </dd>
                <dt>ACE4_WRITE_ATTRIBUTES</dt>
                <dd>
                  <dl newline="true" spacing="normal">
                    <dt>Operation(s) affected:</dt>
                    <dd>
                      <dl newline="false" spacing="normal">
                        <dt>SETATTR of time_access_set, time_backup,</dt>
                        <dd/>
                        <dt>time_create, time_modify_set, mimetype, hidden, system</dt>
                        <dd/>
                      </dl>
                    </dd>
                    <dt>Discussion:</dt>
                    <dd>
		      Permission to change the times associated with a
		      file or directory to an arbitrary value.  Also
		      permission to change the mimetype, hidden, and
		      system attributes.  A user having
		      ACE4_WRITE_DATA or ACE4_WRITE_ATTRIBUTES will be
		      allowed to set the times associated with a file
		      to the current server time.
		    </dd>
                  </dl>
                </dd>
                <dt>ACE4_WRITE_RETENTION</dt>
                <dd>
                  <dl newline="true" spacing="normal">
                    <dt>Operation(s) affected:</dt>
                    <dd>
                      <dl newline="false" spacing="normal">
                        <dt>SETATTR of retention_set, retentevt_set.</dt>
                        <dd/>
                      </dl>
                    </dd>
                    <dt>Discussion:</dt>
                    <dd>
		      Permission to modify the durations of event and
		      non-event-based retention. Also permission to
		      enable event and non-event-based retention. A
		      server MAY behave such that setting
		      ACE4_WRITE_ATTRIBUTES allows
		      ACE4_WRITE_RETENTION.
		    </dd>
                  </dl>
                </dd>
                <dt>ACE4_WRITE_RETENTION_HOLD</dt>
                <dd>
                  <dl newline="true" spacing="normal">
                    <dt>Operation(s) affected:</dt>
                    <dd>
                      <dl newline="false" spacing="normal">
                        <dt>SETATTR of retention_hold.</dt>
                        <dd/>
                      </dl>
                    </dd>
                    <dt>Discussion:</dt>
                    <dd>
		      Permission to modify the administration
		      retention holds.  A server MAY map
		      ACE4_WRITE_ATTRIBUTES to
		      ACE_WRITE_RETENTION_HOLD.
		    </dd>
                  </dl>
                </dd>
                <dt>ACE4_DELETE</dt>
                <dd>
                  <dl newline="true" spacing="normal">
                    <dt>Operation(s) affected:</dt>
                    <dd>
                      <dl newline="false" spacing="normal">
                        <dt>REMOVE</dt>
                        <dd/>
                      </dl>
                    </dd>
                    <dt>Discussion:</dt>
                    <dd>

		      Permission to delete the
		      file or directory.

		      See <xref target="delete-delete_child" format="default"/>
		      for information on ACE4_DELETE and
		      ACE4_DELETE_CHILD interact.

		    </dd>
                  </dl>
                </dd>
                <dt>ACE4_READ_ACL</dt>
                <dd>
                  <dl newline="true" spacing="normal">
                    <dt>Operation(s) affected:</dt>
                    <dd>
                      <dl newline="false" spacing="normal">
                        <dt>GETATTR of acl, dacl, or sacl</dt>
                        <dd/>
                        <dt>NVERIFY</dt>
                        <dd/>
                        <dt>VERIFY</dt>
                        <dd/>
                      </dl>
                    </dd>
                    <dt>Discussion:</dt>
                    <dd>
		      Permission to read the ACL.
		    </dd>
                  </dl>
                </dd>
                <dt>ACE4_WRITE_ACL</dt>
                <dd>
                  <dl newline="true" spacing="normal">
                    <dt>Operation(s) affected:</dt>
                    <dd>
                      <dl newline="false" spacing="normal">
                        <dt>SETATTR of acl and mode</dt>
                        <dd/>
                      </dl>
                    </dd>
                    <dt>Discussion:</dt>
                    <dd>
		      Permission to write the acl and mode attributes.
		    </dd>
                  </dl>
                </dd>
                <dt>ACE4_WRITE_OWNER</dt>
                <dd>
                  <dl newline="true" spacing="normal">
                    <dt>Operation(s) affected:</dt>
                    <dd>
                      <dl newline="false" spacing="normal">
                        <dt>SETATTR of owner and owner_group</dt>
                        <dd/>
                      </dl>
                    </dd>
                    <dt>Discussion:</dt>
                    <dd>
		      Permission to write the owner and owner_group
		      attributes.  On UNIX systems, this is the
		      ability to execute chown() and chgrp().
		    </dd>
                  </dl>
                </dd>
                <dt>ACE4_SYNCHRONIZE</dt>
                <dd>
                  <dl newline="true" spacing="normal">
                    <dt>Operation(s) affected:</dt>
                    <dd>
                      <dl newline="false" spacing="normal">
                        <dt>NONE</dt>
                        <dd/>
                      </dl>
                    </dd>
                    <dt>Discussion:</dt>
                    <dd>
                      <t>
		      Permission to use the file object as a
		      synchronization primitive for interprocess
		      communication. This permission is not enforced
		      or interpreted by the NFSv4.1 server on behalf of
		      the client.

                      </t>
                      <t>

                      Typically, the ACE4_SYNCHRONIZE permission is
                      only meaningful on local file systems, i.e.,
                      file systems not accessed via NFSv4.1. The reason
                      that the permission bit exists is that some operating
                      environments, such as Windows, use ACE4_SYNCHRONIZE.

                      </t>
                      <t>

                      For example, if a client copies a file that has
                      ACE4_SYNCHRONIZE set from a local file system to
                      an NFSv4.1 server, and then later copies the file
                      from the NFSv4.1 server to a local file system,
                      it is likely that if ACE4_SYNCHRONIZE was set
                      in the original file, the client will want it
                      set in the second copy.  The first copy will not
                      have the permission set unless the NFSv4.1 server
                      has the means to set the ACE4_SYNCHRONIZE bit. The
                      second copy will not have the permission set unless
                      the NFSv4.1 server has the means to retrieve the
                      ACE4_SYNCHRONIZE bit.

                      </t>
                    </dd>
                  </dl>
                </dd>
              </dl>
              <t>
              Server implementations need not provide the granularity
              of control that is implied by this list of masks. For
              example, POSIX-based systems might not distinguish
              ACE4_APPEND_DATA (the ability to append to a file) from
              ACE4_WRITE_DATA (the ability to modify existing
              contents); both masks would be tied to a single "write"
              permission <xref target="chmod" format="default"/>. When such a server returns attributes to the
              client, it would show both ACE4_APPEND_DATA and
              ACE4_WRITE_DATA if and only if the write permission is
              enabled.
              </t>
              <t>
              If a server receives a SETATTR request that it cannot
              accurately implement, it should err in the direction of
              more restricted access, except in the previously
              discussed cases of execute and read. For example,
              suppose a server cannot distinguish overwriting data
              from appending new data, as described in the previous
              paragraph.  If a client submits an ALLOW ACE where
              ACE4_APPEND_DATA is set but ACE4_WRITE_DATA is not (or
              vice versa), the server should either turn off
              ACE4_APPEND_DATA or reject the request with
              NFS4ERR_ATTRNOTSUPP.
              </t>
            </section>
            <section anchor="delete-delete_child" numbered="true" toc="default">
              <name>ACE4_DELETE vs. ACE4_DELETE_CHILD</name>
              <t>
              Two access mask bits govern the ability to delete a
              directory entry: ACE4_DELETE on the object
              itself (the "target") and ACE4_DELETE_CHILD on
              the containing directory (the "parent").
              </t>
              <t>
              Many systems also take the "sticky bit" (MODE4_SVTX)
              on a directory to allow unlink only to a user that
              owns either the target or the parent; on some
              such systems the decision also depends on
              whether the target is writable.
              </t>
              <t>
              Servers SHOULD allow unlink if either ACE4_DELETE
              is permitted on the target, or ACE4_DELETE_CHILD is
              permitted on the parent.  (Note that this is
              true even if the parent or target explicitly
              denies one of these permissions.)
              </t>
              <t>
              If the ACLs in question neither explicitly ALLOW
              nor DENY either of the above, and if MODE4_SVTX is
              not set on the parent, then the server SHOULD allow
              the removal if and only if ACE4_ADD_FILE is permitted.
              In the case where MODE4_SVTX is set, the server
              may also require the remover to own either the parent
              or the target, or may require the target to be
              writable.
              </t>
              <t>
              This allows servers to support something close to
              traditional UNIX-like semantics, with ACE4_ADD_FILE
              taking the place of the write bit.
              </t>
            </section>
          </section>
          <section anchor="aceflag" numbered="true" toc="default">
            <name>ACE flag</name>
            <t>
            The bitmask constants used for the flag field are as
            follows:
</t>
            <artwork name="" type="" align="left" alt=""><![CDATA[
const ACE4_FILE_INHERIT_ACE             = 0x00000001;
const ACE4_DIRECTORY_INHERIT_ACE        = 0x00000002;
const ACE4_NO_PROPAGATE_INHERIT_ACE     = 0x00000004;
const ACE4_INHERIT_ONLY_ACE             = 0x00000008;
const ACE4_SUCCESSFUL_ACCESS_ACE_FLAG   = 0x00000010;
const ACE4_FAILED_ACCESS_ACE_FLAG       = 0x00000020;
const ACE4_IDENTIFIER_GROUP             = 0x00000040;
const ACE4_INHERITED_ACE                = 0x00000080;
 ]]></artwork>
            <t>

            A server need not support any of these flags. If the
            server supports flags that are similar to, but not
            exactly the same as, these flags, the implementation
            may define a mapping between the protocol-defined
            flags and the implementation-defined flags.
            </t>
            <t>
            For example, suppose a client tries to set an ACE with
            ACE4_FILE_INHERIT_ACE set but not
            ACE4_DIRECTORY_INHERIT_ACE. If the server does not
            support any form of ACL inheritance, the server should
            reject the request with NFS4ERR_ATTRNOTSUPP. If the
            server supports a single "inherit ACE" flag that
            applies to both files and directories, the server may
            reject the request (i.e., requiring the client to set
            both the file and directory inheritance flags). The
            server may also accept the request and silently turn
            on the ACE4_DIRECTORY_INHERIT_ACE flag.
            </t>
            <section numbered="true" toc="default">
              <name>Discussion of Flag Bits</name>
              <dl newline="true" spacing="normal">
                <dt>ACE4_FILE_INHERIT_ACE</dt>
                <dd>
                  Any non-directory file in any
                  sub-directory will get this ACE
                  inherited.
                </dd>
                <dt>ACE4_DIRECTORY_INHERIT_ACE</dt>
                <dd>
                  <t>
                  Can be placed on a directory and indicates
                  that this ACE should be added to each new
                  directory created.
                  </t>
                  <t>
                  If this flag is set in an ACE in an ACL
                  attribute to be set on a non-directory
                  file system object, the operation
                  attempting to set the ACL SHOULD fail
                  with NFS4ERR_ATTRNOTSUPP.
                  </t>
                </dd>
                <dt>ACE4_NO_PROPAGATE_INHERIT_ACE</dt>
                <dd>
                  Can be placed on a directory.  This flag
                  tells the server that inheritance of this
                  ACE should stop at newly created child
                  directories.
                </dd>
                <dt>ACE4_INHERIT_ONLY_ACE</dt>
                <dd>
                  <t>
                  Can be placed on a directory but does not
                  apply to the directory; ALLOW and DENY ACEs
                  with this bit set do not affect access to
                  the directory, and AUDIT and ALARM ACEs
                  with this bit set do not trigger log or
                  alarm events.  Such ACEs only take effect
                  once they are applied (with this bit
                  cleared) to newly created files and
                  directories as specified by the
                  ACE4_FILE_INHERIT_ACE and ACE4_DIRECTORY_INHERIT_ACE
                  flags.
                  </t>
                  <t>
                  If this flag is present on an ACE, but
                  neither ACE4_DIRECTORY_INHERIT_ACE nor
                  ACE4_FILE_INHERIT_ACE is present, then
                  an operation attempting to set such an
                  attribute SHOULD fail with
                  NFS4ERR_ATTRNOTSUPP.
                  </t>
                </dd>
                <dt>ACE4_SUCCESSFUL_ACCESS_ACE_FLAG</dt>
                <dd/>
                <dt>ACE4_FAILED_ACCESS_ACE_FLAG</dt>
                <dd>
                  The ACE4_SUCCESSFUL_ACCESS_ACE_FLAG
                  (SUCCESS) and ACE4_FAILED_ACCESS_ACE_FLAG
                  (FAILED) flag bits may be set only on
                  ACE4_SYSTEM_AUDIT_ACE_TYPE (AUDIT) and
                  ACE4_SYSTEM_ALARM_ACE_TYPE (ALARM) ACE
                  types. If during the processing of the
                  file's ACL, the server encounters an AUDIT
                  or ALARM ACE that matches the principal
                  attempting the OPEN, the server notes that
                  fact, and the presence, if any, of the
                  SUCCESS and FAILED flags encountered in
                  the AUDIT or ALARM ACE. Once the server
                  completes the ACL processing, it then
                  notes if the operation succeeded or
                  failed. If the operation succeeded, and if
                  the SUCCESS flag was set for a matching
                  AUDIT or ALARM ACE, then the appropriate
                  AUDIT or ALARM event occurs. If the
                  operation failed, and if the FAILED flag
                  was set for the matching AUDIT or ALARM
                  ACE, then the appropriate AUDIT or ALARM
                  event occurs.  Either or both of the
                  SUCCESS or FAILED can be set, but if
                  neither is set, the AUDIT or ALARM ACE is
                  not useful.
                </dd>
                <dt></dt>
                <dd>
                  The previously described processing
                  applies to ACCESS operations even when
                  they return NFS4_OK.  For the purposes of
                  AUDIT and ALARM, we consider an ACCESS
                  operation to be a "failure" if it fails
                  to return a bit that was requested and
                  supported.
                </dd>
                <dt>ACE4_IDENTIFIER_GROUP</dt>
                <dd>
                  Indicates that the "who" refers to a GROUP
                  as defined under UNIX or a GROUP ACCOUNT
                  as defined under Windows. Clients and
                  servers MUST ignore the
                  ACE4_IDENTIFIER_GROUP flag on ACEs with a
                  who value equal to one of the special
                  identifiers outlined in
                  <xref target="acewho" format="default"/>.
                </dd>
                <dt>ACE4_INHERITED_ACE</dt>
                <dd>
                  Indicates that this ACE is inherited from
                  a parent directory.  A server that supports
                  automatic inheritance will place
                  this flag on any ACEs inherited from the
                  parent directory when creating a new
                  object.  Client applications will use this
                  to perform automatic inheritance.
                  Clients and servers MUST clear this
                  bit in the acl attribute; it may only
                  be used in the dacl and sacl attributes.
                </dd>
              </dl>
            </section>
          </section>
          <section anchor="acewho" numbered="true" toc="default">
            <name>ACE Who</name>
            <t>
            The "who" field of an ACE is an identifier that
            specifies the principal or principals to whom the ACE
            applies. It may refer to a user or a group, with the flag
            bit ACE4_IDENTIFIER_GROUP specifying which.
            </t>
            <t>
            There are several special identifiers that need to be
            understood universally, rather than in the context of a
            particular DNS domain. Some of these identifiers cannot be
            understood when an NFS client accesses the server, but
            have meaning when a local process accesses the file. The
            ability to display and modify these permissions is
            permitted over NFS, even if none of the access methods on
            the server understands the identifiers.
            </t>
            <table anchor="specialwho" align="center">
              <thead>
                <tr>
                  <th align="left">Who</th>
                  <th align="left">Description</th>
                </tr>
              </thead>
              <tbody>
                <tr>
                  <td align="left">OWNER</td>
                  <td align="left">
              The owner of the file.
            </td>
                </tr>
                <tr>
                  <td align="left">GROUP</td>
                  <td align="left">
              The group associated with the file.
            </td>
                </tr>
                <tr>
                  <td align="left">EVERYONE</td>
                  <td align="left">
              The world, including the owner and owning group.
            </td>
                </tr>
                <tr>
                  <td align="left">INTERACTIVE</td>
                  <td align="left">
              Accessed from an interactive terminal.
            </td>
                </tr>
                <tr>
                  <td align="left">NETWORK</td>
                  <td align="left">
              Accessed via the network.
            </td>
                </tr>
                <tr>
                  <td align="left">DIALUP</td>
                  <td align="left">
              Accessed as a dialup user to the server.
            </td>
                </tr>
                <tr>
                  <td align="left">BATCH</td>
                  <td align="left">
              Accessed from a batch job.
            </td>
                </tr>
                <tr>
                  <td align="left">ANONYMOUS</td>
                  <td align="left">
              Accessed without any authentication.
            </td>
                </tr>
                <tr>
                  <td align="left">AUTHENTICATED</td>
                  <td align="left">
              Any authenticated user (opposite of
              ANONYMOUS).
            </td>
                </tr>
                <tr>
                  <td align="left">SERVICE</td>
                  <td align="left">
              Access from a system service.
            </td>
                </tr>
              </tbody>
            </table>
            <t>
            To avoid conflict, these special identifiers are
            distinguished by an appended "@" and should appear in the
            form "xxxx@" (with no domain name after the "@"), for
            example, ANONYMOUS@.
            </t>
            <t>
            The ACE4_IDENTIFIER_GROUP flag MUST be ignored on
            entries with these special identifiers.  When encoding
            entries with these special identifiers, the
            ACE4_IDENTIFIER_GROUP flag SHOULD be set to zero.
            </t>
            <section numbered="true" toc="default">
              <name>Discussion of EVERYONE@</name>
              <t>
              It is important to note that "EVERYONE@" is not
              equivalent to the UNIX "other" entity. This is
              because, by definition, UNIX "other" does not include
              the owner or owning group of a file. "EVERYONE@" means
              literally everyone, including the owner or owning
              group.
              </t>
            </section>
          </section>
        </section>
        <section anchor="attrdef_dacl" numbered="true" toc="default">
          <name>Attribute 58: dacl</name>
          <t>
          The dacl attribute is like the acl attribute,
          but dacl allows
          just ALLOW and DENY ACEs.  The dacl
          attribute supports automatic inheritance (see
          <xref target="auto_inherit" format="default"/>).
          </t>
        </section>
        <section anchor="attrdef_sacl" numbered="true" toc="default">
          <name>Attribute 59: sacl</name>
          <t>
          The sacl attribute is like the acl attribute,
          but sacl allows
          just AUDIT and ALARM ACEs. The sacl
          attribute supports automatic inheritance (see
          <xref target="auto_inherit" format="default"/>).
          </t>
        </section>
        <section anchor="attrdef_mode" numbered="true" toc="default">
          <name>Attribute 33: mode</name>
          <t>
          The NFSv4.1 mode attribute is based on the UNIX mode
          bits. The following bits are defined:
          </t>
          <artwork name="" type="" align="left" alt=""><![CDATA[
const MODE4_SUID = 0x800;  /* set user id on execution */
const MODE4_SGID = 0x400;  /* set group id on execution */
const MODE4_SVTX = 0x200;  /* save text even after use */
const MODE4_RUSR = 0x100;  /* read permission: owner */
const MODE4_WUSR = 0x080;  /* write permission: owner */
const MODE4_XUSR = 0x040;  /* execute permission: owner */
const MODE4_RGRP = 0x020;  /* read permission: group */
const MODE4_WGRP = 0x010;  /* write permission: group */
const MODE4_XGRP = 0x008;  /* execute permission: group */
const MODE4_ROTH = 0x004;  /* read permission: other */
const MODE4_WOTH = 0x002;  /* write permission: other */
const MODE4_XOTH = 0x001;  /* execute permission: other */
 ]]></artwork>
          <t>
          Bits MODE4_RUSR, MODE4_WUSR, and MODE4_XUSR apply to the
          principal identified in the owner attribute. Bits MODE4_RGRP,
          MODE4_WGRP, and MODE4_XGRP apply to principals identified in
          the owner_group attribute but who are not identified in the
          owner attribute. Bits MODE4_ROTH, MODE4_WOTH, and MODE4_XOTH apply
          to any principal that does not match that in the owner
          attribute and does not have a group matching that of the
          owner_group attribute.
          </t>
          <t>
          Bits within a mode other than those specified above
          are not defined by this protocol. A server
          MUST NOT return bits other than those defined above in a
          GETATTR or READDIR operation, and it MUST return NFS4ERR_INVAL
          if bits other than those defined above are set in a SETATTR,
          CREATE, OPEN, VERIFY, or NVERIFY operation.
          </t>
        </section>
        <section anchor="attrdef_mode_set_masked" numbered="true" toc="default">
          <name>Attribute 74: mode_set_masked</name>
          <t>
          The mode_set_masked attribute is a write-only attribute
          that allows individual bits in the mode attribute to be
          set or reset, without changing others.  It allows, for
          example, the bits MODE4_SUID, MODE4_SGID, and MODE4_SVTX
          to be modified while leaving unmodified any of the
          nine low-order mode bits devoted to permissions.
          </t>
          <t>
          In such instances that the nine low-order bits are left
          unmodified, then neither the acl nor the dacl attribute
          should be automatically modified as discussed in
	  <xref target="setattr" format="default"/>.
          </t>
          <t>
          The mode_set_masked attribute consists of two words,
          each in the form of a mode4.  The first consists of the
          value to be applied to the current mode value and the
          second is a mask.  Only bits set to one in the mask word
          are changed (set or reset) in the file's mode.  All
          other bits in the mode remain unchanged.  Bits in the
          first word that correspond to bits that are zero in
          the mask are ignored, except that undefined bits are
          checked for validity and can result in NFS4ERR_INVAL as
          described below.
          </t>
          <t>
          The mode_set_masked attribute is only valid in a SETATTR
          operation.  If it is used in a CREATE or OPEN operation, the
          server MUST return NFS4ERR_INVAL.
          </t>
          <t>
          Bits not defined as valid in the mode attribute are not
          valid in either word of the mode_set_masked attribute.
          The server MUST return NFS4ERR_INVAL
          if any such bits are set to one in a SETATTR.
If the mode and
          mode_set_masked attributes are both specified in the
          same SETATTR, the server MUST also return NFS4ERR_INVAL.
          </t>
        </section>
      </section>
      <section numbered="true" toc="default">
        <name>Common Methods</name>
        <t>
        The requirements in this section will be referred to in future
        sections, especially <xref target="aclreqs" format="default"/>.
        </t>
        <section anchor="useacl" numbered="true" toc="default">
          <name>Interpreting an ACL</name>
          <section anchor="serverinterp" numbered="true" toc="default">
            <name>Server Considerations</name>
            <t>
	    The server uses the algorithm described in
	    <xref target="attrdef_acl" format="default"/> to determine whether an ACL
	    allows access to an object.  However, the ACL might not be
	    the sole determiner of access.  For example:
            </t>
            <ul spacing="normal">
              <li>
                In the case of a file system exported as read-only,
                the server may deny write access even though
                an object's ACL grants it.
              </li>
              <li>
                Server implementations MAY grant ACE4_WRITE_ACL
                and ACE4_READ_ACL permissions to prevent
                a situation from arising in which there is no valid
                way to ever modify the ACL.
              </li>
              <li>
                All servers will allow a user the ability to read
                the data of the file when only the execute
                permission is granted (i.e., if the ACL denies the
                user the ACE4_READ_DATA access and allows the user
                ACE4_EXECUTE, the server will allow the user to
                read the data of the file).
              </li>
              <li>
                Many servers have the notion of owner-override in
                which the owner of the object is allowed to
                override accesses that are denied by the ACL.
                This may be helpful, for example, to allow users
                continued access to open files on which the
                permissions have changed.
              </li>
              <li>
                Many servers have the notion of a
                "superuser" that has privileges beyond
                an ordinary user.  The superuser may be able
                to read or write data or metadata in ways that would
                not be permitted by the ACL.
              </li>
              <li>
                A retention attribute might also block access otherwise
                allowed by ACLs (see <xref target="retention" format="default"/>).
              </li>
            </ul>
          </section>
          <section anchor="clientinterp" numbered="true" toc="default">
            <name>Client Considerations</name>
            <t>
            Clients SHOULD NOT do their own access checks based on
            their interpretation of the ACL, but rather use the OPEN and
            ACCESS operations to do access checks. This allows the
            client to act on the results of having the server
            determine whether or not access should be granted based on
            its interpretation of the ACL.
            </t>
            <t>
            Clients must be aware of situations in which an object's
            ACL will define a certain access even though the server
            will not enforce it. In general, but especially in these
            situations, the client needs to do its part in the
            enforcement of access as defined by the ACL. To do this,
            the client MAY send the appropriate ACCESS operation
            prior to servicing the request of the user or application
            in order to determine whether the user or application
            should be granted the access requested. For examples in
            which the ACL may define accesses that the server doesn't
            enforce, see <xref target="serverinterp" format="default"/>.
            </t>
          </section>
        </section>
        <section anchor="computemode" numbered="true" toc="default">
          <name>Computing a Mode Attribute from an ACL</name>
          <t>
          The following method can be used to calculate the MODE4_R*,
          MODE4_W*, and MODE4_X* bits of a mode attribute, based upon
          an ACL.
          </t>
          <t>
          First, for each of the special identifiers OWNER@, GROUP@, and
          EVERYONE@, evaluate the ACL in order, considering only ALLOW
          and DENY ACEs for the identifier EVERYONE@ and for the
          identifier under consideration.  The result of the evaluation
          will be an NFSv4 ACL mask showing exactly which bits are
          permitted to that identifier.
          </t>
          <t>
          Then translate the calculated mask for OWNER@, GROUP@, and
          EVERYONE@ into mode bits for, respectively, the user, group,
          and other, as follows:

          </t>
          <ol spacing="normal" type="1">
            <li>
              Set the read bit (MODE4_RUSR, MODE4_RGRP, or
              MODE4_ROTH) if and only if ACE4_READ_DATA is set in
              the corresponding mask.
            </li>
            <li>
              Set the write bit (MODE4_WUSR, MODE4_WGRP, or
              MODE4_WOTH) if and only if ACE4_WRITE_DATA and
              ACE4_APPEND_DATA are both set in the corresponding
              mask.
            </li>
            <li>
              Set the execute bit (MODE4_XUSR, MODE4_XGRP, or
              MODE4_XOTH), if and only if ACE4_EXECUTE is set in the
              corresponding mask.
            </li>
          </ol>
          <section numbered="true" toc="default">
            <name>Discussion</name>
            <t>
            Some server implementations also add bits permitted to
            named users and groups to the group bits (MODE4_RGRP,
            MODE4_WGRP, and MODE4_XGRP).
            </t>
            <t>
            Implementations are discouraged from doing this, because
            it has been found to cause confusion for users who see
            members of a file's group denied access that the mode
            bits appear to allow.  (The presence of DENY ACEs may also
            lead to such behavior, but DENY ACEs are expected to be
            more rarely used.)
            </t>
            <t>
            The same user confusion seen when fetching the mode also
            results if setting the mode does not effectively control
            permissions for the owner, group, and other users; this
            motivates some of the requirements that follow.
            </t>
          </section>
        </section>
      </section>
      <section anchor="aclreqs" numbered="true" toc="default">
        <name>Requirements</name>
        <t>
        The server that supports both mode and ACL must take care to
        synchronize the MODE4_*USR, MODE4_*GRP, and MODE4_*OTH bits with
        the ACEs that have respective who fields of "OWNER@", "GROUP@",
        and "EVERYONE@". This way, the client can see if semantically equivalent
        access permissions exist whether the client asks for the owner,
        owner_group, and mode attributes or for just the ACL.
        </t>
        <t>
        In this section, much is made of the methods in <xref target="computemode" format="default"/>. Many requirements refer to this section.
        But note that the methods have behaviors specified with
        "SHOULD". This is intentional, to avoid invalidating
        existing implementations that compute the mode according to the
        withdrawn POSIX ACL draft (1003.1e draft 17), rather than by
        actual permissions on owner, group, and other.
        </t>
        <section anchor="setattr" numbered="true" toc="default">
          <name>Setting the Mode and/or ACL Attributes</name>
          <t>
          In the case where a server supports the sacl or
          dacl attribute, in addition to the acl attribute,
          the server MUST fail a request to set the acl
          attribute simultaneously with a dacl or sacl
          attribute.  The error to be given is NFS4ERR_ATTRNOTSUPP.
          </t>
          <section anchor="setmode" numbered="true" toc="default">
            <name>Setting Mode and not ACL</name>
            <t>
            When any of the nine low-order mode bits
            are subject to change, either because the mode
            attribute was set or because the mode_set_masked
            attribute was set and the mask included one or more
            bits from the nine low-order mode bits,
            and no ACL attribute is explicitly
            set, the acl and dacl attributes must be modified
            in accordance with the updated value of those bits.
            This must happen
            even if the value of the low-order bits
            is the same after the mode is set as before.
            </t>
            <t>
            Note that any AUDIT or ALARM ACEs (hence any ACEs in the
            sacl attribute) are unaffected by changes to the mode.
            </t>
            <t>
            In cases in which the permissions bits are subject to
            change, the acl and dacl attributes
            MUST be modified such that the mode computed via the
            method in
            <xref target="computemode" format="default"/>
            yields the low-order nine bits (MODE4_R*, MODE4_W*,
            MODE4_X*) of the mode attribute as modified by the
            attribute change.  The ACL attributes
            SHOULD also be modified such that:
            </t>
            <ol spacing="normal" type="1">
              <li>
                If MODE4_RGRP is not set, entities explicitly
                listed in the ACL other than OWNER@ and EVERYONE@
                SHOULD NOT be granted ACE4_READ_DATA.
              </li>
              <li>
                If MODE4_WGRP is not set, entities explicitly
                listed in the ACL other than OWNER@ and
                EVERYONE@ SHOULD NOT be granted
                ACE4_WRITE_DATA or ACE4_APPEND_DATA.
              </li>
              <li>
                If MODE4_XGRP is not set, entities explicitly
                listed in the ACL other than OWNER@ and EVERYONE@
                SHOULD NOT be granted ACE4_EXECUTE.
              </li>
            </ol>
            <t>
            Access mask bits other than those listed above, appearing
            in ALLOW ACEs, MAY also be disabled.
            </t>
            <t>
            Note that ACEs with the flag ACE4_INHERIT_ONLY_ACE set do
            not affect the permissions of the ACL itself, nor do ACEs
            of the type AUDIT and ALARM. As such, it is desirable to
            leave these ACEs unmodified when modifying the ACL
            attributes.
            </t>
            <t>
            Also note that the requirement may be met by
            discarding the acl and dacl, in favor of an ACL
            that represents the mode and only the mode. This is
            permitted, but it is preferable for a server to
            preserve as much of the ACL as possible without
            violating the above requirements. Discarding the
            ACL makes it effectively impossible for a file
            created with a mode attribute to inherit an ACL
            (see <xref target="aclcreate" format="default"/>).
            </t>
          </section>
          <section anchor="settingacl" numbered="true" toc="default">
            <name>Setting ACL and Not Mode</name>
            <t>
            When setting the acl or dacl and not setting the
            mode or mode_set_masked attributes, the permission
            bits of the mode need to be derived from the ACL.
            In this case, the ACL attribute SHOULD be set as
            given. The nine low-order bits of the mode
            attribute (MODE4_R*, MODE4_W*, MODE4_X*) MUST be
            modified to match the result of the method in
	    <xref target="computemode" format="default"/>. The three high-order bits
            of the mode (MODE4_SUID, MODE4_SGID, MODE4_SVTX)
            SHOULD remain unchanged.
            </t>
          </section>
          <section anchor="setboth" numbered="true" toc="default">
            <name>Setting Both ACL and Mode</name>
            <t>
            When setting both the mode (includes use of either the
            mode attribute or the mode_set_masked attribute)
            and the acl or dacl attributes in the
            same operation, the attributes MUST be applied in this
            order: mode (or mode_set_masked), then ACL.  The
            mode-related attribute is set as given,
            then the ACL attribute is set as given, possibly changing
            the final mode, as described above in
            <xref target="settingacl" format="default"/>.
            </t>
          </section>
        </section>
        <section numbered="true" toc="default">
          <name>Retrieving the Mode and/or ACL Attributes</name>
          <t>
          This section applies only to servers that support both the
          mode and ACL attributes.
          </t>
          <t>
          Some server implementations may have a concept of
          "objects without ACLs", meaning that all permissions
          are granted and denied according to the mode attribute and
          that no ACL attribute is stored for that object. If an ACL
          attribute is requested of such a server, the server SHOULD
          return an ACL that does not conflict with the mode; that is to
          say, the ACL returned SHOULD represent the nine low-order bits
          of the mode attribute (MODE4_R*, MODE4_W*, MODE4_X*) as
          described in <xref target="computemode" format="default"/>.
          </t>
          <t>
          For other server implementations, the ACL attribute is always
          present for every object. Such servers SHOULD store at least
          the three high-order bits of the mode attribute (MODE4_SUID,
          MODE4_SGID, MODE4_SVTX). The server SHOULD return a mode
          attribute if one is requested, and the low-order nine bits of
          the mode (MODE4_R*, MODE4_W*, MODE4_X*) MUST match the result
          of applying the method in
          <xref target="computemode" format="default"/> to the ACL attribute.
          </t>
        </section>
        <section anchor="aclcreate" numbered="true" toc="default">
          <name>Creating New Objects</name>
          <t>
          If a server supports any ACL attributes, it may use the ACL
          attributes on the parent directory to compute an initial ACL
          attribute for a newly created object. This will be referred to
          as the inherited ACL within this section. The act of adding
          one or more ACEs to the inherited ACL that are based upon ACEs
          in the parent directory's ACL will be referred to as
          inheriting an ACE within this section.
          </t>
          <t>
          Implementors should standardize what the behavior of CREATE
          and OPEN must be depending on the presence or absence of the
          mode and ACL attributes.
          </t>
          <ol spacing="normal" type="1">
            <li>
              <t>If just the mode is given in the call:
              </t>
              <t> In this case, inheritance
              SHOULD take place, but the mode MUST be applied to the
              inherited ACL as described in <xref target="setmode" format="default"/>, thereby modifying the ACL.

              </t>
            </li>
            <li>
              <t>If just the ACL is given in the call:
              </t>
              <t>
              In this case, inheritance SHOULD NOT take place, and
              the ACL as defined in the CREATE or OPEN will be set
              without modification, and the mode modified as in
              <xref target="settingacl" format="default"/>.

              </t>
            </li>
            <li>
              <t>If both mode and ACL are given in the call:
              </t>
              <t> In this case, inheritance
              SHOULD NOT take place, and both attributes will be set
              as described in <xref target="setboth" format="default"/>.

              </t>
            </li>
            <li>
              <t>
              If neither mode nor ACL is given in the call:
              </t>
              <t>
              In the case where an object is being created without
              any initial attributes at all, e.g., an OPEN operation
              with an opentype4 of OPEN4_CREATE and a createmode4 of
              EXCLUSIVE4, inheritance SHOULD NOT take place (note that
              EXCLUSIVE4_1 is a better choice of createmode4, since it
              does permit initial attributes).
              Instead, the server SHOULD set permissions to deny all
              access to the newly created object. It is expected
              that the appropriate client will set the desired
              attributes in a subsequent SETATTR operation, and the
              server SHOULD allow that operation to succeed,
              regardless of what permissions the object is created
              with. For example, an empty ACL denies all
              permissions, but the server should allow the owner's
              SETATTR to succeed even though WRITE_ACL is implicitly
              denied.
              </t>
              <t>
              In other cases, inheritance SHOULD take place, and no
              modifications to the ACL will happen. The mode
              attribute, if supported, MUST be as computed in
	      <xref target="computemode" format="default"/>, with the MODE4_SUID,
              MODE4_SGID, and MODE4_SVTX bits clear.
              If no inheritable ACEs exist on the parent directory,
              the rules for creating acl, dacl, or sacl attributes
              are implementation defined.
              If either the dacl or sacl attribute is supported,
              then the ACL4_DEFAULTED flag SHOULD be set on the
              newly created attributes.
              </t>
              <t/>
            </li>
          </ol>
          <section anchor="inheritreq" numbered="true" toc="default">
            <name>The Inherited ACL</name>
            <t>
            If the object being created is not a directory, the
            inherited ACL SHOULD NOT inherit ACEs from the parent
            directory ACL unless the ACE4_FILE_INHERIT_FLAG is set.
            </t>
            <t>
            If the object being created is a directory, the inherited
            ACL should inherit all inheritable ACEs from the parent
            directory, that is, those that have the ACE4_FILE_INHERIT_ACE or
            ACE4_DIRECTORY_INHERIT_ACE flag set.
If the inheritable
            ACE has ACE4_FILE_INHERIT_ACE set but
            ACE4_DIRECTORY_INHERIT_ACE is clear, the inherited ACE on
            the newly created directory MUST have the
            ACE4_INHERIT_ONLY_ACE flag set to prevent the directory
            from being affected by ACEs meant for non-directories.
            </t>
            <t>
            When a new directory is created, the server MAY split
            any inherited ACE that is both inheritable and effective
            (in other words, that has neither ACE4_INHERIT_ONLY_ACE
            nor ACE4_NO_PROPAGATE_INHERIT_ACE set), into two ACEs,
            one with no inheritance flags and one with
            ACE4_INHERIT_ONLY_ACE set.  (In the case of a dacl or
            sacl attribute, both of those ACEs SHOULD also have the
            ACE4_INHERITED_ACE flag set.)  This makes it simpler to
            modify the effective permissions on the directory
            without modifying the ACE that is to be inherited to the
            new directory's children.
            </t>
          </section>
          <section anchor="auto_inherit" numbered="true" toc="default">
            <name>Automatic Inheritance</name>
            <t>
            The acl attribute consists only of an array of ACEs, but
            the <xref target="attrdef_sacl" format="default">sacl</xref>
            and <xref target="attrdef_dacl" format="default">dacl</xref> attributes
            also include an additional flag field.

</t>
            <artwork name="" type="" align="left" alt=""><![CDATA[
struct nfsacl41 {
        aclflag4        na41_flag;
        nfsace4         na41_aces<>;
};
 ]]></artwork>
            <t>

            The flag field
            applies to the entire sacl or dacl; three flag values are
            defined:

</t>
            <artwork name="" type="" align="left" alt=""><![CDATA[
const ACL4_AUTO_INHERIT         = 0x00000001;
const ACL4_PROTECTED            = 0x00000002;
const ACL4_DEFAULTED            = 0x00000004;
 ]]></artwork>
            <t>

            and all other bits must be cleared.  The
            ACE4_INHERITED_ACE flag may be set in the ACEs of the sacl
            or dacl (whereas it must always be cleared in the acl).
            </t>
            <t>
            Together these features allow a server to support automatic
            inheritance, which we now explain in more detail.
            </t>
            <t>
            Inheritable ACEs are normally inherited by child objects only
            at the time that the child objects are created; later
            modifications to inheritable ACEs do not result in
            modifications to inherited ACEs on descendants.
            </t>
            <t>
            However, the dacl and sacl provide an OPTIONAL mechanism
            that allows a client application to propagate changes to
            inheritable ACEs to an entire directory hierarchy.
            </t>
            <t>
            A server that supports this performs inheritance at object
            creation time in the normal way, and SHOULD  set the
            ACE4_INHERITED_ACE flag on any inherited ACEs as they are
            added to the new object.
            </t>
            <t>
            A client application such as an ACL editor may then propagate
            changes to inheritable ACEs on a directory by recursively
            traversing that directory's descendants and modifying each ACL
            encountered to remove any ACEs with the ACE4_INHERITED_ACE flag
            and to replace them by the new inheritable ACEs (also with the
            ACE4_INHERITED_ACE flag set).  It uses the existing ACE
            inheritance flags in the obvious way to decide which ACEs to
            propagate.  (Note that it may encounter further inheritable
            ACEs when descending the directory hierarchy and that those
            will also need to be taken into account when propagating
            inheritable ACEs to further descendants.)
            </t>
            <t>
            The reach of this propagation may be limited in two ways:
            first, automatic inheritance is not performed from any
            directory ACL that has the ACL4_AUTO_INHERIT flag
            cleared; and second, automatic inheritance stops wherever
            an ACL with the ACL4_PROTECTED flag is set, preventing
            modification of that ACL and also (if the ACL is set on
            a directory) of the ACL on any of the object's descendants.
            </t>
            <t>
            This propagation is performed independently for the sacl
            and the dacl attributes; thus, the ACL4_AUTO_INHERIT and
            ACL4_PROTECTED flags may be independently set for the sacl
            and the dacl, and propagation of one type of acl may continue
            down a hierarchy even where propagation of the other acl has
            stopped.
            </t>
            <t>
            New objects should be created with a dacl and a sacl that
            both have the ACL4_PROTECTED flag cleared and the
            ACL4_AUTO_INHERIT flag set to the same value as that on,
            respectively, the sacl or dacl of the parent object.
            </t>
            <t>
            Both the dacl and sacl attributes are RECOMMENDED, and a server
            may support one without supporting the other.
            </t>
            <t>
            A server that supports both the old acl attribute and
            one or both of the new dacl or sacl attributes must do so
            in such a way as to keep all three attributes consistent
            with each other.  Thus, the ACEs reported in the acl attribute
            should be the union of the ACEs reported in the dacl and
            sacl attributes, except that the ACE4_INHERITED_ACE flag must
            be cleared from the ACEs in the acl.  And of course a
            client that queries only the acl will be unable to determine
            the values of the sacl or dacl flag fields.
            </t>
            <t>
            When a client performs a SETATTR for the acl attribute,
            the server SHOULD set the ACL4_PROTECTED flag to true on
            both the sacl and the dacl.  By using the acl attribute,
            as opposed to the dacl or sacl attributes, the client signals
            that it may not understand automatic inheritance, and thus
            cannot be trusted to set an ACL for which automatic
            inheritance would make sense.
            </t>
            <t>
            When a client application queries an ACL, modifies it, and sets
            it again, it should leave any ACEs marked with
            ACE4_INHERITED_ACE unchanged, in their original order, at the
            end of the ACL.  If the application is unable to do this, it
            should set the ACL4_PROTECTED flag.  This behavior
            is not enforced by servers, but violations of this rule may
            lead to unexpected results when applications perform automatic
            inheritance.
            </t>
            <t>
            If a server also supports the mode attribute, it SHOULD set the
            mode in such a way that leaves inherited ACEs unchanged, in
            their original order, at the end of the ACL.  If it is unable
            to do so, it SHOULD set the ACL4_PROTECTED flag on the file's
            dacl.
            </t>
            <t>Finally, in the case where the request that creates a new file
            or directory does not also set permissions for that file or
            directory, and there are also no ACEs to inherit from the
            parent's directory, then the server's choice of ACL for the new
            object is implementation-dependent.  In this case, the server
            SHOULD set the ACL4_DEFAULTED flag on the ACL it chooses for
            the new object.  An application performing automatic
            inheritance takes the ACL4_DEFAULTED flag as a sign that the
            ACL should be completely replaced by one generated using the
            automatic inheritance rules.
            </t>
          </section>
        </section>
      </section>
    </section>
    <!-- 	$Id: 2009-12-20-TO-rfc5661.xml,v 1.2 2009/12/21 05:59:32 shepler.mre Exp $	 -->
<!-- Copyright (C) The IETF Trust (2007) -->
<!-- Copyright (C) The Internet Society (2006) -->
<section anchor="single_server_namespace" numbered="true" toc="default">
      <name>Single-Server Namespace</name>
      <t>
    This section describes the NFSv4 single-server namespace.
    Single-server namespaces may be presented directly to clients,
    or they may be used as a basis to form larger multi-server
    namespaces (e.g., site-wide or organization-wide) to be presented
    to clients, as described in <xref target="NEW11" format="default"/>.
      </t>
      <section anchor="server_exports" numbered="true" toc="default">
        <name>Server Exports</name>
        <t>
      On a UNIX server, the namespace describes all the files reachable by
      pathnames under the root directory or "/".  On a Windows server, the
      namespace constitutes all the files on disks named by mapped disk
      letters.  NFS server administrators rarely make the entire server's
      file system namespace available to NFS clients.  More often, portions
      of the namespace are made available via an "export" feature.  In
      previous versions of the NFS protocol, the root filehandle for each
      export is obtained through the MOUNT protocol; the client sent a
      string that identified the export name within the namespace and
      the server returned the root filehandle
      for that export.  The MOUNT protocol also provided an EXPORTS
      procedure that enumerated the server's exports.
        </t>
      </section>
      <section anchor="browsing_exports" numbered="true" toc="default">
        <name>Browsing Exports</name>
        <t>
      The NFSv4.1 protocol provides a root filehandle that clients can
      use to obtain filehandles for the exports of a particular server,
      via a series of LOOKUP operations within a COMPOUND, to traverse
      a path.  A common user experience is to use a graphical user interface
      (perhaps a file "Open" dialog window) to find a file via progressive
      browsing through a directory tree.  The client must be able to move
      from one export to another export via single-component, progressive
      LOOKUP operations.
        </t>
        <t>
      This style of browsing is not well supported by the NFSv3 protocol.  In NFSv3, the client expects all
      LOOKUP operations to remain
      within a single server file system.  For example, the device attribute
      will not change.  This prevents a client from taking namespace paths
      that span exports.
        </t>
        <t>
      In the case of NFSv3, an automounter on the client
      can obtain a snapshot of the server's namespace
      using the EXPORTS procedure of the MOUNT protocol.
      If it understands the server's pathname syntax,
      it can create an image of the server's namespace
      on the client.  The parts of the namespace that
      are not exported by the server are filled in
      with directories that might be constructed similarly
      to an NFSv4.1 "pseudo file system" (see <xref target="server_pseudo_file_system" format="default"/>) that
      allows the user to browse from one mounted file
      system to another.  There is a drawback to this
      representation of the server's namespace on the
      client: it is static.  If the server administrator
      adds a new export, the client will be unaware of it.

        </t>
      </section>
      <section anchor="server_pseudo_file_system" numbered="true" toc="default">
        <name>Server Pseudo File System</name>
        <t>
      NFSv4.1 servers avoid this namespace inconsistency by
      presenting all the exports for a given server within the
      framework of a single namespace for that server.
      An NFSv4.1 client uses LOOKUP and READDIR
      operations to browse seamlessly from one export to another.
        </t>
        <t>
      Where there are portions of the server namespace that are not
      exported, clients require some way of traversing those portions
      to reach actual exported file systems.  A technique that servers
      may use to provide for this is to bridge the unexported portion of
      the namespace via a
      "pseudo file system" that provides a view of exported directories
      only.  A pseudo file system has a unique fsid and behaves like a
      normal, read-only file system.
        </t>
        <t>
      Based on the construction of the server's namespace, it is possible
      that multiple pseudo file systems may exist.  For example,
        </t>
        <artwork name="" type="" align="left" alt=""><![CDATA[
        /a              pseudo file system
        /a/b            real file system
        /a/b/c          pseudo file system
        /a/b/c/d        real file system
      ]]></artwork>
        <t>
      Each of the pseudo file systems is considered a separate entity and
      therefore MUST have its own fsid, unique among all the fsids for that
      server.
        </t>
      </section>
      <section numbered="true" toc="default">
        <name>Multiple Roots</name>
        <t>
      Certain operating environments are sometimes described as
      having "multiple roots".  In such environments, individual file
      systems are commonly represented by disk or volume names.
      NFSv4 servers for these platforms can construct a pseudo file
      system above these root names so that disk letters or volume names are
      simply directory names in the pseudo root.
        </t>
      </section>
      <section anchor="pseudo_fs_volatility" numbered="true" toc="default">
        <name>Filehandle Volatility</name>
        <t>
      The nature of the server's pseudo file system is that it is a logical
      representation of file system(s) available from the server.
      Therefore, the pseudo file system is most likely constructed
      dynamically when the server is first instantiated.  It is expected
      that the pseudo file system may not have an on-disk counterpart from
      which persistent filehandles could be constructed.  Even though it is
      preferable that the server provide persistent filehandles for the
      pseudo file system, the NFS client should expect that pseudo file
      system filehandles are volatile.  This can be confirmed by checking
      the associated "fh_expire_type" attribute for those filehandles in
      question.  If the filehandles are volatile, the NFS client must be
      prepared to recover a filehandle value (e.g., with a series of
      LOOKUP operations) when receiving an error of NFS4ERR_FHEXPIRED.
        </t>
        <t>
      Because it is quite likely that servers will implement pseudo
      file systems using volatile filehandles, clients need to be
      prepared for them, rather than assuming that all filehandles
      will be persistent.
        </t>
      </section>
      <section numbered="true" toc="default">
        <name>Exported Root</name>
        <t>
      If the server's root file system is exported, one might conclude that
      a pseudo file system is unneeded.  This is not necessarily so.  Assume the
      following file systems on a server:
        </t>
        <artwork name="" type="" align="left" alt=""><![CDATA[
        /       fs1  (exported)
        /a      fs2  (not exported)
        /a/b    fs3  (exported)
      ]]></artwork>
        <t>
      Because fs2 is not exported, fs3 cannot be reached with simple
      LOOKUPs.  The server must bridge the gap with a pseudo file system.
        </t>
      </section>
      <section numbered="true" toc="default">
        <name>Mount Point Crossing</name>
        <t>
      The server file system environment may be constructed in such a way
      that one file system contains a directory that is 'covered' or
      mounted upon by a second file system.  For example:
        </t>
        <artwork name="" type="" align="left" alt=""><![CDATA[
        /a/b            (file system 1)
        /a/b/c/d        (file system 2)
      ]]></artwork>
        <t>
      The pseudo file system for this server may be constructed to look
      like:
        </t>
        <artwork name="" type="" align="left" alt=""><![CDATA[
        /               (place holder/not exported)
        /a/b            (file system 1)
        /a/b/c/d        (file system 2)
      ]]></artwork>
        <t>
      It is the server's responsibility to present the pseudo file system
      that is complete to the client.  If the client sends a LOOKUP request
      for the path /a/b/c/d, the server's response is the filehandle of
      the root of the file system /a/b/c/d.  In previous versions of the
      NFS protocol,
      the server would respond with the filehandle of directory
      /a/b/c/d within the file system /a/b.

        </t>
        <t>
      The NFS client will be able to determine if it crosses a server mount
      point by a change in the value of the "fsid" attribute.
        </t>
      </section>
      <section numbered="true" toc="default">
        <name>Security Policy and Namespace Presentation</name>
        <t>
      Because NFSv4 clients possess the ability to change the security
      mechanisms used, after determining what is allowed,
      by using SECINFO and SECINFO_NONAME, the server
      SHOULD NOT present a different view of the namespace based on
      the security mechanism being used by a client.  Instead, it
      should present a consistent view and return NFS4ERR_WRONGSEC
      if an attempt is made to access data with an inappropriate
      security mechanism.
        </t>
        <t>
      If security considerations make it necessary to hide the existence
      of a particular file system, as opposed to all of the data within
      it, the server can apply the security policy of
      a shared resource in the server's namespace to components of the
      resource's ancestors. For example:
        </t>
        <artwork name="" type="" align="left" alt=""><![CDATA[
        /                           (place holder/not exported)
        /a/b                        (file system 1)
        /a/b/MySecretProject        (file system 2)

      ]]></artwork>
        <t>
      The /a/b/MySecretProject directory is a real file system and
      is the shared resource.
      Suppose the security policy for /a/b/MySecretProject is Kerberos
      with integrity and it is desired to limit knowledge of the existence
      of this file system.  In this case, the
      server should apply the same security policy to /a/b.  This allows
      for knowledge of the existence of a file system to be secured
      when desirable.
        </t>
        <t>
      For the case of the use of multiple, disjoint security mechanisms in
      the server's resources, applying that sort of policy would result
      in the higher-level file system not being accessible using any
      security flavor.
Therefore, that sort of configuration is not compatible
      with hiding the existence (as opposed to the contents) from clients
      using multiple disjoint sets of security flavors.
        </t>
        <t>
      In other circumstances, a desirable policy is for the security of a
      particular object in the
      server's namespace to include the union of all security mechanisms of
      all direct descendants.  A common and convenient practice, unless
      strong security requirements dictate otherwise, is to make the
      entire the pseudo file system accessible by all of the valid security
      mechanisms.
        </t>
        <t>
      Where there is concern about the security of data on the network,
      clients should use strong security mechanisms to access the pseudo
      file system in order to prevent man-in-the-middle attacks.
        </t>
      </section>
    </section>
    <!-- 	$Id: 2009-12-20-TO-rfc5661.xml,v 1.2 2009/12/21 05:59:32 shepler.mre Exp $	 -->
<!-- Copyright (C) The IETF Trust (2007-2008) -->
<!-- Copyright (C) The Internet Society (2006) -->
<section numbered="true" toc="default">
      <name>State Management</name>
      <t>
    Integrating locking into the NFS protocol necessarily causes it to be
    stateful.  With the inclusion of such features as share reservations,
    file and directory delegations, recallable layouts, and support for
    mandatory byte-range locking, the protocol becomes substantially more
    dependent on proper management of state than the traditional
    combination of NFS and NLM (Network Lock Manager)
    <xref target="xnfs" format="default"/>. These features include expanded
    locking facilities, which provide some measure of inter-client
    exclusion, but the state also offers
    features not readily providable using a stateless model.
    There are three components to
    making this state manageable:
      </t>
      <ul spacing="normal">
        <li>
        clear division between client and server
      </li>
        <li>
        ability to reliably detect inconsistency in state between client
        and server
      </li>
        <li>
        simple and robust recovery mechanisms
      </li>
      </ul>
      <t>
    In this model, the server owns the state information.  The client
    requests changes in locks and the server responds with the changes
    made.  Non-client-initiated changes in locking state are infrequent.
    The client receives prompt notification of such changes and can adjust
    its view of the locking state to reflect the server's changes.
      </t>
      <t>
    Individual pieces of state created by the server and passed to the
    client at its request are represented by 128-bit stateids.  These
    stateids may represent a particular open file, a set of
    byte-range locks held
    by a particular owner, or a recallable delegation of privileges
    to access a file in particular ways or at a particular location.
      </t>
      <t>
    In all cases, there is a transition from the most general
    information that represents a client as a whole to the eventual
    lightweight stateid used for most client and server
    locking interactions.  The details of this transition will vary
    with the type of object but it always starts with a client ID.
      </t>
      <section anchor="client_id" numbered="true" toc="default">
        <name>Client and Session ID</name>
        <t>
      A client must establish a client ID (see <xref target="Client_Identifiers" format="default"/>)
      and then one or more sessionids (see <xref target="Session" format="default"/>) before
      performing any operations to open, byte-range lock, delegate, or obtain
      a layout for a file object.
      Each session ID is associated with a specific client ID, and thus
      serves as a shorthand reference to an NFSv4.1 client.
        </t>
        <t>
       For some types of locking interactions, the client will represent
       some number of internal locking entities called "owners", which
       normally correspond to processes internal to the client.  For
       other types of locking-related objects, such as delegations and
       layouts, no such intermediate entities are provided for, and the
       locking-related objects are considered to be transferred
       directly between the server and a unitary client.
        </t>
      </section>
      <!-- "Client and Session ID" -->
    <section anchor="stateid" numbered="true" toc="default">
        <name>Stateid Definition</name>
        <t>
        When the server grants a lock of any type (including opens,
        byte-range locks, delegations, and layouts), it responds with a
        unique stateid that represents a set of locks (often a single
        lock) for the same file, of the same type, and sharing the same
        ownership characteristics.  Thus, opens of the same file by
        different open-owners each have an identifying stateid.  Similarly,
        each set of byte-range locks on a file owned by a specific lock-owner
        has its own
        identifying stateid.  Delegations and layouts also have
        associated stateids by which they may be referenced.
        The stateid is used as a shorthand reference to a lock or set
        of locks, and given a stateid, the server can determine the associated
        state-owner or state-owners (in the case of an open-owner/lock-owner pair)
        and the associated filehandle.  When stateids are used, the current
        filehandle must be the one associated with that stateid.
        </t>
        <t>
        All stateids associated with a given client ID are associated with
        a common lease that represents the claim of those stateids
        and the objects they represent to be maintained
        by the server.  See <xref target="lease_renewal" format="default"/> for a
        discussion of the lease.
        </t>
        <t>
        The server may assign stateids independently for different clients.
        A stateid with the same bit pattern for one client may designate
        an entirely different set of locks for a different client.  The
        stateid is always interpreted with respect to the client ID associated
        with the current session.  Stateids apply to all sessions associated
        with the given client ID, and the client may use a stateid obtained from
        one session on another session associated with the same client ID.
        </t>
        <section anchor="stateid_types" numbered="true" toc="default">
          <name>Stateid Types</name>
          <t>
          With the exception of special stateids (see <xref target="special_stateid" format="default"/>),
          each stateid
          represents locking objects of one of a set of types defined
          by the NFSv4.1 protocol.  Note that in all these cases, where
          we speak of guarantee, it is understood there are
          situations such as a client restart, or lock revocation,
          that allow the guarantee to be voided.
          </t>
          <ul spacing="normal">
            <li>
              <t>
              Stateids may represent opens of files.
              </t>
              <t>
              Each stateid in this case represents the OPEN state for a
              given client ID/open-owner/filehandle triple.  Such
              stateids are subject to change (with consequent
              incrementing of the stateid's seqid) in response to OPENs that
              result in upgrade and OPEN_DOWNGRADE operations.
              </t>
            </li>
            <li>
              <t>
              Stateids may represent sets of byte-range locks.
              </t>
              <t>
              All locks held on a particular file by a particular owner and
              gotten under the aegis of a particular open file
              are associated with a single stateid with the seqid
              being incremented whenever LOCK and LOCKU operations affect that
              set of locks.
              </t>
            </li>
            <li>
              <t>
              Stateids may represent file delegations, which are
              recallable guarantees by the server to the client
              that other clients will not reference or
              modify a particular file, until the delegation
              is returned.  In NFSv4.1, file delegations may be
              obtained on both regular and non-regular files.
              </t>
              <t>
              A stateid represents a single delegation held by
              a client for a particular filehandle.
              </t>
            </li>
            <li>
              <t>
              Stateids may represent directory delegations, which
              are recallable guarantees by the server to the client
              that other clients will not modify the directory,
              until the delegation is returned.
              </t>
              <t>
              A stateid represents a single delegation held by
              a client for a particular directory filehandle.
              </t>
            </li>
            <li>
              <t>
              Stateids may represent layouts, which are recallable
              guarantees by the server to the client that particular
              files may be accessed via an alternate data access
              protocol at specific locations.  Such access is
              limited to particular sets of byte-ranges and may
              proceed until those byte-ranges are reduced or the
              layout is returned.
              </t>
              <t>
              A stateid represents the set of all layouts held by a particular
              client for a particular filehandle with a given
              layout type.  The seqid is updated as the layouts
              of that set of byte-ranges change, via layout stateid changing operations such
              as LAYOUTGET and LAYOUTRETURN.
              </t>
            </li>
          </ul>
        </section>
        <section anchor="stateid_structure" numbered="true" toc="default">
          <name>Stateid Structure</name>
          <t>
	  Stateids are divided into two fields, a 96-bit
	  "other" field identifying the specific set
	  of locks and a 32-bit "seqid" sequence value.
	  Except in the case of special stateids
          (see <xref target="special_stateid" format="default"/>),
	  a particular value of the
          "other" field denotes a
          set of locks of the same type (for example,
          byte-range locks, opens, delegations, or layouts),
          for a specific file or directory, and sharing
          the same ownership characteristics.  The seqid
          designates a specific instance of such a set of
          locks, and is incremented to indicate changes in
          such a set of locks, either by the addition or
          deletion of locks from the set, a change in the
          byte-range they apply to, or an upgrade or downgrade
          in the type of one or more locks.
          </t>
          <t>
          When such a set of locks is first created, the server returns a
          stateid with seqid value of one.  On subsequent
          operations that modify the set of locks, the server
          is required to increment the "seqid" field by one
          whenever it returns a stateid for the same
          state-owner/file/type  combination and there is some
          change in the set of locks actually designated.
          In this case, the server will return a stateid with an "other" field
          the same as previously used for that
          state-owner/file/type  combination, with an
          incremented "seqid" field.
          This pattern continues until the seqid is incremented
          past NFS4_UINT32_MAX, and one
          (not zero) is the next seqid value.
          </t>
          <t>
	  The purpose of the incrementing of the seqid
	  is to allow the server to
	  communicate to the client the order in which
	  operations that modified locking state associated
	  with a stateid have been processed and to make
          it possible for the client to send requests
          that are conditional on the set of locks not
          having changed since the stateid in question
          was returned.
          </t>
          <t>
	  Except for layout stateids (<xref target="layout_stateid" format="default"/>),
          when a client sends a stateid to the server, it has two
          choices with regard to the seqid sent.  It may set the seqid
          to zero to indicate to the server that it wishes the most
          up-to-date seqid for that stateid's "other" field to be
          used.  This would be the common choice in the case of a
          stateid sent with a READ or WRITE operation.  It also may
          set a non-zero value, in which case the server checks if that
          seqid is the correct one.  In that case, the server is
          required to return NFS4ERR_OLD_STATEID if the seqid is lower
          than the most current value and NFS4ERR_BAD_STATEID if the
          seqid is greater than the most current value.  This would be
          the common choice in the case of stateids sent with a CLOSE
          or OPEN_DOWNGRADE.  Because OPENs may be sent in parallel
          for the same owner, a client might close a file without
          knowing that an OPEN upgrade had been done by the server,
          changing the lock in question.  If CLOSE were sent with a
          zero seqid, the OPEN upgrade would be cancelled before the
          client even received an indication that an upgrade had
          happened.
          </t>
          <t>
          When a stateid is sent by the server to the client as part of
          a callback operation, it is not subject to checking for
          a current seqid and returning NFS4ERR_OLD_STATEID.  This
          is because the client is not in a position to know the
          most up-to-date seqid and thus cannot verify it.  Unless
          specially noted, the seqid value for a stateid sent by the
          server to the client as part of a callback is required
          to be zero with NFS4ERR_BAD_STATEID returned if it is
          not.
          </t>
          <t>
          In making comparisons between seqids, both by the client
	  in determining the order of operations and by the server
	  in determining whether the NFS4ERR_OLD_STATEID is to be
          returned, the possibility of the seqid being swapped
	  around past the NFS4_UINT32_MAX value needs to be taken
	  into account.  When two seqid values are being compared,
  	  the total count of slots for all sessions associated
	  with the current client is used to do this.  When one
	  seqid value is less than this total slot count and
	  another seqid value is greater than NFS4_UINT32_MAX
	  minus the total slot count, the former is to be treated
	  as lower than the latter, despite the fact that it is
	  numerically greater.
          </t>
        </section>
        <!-- "Stateid Structure" -->
      <section anchor="special_stateid" numbered="true" toc="default">
          <name>Special Stateids</name>
          <t>
          Stateid values whose "other" field is either all zeros or all
          ones are reserved.  They may not be assigned by the server but
          have special meanings defined by the protocol.  The particular
          meaning depends on whether the "other" field is all zeros or
          all ones and the specific value of the "seqid" field.
          </t>
          <t>
          The following combinations of "other" and "seqid" are defined
          in NFSv4.1:
          </t>
          <ul spacing="normal">
            <li>
              When "other" and "seqid" are both zero, the
              stateid is treated as a special anonymous
              stateid, which can be used in READ, WRITE,
              and SETATTR requests to indicate the absence
              of any OPEN state associated with the
              request.  When an anonymous stateid value is
              used and an existing open denies the form of
              access requested, then access will be denied
              to the request.  This stateid MUST NOT be
              used on operations to data servers (<xref target="ds_ops" format="default"/>).
            </li>
            <li>
              When "other" and "seqid" are both all ones,
              the stateid is a special READ bypass stateid.
              When this value is used in WRITE or SETATTR,
              it is treated like the anonymous value.
              When used in READ, the server MAY grant
              access, even if access would normally be
              denied to READ operations.  This stateid MUST
              NOT be used on operations to data servers.
            </li>
            <li>
              When "other" is zero and "seqid" is one,
              the stateid represents the current stateid,
              which is whatever value is the last stateid
              returned by an operation within the COMPOUND.
              In the case of an OPEN, the stateid returned
              for the open file and not the delegation is
              used.  The stateid passed to the operation in
              place of the special value has its "seqid"
              value set to zero, except when the current
              stateid is used by the operation CLOSE or
              OPEN_DOWNGRADE.  If there is no operation
              in the COMPOUND that has returned a stateid
              value, the server MUST return the error
	      NFS4ERR_BAD_STATEID. As illustrated  in <xref target="csid_example4" format="default"/>, if the value of a
	      current stateid is a special stateid and the
	      stateid of an operation's arguments has
	      "other" set to zero and "seqid" set to one,
	      then the server MUST return the error
	      NFS4ERR_BAD_STATEID.

            </li>
            <li>
              When "other" is zero and "seqid" is NFS4_UINT32_MAX,
              the stateid represents a reserved stateid
              value defined to be invalid.  When this
              stateid is used, the server MUST return the error
              NFS4ERR_BAD_STATEID.
            </li>
          </ul>
          <t>
          If a stateid value is used that has all zeros or all ones in the
          "other" field but does not match one of the cases above, the server
          MUST return the error NFS4ERR_BAD_STATEID.
          </t>
          <t>
          Special stateids, unlike other stateids, are not associated with
          individual client IDs or filehandles and can be used with all valid
          client IDs and filehandles.  In the case of a special
          stateid designating the current stateid, the current stateid
          value substituted for the special stateid is associated with a
          particular client ID and filehandle, and so, if it is used
          where the current filehandle does not match that associated with the current
          stateid, the operation to which the stateid is passed will return
          NFS4ERR_BAD_STATEID.
          </t>
        </section>
        <!-- "Special Stateids" -->
      <section anchor="stateid_lifetime" numbered="true" toc="default">
          <name>Stateid Lifetime and Validation</name>
          <t>
          Stateids must remain valid until either a client restart or a
          server restart or until the client returns all of the locks
          associated with the stateid by means of an operation such as
          CLOSE or DELEGRETURN.

          If the locks are lost due to revocation, as long
          as the client ID is valid, the stateid remains
          a valid designation of that revoked state until
          the client frees it by using FREE_STATEID.

          Stateids associated
          with byte-range locks are an exception.  They remain valid even
          if a LOCKU frees all remaining locks, so long as the open file
          with which they are associated remains open, unless the client
          frees the stateids via the FREE_STATEID operation.
          </t>
          <t>
          It should be noted that there are situations in which the
          client's locks become invalid, without the client requesting
          they be returned.  These include lease expiration and a number
          of forms of lock revocation within the lease period.  It is
          important to note that in these situations, the stateid remains
          valid and the client can use it to determine the disposition of
          the associated lost locks.
          </t>
          <t>
          An "other" value must never be reused for a different purpose
          (i.e., different filehandle, owner, or type of locks) within the
          context of a single client ID.  A server may retain the "other"
          value for the same purpose beyond the point where it may otherwise
          be freed, but if it does so, it must maintain "seqid" continuity
          with previous values.
          </t>
          <t>
          One mechanism that may be used to satisfy the requirement that the
          server recognize invalid and out-of-date stateids is for
          the server to divide the "other" field of the stateid into two
          fields.
          </t>
          <ul spacing="normal">
            <li>
              an index into a table of locking-state structures.
            </li>
            <li>
              a generation number that is incremented on each allocation
              of a table entry for a particular use.
            </li>
          </ul>
          <t>
          And then store in each table entry,
          </t>
          <ul spacing="normal">
            <li>
               the client ID with which the stateid is associated.
             </li>
            <li>
               the current generation number for the (at most one)
               valid stateid sharing this index value.
             </li>
            <li>
               the filehandle of the file on which the locks are taken.
             </li>
            <li>
               an indication of the type of stateid (open, byte-range lock,
               file delegation, directory delegation, layout).
             </li>
            <li>
               the last "seqid" value returned corresponding to the current
               "other" value.
             </li>
            <li>
               an indication of the current status of the locks
               associated with this stateid, in particular,
               whether these have been revoked and if so, for what reason.
             </li>
          </ul>
          <t>
          With this information, an incoming stateid can be validated and
          the appropriate error returned when necessary.  Special and
          non-special stateids are handled separately. (See
          <xref target="special_stateid" format="default"/> for a discussion of special
          stateids.)
          </t>
          <t>
          Note that stateids are implicitly qualified by the current client
          ID, as derived from the client ID associated with the current
          session.  Note, however, that the semantics of the session will
          prevent stateids associated with a previous client or server
          instance from being analyzed by this procedure.
          </t>
          <t>
          If server restart has resulted in an invalid
          client ID or a session ID that is invalid, SEQUENCE will return
          an error and the operation that takes a stateid as an argument will never
          be processed.
          </t>
          <t>
          If there has been a server restart where there is a persistent
          session and all leased state has been lost, then the session
          in question will, although valid, be marked as dead, and any
          operation not satisfied by means of the reply cache will
          receive the error NFS4ERR_DEADSESSION, and thus not be
          processed as indicated below.
          </t>
          <t>
          When a stateid is being tested and the "other" field is all
          zeros or all ones, a check that
          the "other" and "seqid" fields match a defined combination for
          a special stateid is done and the results determined as follows:
          </t>
          <ul spacing="normal">
            <li>
              If the "other" and "seqid" fields do not match a defined
              combination associated with a special stateid, the error
              NFS4ERR_BAD_STATEID is returned.
            </li>
            <li>
              If the special stateid is one designating the current
              stateid and there is a current stateid, then the current
              stateid is substituted for the special stateid and the
              checks appropriate to non-special stateids are performed.
            </li>
            <li>
              If the combination is valid in general but is not
              appropriate to the context in which the stateid is used
              (e.g., an all-zero stateid is used when an OPEN stateid
              is required in a LOCK operation), the error
              NFS4ERR_BAD_STATEID is also returned.
            </li>
            <li>
              Otherwise, the check is completed and the special stateid
              is accepted as valid.
            </li>
          </ul>
          <t>
          When a stateid is being tested,
          and the "other" field is neither all zeros nor all ones, the
          following procedure could be used to
          validate an incoming stateid and return an appropriate error,
          when necessary, assuming that the "other" field would be divided
          into a table index and an entry generation.
          </t>
          <ul spacing="normal">
            <li>
              If the table index field is outside the range of the
              associated table, return NFS4ERR_BAD_STATEID.
            </li>
            <li>
              If the selected table entry is of a different generation than
              that specified in the incoming stateid, return
              NFS4ERR_BAD_STATEID.
            </li>
            <li>
              If the selected table entry does not match the current
              filehandle, return NFS4ERR_BAD_STATEID.
            </li>
            <li>
              If the client ID in the table entry does not match the
              client ID associated with the current session,
              return NFS4ERR_BAD_STATEID.
            </li>
            <li>
              If the stateid represents revoked state, then return
              NFS4ERR_EXPIRED, NFS4ERR_ADMIN_REVOKED, or
              NFS4ERR_DELEG_REVOKED, as appropriate.
            </li>
            <li>
              If the stateid type is not valid for the context in which the
              stateid appears, return NFS4ERR_BAD_STATEID.
              Note that a stateid may be valid in general, as would be
              reported by the TEST_STATEID operation, but be invalid for
              a particular operation, as, for example, when a stateid
              that doesn't represent byte-range locks is passed to
              the non-from_open case of LOCK or to LOCKU, or when a stateid
              that does not represent an open is passed to CLOSE or
              OPEN_DOWNGRADE.  In such cases, the server MUST return
              NFS4ERR_BAD_STATEID.
            </li>
            <li>
              If the "seqid" field is not zero and it is greater
              than the current sequence value corresponding to the
              current "other" field, return NFS4ERR_BAD_STATEID.
            </li>
            <li>
              If the "seqid" field is not zero and it is less
              than the current sequence value corresponding to the
              current "other" field, return NFS4ERR_OLD_STATEID.
            </li>
            <li>
              Otherwise, the stateid is valid and the table entry
              should contain any additional information about the
              type of stateid and information associated with that
              particular type of stateid, such as the associated
              set of locks, e.g., open-owner and
              lock-owner information, as well as information on the
              specific locks, e.g., open modes and byte-ranges.
            </li>
          </ul>
        </section>
        <!-- "Stateid Lifetime and Validation" -->
      <section anchor="stateid_use" numbered="true" toc="default">
          <name>Stateid Use for I/O Operations</name>
          <t>
          Clients performing I/O operations need to select an
          appropriate stateid based on the
          locks (including opens and delegations) held by the client and
          the various types of state-owners sending the I/O requests.
          SETATTR operations that change the file size are treated
          like I/O operations in this regard.
          </t>
          <t>
          The following rules, applied in order of decreasing priority,
          govern the selection of the appropriate stateid.  In following
          these rules, the client will only consider locks of which it
          has actually received notification by an appropriate operation
          response or callback.  Note that the
          rules are slightly different in the case of I/O to data servers
          when file layouts are being
          used (see <xref target="global_stateid" format="default"/>).
          </t>
          <ul spacing="normal">
            <li>
              If the client holds a delegation for the file in question, the
              delegation stateid SHOULD be used.
            </li>
            <li>
              Otherwise, if the entity corresponding to the lock-owner (e.g., a process)
              sending the I/O has a byte-range lock stateid for the associated open file,
              then the byte-range lock stateid for that lock-owner and open file SHOULD
              be used.
            </li>
            <li>
              If there is no byte-range lock stateid, then the OPEN stateid for the open
              file in question SHOULD be used.
            </li>
            <li>
              Finally, if none of the above apply, then a special stateid
              SHOULD be used.
            </li>
          </ul>
          <t>
          Ignoring these rules may result in situations in which the server
          does not have information necessary to properly process the request.
          For example, when mandatory byte-range locks are in effect, if the
          stateid does not indicate the proper lock-owner, via a lock stateid,
          a request might be avoidably rejected.
          </t>
          <t>
          The server however should not try to enforce these ordering rules
          and should use whatever information is available to properly process
          I/O requests. In particular, when a client has a delegation for a given file, it
          SHOULD take note of this fact in processing a request, even if it is
          sent with a special stateid.
          </t>
        </section>
        <!-- "Stateid Use for I/O Operations" -->
      <section anchor="stateid_use_sa" numbered="true" toc="default">
          <name>Stateid Use for SETATTR Operations</name>
          <t>
          Because each operation is associated with a session ID and from that
          the clientid can be determined, operations do not need to
          include a stateid for the server to be able to determine whether
          they should cause a delegation to be recalled or are to be
          treated as done within the scope of the delegation.
          </t>
          <t>
          In the case of SETATTR operations, a stateid is present.  In cases
          other than those that set the file size, the client may send either
          a special stateid or, when a delegation is held for the file in
          question, a delegation stateid.  While the server SHOULD validate
          the stateid and may use the stateid to optimize the determination
          as to whether a delegation is held, it SHOULD note the presence of
          a delegation even when a special stateid is sent, and MUST accept a
          valid delegation stateid when sent.
          </t>
        </section>
        <!-- "Stateid Use for SETATTR Operations" -->
    </section>
      <!-- "Stateid Definition" -->
  <section anchor="lease_renewal" numbered="true" toc="default">
        <name>Lease Renewal</name>
        <t>
      Each client/server pair, as represented by a client ID, has a single
      lease.
      The purpose of the lease is to allow the client to indicate
      to the server, in a low-overhead way, that it is active, and
      thus that the server is to retain the client's locks.  This arrangement
      allows the server to remove stale locking-related objects
      that are held by a client that has crashed or is otherwise
      unreachable, once the relevant lease expires.  This in turn allows
      other clients to obtain conflicting locks without being
      delayed indefinitely by inactive or unreachable clients.
      It is not a
      mechanism for cache consistency and lease
      renewals may not be denied if the lease interval has not expired.
        </t>
        <t>
      Since each session is associated with a specific
      client (identified by the client's client ID), any
      operation sent on that session is an indication
      that the associated client is reachable.  When a
      request is sent for a given session, successful
      execution of a SEQUENCE operation (or successful
      retrieval of the result of SEQUENCE from the reply
      cache) on an unexpired lease will result in the
      lease being implicitly renewed, for the standard
      renewal period (equal to the lease_time attribute).

        </t>
        <t>
      If the client ID's lease has not expired when the
      server receives a SEQUENCE operation, then the server
      MUST renew the lease.  If the client ID's lease has expired
      when the server receives a SEQUENCE operation, the
      server MAY renew the lease; this depends on whether
      any state was revoked as a result of the client's
      failure to renew the lease before expiration.

        </t>
        <t>
      Absent other activity that would renew the lease, a COMPOUND
      consisting of a single SEQUENCE operation will suffice.  The
      client should also take communication-related delays into
      account and take steps to ensure that the renewal messages
      actually reach the server in good time.  For example:
        </t>
        <ul spacing="normal">
          <li>
          When trunking is in effect, the client should
          consider sending multiple requests on different
          connections, in order to ensure that renewal
          occurs, even in the event of blockage in the
          path used for one of those connections.
        </li>
          <li>
            <t>
	  Transport retransmission delays might become
	  so large as to approach or exceed the length
	  of the lease period.	This may be particularly
	  likely when the server is unresponsive due to
	  a restart; see <xref target="reclaim_locks" format="default"/>. If the client implementation is not careful,
	  transport retransmission delays can result in the
	  client failing to detect a server restart before
	  the grace period ends. The scenario is that the
	  client is using a transport with exponential
	  backoff, such that the maximum retransmission
	  timeout exceeds both the grace period and the
	  lease_time attribute. A network partition causes
	  the client's connection's retransmission interval
	  to back off, and even after the partition heals,
	  the next transport-level retransmission is sent
	  after the server has restarted and its grace
	  period ends.

            </t>
            <t>

          The client MUST either recover from the ensuing
          NFS4ERR_NO_GRACE errors or it MUST ensure that,
          despite transport-level retransmission intervals
          that exceed the lease_time, a SEQUENCE operation is sent
          that renews the lease before expiration. The client can achieve this
          by associating a new connection with the session,
          and sending a SEQUENCE operation on it. However, if
          the attempt to establish a new connection is delayed
          for some reason (e.g., exponential backoff of the connection
          establishment packets), the client will have to
          abort the connection establishment attempt before
          the lease expires, and attempt to reconnect.

            </t>
          </li>
        </ul>
        <t>
      If the server renews the lease upon receiving
      a SEQUENCE operation, the server MUST NOT allow the lease
      to expire while the rest of the operations
      in the COMPOUND procedure's request are still
      executing. Once the last operation has finished, and
      the response to COMPOUND has been sent, the server
      MUST set the lease to expire no sooner than the
      sum of current time and the value of the lease_time attribute.

        </t>
        <t>
      A client ID's lease can expire when it has been
      at least the lease interval (lease_time) since the
      last lease-renewing SEQUENCE operation was sent
      on any of the client ID's sessions and there
      are no active COMPOUND operations on any such sessions.

        </t>
        <t>
      Because the SEQUENCE operation is the basic mechanism to renew
      a lease, and because it must be done at least once for each
      lease period, it is the natural mechanism whereby the server
      will inform the client of changes in the lease status that the
      client needs to be informed of.  The client should inspect the
      status flags (sr_status_flags) returned by sequence and take
      the appropriate action (see
      <xref target="OP_SEQUENCE_DESCRIPTION" format="default"/> for details).
        </t>
        <ul spacing="normal">
          <li>
          The status bits SEQ4_STATUS_CB_PATH_DOWN and
          SEQ4_STATUS_CB_PATH_DOWN_SESSION indicate problems with
          the backchannel that the client may need to address
          in order to receive callback requests.
        </li>
          <li>
          The status bits SEQ4_STATUS_CB_GSS_CONTEXTS_EXPIRING and
          SEQ4_STATUS_CB_GSS_CONTEXTS_EXPIRED indicate
          problems with GSS contexts or RPCSEC_GSS handles
          for the backchannel that the
          client might have to address in order to allow callback requests
          to be sent.
        </li>
          <li>
          The status bits SEQ4_STATUS_EXPIRED_ALL_STATE_REVOKED,
          SEQ4_STATUS_EXPIRED_SOME_STATE_REVOKED,
          SEQ4_STATUS_ADMIN_STATE_REVOKED, and
          SEQ4_STATUS_RECALLABLE_STATE_REVOKED notify the
          client of lock revocation events.  When these bits
          are set, the client should use TEST_STATEID to find
          what stateids have been revoked and use FREE_STATEID
          to acknowledge loss of the associated state.
        </li>
          <li>
          The status bit SEQ4_STATUS_LEASE_MOVE
          indicates that
          responsibility for lease renewal has been transferred to
          one or more new servers.
        </li>
          <li>
          The status bit SEQ4_STATUS_RESTART_RECLAIM_NEEDED
	  indicates that due to server
	  restart the client must reclaim locking state.
        </li>
          <li>
          The status bit SEQ4_STATUS_BACKCHANNEL_FAULT
          indicates that the server has encountered an unrecoverable fault
          with the backchannel (e.g., it has lost track of a
          sequence ID for a slot in the backchannel).
        </li>
        </ul>
      </section>
      <!-- "Lease Renewal" -->
  <section anchor="lock_crash_recovery" numbered="true" toc="default">
        <name>Crash Recovery</name>
        <t>
      A critical requirement in crash recovery is that both the client
      and the server know when the other has failed. Additionally, it
      is required that a client sees a consistent view of data across
      server restarts. All READ and WRITE operations that
      may have been queued within the client or network buffers must
      wait until the client has successfully recovered the locks
      protecting the READ and WRITE operations. Any that reach the
      server before the server can safely determine that the client
      has recovered enough locking state to be sure that such
      operations can be safely processed must be rejected.
      This will happen because either:
        </t>
        <ul spacing="normal">
          <li>
          The state presented is no longer valid since it is
          associated with a now invalid client ID.  In this case, the
          client will receive either an NFS4ERR_BADSESSION or
          NFS4ERR_DEADSESSION error, and any attempt to attach a new
          session to that invalid client ID will result in an
          NFS4ERR_STALE_CLIENTID error.
        </li>
          <li>
          Subsequent recovery of locks may make execution of the
          operation inappropriate (NFS4ERR_GRACE).
        </li>
        </ul>
        <section numbered="true" toc="default">
          <name>Client Failure and Recovery</name>
          <t>
        In the event that a client fails, the server may release the
        client's locks when the associated lease has expired.  Conflicting
        locks from another client may only be granted after this lease
        expiration.  As discussed in <xref target="lease_renewal" format="default"/>, when
        a client has not failed and re-establishes its lease before expiration
        occurs, requests for conflicting locks will not be granted.
          </t>
          <t>
        To minimize client delay upon restart, lock requests are associated
        with an instance of the client by a client-supplied verifier.  This
        verifier is part of the client_owner4 sent in the initial
        EXCHANGE_ID call made by the client.
        The server returns a client ID as a result of the EXCHANGE_ID
        operation.  The client then confirms the use of the client ID by
        establishing a session associated with that client ID  (see
        <xref target="OP_CREATE_SESSION_DESCRIPTION" format="default"/> for a
        description of how this is done).  All locks,
        including opens, byte-range locks, delegations, and layouts obtained
        by sessions using that client ID, are associated with that client ID.
          </t>
          <t>
        Since the verifier will be changed by the client upon each
        initialization, the server can compare a new verifier to the verifier
        associated with currently held locks and determine that they do not
        match.  This signifies the client's new instantiation and subsequent
        loss (upon confirmation of the new client ID) of locking
        state.  As a result, the server is free to release all
        locks held that are associated with the old client ID that was
        derived from the old verifier.  At this point, conflicting locks from
        other clients, kept waiting while the lease had not yet expired, can
        be granted.  In addition, all stateids associated with the old client ID
        can also be freed, as they are no longer reference-able.
          </t>
          <t>
        Note that the verifier must have the same uniqueness properties as the
        verifier for the COMMIT operation.
          </t>
        </section>
        <!-- "Client Failure and Recovery" -->
    <section anchor="server_failure" numbered="true" toc="default">
          <name>Server Failure and Recovery</name>
          <t>
        If the server loses locking state (usually as a result of a restart), it must allow clients time to discover this fact and
        re-establish the lost locking state.  The client must be able to
        re-establish the locking state without having the server deny valid
        requests because the server has granted conflicting access to another
        client.  Likewise, if there is a possibility that clients have not
        yet re-established their locking state for a file and that
        such locking state might make it invalid to perform READ or
        WRITE operations. For example, if mandatory locks are a possibility,
        the server must disallow READ and WRITE operations for that file.
          </t>
          <t>
        A client can determine that loss of locking
        state has occurred via several methods.
          </t>
          <ol spacing="normal" type="1">
            <li>
	When a SEQUENCE (most common) or other operation returns
	NFS4ERR_BADSESSION, this may mean that the session has
	been destroyed but the client ID is still valid.
	The client sends a CREATE_SESSION request with the
	client ID to re-establish the session. If
	CREATE_SESSION fails with NFS4ERR_STALE_CLIENTID,
	the client must establish a new client ID (see
	<xref target="client_id" format="default"/>) and re-establish its
	lock state with the new client ID, after the CREATE_SESSION
        operation succeeds (see <xref target="reclaim_locks" format="default"/>).

        </li>
            <li>
        When a SEQUENCE (most common) or other operation on a
        persistent session returns NFS4ERR_DEADSESSION, this indicates
        that a session is no longer usable for new, i.e., not satisfied
        from the reply cache, operations.  Once all pending operations
        are determined to be either performed before the retry or not
        performed, the client sends a CREATE_SESSION request with the
	client ID to re-establish the session. If
	CREATE_SESSION fails with NFS4ERR_STALE_CLIENTID,
	the client must establish a new client ID (see
	<xref target="client_id" format="default"/>) and re-establish its
	lock state after the CREATE_SESSION, with the
        new client ID, succeeds
        (<xref target="reclaim_locks" format="default"/>).
        </li>
            <li>
	When an operation, neither SEQUENCE nor preceded by SEQUENCE (for
	example, CREATE_SESSION, DESTROY_SESSION), returns
	NFS4ERR_STALE_CLIENTID, the client MUST establish
	a new client ID (<xref target="client_id" format="default"/>) and
	re-establish its lock state (<xref target="reclaim_locks" format="default"/>).
        </li>
          </ol>
          <section anchor="reclaim_locks" numbered="true" toc="default">
            <name>State Reclaim</name>
            <t>
        When state information and the associated locks are lost
        as a result of a server restart, the protocol must provide
        a way to cause that state to be re-established.  The
        approach used is to define, for most types of locking
        state (layouts are an exception), a request whose function
        is to allow the client to
        re-establish on the server a lock first obtained from a
        previous instance.  Generally, these requests are variants
        of the requests normally used to create locks of that type
        and are referred to as "reclaim-type" requests, and the process
        of re-establishing such locks is referred to as "reclaiming"
        them.
            </t>
            <t anchor="read_write_grace">
        Because each client must have an opportunity to reclaim
        all of the locks that it has without the possibility that
        some other client will be granted a conflicting lock,
        a "grace period" is devoted
        to the reclaim process.  During this period, requests
        creating client IDs and
        sessions are handled normally, but locking requests are
        subject to special restrictions.  Only
        reclaim-type locking requests are allowed, unless the
        server can reliably determine (through state
        persistently maintained across restart instances) that
        granting any such lock cannot possibly conflict with a
        subsequent reclaim.
        When a request is made to obtain
        a new lock (i.e., not a reclaim-type request) during the
        grace period and such a determination cannot be made,
        the server must return the error NFS4ERR_GRACE.
            </t>
            <t>
        Once a session is established using the new client ID, the
        client will use reclaim-type locking requests (e.g., LOCK
        operations with reclaim set to TRUE and OPEN operations with a
        claim type of CLAIM_PREVIOUS;  see
        <xref target="open_br_reclaim" format="default"/>) to re-establish its locking
        state.  Once this is done, or if there is no such locking
        state to reclaim, the client sends a global RECLAIM_COMPLETE
        operation, i.e., one with the rca_one_fs argument set to FALSE, to
        indicate that it has reclaimed all of the locking state that
        it will reclaim.  Once a client sends such a RECLAIM_COMPLETE
        operation, it may attempt non-reclaim locking operations,
        although it might get an NFS4ERR_GRACE status result from each such operation until
        the period of special handling is over.
        See <xref target="SEC11-EFF-lock" format="default"/> for a discussion of the
        analogous handling lock reclamation in the case of file systems
        transitioning from server to server.
            </t>
            <t>
        During the grace period, the server must reject READ
        and WRITE operations
        and non-reclaim locking requests (i.e., other LOCK
        and OPEN operations) with an error of NFS4ERR_GRACE,
        unless it can guarantee that these may be done
        safely, as described below.
            </t>
            <t>
        The grace period may last until all clients that are known to
        possibly have had locks have done a global RECLAIM_COMPLETE operation, indicating
        that they have finished reclaiming the locks they held before
        the server restart.  This means that a client that has done a
        RECLAIM_COMPLETE must be prepared to receive an NFS4ERR_GRACE
        when attempting to acquire new locks.
        In order for the server to know that all clients with possible prior
        lock state have done a RECLAIM_COMPLETE,
        the server must maintain in stable
        storage a list clients that may have such locks.  The server
        may also terminate the grace period before all clients have
        done a global RECLAIM_COMPLETE.  The server SHOULD NOT terminate the
        grace period before a time equal to the lease period in order
        to give clients an opportunity to find out about the server
        restart, as a result of sending requests on associated
        sessions with a frequency governed by the lease time.
        Note that when a client does not send such requests (or they
        are sent by the client but not received by the server),
        it is possible for the grace period to expire before the client
        finds out that the server restart has occurred.
            </t>
            <t>
        Some additional time in
        order to allow a client to
        establish a new client ID and session and to effect lock
        reclaims may be added to the lease time.  Note that
        analogous rules apply to
        file system-specific grace periods discussed in
        <xref target="SEC11-EFF-lock" format="default"/>.
            </t>
            <t>
        If the server can reliably determine that granting a non-reclaim
        request will not conflict with reclamation of locks by other
        clients, the NFS4ERR_GRACE error does not have to be returned
        even within the grace period, although NFS4ERR_GRACE must always
        be returned to clients attempting a non-reclaim lock request
        before doing their own global RECLAIM_COMPLETE.
        For the server to be able
        to service READ and WRITE operations during the grace period, it must
        again be able to guarantee that no possible conflict could arise
        between a potential reclaim locking request and the READ or WRITE
        operation.  If the server is unable to offer that guarantee, the
        NFS4ERR_GRACE error must be returned to the client.
            </t>
            <t>
        For a server to provide simple, valid handling during the grace
        period, the easiest method is to simply reject all non-reclaim locking
        requests and READ and WRITE operations by returning the NFS4ERR_GRACE
        error.  However, a server may keep information about granted locks in
        stable storage.  With this information, the server could determine if
        a locking, READ or WRITE operation can be safely processed.
            </t>
            <t>
        For example, if the server maintained on stable storage summary
        information on whether mandatory locks exist, either mandatory
        byte-range locks, or share reservations specifying deny modes,
        many requests could be allowed during the grace period.  If it
        is known that no such share reservations exist, OPEN request that
        do not specify deny modes may be safely granted.  If, in addition,
        it is known that no mandatory byte-range locks exist, either
        through information stored on stable storage or simply because
        the server does not support such locks, READ and WRITE operations
        may be safely processed during the grace period.
        Another important case is where it is known that no mandatory
        byte-range locks exist, either because the server does not
        provide support for them or because their absence is known
        from persistently recorded data.  In this case, READ and
        WRITE operations specifying stateids derived from reclaim-type
        operations may be validly processed during the grace period
        because of the fact that the valid reclaim ensures that no lock
        subsequently granted can prevent the I/O.
            </t>
            <t>
        To reiterate, for a server that allows non-reclaim lock and I/O
        requests to be processed during the grace period, it MUST determine
        that no lock subsequently reclaimed will be rejected and that no lock
        subsequently reclaimed would have prevented any I/O operation
        processed during the grace period.
            </t>
            <t>
        Clients should be prepared for the return of NFS4ERR_GRACE errors for
        non-reclaim lock and I/O requests.  In this case, the client should
        employ a retry mechanism for the request.  A delay (on the order of
        several seconds) between retries should be used to avoid overwhelming
        the server.  Further discussion of the general issue is included in
        <xref target="Floyd" format="default"/>.  The client must account for the server that
        can perform I/O and non-reclaim locking requests within the grace period
        as well as those that cannot do so.
            </t>
            <t>
        A reclaim-type locking request outside the server's grace period
        can only succeed if the server can guarantee that no conflicting
        lock or I/O request has been granted since restart.
            </t>
            <t>
        A server may, upon restart, establish a new value for the lease
        period.  Therefore, clients should, once a new client ID is
        established, refetch the lease_time attribute and use it as the basis
        for lease renewal for the lease associated with that server. However,
        the server must establish, for this restart event, a grace period at
        least as long as the lease period for the previous server
        instantiation. This allows the client state obtained during the
        previous server instance to be reliably re-established.
            </t>
            <t>
        The possibility exists that, because of server configuration
        events, the client will be communicating with a server
        different than the one on which the locks were obtained, as
        shown by the combination of eir_server_scope and
        eir_server_owner.  This leads to the issue of if and when
        the client should attempt to reclaim locks previously obtained
        on what is being reported as a different server.  The rules
        to resolve this question are as follows:
            </t>
            <ul spacing="normal">
              <li>
            If the server scope is different, the client should not
            attempt to reclaim locks.  In this situation, no lock
            reclaim is possible.  Any attempt to re-obtain the locks
            with non-reclaim operations is problematic since there is
            no guarantee that the existing filehandles will be recognized
            by the new server, or that if recognized, they denote the
            same objects.  It is best to treat the locks as having been
            revoked by the reconfiguration event.
          </li>
              <li>
            If the server scope is the same, the client should attempt
            to reclaim locks, even if the eir_server_owner value is
            different.  In this situation, it is the responsibility
            of the server to return NFS4ERR_NO_GRACE if it cannot
            provide correct support for lock reclaim operations,
            including the prevention of edge conditions.
          </li>
            </ul>
            <t>
        The eir_server_owner field is not used in making this
        determination.  Its function is to specify trunking
        possibilities for the client (see <xref target="Trunking" format="default"/>)
        and not to control lock reclaim.
            </t>
            <section anchor="reclaim_security_considerations" numbered="true" toc="default">
              <name>Security Considerations for State Reclaim</name>
              <t>
          During the grace period, a client can reclaim state that it believes or
          asserts it had before the server restarted. Unless the server
          maintained a complete record of all the state the client had,
          the server has little choice but to trust the client. (Of course,
          if the server maintained a complete record, then it would not
          have to force the client to reclaim state after server restart.)
          While the server has to trust the client to tell the truth, the
          negative consequences for security are limited to enabling
	  denial-of-service attacks in situations in which AUTH_SYS is
	  supported.   The
          fundamental rule for the server when processing reclaim requests
          is that it MUST NOT grant the reclaim if an equivalent non-reclaim
          request would not be granted during steady state due to access
          control or access conflict issues. For example, an OPEN request
	  during a reclaim will be refused with NFS4ERR_ACCESS if the principal making
	  the request does not have access to open the file according to the
	  discretionary ACL (<xref target="attrdef_dacl" format="default"/>) on the file.

              </t>
              <t>
          Nonetheless, it is possible that a client operating in error or
          maliciously could, during reclaim, prevent another client from
          reclaiming access to state. For example, an attacker could
          send an OPEN reclaim operation with a deny mode that prevents
          another client from reclaiming the OPEN state it had before the
          server restarted.
          The attacker could perform the same denial of service during
          steady state prior to server restart, as long as the
          attacker had permissions. Given that the attack
          vectors are equivalent, the grace period does not offer any
          additional opportunity for denial of service, and any concerns
          about this attack vector, whether during grace or steady state,
          are addressed the same way: use RPCSEC_GSS for authentication
          and limit access to the file only to principals that the owner of
          the file trusts.

              </t>
              <t>
           Note that if prior to restart the server had client
           IDs with the  EXCHGID4_FLAG_BIND_PRINC_STATEID (<xref target="OP_EXCHANGE_ID" format="default"/>) capability set, then the server
           SHOULD record in stable storage the client owner and the
           principal that established the client ID via EXCHANGE_ID.
           If the server does not, then there is a risk a client will
           be unable to reclaim state if it does not have a credential
           for a principal that was originally authorized to
           establish the state.

              </t>
            </section>
            <!-- "Security Considerations for State Reclaim" -->
      </section>
          <!-- "State Reclaim" -->
    </section>
        <!-- "Server Failure and Recovery" -->
    <section anchor="network_partitions_and_recovery" numbered="true" toc="default">
          <name>Network Partitions and Recovery</name>
          <t>
        If the duration of a network partition is greater than the lease
        period provided by the server, the server will not have received a
        lease renewal from the client.  If this occurs, the server may free
        all locks held for the client or it may allow the lock state to
        remain for a considerable period, subject to the constraint that
        if a request for a conflicting lock is made, locks associated with
        an expired lease do not prevent such a conflicting lock from being
        granted but MUST be revoked as necessary so as to avoid interfering with
        such conflicting requests.
          </t>
          <t>
        If the server chooses to delay freeing of lock state until there
        is a conflict, it may either free all of the client's locks once
        there is a conflict or it may only revoke the minimum set of locks
        necessary to allow conflicting requests.  When it adopts the
        finer-grained approach, it must revoke all locks associated with a
        given stateid, even if the conflict is with only a subset of locks.
          </t>
          <t>
        When the server chooses to free all of a client's lock state, either
        immediately upon lease expiration or as a result of the first
        attempt to obtain a conflicting a lock, the server may report the
        loss of lock state in a number of ways.
          </t>
          <t>
        The server may choose to invalidate the session and the associated
        client ID.  In this case, once the client can communicate
        with the server, it will receive an NFS4ERR_BADSESSION error.  Upon
        attempting to create a new session, it would get an
        NFS4ERR_STALE_CLIENTID.  Upon creating the new client ID and new
        session, the client will attempt to reclaim locks. Normally, the
        server will not allow the client to reclaim locks, because the
        server will not be in its recovery grace period.
          </t>
          <t>
        Another possibility is for the server to maintain the session and
        client ID but for all stateids held by the
        client to become invalid or stale.  Once the client can reach
        the server after such a network partition, the status returned by
        the SEQUENCE operation will indicate a loss of locking state; i.e.,
        the flag SEQ4_STATUS_EXPIRED_ALL_STATE_REVOKED will be set in
        sr_status_flags. In
        addition, all I/O submitted by the
        client with the now invalid stateids will fail with the server
        returning the error NFS4ERR_EXPIRED.  Once the client learns of
        the loss of locking state, it
        will suitably notify the applications that held the invalidated
        locks.  The client should then take action to free invalidated
        stateids, either by establishing a new client ID using a new
        verifier or by doing a FREE_STATEID operation to release each
        of the invalidated stateids.
          </t>
          <t>
        When the server adopts a finer-grained approach to revocation
        of locks when a client's lease has expired, only a subset of stateids
        will normally become invalid during a network partition.
        When the client can communicate with the server after such a
        network partition heals, the status returned by the SEQUENCE
        operation will indicate a partial loss of locking state
        (SEQ4_STATUS_EXPIRED_SOME_STATE_REVOKED).
        In addition, operations, including I/O submitted by the
        client, with the now invalid stateids will fail with the server
        returning the error NFS4ERR_EXPIRED.  Once the client learns of
        the loss of locking state, it will use the TEST_STATEID operation
        on all of its stateids to
        determine which locks have been lost and then
        suitably notify the applications that held the invalidated
        locks.  The client can then release the invalidated locking
        state and acknowledge the revocation of the associated locks
        by doing a FREE_STATEID operation on each of the invalidated
        stateids.
          </t>
          <t>
        When a network partition is combined with a server restart, there are
        edge conditions that place requirements on the server in order to
        avoid silent data corruption following the server restart. Two of these
        edge conditions are known, and are discussed below.
          </t>
          <t>
        The first edge condition arises as a result of the scenarios such as
        the following:
          </t>
          <ol spacing="normal" type="1">
            <li>
            Client A acquires a lock.
          </li>
            <li>
            Client A and server experience mutual network partition, such that
            client A is unable to renew its lease.
          </li>
            <li>
            Client A's lease expires, and the server releases the lock.
          </li>
            <li>
            Client B acquires a lock that would have conflicted
            with that of client A.
          </li>
            <li>
            Client B releases its lock.
          </li>
            <li>
            Server restarts.
          </li>
            <li>
            Network partition between client A and server heals.
          </li>
            <li>
            Client A connects to a new server instance and finds out about
            server restart.
          </li>
            <li>
            Client A reclaims its lock within the server's grace period.
          </li>
          </ol>
          <t>
        Thus, at the final step, the server has erroneously granted client A's
        lock reclaim. If client B modified the object the lock was protecting,
        client A will experience object corruption.
          </t>
          <t>
        The second known edge condition arises in situations such as the following:
          </t>
          <ol spacing="normal" type="1">
            <li>
            Client A acquires one or more locks.
          </li>
            <li>
            Server restarts.
          </li>
            <li>
            Client A and server experience mutual network
            partition, such that client A is unable to reclaim
            all of its locks within the grace period.
          </li>
            <li>
            Server's reclaim grace period ends. Client A has either
            no locks or an incomplete set of locks known to the server.
          </li>
            <li>
            Client B acquires a lock that would have conflicted
            with a lock of client A that was not reclaimed.
          </li>
            <li>
            Client B releases the lock.
          </li>
            <li>
            Server restarts a second time.
          </li>
            <li>
            Network partition between client A and server heals.
          </li>
            <li>
            Client A connects to new server instance and finds out about
            server restart.
          </li>
            <li>
            Client A reclaims its lock within the server's
            grace period.
          </li>
          </ol>
          <t>
        As with the first edge condition, the final step of the scenario of
        the second edge condition has the server erroneously granting client
        A's lock reclaim.
          </t>
          <t>
        Solving the first and second edge conditions requires either that the server
        always assumes after it restarts that some edge condition
        occurs, and thus returns NFS4ERR_NO_GRACE for all reclaim attempts, or that the server
        record some information in stable storage.  The amount
        of information the
        server records in stable storage is in inverse proportion to how harsh
        the server intends to be whenever edge conditions arise.
        The server
        that is completely tolerant of all edge conditions will record in
        stable storage every lock that is acquired, removing the lock record
        from stable storage only when the lock is released.
        For the two edge conditions discussed above, the harshest a
        server can be, and still support a grace period for reclaims, requires
        that the server record in stable storage some minimal
        information.  For example, a server implementation could, for each
        client, save in stable storage a record containing:
          </t>
          <ul spacing="normal">
            <li>
            the co_ownerid field from the client_owner4 presented in the
            EXCHANGE_ID operation.
          </li>
            <li>
            a boolean that indicates if the client's lease expired
            or if there was administrative intervention (see
            <xref target="server_revocation" format="default"/>) to revoke
            a byte-range lock, share reservation, or delegation and
            there has been no acknowledgment, via FREE_STATEID,
            of such revocation.
          </li>
            <li>
            a boolean that indicates whether the client may have locks
            that it believes to be reclaimable in situations in which the
            grace period was terminated, making the server's view of
            lock reclaimability suspect.  The server will set this for
            any client record in stable storage where the client has
            not done a suitable RECLAIM_COMPLETE (global or file
            system-specific depending on the target of the lock
            request) before it grants any new (i.e., not reclaimed)
            lock to any client.
          </li>
          </ul>
          <t>
        Assuming the above record keeping, for the first edge condition, after
        the server restarts, the record that client A's lease expired means
        that another client could have acquired a conflicting byte-range lock,
        share reservation, or delegation. Hence, the server must reject a
        reclaim from client A with the error NFS4ERR_NO_GRACE.
          </t>
          <t>
        For the second edge condition, after the server restarts for a second
        time, the indication that the client had not completed its
        reclaims at the time at which the grace period ended
        means that the server must reject a reclaim from client A
        with the error NFS4ERR_NO_GRACE.
          </t>
          <t>
        When either edge condition occurs, the client's attempt to reclaim
        locks will result in the error NFS4ERR_NO_GRACE.  When this is
        received, or after the client restarts with no lock state, the
        client will send a global RECLAIM_COMPLETE.  When
        the RECLAIM_COMPLETE is received, the server and client are
        again in agreement regarding reclaimable locks and both booleans in persistent
        storage can be reset, to be set again only when there is a subsequent
        event that causes lock reclaim operations to be questionable.
          </t>
          <t>
        Regardless of the level and approach to record keeping, the server
        MUST implement one of the following strategies (which apply to
        reclaims of share reservations, byte-range locks, and delegations):
          </t>
          <ol spacing="normal" type="1">
            <li>
            Reject all reclaims with NFS4ERR_NO_GRACE. This
            is extremely unforgiving, but necessary if the server does not
            record lock state in stable storage.
          </li>
            <li>
              <t>
            Record sufficient state in stable storage such that
            all known edge conditions involving server restart,
            including the two noted in this section, are
            detected.  It is acceptable to erroneously recognize an edge condition
            and not allow a reclaim, when, with sufficient knowledge, it
            would be allowed. The error the server would return in this
            case is NFS4ERR_NO_GRACE.  Note that it is not known if there are other
            edge conditions.
              </t>
              <t>
            In the event that, after a server restart, the server
            determines there is unrecoverable damage or
            corruption to the information in stable storage, then for
            all clients and/or locks that may be affected, the server MUST
            return NFS4ERR_NO_GRACE.
              </t>
            </li>
          </ol>
          <t>
        A mandate for the client's handling of the NFS4ERR_NO_GRACE error is
        outside the scope of this specification, since the strategies for such
        handling are very dependent on the client's operating environment.
        However, one potential approach is described below.
          </t>
          <t>
        When the client receives NFS4ERR_NO_GRACE, it could examine the change
        attribute of the objects  for which the client is trying to reclaim state,
        and use that to determine whether to re-establish the state via normal
        OPEN or LOCK operations. This is acceptable provided that the client's
        operating environment allows it.  In other words, the client
        implementor is advised to document for his users the behavior. The
        client could also inform the application that its byte-range lock or share
        reservations (whether or not they were delegated) have been lost, such
        as via a UNIX signal, a Graphical User Interface (GUI) pop-up window, etc.
        See <xref target="data_caching_revocation" format="default"/>
        for a discussion of what the client should do
        for dealing with unreclaimed delegations on client state.
          </t>
          <t>
        For further discussion of revocation of locks, see
        <xref target="server_revocation" format="default"/>.
          </t>
        </section>
        <!-- "Network Partitions and Recovery" -->
  </section>
      <!-- "Crash Recovery" -->
  <section anchor="server_revocation" numbered="true" toc="default">
        <name>Server Revocation of Locks</name>
        <t>
      At any point, the server can revoke locks held by a client, and the
      client must be prepared for this event.  When the client detects that
      its locks have been or may have been revoked, the client is
      responsible for validating the state information between itself and
      the server.  Validating locking state for the client means that it
      must verify or reclaim state for each lock currently held.
        </t>
        <t>
      The first occasion of lock revocation is upon server
      restart.  Note that this includes situations
      in which sessions are persistent and locking state is
      lost.  In this class of instances, the client will
      receive an error (NFS4ERR_STALE_CLIENTID) on an
      operation that takes client ID, usually as part of
      recovery in response to a problem with the current
      session), and the client will proceed
      with normal crash recovery as described in the <xref target="reclaim_locks" format="default"/>.
        </t>
        <t>
      The second occasion of lock revocation is the inability to renew the lease
      before expiration, as discussed in
      <xref target="network_partitions_and_recovery" format="default"/>. While this is
      considered a rare or unusual event,
      the client must be prepared to recover.  The server is responsible
      for determining the precise consequences of the lease expiration,
      informing the client of the scope of the lock revocation decided
      upon.  The client then uses the status information provided
      by the server in the SEQUENCE results (field sr_status_flags,
      see <xref target="OP_SEQUENCE_DESCRIPTION" format="default"/>)
      to synchronize its locking state with that of the
      server, in order to recover.
        </t>
        <t>
      The third occasion of lock revocation can occur as a result of
      revocation of locks within the lease period, either because of
      administrative intervention or because a recallable lock (a
      delegation or layout) was not returned within the lease period
      after having been recalled.  While these are
      considered rare events, they are possible, and the client must be
      prepared to deal with them.  When either of these events occurs,
      the client finds out about the situation through the status returned
      by the SEQUENCE operation.  Any use of stateids associated with
      locks revoked during the lease period will receive the error
      NFS4ERR_ADMIN_REVOKED or NFS4ERR_DELEG_REVOKED, as appropriate.
        </t>
        <t>
      In all situations in which a subset of locking state may have been
      revoked, which include all cases in which locking state is revoked
      within the lease period, it is up to the client to determine which
      locks have been revoked and which have not.  It does this by
      using the TEST_STATEID operation on the appropriate set of stateids.
      Once the set of revoked locks has been determined, the applications
      can be notified, and the invalidated stateids can be freed and
      lock revocation acknowledged by using FREE_STATEID.
        </t>
      </section>
      <!-- "Server Revocation of Locks" -->
  <section numbered="true" toc="default">
        <name>Short and Long Leases</name>
        <t>
      When determining the time period for the server lease, the usual lease
      tradeoffs apply.  A short lease is good for fast server recovery at a
      cost of increased operations to effect lease renewal (when there are
      no other operations during the period to effect lease renewal as a
      side effect).  A long lease is certainly kinder and gentler to
      servers trying to handle very large numbers of clients.  The number of extra requests
      to effect lock renewal drops in inverse
      proportion to the lease time.  The disadvantages of a long lease
      include the possibility of slower recovery after certain failures.
      After server failure, a longer grace period may be required when
      some clients do not promptly reclaim their locks and do a
      global RECLAIM_COMPLETE.  In the event of client failure,
      the longer period for a lease to expire will force conflicting
      requests to wait longer.
        </t>
        <t>
      A long lease is practical if the server can store lease state in
      stable storage.  Upon recovery, the server can reconstruct the
      lease state from its stable storage and continue operation with
      its clients.
        </t>
      </section>
      <!-- "Short and Long Leases" -->
  <section anchor="lease_propagation_delay" numbered="true" toc="default">
        <name>Clocks, Propagation Delay, and Calculating Lease Expiration</name>
        <t>
      To avoid the need for synchronized clocks, lease times are granted by
      the server as a time delta.  However, there is a requirement that the
      client and server clocks do not drift excessively over the duration of
      the lease.  There is also the issue of propagation delay across the
      network, which could easily be several hundred milliseconds, as well as
      the possibility that requests will be lost and need to be
      retransmitted.
        </t>
        <t>
      To take propagation delay into account, the client should
      subtract it from lease times (e.g., if the client estimates the
      one-way propagation delay as 200 milliseconds, then it can
      assume that the lease is already 200 milliseconds old when it
      gets it).  In addition, it will take another 200 milliseconds to
      get a response back to the server.  So the client must send a
      lease renewal or write data back to the server at least 400
      milliseconds before the lease would expire. If the propagation delay
      varies over the life of the lease (e.g., the client is on a mobile
      host), the client will need to continuously subtract the increase
      in propagation delay from the lease times.
        </t>
        <t>
      The server's lease period configuration should take into account the
      network distance of the clients that will be accessing the server's
      resources.  It is expected that the lease period will take into
      account the network propagation delays and other network delay factors
      for the client population.  Since the protocol does not allow for an
      automatic method to determine an appropriate lease period, the
      server's administrator may have to tune the lease period.

        </t>
      </section>
      <!-- "Clocks, Propagation Delay, and Calculating Lease Expiration" -->
  <section anchor="vestigial_locking" numbered="true" toc="default">
        <name>Obsolete Locking Infrastructure from NFSv4.0</name>
        <t>
      There are a number of operations and fields within existing
      operations that no longer have a function in NFSv4.1.
      In one way or another, these changes are all due to
      the implementation of sessions that provide client context
      and exactly once semantics as a base feature of the protocol,
      separate from locking itself.
        </t>
        <t>
      The following NFSv4.0 operations MUST NOT be implemented in NFSv4.1.
      The server MUST return NFS4ERR_NOTSUPP if these operations are
      found in an NFSv4.1 COMPOUND.
        </t>
        <ul spacing="normal">
          <li>
          SETCLIENTID since its function has been replaced by
          EXCHANGE_ID.
        </li>
          <li>
          SETCLIENTID_CONFIRM since client ID confirmation now
          happens by means of CREATE_SESSION.
        </li>
          <li>
          OPEN_CONFIRM because state-owner-based seqids
          have been replaced by the sequence ID in the
          SEQUENCE operation.
        </li>
          <li>
          RELEASE_LOCKOWNER because lock-owners with no associated
          locks do not have any sequence-related state and so can
          be deleted by the server at will.
        </li>
          <li>
          RENEW because every SEQUENCE operation for a session causes
          lease renewal, making a separate operation superfluous.
        </li>
        </ul>
        <t>
      Also, there are a number of fields, present in existing operations,
      related to locking that have no use in minor version 1.  They
      were used in minor version 0 to perform functions now provided
      in a different
      fashion.
        </t>
        <ul spacing="normal">
          <li>
          Sequence ids used to sequence requests for a given state-owner
          and to provide retry protection, now provided
          via sessions.
        </li>
          <li>
          Client IDs used to identify the client associated with a given
          request.  Client identification is now available using the client ID
          associated with the current session, without needing an explicit
          client ID field.
        </li>
        </ul>
        <t>
      Such vestigial fields in existing operations have no function in
      NFSv4.1 and are ignored by the server.  Note that client IDs in
      operations new to NFSv4.1 (such as CREATE_SESSION and DESTROY_CLIENTID)
      are not ignored.
        </t>
      </section>
      <!-- "Vestigial Locking Infrastructure From V4.0" -->
</section>
    <!-- "State Management" -->
<!--  $Id: 2009-12-20-TO-rfc5661.xml,v 1.2 2009/12/21 05:59:32 shepler.mre Exp $       -->
<!-- Copyright (C) The IETF Trust (2007-2008) -->
<!-- Copyright (C) The Internet Society (2006) -->

<section anchor="file_locking" numbered="true" toc="default">
      <name>File Locking and Share Reservations</name>
      <t>
    To support Win32 share reservations, it is necessary to provide
    operations that atomically open or create files.  Having a
    separate share/unshare operation would not allow correct
    implementation of the Win32 OpenFile API.  In order to
    correctly implement share semantics, the previous NFS protocol
    mechanisms used when a file is opened or created (LOOKUP, CREATE,
    ACCESS) need to be replaced.  The NFSv4.1 protocol defines
    an OPEN operation that is capable of atomically looking up, creating,
    and locking a file on the server.

      </t>
      <section numbered="true" toc="default">
        <name>Opens and Byte-Range Locks</name>
        <t>
      It is assumed that manipulating a byte-range lock is rare when
      compared to READ
      and WRITE operations.  It is also assumed that server restarts and network
      partitions are relatively rare.  Therefore, it is important that the
      READ and WRITE operations have a lightweight mechanism to indicate if
      they possess a held lock.  A LOCK operation contains the
      heavyweight information required to establish a byte-range lock and uniquely
      define the owner of the lock.
        </t>
        <section anchor="state-owner" numbered="true" toc="default">
          <name>State-Owner Definition</name>
          <t>
        When opening a file or requesting a byte-range lock, the
        client must specify an identifier that represents the owner of
        the requested lock.  This identifier is in the form of a
        state-owner, represented in the protocol by a state_owner4, a
        variable-length opaque array that, when concatenated with the
        current client ID, uniquely defines the owner of a lock managed
        by the client. This may be a thread ID, process ID, or other
        unique value.
          </t>
          <t>
        Owners of opens and owners of byte-range locks are separate
        entities and remain separate even if the same opaque arrays
        are used to designate owners of each.  The protocol distinguishes
        between open-owners (represented by open_owner4 structures)
        and lock-owners (represented by lock_owner4 structures).
          </t>
          <t>
        Each open is associated with a specific open-owner while each
        byte-range lock is associated with a lock-owner and an
        open-owner, the latter being the open-owner associated with the
        open file under which the LOCK operation was done.  Delegations
        and layouts, on the other hand, are not associated with a
        specific owner but are associated with the client as a whole
        (identified by a client ID).
          </t>
        </section>
        <!-- "State-owner Definition" -->
    <section numbered="true" toc="default">
          <name>Use of the Stateid and Locking</name>
          <t>
        All READ, WRITE, and SETATTR operations contain a stateid.  For the
        purposes of this section, SETATTR operations that change the size
        attribute of a file are treated as if they are writing the area
        between the old and new sizes (i.e., the byte-range truncated or added to the
        file by means of the SETATTR), even where SETATTR is not explicitly
        mentioned in the text.  The stateid passed to one of these operations must
        be one that represents an open, a set of byte-range locks, or a
        delegation, or it may be a special stateid representing anonymous
        access or the special bypass stateid.
          </t>
          <t>
        If the state-owner performs a READ or WRITE operation in a situation in which
        it has established a byte-range lock or share reservation
        on the server (any OPEN constitutes a share reservation), the
        stateid (previously returned by the server) must be used to
        indicate what locks, including both byte-range
        locks and share reservations, are held by the state-owner.  If no state
        is established by the client, either a byte-range lock or a share reservation,
        a special stateid for anonymous state (zero as the value for "other" and "seqid")
        is used.  (See <xref target="special_stateid" format="default"/> for a description of
        'special' stateids in general.)
        Regardless of whether a stateid for anonymous state
        or a stateid returned by the server is used, if there is a
        conflicting share reservation or mandatory byte-range lock held on the
        file, the server MUST refuse to service the READ or WRITE operation.
          </t>
          <t>
        Share reservations are established by OPEN operations and by their
        nature are mandatory in that when the OPEN denies READ or WRITE
        operations, that denial results in such operations being rejected with
        error NFS4ERR_LOCKED.  Byte-range locks may be implemented by the server
        as either mandatory or advisory, or the choice of mandatory or
        advisory behavior may be determined by the server on the basis of the
        file being accessed (for example, some UNIX-based servers support a
        "mandatory lock bit" on the mode attribute such that if set, byte-range
        locks are required on the file before I/O is possible).  When byte-range
        locks are advisory, they only prevent the granting of conflicting lock
        requests and have no effect on READs or WRITEs.  Mandatory byte-range
        locks, however, prevent conflicting I/O operations.  When they are
        attempted, they are rejected with NFS4ERR_LOCKED.  When the client
        gets NFS4ERR_LOCKED on a file for which it knows it has the proper share
        reservation, it will need to send a LOCK operation on the byte-range of
        the file that includes the byte-range the I/O was to be performed on, with
        an appropriate locktype field of the LOCK operation's arguments (i.e., READ*_LT for a READ operation, WRITE*_LT
        for a WRITE operation).
          </t>
          <t>
        Note that for UNIX environments that support mandatory byte-range locking,
        the distinction between advisory and mandatory locking is subtle.  In
        fact, advisory and mandatory byte-range locks are exactly the same as
        far as the APIs and requirements on implementation. If the mandatory
        lock attribute is set on the file, the server checks to see if the
        lock-owner has an appropriate shared (READ_LT) or exclusive (WRITE_LT) byte-range
        lock on the byte-range it wishes to READ from or WRITE to. If there is no
        appropriate lock, the server checks if there is a conflicting lock
        (which can be done by attempting to acquire the conflicting lock on
        behalf of the lock-owner, and if successful, release the lock after
        the READ or WRITE operation is done), and if there is, the server returns
        NFS4ERR_LOCKED.
          </t>
          <t>
        For Windows environments, byte-range locks are always mandatory, so the
        server always checks for byte-range locks during I/O requests.
          </t>
          <t>
        Thus, the LOCK operation does not need to distinguish
        between advisory and mandatory byte-range locks. It is the
        server's processing of the READ and WRITE operations that introduces
        the distinction.
          </t>
          <t>
        Every stateid that is validly passed to READ, WRITE, or SETATTR,
        with the exception of special stateid values,
        defines an access mode for the file (i.e.,
        OPEN4_SHARE_ACCESS_READ, OPEN4_SHARE_ACCESS_WRITE, or
        OPEN4_SHARE_ACCESS_BOTH).
          </t>
          <ul spacing="normal">
            <li>
            For stateids associated with opens, this is the mode defined by
            the original OPEN that caused the
            allocation of the OPEN stateid
            and as modified by subsequent OPENs and OPEN_DOWNGRADEs for the
            same open-owner/file pair.
          </li>
            <li>
            For stateids returned by byte-range LOCK operations,
            the appropriate mode is the access mode for the OPEN
            stateid associated with the lock set represented by the stateid.
          </li>
            <li>
            For delegation stateids, the access mode is based on the type of delegation.
          </li>
          </ul>
          <t>
        When a READ, WRITE, or SETATTR (that specifies the
        size attribute) operation is done, the operation is subject to checking against
        the access mode to verify that the operation is appropriate given the
        stateid with which the operation is associated.
          </t>
          <t>
        In the case of WRITE-type operations (i.e., WRITEs and SETATTRs that
        set size), the server MUST verify that the access mode allows writing
        and MUST return an NFS4ERR_OPENMODE error if it does not.  In the case of
        READ, the server may perform the corresponding check on the access
        mode, or it may choose to allow READ on OPENs for OPEN4_SHARE_ACCESS_WRITE, to
        accommodate clients whose WRITE implementation may unavoidably do
        reads (e.g., due to buffer cache constraints).  However, even if READs
        are allowed in these circumstances, the server MUST still check for
        locks that conflict with the READ (e.g., another OPEN specified OPEN4_SHARE_DENY_READ or OPEN4_SHARE_DENY_BOTH).  Note that a server that does enforce the access mode check
        on READs need not explicitly check for conflicting share reservations
        since the existence of OPEN for OPEN4_SHARE_ACCESS_READ guarantees that no
        conflicting share reservation can exist.
          </t>
          <t>
        The READ bypass special stateid (all bits of "other" and "seqid" set
        to one)
        indicates a desire to bypass locking checks.  The server MAY
        allow READ operations to bypass
        locking checks at the server, when this special stateid is used.
        However, WRITE operations with
        this special stateid value MUST NOT bypass locking checks and are
        treated exactly the same as if a special stateid for anonymous state
        were used.
          </t>
          <t>
        A lock may not be granted while a READ or WRITE operation using one of
        the special stateids is being performed and the scope of the lock
        to be granted would conflict with the READ or WRITE operation.
        This can occur when:
          </t>
          <ul spacing="normal">
            <li>
            A mandatory byte-range lock is requested with a byte-range that
            conflicts with the byte-range of the READ or WRITE operation.
            For the purposes of this paragraph, a conflict occurs when
            a shared lock is requested and a WRITE operation is being
            performed, or an exclusive lock is requested and either a
            READ or a WRITE operation is being performed.
          </li>
            <li>
            A share reservation is requested that denies reading and/or
            writing and the corresponding operation is being performed.
          </li>
            <li>
            A delegation is to be granted and the delegation type would
            prevent the I/O operation, i.e., READ and WRITE conflict with
            an OPEN_DELEGATE_WRITE delegation and WRITE conflicts with an OPEN_DELEGATE_READ delegation.
          </li>
          </ul>
          <t>
        When a client holds a delegation, it needs to ensure
        that the stateid sent conveys the association of
        operation with the delegation, to avoid the delegation from
        being avoidably recalled.  When the delegation stateid,
        a stateid open associated with that delegation, or a stateid
        representing byte-range locks derived from such an open is
        used, the server knows that the READ, WRITE, or SETATTR
        does not conflict with the delegation but is sent under
        the aegis of the delegation.  Even though it is possible
        for the server to determine from the client ID (via
        the session ID) that the client does in fact have a
        delegation, the server is not obliged to check this, so
        using a special stateid can result in avoidable recall
        of the delegation.
          </t>
        </section>
        <!-- "Use of the Stateid and Locking" -->
  </section>
      <!-- "Opens and Byte-Range Locks" -->
  <section numbered="true" toc="default">
        <name>Lock Ranges</name>
        <t>
      The protocol allows a lock-owner to request a lock with a byte-range
      and then either upgrade, downgrade, or unlock a sub-range of
      the initial lock, or a byte-range that
      overlaps -- fully or partially -- either with that initial lock or a
      combination of a set of existing locks for the same lock-owner.  It
      is expected that this will be an uncommon type of request.  In any
      case, servers or server file systems may not be able to support
      sub-range lock semantics.  In the event that a server receives a
      locking request that represents a sub-range of current locking state
      for the lock-owner, the server is allowed to return the error
      NFS4ERR_LOCK_RANGE to signify that it does not support sub-range lock
      operations.  Therefore, the client should be prepared to receive this
      error and, if appropriate, report the error to the requesting
      application.
        </t>
        <t>
      The client is discouraged from combining multiple independent locking
      ranges that happen to be adjacent into a single request since the
      server may not support sub-range requests for reasons related to
      the recovery of byte-range locking state in the event of server failure.  As
      discussed in <xref target="server_failure" format="default"/>, the
      server may employ certain optimizations during recovery that work
      effectively only when the client's behavior during lock recovery is
      similar to the client's locking behavior prior to server failure.
        </t>
      </section>
      <!-- "Lock Ranges" -->
  <section numbered="true" toc="default">
        <name>Upgrading and Downgrading Locks</name>
        <t>
      If a client has a WRITE_LT lock on a byte-range, it can request an atomic
      downgrade of the lock to a READ_LT lock via the LOCK operation, by setting
      the type to READ_LT. If the server supports atomic downgrade, the
      request will succeed. If not, it will return NFS4ERR_LOCK_NOTSUPP. The
      client should be prepared to receive this error and, if appropriate,
      report the error to the requesting application.
        </t>
        <t>
      If a client has a READ_LT lock on a byte-range, it can request an atomic
      upgrade of the lock to a WRITE_LT lock via the LOCK operation by setting
      the type to WRITE_LT or WRITEW_LT.  If the server does not support
      atomic upgrade, it will return NFS4ERR_LOCK_NOTSUPP.  If the upgrade
      can be achieved without an existing conflict, the request will
      succeed.  Otherwise, the server will return either NFS4ERR_DENIED or
      NFS4ERR_DEADLOCK.  The error NFS4ERR_DEADLOCK is returned if the client
      sent the LOCK operation with the type set to WRITEW_LT and the server
      has detected a deadlock. The client should be prepared to receive such
      errors and, if appropriate, report the error to the requesting
      application.

        </t>
      </section>
      <!-- "Upgrading and Downgrading Locks" -->
  <section anchor="byte_range_seqid" numbered="true" toc="default">
        <name>Stateid Seqid Values and Byte-Range Locks</name>
        <t>
      When a LOCK or LOCKU operation is performed,
      the stateid returned has the same "other" value as the argument's
      stateid, and a
      "seqid" value that is incremented (relative to the argument's
      stateid) to reflect the occurrence
      of the LOCK or LOCKU operation.  The server MUST increment
      the value of the "seqid" field whenever there is any change
      to the locking status of any byte offset as described by
      any of the locks covered by the stateid.  A change in locking
      status includes a change from locked to unlocked or the reverse or
      a change from being locked for READ_LT to being locked for WRITE_LT
      or the reverse.
        </t>
        <t>
      When there is no such change, as, for example, when a range
      already locked for WRITE_LT is locked again for WRITE_LT, the
      server MAY increment the "seqid" value.
        </t>
      </section>
      <!-- "Stateid Sequence Values and Byte-Range Locks" -->
  <section anchor="multiple_openowners" numbered="true" toc="default">
        <name>Issues with Multiple Open-Owners</name>
        <t>

      When the same file is opened by multiple open-owners,
      a client will have multiple OPEN stateids for that
      file, each associated with a different open-owner.
      In that case, there can be multiple LOCK and LOCKU
      requests for the same lock-owner sent using the
      different OPEN stateids, and so a situation may
      arise in which there are multiple stateids, each
      representing byte-range locks on the same file and
      held by the same lock-owner but each associated with
      a different open-owner.

        </t>
        <t>
      In such a situation, the locking status of each byte
      (i.e., whether it is locked, the READ_LT or WRITE_LT type of
      the lock, and the lock-owner holding the lock) MUST
      reflect the last LOCK or LOCKU operation done for the
      lock-owner in question, independent of the stateid through
      which the request was sent.
        </t>
        <t>
      When a byte is locked by the lock-owner in question, the
      open-owner to which that byte-range lock is assigned SHOULD be that
      of the open-owner associated with the stateid through
      which the last LOCK of that byte was done.  When there
      is a change in the open-owner associated with locks for
      the stateid through which a LOCK or LOCKU was done, the
      "seqid" field of the stateid MUST be incremented, even
      if the locking, in terms of lock-owners has not changed.
      When there is a change to the set of locked bytes associated
      with a different stateid for the same lock-owner, i.e.,
      associated with a different open-owner, the "seqid" value
      for that stateid MUST NOT be incremented.
        </t>
      </section>
      <!-- "Issues with Multiple Open-Owners" -->
  <section anchor="blocking_locks" numbered="true" toc="default">
        <name>Blocking Locks</name>
        <t>
      Some clients require the support of blocking locks.  While NFSv4.1
      provides a callback when a previously unavailable lock becomes
      available, this is an OPTIONAL feature and clients cannot
      depend on its presence.  Clients need to be prepared to continually
      poll for the lock.  This presents a fairness problem.  Two of
      the lock types, READW_LT and WRITEW_LT, are used to indicate to the
      server that the client is requesting a blocking lock.  When the
      callback is not used, the server should maintain an ordered
      list of pending blocking locks.  When the conflicting lock is
      released, the server may wait for the period of time equal to
      lease_time for the first waiting
      client to re-request the lock.  After the lease period expires, the
      next waiting client request is allowed the lock.  Clients are required
      to poll at an interval sufficiently small that it is likely to acquire
      the lock in a timely manner.  The server is not required to maintain a
      list of pending blocked locks as it is used to increase fairness and
      not correct operation.  Because of the unordered nature of crash
      recovery, storing of lock state to stable storage would be required to
      guarantee ordered granting of blocking locks.
        </t>
        <t>
      Servers may also note the lock types and delay returning denial of the
      request to allow extra time for a conflicting lock to be released,
      allowing a successful return.  In this way, clients can avoid the
      burden of needless frequent polling for blocking locks.  The server
      should take care in the length of delay in the event the client
      retransmits the request.
        </t>
        <t>
      If a server receives a blocking LOCK operation, denies it, and then
      later receives a nonblocking request for the same lock, which is
      also denied, then it should remove the lock in question from its list of
      pending blocking locks.  Clients should use such a nonblocking request
      to indicate to the server that this is the last time they intend to poll
      for the lock, as may happen when the process requesting the lock is
      interrupted.  This is a courtesy to the server, to prevent it from
      unnecessarily waiting a lease period before granting other LOCK operations.
      However, clients are not required to perform this courtesy, and servers
      must not depend on them doing so.  Also, clients must be prepared for
      the possibility that this final locking request will be accepted.
        </t>
        <t>
      When a server indicates, via the flag OPEN4_RESULT_MAY_NOTIFY_LOCK, that
      CB_NOTIFY_LOCK callbacks might be done for the current open file, the
      client should take notice of this, but, since this is a hint, cannot
      rely on a CB_NOTIFY_LOCK always being done.  A client may reasonably
      reduce the frequency with which it polls for a denied lock, since the
      greater latency that might occur is likely to be eliminated given a
      prompt callback, but it still needs to poll.  When it receives a
      CB_NOTIFY_LOCK, it should promptly try to obtain the lock, but it
      should be aware that other clients may be polling and that the server is under
      no obligation to reserve the lock for that particular client.
        </t>
      </section>
      <!-- title="Blocking Locks" -->
  <section anchor="share_reserve" numbered="true" toc="default">
        <name>Share Reservations</name>
        <t>
      A share reservation is a mechanism to control access to a file.  It is
      a separate and independent mechanism from byte-range locking.  When a
      client opens a file, it sends an OPEN operation to the server
      specifying the type of access required (READ, WRITE, or BOTH) and the
      type of access to deny others (OPEN4_SHARE_DENY_NONE,
      OPEN4_SHARE_DENY_READ, OPEN4_SHARE_DENY_WRITE, or OPEN4_SHARE_DENY_BOTH).  If
      the OPEN fails, the client will fail the application's open request.
        </t>
        <t>
      Pseudo-code definition of the semantics:
        </t>
        <artwork name="" type="" align="left" alt=""><![CDATA[
        if (request.access == 0) {
          return (NFS4ERR_INVAL)
        } else {
          if ((request.access & file_state.deny)) ||
             (request.deny & file_state.access)) {
            return (NFS4ERR_SHARE_DENIED)
        }
        return (NFS4ERR_OK);
      ]]></artwork>
        <t>
      When doing this checking of share reservations on OPEN, the current
      file_state used in the algorithm includes bits that reflect all
      current opens, including those for the open-owner making the
      new OPEN request.
        </t>
        <t>
      The constants used for the OPEN and OPEN_DOWNGRADE operations for the
      access and deny fields are as follows:
        </t>
        <artwork name="" type="" align="left" alt=""><![CDATA[
const OPEN4_SHARE_ACCESS_READ   = 0x00000001;
const OPEN4_SHARE_ACCESS_WRITE  = 0x00000002;
const OPEN4_SHARE_ACCESS_BOTH   = 0x00000003;

const OPEN4_SHARE_DENY_NONE     = 0x00000000;
const OPEN4_SHARE_DENY_READ     = 0x00000001;
const OPEN4_SHARE_DENY_WRITE    = 0x00000002;
const OPEN4_SHARE_DENY_BOTH     = 0x00000003;
 ]]></artwork>
      </section>
      <!-- "Share Reservations" -->
  <section numbered="true" toc="default">
        <name>OPEN/CLOSE Operations</name>
        <t>
      To provide correct share semantics, a client MUST use the OPEN
      operation to obtain the initial filehandle and indicate the desired
      access and what access, if any, to deny.  Even if the client intends to
      use a special stateid for anonymous state or READ bypass,
      it must still obtain the
      filehandle for the regular file with the OPEN operation so the
      appropriate share semantics can be applied.  Clients that do not
      have a deny mode built into their programming interfaces for opening
      a file should request a deny mode of
      OPEN4_SHARE_DENY_NONE.
        </t>
        <t>
      The OPEN operation with the CREATE flag also subsumes the CREATE
      operation for regular files as used in previous versions of the NFS
      protocol.  This allows a create with a share to be done atomically.
        </t>
        <t>
      The CLOSE operation removes all share reservations held by the
      open-owner on that file.  If byte-range locks are held, the client
      SHOULD release all locks before sending a CLOSE operation.  The server MAY free
      all outstanding locks on CLOSE, but some servers may not support the
      CLOSE of a file that still has byte-range locks held.  The server MUST
      return failure, NFS4ERR_LOCKS_HELD, if any locks would exist after the
      CLOSE.
        </t>
        <t>
      The LOOKUP operation will return a filehandle without establishing any
      lock state on the server.  Without a valid stateid, the server will
      assume that the client has the least access.  For example, if one
      client opened a file with OPEN4_SHARE_DENY_BOTH and another client
      accesses the file via a filehandle obtained through LOOKUP, the
      second client could only read the file using the special read
      bypass stateid. The second client could not WRITE the file
      at all because it would
      not have a valid stateid from OPEN and the special anonymous stateid would
      not be allowed access.
        </t>
      </section>
      <!-- "OPEN/CLOSE Operations" -->
  <section anchor="open_upgrade" numbered="true" toc="default">
        <name>Open Upgrade and Downgrade</name>
        <t>
      When an OPEN is done for a file and the open-owner for which the OPEN
      is being done already has the file open, the result is to upgrade the
      open file status maintained on the server to include the access and
      deny bits specified by the new OPEN as well as those for the existing
      OPEN.  The result is that there is one open file, as far as the
      protocol is concerned, and it includes the union of the access and
      deny bits for all of the OPEN requests completed.  The OPEN
      is represented by a single stateid whose "other" value matches
      that of the original open, and whose "seqid" value is incremented
      to reflect the occurrence of the upgrade.  The increment is required
      in cases in which the "upgrade" results in no change to the open mode (e.g., an OPEN
      is done for read when the existing open file is opened for
      OPEN4_SHARE_ACCESS_BOTH).  Only a single CLOSE will be done to reset the
      effects of both OPENs.  The client may use the stateid returned
      by the OPEN effecting the upgrade or with a stateid sharing the
      same "other" field and a seqid of zero,
      although care needs to be taken as far as upgrades that happen
      while the CLOSE is pending.  Note that the
      client, when sending the OPEN, may not know that the same file is in
      fact being opened.  The above only applies if both OPENs result in
      the OPENed object being designated by the same filehandle.
        </t>
        <t>
      When the server chooses to export multiple filehandles corresponding
      to the same file object and returns different filehandles on two
      different OPENs of the same file object, the server MUST NOT "OR"
      together the access and deny bits and coalesce the two open files.
      Instead, the server must maintain separate OPENs with separate
      stateids and will require separate CLOSEs to free them.
        </t>
        <t>
      When multiple open files on the client are merged into a single OPEN
      file object on the server, the close of one of the open files (on the
      client) may necessitate change of the access and deny status of the
      open file on the server.  This is because the union of the access and
      deny bits for the remaining opens may be smaller (i.e., a proper
      subset) than previously.  The OPEN_DOWNGRADE operation is used to make
      the necessary change and the client should use it to update the server
      so that share reservation requests by other clients are handled
      properly.  The stateid returned has the same "other" field as
      that passed to the server.  The "seqid" value in the returned
      stateid MUST be incremented, even in situations in which there is
      no change to the access and deny bits for the file.
        </t>
      </section>
      <!-- "Open Upgrade and Downgrade" -->
  <section anchor="parallel_opens" numbered="true" toc="default">
        <name>Parallel OPENs</name>
        <t>
      Unlike the case of NFSv4.0, in which OPEN operations for the same
      open-owner are inherently serialized because of the owner-based seqid,
      multiple OPENs for the same open-owner may be done in parallel.  When
      clients do this, they may encounter situations in which, because
      of the existence of hard links, two OPEN operations may turn out
      to open the same file, with a later OPEN performed being an upgrade of
      the first, with this fact only visible to the
      client once the operations complete.
        </t>
        <t>
      In this situation, clients may determine the order in which the
      OPENs were performed by examining the stateids returned by the OPENs.
      Stateids that share a common value of the "other" field can be
      recognized as having opened the same file, with the order of the
      operations determinable from the order of the "seqid" fields, mod
      any possible wraparound of the 32-bit field.
        </t>
        <t>
      When the possibility exists that the client will send multiple
      OPENs for the same open-owner in parallel, it may be the case that
      an open upgrade may happen without the client knowing beforehand
      that this could happen.  Because of this possibility, CLOSEs and
      OPEN_DOWNGRADEs should generally be sent with a non-zero seqid
      in the stateid, to avoid the possibility that the status change
      associated with an open upgrade is not inadvertently lost.
        </t>
      </section>
      <!-- "Parallel OPENs" -->
  <section anchor="open_br_reclaim" numbered="true" toc="default">
        <name>Reclaim of Open and Byte-Range Locks</name>
        <t>
      Special forms of the LOCK and OPEN operations are provided when it
      is necessary to re-establish byte-range locks or opens after a
      server failure.
        </t>
        <ul spacing="normal">
          <li>
          To reclaim existing opens, an OPEN operation is performed
          using a CLAIM_PREVIOUS.  Because the client, in this type
          of situation, will have already opened the file and have
          the filehandle of the target file, this operation requires
          that the current filehandle be the target file, rather than
          a directory, and no file name is specified.
        </li>
          <li>
          To reclaim byte-range locks, a LOCK operation with the
          reclaim parameter set to true is used.
        </li>
        </ul>
        <t>
      Reclaims of opens associated with delegations are discussed in
      <xref target="delegation_recovery" format="default"/>.
        </t>
      </section>
    </section>
    <!-- "File Locking and Share Reservations" -->
<!-- 	$Id: 2009-12-20-TO-rfc5661.xml,v 1.2 2009/12/21 05:59:32 shepler.mre Exp $	 -->
<!-- Copyright (C) The IETF Trust (2007-2008) -->
<!-- Copyright (C) The Internet Society (2006) -->
<section numbered="true" toc="default">
      <name>Client-Side Caching</name>
      <t>
    Client-side caching of data, of file attributes, and of file names is
    essential to providing good performance with the NFS protocol.
    Providing distributed cache coherence is a difficult problem, and
    previous versions of the NFS protocol have not attempted it.  Instead,
    several NFS client implementation techniques have been used to reduce
    the problems that a lack of coherence poses for users.  These
    techniques have not been clearly defined by earlier protocol
    specifications, and it is often unclear what is valid or invalid client
    behavior.
      </t>
      <t>
    The NFSv4.1 protocol uses many techniques similar to those that
    have been used in previous protocol versions.  The NFSv4.1
    protocol does not provide distributed cache coherence.  However, it
    defines a more limited set of caching guarantees to allow locks and
    share reservations to be used without destructive interference from
    client-side caching.
      </t>
      <t>
    In addition, the NFSv4.1 protocol introduces a delegation
    mechanism, which allows many decisions normally made by the server to
    be made locally by clients.  This mechanism provides efficient support
    of the common cases where sharing is infrequent or where sharing is
    read-only.

      </t>
      <section numbered="true" toc="default">
        <name>Performance Challenges for Client-Side Caching</name>
        <t>
      Caching techniques used in previous versions of the NFS protocol have
      been successful in providing good performance.  However, several
      scalability challenges can arise when those techniques are used with
      very large numbers of clients.  This is particularly true when clients
      are geographically distributed, which classically increases the latency
      for cache revalidation requests.
        </t>
        <t>
      The previous versions of the NFS protocol repeat their file data cache
      validation requests at the time the file is opened.  This behavior can
      have serious performance drawbacks.  A common case is one in which a
      file is only accessed by a single client.  Therefore, sharing is
      infrequent.
        </t>
        <t>
      In this case, repeated references to the server to find that no
      conflicts exist are expensive.  A better option with regards to
      performance is to allow a client that repeatedly opens a file to do so
      without reference to the server.  This is done until potentially
      conflicting operations from another client actually occur.
        </t>
        <t>
      A similar situation arises in connection with byte-range locking.  Sending
      LOCK and LOCKU operations as well as the READ and
      WRITE operations necessary to make data caching consistent with the
      locking semantics (see <xref target="dc_file_locking" format="default"/>)
      can severely limit performance.  When locking is used to provide
      protection against infrequent conflicts, a large penalty is incurred.
      This penalty may discourage the use of byte-range locking by applications.
        </t>
        <t>
      The NFSv4.1 protocol provides more aggressive caching strategies
      with the following design goals:
        </t>
        <ul spacing="normal">
          <li>
          Compatibility with a large range of server semantics.
        </li>
          <li>
          Providing the same caching benefits as previous versions of
          the NFS protocol when unable to support the more aggressive model.
        </li>
          <li>
          Requirements for aggressive caching are organized so that a
         large portion of the benefit can be obtained even when not
         all of the requirements can be met.
        </li>
        </ul>
        <t>
      The appropriate requirements for the server are discussed in later
      sections in which specific forms of caching are covered (see
      <xref target="open_delegation" format="default"/>).
        </t>
      </section>
      <section anchor="deleg_and_cb" numbered="true" toc="default">
        <name>Delegation and Callbacks</name>
        <t>
      Recallable delegation of server responsibilities for a file to a
      client improves performance by avoiding repeated requests to the
      server in the absence of inter-client conflict.  With the use of a
      "callback" RPC from server to client, a server recalls delegated
      responsibilities when another client engages in sharing of a delegated
      file.
        </t>
        <t>
      A delegation is passed from the server to the client, specifying the
      object of the delegation and the type of delegation.  There are
      different types of delegations, but each type contains a stateid to be
      used to represent the delegation when performing operations that
      depend on the delegation.  This stateid is similar to those associated
      with locks and share reservations but differs in that the stateid for
      a delegation is associated with a client ID and may be used on behalf
      of all the open-owners for the given client.  A delegation is made
      to the client as a whole and not to any specific process or thread of
      control within it.
        </t>
        <t>
      The backchannel is established by CREATE_SESSION and
      BIND_CONN_TO_SESSION, and the client is required
      to maintain it. Because the backchannel may be down, even
      temporarily,
      correct protocol operation does not depend on
      them.  Preliminary testing of backchannel functionality by means of a
      CB_COMPOUND procedure with a single operation, CB_SEQUENCE,
      can be used to check the continuity of the backchannel.  A
      server avoids delegating responsibilities until it has
      determined that the backchannel exists.  Because the granting of a
      delegation is always conditional upon the absence of conflicting
      access, clients MUST NOT assume that a delegation will be granted and
      they MUST always be prepared for OPENs, WANT_DELEGATIONs, and
      GET_DIR_DELEGATIONs to be processed without any
      delegations being granted.
        </t>
        <t>
      Unlike locks, an operation by a second client to a delegated file will
      cause the server to recall a delegation through a callback.  For
      individual operations, we will describe, under IMPLEMENTATION, when
      such operations are required to effect a recall.  A number of
      points should be noted, however.
        </t>
        <ul spacing="normal">
          <li>
          The server is free to recall a delegation
          whenever it feels it is desirable and may do so even if no
          operations requiring recall are being done.
        </li>
          <li>
          Operations done outside the NFSv4.1 protocol, due to, for
          example, access by other protocols, or by local access,
          also need to result in delegation recall when they make
          analogous changes to file system data.  What is crucial
          is if the change would invalidate the guarantees provided
          by the delegation.  When this is possible, the
          delegation needs to be recalled and MUST be returned or
          revoked  before allowing the operation to proceed.
        </li>
          <li>
          The semantics of the file system are crucial in defining
          when delegation recall is required.  If a particular change
          within a specific implementation causes change to a
          file attribute, then delegation recall is required, whether
          that operation has been specifically listed as requiring
          delegation recall.  Again, what is critical is whether the
          guarantees provided by the delegation are being invalidated.
        </li>
        </ul>
        <t>
      Despite those caveats, the implementation sections for a number
      of operations describe situations in which delegation recall
      would be required under some common circumstances:
        </t>
        <ul spacing="normal">
          <li>
          For GETATTR, see <xref target="OP_GETATTR_IMPLEMENTATION" format="default"/>.
        </li>
          <li>
          For OPEN, see <xref target="OP_OPEN_IMPLEMENTATION" format="default"/>.
        </li>
          <li>
          For READ, see <xref target="OP_READ_IMPLEMENTATION" format="default"/>.
        </li>
          <li>
          For REMOVE, see <xref target="OP_REMOVE_IMPLEMENTATION" format="default"/>.
        </li>
          <li>
          For RENAME, see <xref target="OP_RENAME_IMPLEMENTATION" format="default"/>.
        </li>
          <li>
          For SETATTR, see <xref target="OP_SETATTR_IMPLEMENTATION" format="default"/>.
        </li>
          <li>
          For WRITE, see <xref target="OP_WRITE_IMPLEMENTATION" format="default"/>.
        </li>
        </ul>
        <t>
      On recall, the client holding the delegation needs to flush modified
      state (such as modified data) to the server and return the
      delegation.  The conflicting request will not be acted on until
      the recall is complete.  The recall is considered complete when
      the client returns the delegation or the server times its wait
      for the delegation to be returned and revokes the delegation as
      a result of the timeout.  In the interim, the server will either
      delay responding to conflicting requests or respond to them with
      NFS4ERR_DELAY.  Following the resolution of the recall, the
      server has the information necessary to grant or deny the second
      client's request.
        </t>
        <t>
      At the time the client receives a delegation recall, it may have
      substantial state that needs to be flushed to the server.  Therefore,
      the server should allow sufficient time for the delegation to be
      returned since it may involve numerous RPCs to the server.  If the
      server is able to determine that the client is diligently flushing
      state to the server as a result of the recall, the server may extend
      the usual time allowed for a recall.  However, the time allowed for
      recall completion should not be unbounded.
        </t>
        <t>
      An example of this is when responsibility to mediate opens on a given
      file is delegated to a client (see <xref target="open_delegation" format="default"/>).
      The server will not know what opens are in effect on the client.
      Without this knowledge, the server will be unable to determine if the
      access and deny states for the file allow any particular open until
      the delegation for the file has been returned.
        </t>
        <t>
      A client failure or a network partition can result in failure to
      respond to a recall callback. In this case, the server will revoke the
      delegation, which in turn will render useless any modified state still
      on the client.
        </t>
        <section anchor="delegation_recovery" numbered="true" toc="default">
          <name>Delegation Recovery</name>
          <t>
        There are three situations that delegation recovery needs to deal with:
          </t>
          <ul spacing="normal">
            <li>
            client restart
          </li>
            <li>
            server restart
          </li>
            <li>
            network partition (full or backchannel-only)
          </li>
          </ul>
          <t>
        In the event the client restarts, the failure to renew
        the lease will result in the revocation of byte-range locks and share
        reservations.  Delegations, however, may be treated a bit differently.
          </t>
          <t>
        There will be situations in which delegations will need to be
        re-established after a client restarts.  The reason for this
        is that the client may have file data stored locally and this data was
        associated with the previously held delegations.  The client will need
        to re-establish the appropriate file state on the server.
          </t>
          <t>
        To allow for this type of client recovery, the server MAY extend the
        period for delegation recovery beyond the typical lease expiration
        period.  This implies that requests from other clients that conflict
        with these delegations will need to wait.  Because the normal recall
        process may require significant time for the client to flush changed
        state to the server, other clients need be prepared for delays that
        occur because of a conflicting delegation.  This longer interval would
        increase the window for clients to restart and consult stable storage
        so that the delegations can be reclaimed.  For OPEN delegations, such
        delegations are reclaimed using OPEN with a claim type of
        CLAIM_DELEGATE_PREV or CLAIM_DELEG_PREV_FH (see Sections
        <xref target="data_caching_revocation" format="counter"/>
        and <xref target="OP_OPEN" format="counter"/> for discussion of OPEN delegation
        and the details of OPEN, respectively).
          </t>
          <t>
        A server MAY support claim types of CLAIM_DELEGATE_PREV and
        CLAIM_DELEG_PREV_FH, and if it
        does, it MUST NOT remove delegations upon a CREATE_SESSION that
        confirm a client ID created by EXCHANGE_ID.
        Instead, the server MUST, for a period of time no less than that of the value of
        the lease_time attribute, maintain the client's delegations to allow
        time for the client to send CLAIM_DELEGATE_PREV and/or CLAIM_DELEG_PREV_FH requests. The server
        that supports CLAIM_DELEGATE_PREV and/or CLAIM_DELEG_PREV_FH MUST support the DELEGPURGE
        operation.
          </t>
          <t>
        When the server restarts, delegations are reclaimed (using
        the OPEN operation with CLAIM_PREVIOUS) in a similar fashion to byte-range
        locks and share reservations.  However, there is a slight semantic
        difference.  In the normal case, if the server decides that a
        delegation should not be granted, it performs the requested action
        (e.g., OPEN) without granting any delegation.  For reclaim, the server
        grants the delegation but a special designation is applied so that the
        client treats the delegation as having been granted but recalled by
        the server.  Because of this, the client has the duty to write all
        modified state to the server and then return the delegation.  This
        process of handling delegation reclaim reconciles three principles of
        the NFSv4.1 protocol:
          </t>
          <ul spacing="normal">
            <li>
            Upon reclaim, a client reporting resources assigned to it by an
            earlier server instance must be granted those resources.
          </li>
            <li>
            The server has unquestionable authority to determine whether
            delegations are to be granted and, once granted, whether they are to
            be continued.
          </li>
            <li>
            The use of callbacks should not be depended upon until the client has
            proven its ability to receive them.
          </li>
          </ul>
          <t>
        When a client needs to reclaim a delegation and there is no associated
        open, the client may use the CLAIM_PREVIOUS variant of the
        WANT_DELEGATION operation.  However, since the server is not required
        to support this operation, an alternative is to reclaim via a dummy OPEN
        together with the delegation
        using an OPEN of type CLAIM_PREVIOUS.  The dummy open file can
        be released using a CLOSE to re-establish the original state to be
        reclaimed, a delegation without an associated open.
          </t>
          <t>
        When a client has more than a single open associated with a delegation,
        state for those additional opens can be established using OPEN
        operations of type CLAIM_DELEGATE_CUR.  When these are used to
        establish opens associated with reclaimed delegations, the
        server MUST allow them when made within the grace period.
          </t>
          <t>
        When a network partition occurs, delegations are subject to freeing by
        the server when the lease renewal period expires.  This is similar to
        the behavior for locks and share reservations.  For delegations,
        however, the server may extend the period in which conflicting
        requests are held off.  Eventually, the occurrence of a conflicting
        request from another client will cause revocation of the delegation.
        A loss of the backchannel (e.g., by later network configuration
        change) will have the same effect.  A recall request will fail and
        revocation of the delegation will result.
          </t>
          <t>
        A client normally finds out about revocation of a delegation when it
        uses a stateid associated with a delegation and receives one of the
        errors NFS4ERR_EXPIRED, NFS4ERR_ADMIN_REVOKED, or NFS4ERR_DELEG_REVOKED.
        It also may find out about delegation revocation
        after a client restart when it attempts to reclaim a delegation and
        receives that same error.  Note that in the case of a revoked OPEN_DELEGATE_WRITE delegation, there are issues because data may have been modified
        by the client whose delegation is revoked and separately by other
        clients.  See <xref target="revocation_recovery_write" format="default"/>
        for a discussion of such issues.  Note also that when
        delegations are revoked, information about the revoked delegation will
        be written by the server to stable storage (as described in
        <xref target="network_partitions_and_recovery" format="default"/>).  This is done
        to deal with the case in
        which a server restarts after revoking a delegation but before the
        client holding the revoked delegation is notified about the
        revocation.
          </t>
        </section>
      </section>
      <section numbered="true" toc="default">
        <name>Data Caching</name>
        <t>
      When applications share access to a set of files, they need to be
      implemented so as to take account of the possibility of conflicting
      access by another application.  This is true whether the applications
      in question execute on different clients or reside on the same client.
        </t>
        <t>
      Share reservations and byte-range locks are the facilities the NFSv4.1 protocol
      provides to allow applications to coordinate access by
      using  mutual exclusion facilities.  The NFSv4.1 protocol's
      data caching must be implemented such that it does not invalidate the
      assumptions on which those using these facilities depend.

        </t>
        <section numbered="true" toc="default">
          <name>Data Caching and OPENs</name>
          <t>
        In order to avoid invalidating the sharing assumptions on which
        applications rely, NFSv4.1 clients should not provide cached
        data to applications or modify it on behalf of an application when it
        would not be valid to obtain or modify that same data via a READ or
        WRITE operation.
          </t>
          <t>
        Furthermore, in the absence of an OPEN delegation
        (see <xref target="open_delegation" format="default"/>),
        two additional rules apply.  Note that these rules are
        obeyed in practice by many NFSv3 clients.
          </t>
          <ul spacing="normal">
            <li>
              <t>
            First, cached data present on a client must be revalidated after doing
            an OPEN. Revalidating means that the client fetches the change
            attribute from the server, compares it with the cached change
            attribute, and if different, declares the cached data (as well as the
            cached attributes) as invalid.  This is to ensure that the data for
            the OPENed file is still correctly reflected in the client's cache.
            This validation must be done at least when the client's OPEN operation
            includes a deny of OPEN4_SHARE_DENY_WRITE or
            OPEN4_SHARE_DENY_BOTH, thus terminating a period in which
            other
            clients may have had the opportunity to open the file with
            OPEN4_SHARE_ACCESS_WRITE/OPEN4_SHARE_ACCESS_BOTH
            access.  Clients may choose to do the revalidation more often (i.e., at
            OPENs specifying a deny mode of OPEN4_SHARE_DENY_NONE) to parallel the NFSv3 protocol's
            practice for the benefit of users assuming this degree of cache
            revalidation.
              </t>
              <t>

            Since the change attribute is updated for data and metadata
            modifications, some client implementors may be tempted to use the
            time_modify attribute and not the change attribute to validate cached data, so that
            metadata changes do not spuriously invalidate clean data.  The
            implementor is cautioned in  this approach. The change attribute is
            guaranteed to change for each update to the file, whereas time_modify
            is guaranteed to change only at the granularity of the time_delta
            attribute. Use by the client's data cache validation logic of
            time_modify and not change runs the risk of the client incorrectly
            marking stale data as valid. Thus, any cache validation approach
            by the client MUST include the use of the change attribute.
              </t>
            </li>
            <li>
            Second, modified data must be flushed to the server before closing a
            file OPENed for OPEN4_SHARE_ACCESS_WRITE.  This is complementary to the first rule.  If
            the data is not flushed at CLOSE, the revalidation done
            after the client OPENs a file is unable to achieve its
            purpose.  The other aspect to flushing the data before
            close is that the data must be committed to stable
            storage, at the server, before the CLOSE operation is
            requested by the client.  In the case of a server restart and a CLOSEd
            file, it may not be possible to retransmit the data to be written to
            the file, hence, this requirement.
          </li>
          </ul>
        </section>
        <section anchor="dc_file_locking" numbered="true" toc="default">
          <name>Data Caching and File Locking</name>
          <t>
        For those applications that choose to use byte-range locking instead of
        share reservations to exclude inconsistent file access, there is an
        analogous set of constraints that apply to client-side data caching.
        These rules are effective only if the byte-range locking is used in a way
        that matches in an equivalent way the actual READ and WRITE operations
        executed.  This is as opposed to byte-range locking that is based on pure
        convention.  For example, it is possible to manipulate a two-megabyte
        file by dividing the file into two one-megabyte ranges and protecting
        access to the two byte-ranges by byte-range locks on bytes zero and one.  A WRITE_LT lock on
        byte zero of the file would represent the right to perform
        READ and WRITE operations on the first byte-range.  A WRITE_LT lock on
        byte one of the file would represent the right to perform READ and WRITE
        operations on the second byte-range.  As long as all applications
        manipulating the file obey this convention, they will work on a local
        file system.  However, they may not work with the NFSv4.1
        protocol unless clients refrain from data caching.
          </t>
          <t>
        The rules for data caching in the byte-range locking environment are:
          </t>
          <ul spacing="normal">
            <li>
            First, when a client obtains a byte-range lock for a particular byte-range, the
            data cache corresponding to that byte-range (if any cache data exists)
            must be revalidated.  If the change attribute indicates that the file
            may have been updated since the cached data was obtained, the client
            must flush or invalidate the cached data for the newly locked byte-range.
            A client might choose to invalidate all of the non-modified cached data
            that it has for the file, but the only requirement for correct
            operation is to invalidate all of the data in the newly locked byte-range.
          </li>
            <li>
            Second, before releasing a WRITE_LT lock for a byte-range, all modified data
            for that byte-range must be flushed to the server.  The modified data must
            also be written to stable storage.
          </li>
          </ul>
          <t>
        Note that flushing data to the server and the invalidation of cached
        data must reflect the actual byte-ranges locked or unlocked.  Rounding
        these up or down to reflect client cache block boundaries will cause
        problems if not carefully done.  For example, writing a modified block
        when only half of that block is within an area being unlocked may
        cause invalid modification to the byte-range outside the unlocked area.
        This, in turn, may be part of a byte-range locked by another client.
        Clients can avoid this situation by synchronously performing portions
        of WRITE operations that overlap that portion (initial or final) that
        is not a full block.  Similarly, invalidating a locked area that is
        not an integral number of full buffer blocks would require the client
        to read one or two partial blocks from the server if the revalidation
        procedure shows that the data that the client possesses may not be
        valid.
          </t>
          <t>
        The data that is written to the server as a prerequisite to the
        unlocking of a byte-range must be written, at the server, to stable
        storage.  The client may accomplish this either with synchronous
        writes or by following asynchronous writes with a COMMIT operation.
        This is required because retransmission of the modified data after a
        server restart might conflict with a lock held by another client.
          </t>
          <t>
        A client implementation may choose to accommodate applications that
        use byte-range locking in non-standard ways (e.g., using a byte-range lock as a
        global semaphore) by flushing to the server more data upon a LOCKU
        than is covered by the locked range.  This may include modified data
        within files other than the one for which the unlocks are being done.
        In such cases, the client must not interfere with applications whose
        READs and WRITEs are being done only within the bounds of byte-range locks
        that the application holds.  For example, an application locks a
        single byte of a file and proceeds to write that single byte.  A
        client that chose to handle a LOCKU by flushing all modified data to
        the server could validly write that single byte in response to an
        unrelated LOCKU operation.  However, it would not be valid to write the entire
        block in which that single written byte was located since it includes
        an area that is not locked and might be locked by another client.
        Client implementations can avoid this problem by dividing files with
        modified data into those for which all modifications are done to areas
        covered by an appropriate byte-range lock and those for which there are
        modifications not covered by a byte-range lock.  Any writes done for the
        former class of files must not include areas not locked and thus not
        modified on the client.

          </t>
        </section>
        <section numbered="true" toc="default">
          <name>Data Caching and Mandatory File Locking</name>
          <t>
        Client-side data caching needs to respect mandatory byte-range locking when
        it is in effect.  The presence of mandatory byte-range locking for a given
        file is indicated when the client gets back NFS4ERR_LOCKED from a READ
        or WRITE operation on a file for which it has an appropriate share reservation.  When
        mandatory locking is in effect for a file, the client must check for
        an appropriate byte-range lock for data being read or written.  If a byte-range lock
        exists for the range being read or written, the client may satisfy the
        request using the client's validated cache.  If an appropriate
        byte-range lock is not held for the range of the read or write, the read or write
        request must not be satisfied by the client's cache and the request
        must be sent to the server for processing.  When a read or write
        request partially overlaps a locked byte-range, the request should be
        subdivided into multiple pieces with each byte-range (locked or not)
        treated appropriately.

          </t>
        </section>
        <section anchor="data_caching_and_file_identity" numbered="true" toc="default">
          <name>Data Caching and File Identity</name>
          <t>
        When clients cache data, the file data needs to be organized according
        to the file system object to which the data belongs.  For NFSv3
        clients, the typical practice has been to assume for the purpose of
        caching that distinct filehandles represent distinct file system
        objects.  The client then has the choice to organize and maintain the
        data cache on this basis.
          </t>
          <t>
        In the NFSv4.1 protocol, there is now the possibility to have
        significant deviations from a "one filehandle per object" model
        because a filehandle may be constructed on the basis of the object's
        pathname.  Therefore, clients need a reliable method to determine if
        two filehandles designate the same file system object.  If clients
        were simply to assume that all distinct filehandles denote distinct
        objects and proceed to do data caching on this basis, caching
        inconsistencies would arise between the distinct client-side objects
        that mapped to the same server-side object.
          </t>
          <t>
        By providing a method to differentiate filehandles, the NFSv4.1
        protocol alleviates a potential functional regression in comparison
        with the NFSv3 protocol.  Without this method, caching
        inconsistencies within the same client could occur, and this has not
        been present in previous versions of the NFS protocol.  Note that it
        is possible to have such inconsistencies with applications executing
        on multiple clients, but that is not the issue being addressed here.
          </t>
          <t>
        For the purposes of data caching, the following steps allow an
        NFSv4.1 client to determine whether two distinct filehandles denote
        the same server-side object:
          </t>
          <ul spacing="normal">
            <li>
            If GETATTR directed to two filehandles returns different values of the
            fsid attribute, then the filehandles represent distinct objects.
          </li>
            <li>
            If GETATTR for any file with an fsid that matches the fsid of the two
            filehandles in question returns a unique_handles attribute with a
            value of TRUE, then the two objects are distinct.
          </li>
            <li>
            If GETATTR directed to the two filehandles does not return the fileid
            attribute for both of the handles, then it cannot be determined
            whether the two objects are the same.  Therefore,
            operations that depend on that knowledge (e.g.,
            client-side data caching) cannot be
            done reliably.  Note that if GETATTR does not return the fileid
	    attribute for both filehandles, it will return it for neither of
	    the filehandles, since the fsid for both filehandles is the same.
          </li>
            <li>
            If GETATTR directed to the two filehandles returns different values
            for the fileid attribute, then they are distinct objects.
          </li>
            <li>
            Otherwise, they are the same object.
          </li>
          </ul>
        </section>
      </section>
      <section anchor="open_delegation" numbered="true" toc="default">
        <name>Open Delegation</name>
        <t>
      When a file is being OPENed, the server may delegate further handling
      of opens and closes for that file to the opening client.  Any such
      delegation is recallable since the circumstances that allowed for the
      delegation are subject to change.  In particular, if the server
      receives a conflicting OPEN from another client, the server must recall
      the delegation before deciding whether the OPEN from the other client
      may be granted.  Making a delegation is up to the server, and clients
      should not assume that any particular OPEN either will or will not
      result in an OPEN delegation.  The following is a typical set of
      conditions that servers might use in deciding whether an OPEN should be
      delegated:
        </t>
        <ul spacing="normal">
          <li>
          The client must be able to respond to the
          server's callback requests.  If a backchannel
          has been established, the server will send
          a CB_COMPOUND request, containing a single
          operation, CB_SEQUENCE, for a test of backchannel
          availability.

        </li>
          <li>
          The client must have responded properly to previous recalls.
        </li>
          <li>
          There must be no current OPEN conflicting with the requested
          delegation.
        </li>
          <li>
          There should be no current delegation that conflicts with the
          delegation being requested.
        </li>
          <li>
          The probability of future conflicting open requests should be
          low based on the recent history of the file.
        </li>
          <li>
          The existence of any server-specific semantics of OPEN/CLOSE
          that would make the required handling incompatible with the
          prescribed handling that the delegated client would apply
          (see below).
        </li>
        </ul>
        <t>
      There are two types of OPEN delegations: OPEN_DELEGATE_READ and OPEN_DELEGATE_WRITE.  An OPEN_DELEGATE_READ
      delegation allows a client to handle, on its own, requests to open a
      file for reading that do not deny OPEN4_SHARE_ACCESS_READ access to others.  Multiple
      OPEN_DELEGATE_READ delegations may be outstanding simultaneously and do not
      conflict.  An OPEN_DELEGATE_WRITE delegation allows the client to handle, on its
      own, all opens.  Only OPEN_DELEGATE_WRITE delegation may exist for a given
      file at a given time, and it is inconsistent with any OPEN_DELEGATE_READ delegations.
        </t>
        <t>
      When a client has an OPEN_DELEGATE_READ delegation, it is assured that
      neither the contents, the attributes (with the exception of
      time_access), nor the names of any
      links to the file will change without its knowledge, so long as the
      delegation is held.  When a client has an OPEN_DELEGATE_WRITE delegation, it
      may modify the file data locally since no other client will be
      accessing the file's data.  The client holding an OPEN_DELEGATE_WRITE delegation
      may only locally affect file attributes that are intimately
      connected with the file data: size, change, time_access,
      time_metadata, and time_modify.
      All other attributes must be reflected on the server.
        </t>
        <t>
      When a client has an OPEN delegation, it does not need to send OPENs or
      CLOSEs to the server. Instead, the client may update the
      appropriate status internally. For an OPEN_DELEGATE_READ delegation, opens
      that cannot be handled locally (opens that are for OPEN4_SHARE_ACCESS_WRITE/OPEN4_SHARE_ACCESS_BOTH or that
      deny OPEN4_SHARE_ACCESS_READ access) must be sent to the server.
        </t>
        <t>
      When an OPEN delegation is made, the reply to the OPEN contains an
      OPEN delegation structure that specifies the following:
        </t>
        <ul spacing="normal">
          <li>
          the type of delegation (OPEN_DELEGATE_READ or OPEN_DELEGATE_WRITE).
        </li>
          <li>
          space limitation information to control flushing of data on close
          (OPEN_DELEGATE_WRITE delegation only;
          see <xref target="open_delegation_caching" format="default"/>)
        </li>
          <li>
          an nfsace4 specifying read and write permissions
        </li>
          <li>
          a stateid to represent the delegation
        </li>
        </ul>
        <t>
      The delegation stateid is separate and distinct from the stateid for
      the OPEN proper.  The standard stateid, unlike the delegation stateid,
      is associated with a particular lock-owner and will continue to be
      valid after the delegation is recalled and the file remains open.
        </t>
        <t>
      When a request internal to the client is made to open a file and an OPEN
      delegation is in effect, it will be accepted or rejected solely on the
      basis of the following conditions.  Any requirement for other checks
      to be made by the delegate should result in the OPEN delegation being
      denied so that the checks can be made by the server itself.
        </t>
        <ul spacing="normal">
          <li>
          The access and deny bits for the request and the file as
          described in <xref target="share_reserve" format="default"/>.
        </li>
          <li>
          The read and write permissions as determined below.
        </li>
        </ul>
        <t>
      The nfsace4 passed with delegation can be used to avoid frequent
      ACCESS calls.  The permission check should be as follows:
        </t>
        <ul spacing="normal">
          <li>
          If the nfsace4 indicates that the open may be done, then it should be
          granted without reference to the server.
        </li>
          <li>
          If the nfsace4 indicates that the open may not be done, then an ACCESS
          request must be sent to the server to obtain the definitive answer.
        </li>
        </ul>
        <t>
      The server may return an nfsace4 that is more restrictive than the
      actual ACL of the file.  This includes an nfsace4 that specifies
      denial of all access.  Note that some common practices such as mapping
      the traditional user "root" to the user "nobody" (see <xref target="owner_owner_group" format="default"/>) may make it incorrect
      to return the actual ACL of the file in the delegation response.
        </t>
        <t>
      The use of a delegation together with various other forms of caching
      creates the possibility that no server authentication and authorization
      will ever be
      performed for a given user since all of the user's requests might be
      satisfied locally.  Where the client is depending on the server for
      authentication and authorization, the client should be sure authentication and authorization occurs for
      each user by use of the ACCESS operation.  This should be the case
      even if an ACCESS operation would not be required otherwise.  As
      mentioned before, the server may enforce frequent authentication by
      returning an nfsace4 denying all access with every OPEN delegation.

        </t>
        <section anchor="open_delegation_caching" numbered="true" toc="default">
          <name>Open Delegation and Data Caching</name>
          <t>
        An OPEN delegation allows much of the message overhead associated with
        the opening and closing files to be eliminated.  An open when an OPEN
        delegation is in effect does not require that a validation
        message be sent to the server.  The continued endurance of the
        "OPEN_DELEGATE_READ delegation" provides a guarantee that no OPEN
        for OPEN4_SHARE_ACCESS_WRITE/OPEN4_SHARE_ACCESS_BOTH, and thus
        no write, has occurred.  Similarly, when closing a file opened
        for OPEN4_SHARE_ACCESS_WRITE/OPEN4_SHARE_ACCESS_BOTH and if an OPEN_DELEGATE_WRITE delegation is in effect,
        the data written does not have to be written to the server until
        the OPEN delegation is recalled.  The continued endurance of
        the OPEN delegation provides a
        guarantee that no open, and thus no READ or WRITE, has been done by
        another client.
          </t>
          <t>
        For the purposes of OPEN delegation, READs and WRITEs done without an
        OPEN are treated as the functional equivalents of a corresponding type
        of OPEN.  Although a client SHOULD NOT use special stateids when
        an open exists, delegation handling on the server can use the
        client ID associated with the current session to determine if the
        operation has been done by the holder of the delegation (in which
        case, no recall is necessary) or by another client (in which case,
        the delegation must be recalled and I/O not proceed until the
        delegation is returned or revoked).
          </t>
          <t>
        With delegations, a client is able to avoid writing data to the server
        when the CLOSE of a file is serviced.  The file close system call is
        the usual point at which the client is notified of a lack of stable
        storage for the modified file data generated by the application.  At
        the close, file data is written to the server and, through normal
        accounting, the server is able to determine if the available file system
        space for the data has been exceeded (i.e., the server returns
        NFS4ERR_NOSPC or NFS4ERR_DQUOT).  This accounting includes quotas.
        The introduction of delegations requires that an alternative method be
        in place for the same type of communication to occur between client
        and server.
          </t>
          <t>
        In the delegation response, the server provides either the limit of
        the size of the file or the number of modified blocks and associated
        block size.  The server must ensure that the client will be able to
        write modified data to the server of a size equal to that provided in the
        original delegation.  The server must make this assurance for all
        outstanding delegations.  Therefore, the server must be careful in its
        management of available space for new or modified data, taking into
        account available file system space and any applicable quotas.  The
        server can recall delegations as a result of managing the available
        file system space.  The client should abide by the server's state
        space limits for delegations.  If the client exceeds the stated limits
        for the delegation, the server's behavior is undefined.
          </t>
          <t>
        Based on server conditions, quotas, or available file system space, the
        server may grant OPEN_DELEGATE_WRITE delegations with very restrictive space
        limitations.  The limitations may be defined in a way that will always
        force modified data to be flushed to the server on close.
          </t>
          <t>
        With respect to authentication, flushing modified data to the server
        after a CLOSE has occurred may be problematic.  For example, the user
        of the application may have logged off the client, and unexpired
        authentication credentials may not be present.  In this case, the
        client may need to take special care to ensure that local unexpired
        credentials will in fact be available.  This may be accomplished by
        tracking the expiration time of credentials and flushing data well in
        advance of their expiration or by making private copies of credentials
        to assure their availability when needed.

          </t>
        </section>
        <section numbered="true" toc="default">
          <name>Open Delegation and File Locks</name>
          <t>
        When a client holds an OPEN_DELEGATE_WRITE delegation, lock operations are
        performed locally.  This includes those required for mandatory byte-range
        locking.  This can be done since the delegation implies that there can
        be no conflicting locks.  Similarly, all of the revalidations that
        would normally be associated with obtaining locks and the flushing of
        data associated with the releasing of locks need not be done.
          </t>
          <t>
        When a client holds an OPEN_DELEGATE_READ delegation, lock operations are not
        performed locally.  All lock operations, including those requesting
        non-exclusive locks, are sent to the server for resolution.

          </t>
        </section>
        <section anchor="handling_cb_getattr" numbered="true" toc="default">
          <name>Handling of CB_GETATTR</name>
          <t>
        The server needs to employ special handling for a GETATTR where the
        target is a file that has an OPEN_DELEGATE_WRITE delegation in effect.  The
        reason for this is that the client holding the OPEN_DELEGATE_WRITE delegation may
        have modified the data, and the server needs to reflect this change to
        the second client that submitted the GETATTR.  Therefore, the client
        holding the OPEN_DELEGATE_WRITE delegation needs to be interrogated.  The server
        will use the CB_GETATTR operation.  The only attributes that the
        server can reliably query via CB_GETATTR are size and change.
          </t>
          <t>
        Since CB_GETATTR is being used to satisfy another client's GETATTR
        request, the server only needs to know if the client holding the
        delegation has a modified version of the file.  If the client's copy
        of the delegated file is not modified (data or size), the server can
        satisfy the second client's GETATTR request from the attributes stored
        locally at the server.  If the file is modified, the server only needs
        to know about this modified state.  If the server determines that the
        file is currently modified, it will respond to the second client's
        GETATTR as if the file had been modified locally at the server.
          </t>
          <t>
        Since the form of the change attribute is determined by the server and
        is opaque to the client, the client and server need to agree on a
        method of communicating the modified state of the file.  For the size
        attribute, the client will report its current view of the file size.
        For the change attribute, the handling is more involved.
          </t>
          <t>
        For the client, the following steps will be taken when receiving an
        OPEN_DELEGATE_WRITE delegation:
          </t>
          <ul spacing="normal">
            <li>
            The value of the change attribute will be obtained from the server and
            cached.  Let this value be represented by c.
          </li>
            <li>
            The client will create a value greater than c that will be used for
            communicating that modified data is held at the client.  Let this value be
            represented by d.
          </li>
            <li>
            When the client is queried via CB_GETATTR for the change attribute, it
            checks to see if it holds modified data.  If the file is modified, the
            value d is returned for the change attribute value.  If this file is
            not currently modified, the client returns the value c for the change
            attribute.
          </li>
          </ul>
          <t>
        For simplicity of implementation, the client MAY for each CB_GETATTR
        return the same value d.  This is true even if, between successive
        CB_GETATTR operations, the client again modifies the file's data or
        metadata in its cache.  The client can return the same value because
        the only requirement is that the client be able to indicate to the
        server that the client holds modified data.  Therefore, the value of d
        may always be c + 1.
          </t>
          <t>
        While the change attribute is opaque to the client in the sense that
        it has no idea what units of time, if any, the server is counting
        change with, it is not opaque in that the client has to treat it as an
        unsigned integer, and the server has to be able to see the results of
        the client's changes to that integer.  Therefore, the server MUST
        encode the change attribute in network order when sending it to the
        client.  The client MUST decode it from network order to its native
        order when receiving it, and the client MUST encode it in network order
        when sending it to the server.  For this reason, change is defined as
        an unsigned integer rather than an opaque array of bytes.
          </t>
          <t>
        For the server, the following steps will be taken when providing an
        OPEN_DELEGATE_WRITE delegation:
          </t>
          <ul spacing="normal">
            <li>
            Upon providing an OPEN_DELEGATE_WRITE delegation, the server will cache a copy of the
            change attribute in the data structure it uses to record the
            delegation.  Let this value be represented by sc.
          </li>
            <li>
            When a second client sends a GETATTR operation on the same file to the
            server, the server obtains the change attribute from the first client.
            Let this value be cc.
          </li>
            <li>
            If the value cc is equal to sc, the file is not modified and the
            server returns the current values for change, time_metadata, and
            time_modify (for example) to the second client.
          </li>
            <li>
            If the value cc is NOT equal to sc, the file is currently modified at
            the first client and most likely will be modified at the server at a
            future time.  The server then uses its current time to construct
            attribute values for time_metadata and time_modify.  A new value of
            sc, which we will call nsc, is computed by the server, such that nsc
            &gt;= sc + 1.  The server then returns the constructed time_metadata,
            time_modify, and nsc values to the requester.  The server replaces sc
            in the delegation record with nsc.  To prevent the possibility of
            time_modify, time_metadata, and change from appearing to go backward
            (which would happen if the client holding the delegation fails to
            write its modified data to the server before the delegation is revoked
            or returned), the server SHOULD update the file's metadata record with
            the constructed attribute values.  For reasons of reasonable
            performance, committing the constructed attribute values to stable
            storage is OPTIONAL.
          </li>
          </ul>
          <t>
        As discussed earlier in this section, the client MAY return the same
        cc value on subsequent CB_GETATTR calls, even if the file was modified
        in the client's cache yet again between successive CB_GETATTR calls.
        Therefore, the server must assume that the file has been modified yet
        again, and MUST take care to ensure that the new nsc it constructs and
        returns is greater than the previous nsc it returned.  An example
        implementation's delegation record would satisfy this mandate by
        including a boolean field (let us call it "modified") that is set to
        FALSE when the delegation is granted, and an sc value set at the time
        of grant to the change attribute value. The modified field would be
        set to TRUE the first time cc != sc, and would stay TRUE until the
        delegation is returned or revoked.  The processing for constructing
        nsc, time_modify, and time_metadata would use this pseudo code:
          </t>
          <artwork name="" type="" align="left" alt=""><![CDATA[
    if (!modified) {
        do CB_GETATTR for change and size;

        if (cc != sc)
            modified = TRUE;
    } else {
        do CB_GETATTR for size;
    }

    if (modified) {
        sc = sc + 1;
        time_modify = time_metadata = current_time;
        update sc, time_modify, time_metadata into file's metadata;
    }

	    ]]></artwork>
          <t>
	  This would return to the client (that sent GETATTR) the attributes
        it requested, but make sure size comes from what
        CB_GETATTR returned. The server would not update the file's
        metadata with the client's modified size.
          </t>
          <t>
        In the case that the file attribute size is different than the
        server's current value, the server treats this as a modification
        regardless of the value of the change attribute retrieved via
        CB_GETATTR and responds to the second client as in the last step.
          </t>
          <t>
        This methodology resolves issues of clock differences between client
        and server and other scenarios where the use of CB_GETATTR break down.
          </t>
          <t>
        It should be noted that the server is under no obligation to use
        CB_GETATTR, and therefore the server MAY simply recall the delegation
        to avoid its use.
          </t>
        </section>
        <section numbered="true" toc="default">
          <name>Recall of Open Delegation</name>
          <t>
        The following events necessitate recall of an OPEN delegation:
          </t>
          <ul spacing="normal">
            <li>
            potentially conflicting OPEN request (or a READ or WRITE operation
            done with a special stateid)
          </li>
            <li>
            SETATTR sent by another client
          </li>
            <li>
            REMOVE request for the file
          </li>
            <li>
            RENAME request for the file as either the source or target of the RENAME
          </li>
          </ul>
          <t>
        Whether a RENAME of a directory in the path leading to the file
        results in recall of an OPEN delegation depends on the semantics of
        the server's file system.  If that file system denies such RENAMEs when
        a file is open, the recall must be performed to determine whether the
        file in question is, in fact, open.
          </t>
          <t>
        In addition to the situations above, the server may choose to recall
        OPEN delegations at any time if resource constraints make it advisable
        to do so.  Clients should always be prepared for the possibility of
        recall.
          </t>
          <t>
        When a client receives a recall for an OPEN delegation, it needs
        to update state on the server before returning the delegation.
        These same updates must be done whenever a client chooses to
        return a delegation voluntarily.  The following items of state
        need to be dealt with:
          </t>
          <ul spacing="normal">
            <li>
            If the file associated with the delegation is no longer open and no
            previous CLOSE operation has been sent to the server, a CLOSE
            operation must be sent to the server.
          </li>
            <li>
            If a file has other open references at the client, then OPEN
            operations must be sent to the server.  The appropriate stateids will
            be provided by the server for subsequent use by the client since the
            delegation stateid will no longer be valid.  These OPEN requests are
            done with the claim type of CLAIM_DELEGATE_CUR.  This will allow the
            presentation of the delegation stateid so that the client can
            establish the appropriate rights to perform the OPEN.  (see
            <xref target="OP_OPEN" format="default"/>, which describes the OPEN operation,
            for details.)
          </li>
            <li>
            If there are granted byte-range locks, the corresponding LOCK operations
            need to be performed.  This applies to the OPEN_DELEGATE_WRITE delegation case
            only.
          </li>
            <li>
            For an OPEN_DELEGATE_WRITE delegation, if
            at the time of recall the file is not open for
            OPEN4_SHARE_ACCESS_WRITE/OPEN4_SHARE_ACCESS_BOTH, all modified
            data for the file must be flushed to the
            server.  If the delegation had not existed, the client would have done
            this data flush before the CLOSE operation.
          </li>
            <li>
            For an OPEN_DELEGATE_WRITE delegation when a file is still open at the time of
            recall, any modified data for the file needs to be flushed to the
            server.
          </li>
            <li>
            With the OPEN_DELEGATE_WRITE delegation in place, it is possible that the file
            was truncated during the duration of the delegation.  For example, the
            truncation could have occurred as a result of an OPEN UNCHECKED with a
            size attribute value of zero.  Therefore, if a truncation of
            the file has occurred and this operation has not been propagated to
            the server, the truncation must occur before any modified data is
            written to the server.
          </li>
          </ul>
          <t>
        In the case of OPEN_DELEGATE_WRITE delegation, byte-range locking imposes some
        additional requirements.  To precisely maintain the associated
        invariant, it is required to flush any modified data in any byte-range for
        which a WRITE_LT lock was released while the OPEN_DELEGATE_WRITE delegation was in
        effect.  However, because the OPEN_DELEGATE_WRITE delegation implies no other
        locking by other clients, a simpler implementation is to flush all
        modified data for the file (as described just above) if any WRITE_LT lock
        has been released while the OPEN_DELEGATE_WRITE delegation was in effect.
          </t>
          <t>
        An implementation need not wait until delegation recall (or
        the decision to voluntarily return a delegation) to perform any of the above
        actions, if implementation considerations (e.g., resource availability
        constraints) make that desirable.  Generally, however, the fact that
        the actual OPEN state of the file may continue to change makes it not
        worthwhile to send information about opens and closes to the server,
        except as part of delegation return.  An exception is
        when the client has no more internal opens of the file. In this
        case, sending a CLOSE is useful because it
        reduces resource utilization on the client
        and server.

Regardless of the client's choices on scheduling these
        actions, all must be performed before the delegation is returned,
        including (when applicable) the close that corresponds to the OPEN
        that resulted in the delegation.  These actions can be performed
        either in previous requests or in previous operations in the same
        COMPOUND request.

          </t>
        </section>
        <section numbered="true" toc="default">
          <name>Clients That Fail to Honor Delegation Recalls</name>
          <t>
        A client may fail to respond to a recall for various reasons, such as
        a failure of the backchannel from server to the client. The client
        may be unaware of a failure in the backchannel.  This lack of
        awareness could result in the client finding out long after the
        failure that its delegation has been revoked, and another client has
        modified the data for which the client had a delegation.  This is
        especially a problem for the client that held an OPEN_DELEGATE_WRITE delegation.
          </t>
          <t>
        Status bits returned by SEQUENCE operations help to provide an
        alternate way of informing the client of issues regarding the
        status of the backchannel and of recalled delegations.  When the
        backchannel is not available, the server returns the status bit
        SEQ4_STATUS_CB_PATH_DOWN on SEQUENCE operations.  The client can
        react by attempting to re-establish the backchannel and by
        returning recallable objects if a backchannel cannot be successfully
        re-established.
          </t>
          <t>
        Whether the backchannel is functioning or not, it may be that the
        recalled delegation is not returned.  Note that the client's lease
        might still be renewed, even though the recalled delegation is not
        returned.  In this situation, servers SHOULD revoke delegations that
        are not returned in a period of time equal to the lease period.  This
        period of time should allow the client time to note the
        backchannel-down status and re-establish the backchannel.
          </t>
          <t>
        When delegations are revoked, the server will return with the
        SEQ4_STATUS_RECALLABLE_STATE_REVOKED status bit set on subsequent
        SEQUENCE operations.  The client should note this and then use
        TEST_STATEID to find which delegations have been revoked.
          </t>
        </section>
        <section numbered="true" toc="default">
          <name>Delegation Revocation</name>
          <t>
        At the point a delegation is revoked, if there are associated opens
        on the client, these opens may or may not be revoked.  If no
        byte-range lock or open is granted that is inconsistent with the existing open,
        the stateid for the open may remain valid and be disconnected
        from the revoked delegation, just as would be the case if the
        delegation were returned.
          </t>
          <t>
        For example, if an OPEN for OPEN4_SHARE_ACCESS_BOTH with a deny of OPEN4_SHARE_DENY_NONE is
        associated with the delegation, granting of another such OPEN
        to a different client will revoke the delegation but need not
        revoke the OPEN, since the two OPENs are consistent with each other.
        On the other hand, if an OPEN denying write access is
        granted, then the existing OPEN must be revoked.
          </t>
          <t>
        When opens and/or locks are revoked,
        the applications holding these opens or locks need to be notified.
        This notification usually occurs by returning errors for READ/WRITE
        operations or when a close is attempted for the open file.
          </t>
          <t>
        If no opens exist for the file at the point the delegation is revoked,
        then notification of the revocation is unnecessary.  However, if there
        is modified data present at the client for the file, the user of the
        application should be notified.  Unfortunately, it may not be possible
        to notify the user since active applications may not be present at the
        client.  See <xref target="revocation_recovery_write" format="default"/>
        for additional details.
          </t>
        </section>
        <section anchor="via_want_delegation" numbered="true" toc="default">
          <name>Delegations via WANT_DELEGATION</name>
          <t>
        In addition to providing delegations as part of the reply
        to OPEN operations, servers MAY provide delegations
        separate from open, via the OPTIONAL WANT_DELEGATION operation.  This
        allows delegations to be obtained in advance of an OPEN that
        might benefit from them, for objects that are not a valid target
        of OPEN, or to deal with cases in which a
        delegation has been recalled and the client wants to make
        an attempt to re-establish it if the absence of use by other
        clients allows that.
          </t>
          <t>
        The WANT_DELEGATION operation may be performed on any type of
        file object other than a directory.
          </t>
          <t>
        When a delegation is obtained using WANT_DELEGATION, any open
        files for the same filehandle held by that client are to be
        treated as subordinate to the delegation, just as if they had
        been created using an OPEN of type CLAIM_DELEGATE_CUR.  They are
        otherwise unchanged as to seqid, access and deny modes, and the
        relationship with byte-range locks.  Similarly, because
        existing byte-range
        locks are subordinate to an open, those byte-range locks also become
        indirectly subordinate to that new delegation.
          </t>
          <t>
        The WANT_DELEGATION operation provides for delivery of delegations
        via callbacks, when the delegations are not immediately available.
        When a requested delegation is available, it is delivered to the
        client via a CB_PUSH_DELEG operation.  When this happens, open files
        for the same filehandle become subordinate to the new delegation
        at the point at which the delegation is delivered, just as if they had
        been created using an OPEN of type CLAIM_DELEGATE_CUR.
        Similarly, this occurs for existing byte-range locks subordinate to an open.
          </t>
        </section>
      </section>
      <section anchor="data_caching_revocation" numbered="true" toc="default">
        <name>Data Caching and Revocation</name>
        <t>
      When locks and delegations are revoked, the assumptions upon which
      successful caching depends are no longer guaranteed.  For any locks or
      share reservations that have been revoked, the corresponding state-owner
      needs to be notified.  This notification includes applications with a
      file open that has a corresponding delegation that has been revoked.
      Cached data associated with the revocation must be removed from the
      client.  In the case of modified data existing in the client's cache,
      that data must be removed from the client without being written to
      the server.  As mentioned, the assumptions made by the client are no
      longer valid at the point when a lock or delegation has been revoked.
      For example, another client may have been granted a conflicting byte-range lock
      after the revocation of the byte-range lock at the first client.  Therefore, the
      data within the lock range may have been modified by the other client.
      Obviously, the first client is unable to guarantee to the application
      what has occurred to the file in the case of revocation.
        </t>
        <t>
      Notification to a state-owner will in many cases consist of simply
      returning an error on the next and all subsequent READs/WRITEs to the
      open file or on the close.  Where the methods available to a client
      make such notification impossible because errors for certain
      operations may not be returned, more drastic action such as signals or
      process termination may be appropriate.  The justification here is
      that an invariant on which an application depends may be violated.
      Depending on how errors are typically treated for the client-operating
      environment, further levels of notification including logging, console
      messages, and GUI pop-ups may be appropriate.
        </t>
        <section anchor="revocation_recovery_write" numbered="true" toc="default">
          <name>Revocation Recovery for Write Open Delegation</name>
          <t>
        Revocation recovery for an OPEN_DELEGATE_WRITE delegation poses the special
        issue of modified data in the client cache while the file is not open.
        In this situation, any client that does not flush modified data to
        the server on each close must ensure that the user receives
        appropriate notification of the failure as a result of the revocation.
        Since such situations may require human action to correct problems,
        notification schemes in which the appropriate user or administrator is
        notified may be necessary.  Logging and console messages are typical
        examples.
          </t>
          <t>
        If there is modified data on the client, it must not be flushed
        normally to the server.  A client may attempt to provide a copy of the
        file data as modified during the delegation under a different name in
        the file system namespace to ease recovery.  Note that when the
        client can determine that the file has not been modified by any other
        client, or when the client has a complete cached copy of the file in
        question, such a saved copy of the client's view of the file may be of
        particular value for recovery.  In another case, recovery using a copy
        of the file based partially on the client's cached data and partially
        on the server's copy as modified by other clients will be anything but
        straightforward, so clients may avoid saving file contents in these
        situations or specially mark the results to warn users of possible
        problems.
          </t>
          <t>
        Saving of such modified data in delegation revocation situations
        may be limited to files of a certain size or might be used only when
        sufficient disk space is available within the target file system.
        Such saving may also be restricted to situations when the client has
        sufficient buffering resources to keep the cached copy available
        until it is properly stored to the target file system.
          </t>
        </section>
      </section>
      <section numbered="true" toc="default">
        <name>Attribute Caching</name>
        <t>
      This section pertains to the caching of a file's attributes on a client
      when that client does not hold a delegation on the file.
        </t>
        <t>
      The attributes discussed in this section do not include named
      attributes.  Individual named attributes are analogous to files, and
      caching of the data for these needs to be handled just as data caching
      is for ordinary files.  Similarly, LOOKUP results from an OPENATTR
      directory (as well as the directory's contents) are to be cached on
      the same basis as any other pathnames.
        </t>
        <t>
      Clients may cache file attributes obtained from the server and use
      them to avoid subsequent GETATTR requests.  Such caching is write
      through in that modification to file attributes is always done by
      means of requests to the server and should not be done locally and
      should not be cached.  The exception to this are modifications to attributes that
      are intimately connected with data caching.  Therefore, extending a
      file by writing data to the local data cache is reflected immediately
      in the size as seen on the client without this change being
      immediately reflected on the server.  Normally, such changes are not
      propagated directly to the server, but when the modified data is
      flushed to the server, analogous attribute changes are made on the
      server.  When OPEN delegation is in effect, the modified attributes
      may be returned to the server in reaction to a CB_RECALL call.
        </t>
        <t>
      The result of local caching of attributes is that the attribute
      caches maintained on individual clients will not be coherent.
      Changes made in one order on the server may be seen in a different
      order on one client and in a third order on another client.
        </t>
        <t>
      The typical file system application programming interfaces do not
      provide means to atomically modify or interrogate attributes for
      multiple files at the same time.  The following rules provide an
      environment where the potential incoherencies mentioned above can be
      reasonably managed.  These rules are derived from the practice of
      previous NFS protocols.
        </t>
        <ul spacing="normal">
          <li>
          All attributes for a given file (per-fsid attributes excepted) are
          cached as a unit at the client so that no non-serializability can
          arise within the context of a single file.
        </li>
          <li>
          An upper time boundary is maintained on how long a client cache entry
          can be kept without being refreshed from the server.
        </li>
          <li>
          When operations are performed that change attributes at the server,
          the updated attribute set is requested as part of the containing RPC.
          This includes directory operations that update attributes indirectly.
          This is accomplished by following the modifying operation with a
          GETATTR operation and then using the results of the GETATTR to update
          the client's cached attributes.
        </li>
        </ul>
        <t>
      Note that if the full set of attributes to be cached is requested by
      READDIR, the results can be cached by the client on the same basis as
      attributes obtained via GETATTR.
        </t>
        <t>
      A client may validate its cached version of attributes for a file by
      fetching both the change and time_access attributes and assuming
      that if the change attribute has the same value as it did when the
      attributes were cached, then no attributes other than time_access have
      changed.  The reason why time_access is also fetched is because many
      servers operate in environments where the operation that updates
      change does not update time_access.  For example, POSIX file semantics
      do not update access time when a file is modified by the write system
      call <xref target="write_atime" format="default"/>.  Therefore, the client that wants a current time_access value
      should fetch it with change during the attribute cache validation
      processing and update its cached time_access.
        </t>
        <t>
      The client may maintain a cache of modified attributes for those
      attributes intimately connected with data of modified regular files
      (size, time_modify, and change). Other than those three attributes,
      the client MUST NOT maintain a cache of modified attributes. Instead,
      attribute changes are immediately sent to the server.
        </t>
        <t>
      In some operating environments, the equivalent to time_access is
      expected to be implicitly updated by each read of the content of the
      file object.  If an NFS client is caching the content of a file
      object, whether it is a regular file, directory, or symbolic link, the
      client SHOULD NOT update the time_access attribute (via SETATTR or a
      small READ or READDIR request) on the server with each read that is
      satisfied from cache.  The reason is that this can defeat the
      performance benefits of caching content, especially since an explicit
      SETATTR of time_access may alter the change attribute on the server.
      If the change attribute changes, clients that are caching the content
      will think the content has changed, and will re-read unmodified data
      from the server.  Nor is the client encouraged to maintain a modified
      version of time_access in its cache, since the client either would
      eventually have to write the access time to the server
      with bad performance effects or never update the
      server's time_access, thereby resulting in a situation where an
      application that caches access time between a close and open of
      the same file observes the access time oscillating between the past and
      present.  The time_access attribute always means the time of last
      access to a file by a read that was satisfied by the server. This way
      clients will tend to see only time_access changes that go forward in
      time.

        </t>
      </section>
      <section numbered="true" toc="default">
        <name>Data and Metadata Caching and Memory Mapped Files</name>
        <t>
      Some operating environments include the capability for an application
      to map a file's content into the application's address space.  Each
      time the application accesses a memory location that corresponds to a
      block that has not been loaded into the address space, a page fault
      occurs and the file is read (or if the block does not exist in the
      file, the block is allocated and then instantiated in the
      application's address space).
        </t>
        <t>
      As long as each memory-mapped access to the file requires a page
      fault, the relevant attributes of the file that are used to detect
      access and modification (time_access, time_metadata, time_modify, and
      change) will be updated.  However, in many operating environments,
      when page faults are not required, these attributes will not be updated
      on reads or updates to the file via memory access (regardless of
      whether the file is local or is accessed remotely).  A client or
      server MAY fail to update attributes of a file that is being accessed
      via memory-mapped I/O.  This has several implications:
        </t>
        <ul spacing="normal">
          <li>
          If there is an application on the server that has memory mapped a file
          that a client is also accessing, the client may not be able to get a
          consistent value of the change attribute to determine
          whether or not its cache is stale.  A server that knows that
          the file is memory-mapped could always pessimistically
          return updated values for change so as to force the
          application to always get the most up-to-date data
          and metadata for the file.  However, due to the negative performance
          implications of this, such behavior is OPTIONAL.
        </li>
          <li>
          If the memory-mapped file is not being modified on the server, and
          instead is just being read by an application via the memory-mapped
          interface, the client will not see an updated time_access attribute.
          However, in many operating environments, neither will any process
          running on the server. Thus, NFS clients are at no disadvantage with
          respect to local processes.
        </li>
          <li>
          If there is another client that is memory mapping the file, and if
          that client is holding an OPEN_DELEGATE_WRITE delegation, the same set of issues as
          discussed in the previous two bullet points apply.  So, when a server
          does a CB_GETATTR to a file that the client has modified in its cache,
          the reply from CB_GETATTR will not necessarily be accurate.  As
          discussed earlier, the client's obligation is to report that the file
          has been modified since the delegation was granted, not whether it has
          been modified again between successive CB_GETATTR calls, and the
          server MUST assume that any file the client has modified in cache has
          been modified again between successive CB_GETATTR calls.  Depending on
          the nature of the client's memory management system, this weak
          obligation may not be possible.  A client MAY return stale information
          in CB_GETATTR whenever the file is memory-mapped.
        </li>
          <li>
            <t>
          The mixture of memory mapping and byte-range locking on the same file is
          problematic. Consider the following scenario, where a page size on
          each client is 8192 bytes.
            </t>
            <ul spacing="normal">
              <li>
              Client A memory maps the first page (8192 bytes) of file X.
            </li>
              <li>
              Client B memory maps the first page (8192 bytes) of file X.
            </li>
              <li>
              Client A WRITE_LT locks the first 4096 bytes.
            </li>
              <li>
              Client B WRITE_LT locks the second 4096 bytes.
            </li>
              <li>
              Client A, via a STORE instruction, modifies part of its locked byte-range.
            </li>
              <li>
              Simultaneous to client A, client B executes a STORE on part of its
              locked byte-range.
            </li>
            </ul>
          </li>
        </ul>
        <t>
      Here the challenge is for each client to resynchronize to get a
      correct view of the first page. In many operating environments, the
      virtual memory management systems on each client only know a page is
      modified, not that a subset of the page corresponding to the
      respective lock byte-ranges has been modified. So it is not possible for
      each client to do the right thing, which is to write to the
      server only that portion of the page that is locked.  For example, if
      client A simply writes out the page, and then client B writes out the
      page, client A's data is lost.
        </t>
        <t>
      Moreover, if mandatory locking is enabled on the file, then we have a
      different problem.  When clients A and B execute the STORE instructions,
      the resulting page faults require a byte-range lock on the entire page.
      Each client then tries to extend their locked range to the entire
      page, which results in a deadlock.  Communicating the NFS4ERR_DEADLOCK
      error to a STORE instruction is difficult at best.
        </t>
        <t>
      If a client is locking the entire memory-mapped file, there is no
      problem with advisory or mandatory byte-range locking, at least until the
      client unlocks a byte-range in the middle of the file.
        </t>
        <t>
      Given the above issues, the following are permitted:
        </t>
        <ul spacing="normal">
          <li>
          Clients and servers MAY deny memory mapping a file for which they know there are
          byte-range locks.
        </li>
          <li>
          Clients and servers MAY deny a byte-range lock on a file they know is
          memory-mapped.
        </li>
          <li>
          A client MAY deny memory mapping a file that it knows requires
          mandatory locking for I/O.  If mandatory locking is enabled after the
          file is opened and mapped, the client MAY deny the application further
          access to its mapped file.
        </li>
        </ul>
      </section>
      <section anchor="without_dir_deleg" numbered="true" toc="default">
        <name>Name and Directory Caching without Directory Delegations</name>
        <t>
      The NFSv4.1 directory delegation facility
      (described in <xref target="dir_deleg" format="default"/> below) is OPTIONAL
      for servers to implement. Even where it is
      implemented, it may not always be functional because of resource
      availability issues or other constraints.  Thus, it is
      important to understand how name and directory caching are done
      in the absence of directory delegations. These topics are
      discussed in the next two subsections.
        </t>
        <section anchor="name_caching" numbered="true" toc="default">
          <name>Name Caching</name>
          <t>
        The results of LOOKUP and READDIR operations may be cached to avoid
        the cost of subsequent LOOKUP operations.  Just as in the case of
        attribute caching, inconsistencies may arise among the various client
        caches.  To mitigate the effects of these inconsistencies and given
        the context of typical file system APIs, an upper time boundary is
        maintained for how long a client name cache entry can be kept without
        verifying that the entry has not been made invalid by a directory
        change operation performed by another client.
          </t>
          <t>
        When a client is not making changes to a directory for which there
        exist name cache entries, the client needs to periodically fetch
        attributes for that directory to ensure that it is not being modified.
        After determining that no modification has occurred, the expiration
        time for the associated name cache entries may be updated to be the
        current time plus the name cache staleness bound.
          </t>
          <t>
        When a client is making changes to a given directory, it needs to
        determine whether there have been changes made to the directory by
        other clients.  It does this by using the change attribute as reported
        before and after the directory operation in the associated
        change_info4 value returned for the operation.  The server is able to
        communicate to the client whether the change_info4 data is provided
        atomically with respect to the directory operation.  If the change
        values are provided atomically, the client has a basis for determining,
        given proper care, whether other clients are modifying the directory
        in question.
          </t>
          <t>
        The simplest way to enable the client to make this determination is
        for the client to serialize all changes made to a specific directory.
        When this is done, and the server provides before and after values of the
        change attribute atomically, the client can simply compare the
        after value of the change attribute from one operation on a
        directory with the before value on the subsequent operation
        modifying that directory.  When these are equal, the client is
        assured that no other client is modifying the directory in question.
          </t>
          <t>
        When such serialization is not used, and there may be multiple
        simultaneous outstanding operations modifying a single directory sent
        from a single client, making this sort of determination can be more
        complicated.  If two such operations
        complete in a different order than they were actually performed,
        that might give an appearance consistent with modification being
        made by another client.  Where this appears to happen, the client
        needs to await the completion of all such modifications that were
        started previously, to see if the outstanding before and after
        change numbers can be sorted into a chain such that the before
        value of one change number matches the after value of a previous
        one, in a chain consistent with this client being the only one
        modifying the directory.
          </t>
          <t>
        In either of these cases, the client is able to determine whether
        the directory is being modified by another client.
        If the comparison indicates that the directory was updated by
        another client, the name cache associated with the modified directory
        is purged from the client.  If the comparison indicates no
        modification, the name cache can be updated on the client to reflect
        the directory operation and the associated timeout can be extended.  The
        post-operation change value needs to be saved as the basis for future
        change_info4 comparisons.
          </t>
          <t>
        As demonstrated by the scenario above, name caching requires that the
        client revalidate name cache data by inspecting the change attribute
        of a directory at the point when the name cache item was cached.  This
        requires that the server update the change attribute for directories
        when the contents of the corresponding directory is modified.  For a
        client to use the change_info4 information appropriately and
        correctly, the server must report the pre- and post-operation change
        attribute values atomically.  When the server is unable to report the
        before and after values atomically with respect to the directory
        operation, the server must indicate that fact in the change_info4
        return value.  When the information is not atomically reported, the
        client should not assume that other clients have not changed the
        directory.
          </t>
        </section>
        <section numbered="true" toc="default">
          <name>Directory Caching</name>
          <t>
        The results of READDIR operations may be used to avoid subsequent
        READDIR operations.  Just as in the cases of attribute and name
        caching, inconsistencies may arise among the various client caches.  To
        mitigate the effects of these inconsistencies, and given the context of
        typical file system APIs, the following rules should be followed:
          </t>
          <ul spacing="normal">
            <li>
            Cached READDIR information for a directory that is not obtained in a
            single READDIR operation must always be a consistent snapshot of
            directory contents.  This is determined by using a GETATTR before the
            first READDIR and after the last READDIR that contributes to the
            cache.
          </li>
            <li>
            An upper time boundary is maintained to indicate the length of time a
            directory cache entry is considered valid before the client must
            revalidate the cached information.
          </li>
          </ul>
          <t>
        The revalidation technique parallels that discussed in the case of
        name caching.  When the client is not changing the directory in
        question, checking the change attribute of the directory with GETATTR
        is adequate.  The lifetime of the cache entry can be extended at these
        checkpoints.  When a client is modifying the directory, the client
        needs to use the change_info4 data to determine whether there are
        other clients modifying the directory.  If it is determined that no
        other client modifications are occurring, the client may update its
        directory cache to reflect its own changes.
          </t>
          <t>
        As demonstrated previously, directory caching requires that the client
        revalidate directory cache data by inspecting the change attribute of
        a directory at the point when the directory was cached.  This requires
        that the server update the change attribute for directories when the
        contents of the corresponding directory is modified.  For a client to
        use the change_info4 information appropriately and correctly, the
        server must report the pre- and post-operation change attribute values
        atomically.  When the server is unable to report the before and after
        values atomically with respect to the directory operation, the server
        must indicate that fact in the change_info4 return value.  When the
        information is not atomically reported, the client should not assume
        that other clients have not changed the directory.
          </t>
        </section>
      </section>
      <section anchor="dir_deleg" numbered="true" toc="default">
        <name>Directory Delegations</name>
        <section numbered="true" toc="default">
          <name>Introduction to Directory Delegations</name>
          <t>
        Directory caching for the NFSv4.1 protocol, as previously
        described, is similar to file
        caching in previous versions.  Clients typically cache
        directory information for
        a duration determined by the client. At the end of a predefined
        timeout, the client will query the server to see if the directory has
        been updated. By caching attributes, clients reduce the number of
        GETATTR calls made to the server to validate attributes. Furthermore,
        frequently accessed files and directories, such as the current
        working directory, have their attributes cached on the client so that
        some NFS operations can be performed without having to make an RPC
        call. By caching name and inode information about most recently
        looked up entries in a Directory Name Lookup Cache (DNLC), clients do
        not need to send LOOKUP calls to the server every time these files
        are accessed.
          </t>
          <t>
        This caching approach works reasonably well at reducing network
        traffic in many environments. However, it does not address
        environments where there are numerous queries for files that do not
        exist. In these cases of "misses", the client sends requests to
        the server in order to provide reasonable application semantics and
        promptly detect the creation of new directory entries. Examples of
        high miss activity are compilation in software development
        environments. The current behavior of NFS limits its potential
        scalability and wide-area sharing effectiveness in these types of
        environments. Other distributed stateful file system architectures
        such as AFS and DFS have proven that adding state around directory
        contents can greatly reduce network traffic in high-miss
        environments.
          </t>
          <t>
        Delegation of directory contents is an OPTIONAL feature of NFSv4.1.
        Directory delegations provide similar traffic reduction
        benefits as with file delegations. By allowing clients to cache
        directory contents (in a read-only fashion) while being notified of
        changes, the client can avoid making frequent requests to interrogate
        the contents of slowly-changing directories, reducing network traffic
        and improving client performance.  It can also simplify the task of
        determining whether other clients are making changes to the directory
        when the client itself is making many changes to the directory and
        changes are not serialized.
          </t>
          <t>
        Directory delegations allow improved namespace cache consistency to be
        achieved through delegations and synchronous recalls, in the absence
        of notifications. In addition, if time-based consistency is
        sufficient, asynchronous notifications can provide performance
        benefits for the client, and possibly the server, under some common
        operating conditions such as slowly-changing and/or very large
        directories.
          </t>
        </section>
        <section numbered="true" toc="default">
          <name>Directory Delegation Design</name>
          <t>
        NFSv4.1 introduces the GET_DIR_DELEGATION
        (<xref target="OP_GET_DIR_DELEGATION" format="default"/>) operation to allow the
        client to ask for a
        directory delegation. The delegation covers directory attributes and
        all entries in the directory. If either of these change, the
        delegation will be recalled synchronously. The operation causing the
        recall will have to wait before the recall is complete. Any changes
        to directory entry attributes will not cause the delegation to be
        recalled.
          </t>
          <t>
        In addition to asking for delegations, a client can also ask for
        notifications for certain events. These events include changes to
        the directory's attributes and/or its contents.  If a client asks for
        notification for a certain event, the server will notify the client
        when that event occurs. This will not result in the delegation being
        recalled for that client.  The notifications are asynchronous and
        provide a way of avoiding recalls in situations where a directory is
        changing enough that the pure recall model may not be effective while
        trying to allow the client to get substantial benefit. In the absence
        of notifications, once the delegation is recalled the client has to
        refresh its directory cache; this might not be very efficient for
        very large directories.
          </t>
          <t>
        The delegation is read-only and the client may not make changes to
        the directory other than by performing NFSv4.1 operations that modify
        the directory or the associated file attributes so that the server
        has knowledge of these changes. In order to keep the client's
        namespace synchronized with that of the server, the server will notify
        the delegation-holding client (assuming it has requested
        notifications) of the changes made as a result of that client's
        directory-modifying operations.  This is to avoid any need for
        that client to send subsequent GETATTR or READDIR operations
        to the server.  If a single client is holding the delegation
        and that client makes any changes to the directory (i.e., the
        changes are made via operations sent on a session
        associated with the client ID holding the delegation), the
        delegation will not be recalled. Multiple clients may hold a delegation
        on the same directory, but if any such client modifies the directory,
        the server MUST recall the delegation from the other clients,
        unless those clients have made provisions to be notified of that
        sort of modification.
          </t>
          <t>
        Delegations can be recalled by the server at any time.  Normally, the
        server will recall the delegation when the directory changes in a way
        that is not covered by the notification, or when the directory
        changes and notifications have not been requested.
        If another client removes the directory for
        which a delegation has been granted, the server will recall the
        delegation.
          </t>
        </section>
        <section numbered="true" toc="default">
          <name>Attributes in Support of Directory Notifications</name>
          <t>
       See <xref target="dir_not_attrs" format="default"/> for a description of the attributes
       associated with directory notifications.
          </t>
        </section>
        <section numbered="true" toc="default">
          <name>Directory Delegation Recall</name>
          <t>
        The server will recall the directory delegation by sending a callback
        to the client. It will use the same callback procedure as used for
        recalling file delegations. The server will recall the delegation
        when the directory changes in a way that is not covered by the
        notification. However, the server need not recall the delegation if
        attributes of an entry within the directory change.
          </t>
          <t>
        If the
        server notices that handing out a delegation for a directory is
        causing too many notifications to be sent out, it may decide to
        not hand out delegations for that directory and/or recall those already
        granted.  If a client tries to remove the directory for which
        a delegation has been granted, the server will recall all associated delegations.
          </t>
          <t>
        The implementation sections for a number
        of operations describe situations in which notification or
        delegation recall would be required under some common circumstances.
        In this regard, a similar set of caveats to those listed
        in <xref target="deleg_and_cb" format="default"/> apply.
          </t>
          <ul spacing="normal">
            <li>
            For CREATE, see <xref target="OP_CREATE_IMPLEMENTATION" format="default"/>.
          </li>
            <li>
            For LINK, see <xref target="OP_LINK_IMPLEMENTATION" format="default"/>.
          </li>
            <li>
            For OPEN, see <xref target="OP_OPEN_IMPLEMENTATION" format="default"/>.
          </li>
            <li>
            For REMOVE, see <xref target="OP_REMOVE_IMPLEMENTATION" format="default"/>.
          </li>
            <li>
            For RENAME, see <xref target="OP_RENAME_IMPLEMENTATION" format="default"/>.
          </li>
            <li>
            For SETATTR, see <xref target="OP_SETATTR_IMPLEMENTATION" format="default"/>.
          </li>
          </ul>
        </section>
        <section numbered="true" toc="default">
          <name>Directory Delegation Recovery</name>
          <t>
        Recovery from client or server restart for state on regular files
        has two main goals: avoiding the necessity of
        breaking application guarantees with respect to locked files and
        delivery of updates cached at the client.  Neither of these
        goals applies to directories protected by OPEN_DELEGATE_READ delegations and
        notifications. Thus, no provision is made for reclaiming
        directory delegations in the event of client or server restart.
        The client can simply establish a directory delegation in the
        same fashion as was done initially.
          </t>
        </section>
      </section>
    </section>
    <!-- 	$Id: 2009-12-20-TO-rfc5661.xml,v 1.2 2009/12/21 05:59:32 shepler.mre Exp $	 -->
<!-- Copyright (C) The IETF Trust (2007-2008) -->
<!-- Copyright (C) The Internet Society (2006) -->
<section anchor="NEW11" numbered="true" toc="default">
      <name>Multi-Server Namespace</name>
      <t>
    NFSv4.1 supports attributes that allow a namespace to extend
    beyond the boundaries of a single server.  It is desirable
    that clients and servers support construction of such
    multi-server namespaces.  Use of such multi-server namespaces
    is OPTIONAL however, and for many purposes,
    single-server namespaces are perfectly acceptable.  Use of
    multi-server namespaces can provide many advantages, by
    separating a file system's logical position in a namespace from
    the (possibly changing) logistical and administrative
    considerations that result in particular file systems being
    located on particular servers via a single network access paths known
    in advance or determined using DNS.
      </t>
      <section anchor="SEC11-TERM" numbered="true" toc="default">
        <name>Terminology</name>
        <t>
      In this section as a whole (i.e. within all of <xref target="NEW11" format="default"/>),
      the phrase "client ID" always refers to the
      64-bit shorthand identifier assigned by the server (a clientid4)
      and never to the structure which the client uses to identify itself
      to the server (called an nfs_client_id4 or client_owner in NFSv4.0
      and NFSv4.1 respectively).  The opaque identifier within those
      structures is referred to as a "client id string".
        </t>
        <section anchor="SEC11-TERM-trunking" numbered="true" toc="default">
          <name>Terminology Related to Trunking</name>
          <t>
      It is particularly important to clarify the distinction
      between trunking detection and
      trunking discovery.  The definitions we present are
      applicable to all
      minor versions of NFSv4, but we will focus on how
      these
      terms apply to NFS version 4.1.
          </t>
          <ul spacing="normal">
            <li>
              <t>
        Trunking detection refers to ways of deciding whether two
        specific network
        addresses are connected to the same NFSv4 server.  The
        means available to make this determination depends on the protocol
        version, and, in some cases, on the client implementation.
              </t>
              <t>
       In the case of NFS version 4.1 and later minor versions, the
       means of
       trunking detection are as described in this document
       and are available
       to every client.  Two network addresses
       connected to the same server can always be used together
       to access a particular server
       but cannot necessarily be used together
       to access a single session.   See below for definitions
       of the terms "server-trunkable" and "session-trunkable"
              </t>
            </li>
            <li>
              <t>
        Trunking discovery is a process by which a client using one
        network address can obtain other addresses that are connected
        to the
	same server.
        Typically, it builds on a trunking detection facility by providing
	one or more methods by which candidate addresses are made
        available to the client
	who can then use trunking detection to appropriately filter them.
              </t>
              <t>
        Despite the support for trunking detection there was no
	description of trunking discovery provided in
        RFC5661 <xref target="RFC5661" format="default"/>, making it necessary to provide
	those means in this document.
              </t>
            </li>
          </ul>
          <t>
	The combination of a server network address and a particular
	connection type to be used by a connection
	is referred to as a "server endpoint".   Although using different
	connection types may result in different ports being used, the
	use of different ports by multiple connections to the same
	network address in such cases is not the essence of the distinction
	between the two endpoints used.   This is in contrast to the case
	of port-specific endpoints,
	in which the explicit specification of port numbers within network
	addresses is used to allow a single server node to support multiple
	NFS servers.
          </t>
          <t>
        Two network addresses connected to the same server are said to
        be server-trunkable.  Two such addresses support the use of
	clientid ID trunking, as described in <xref target="Trunking" format="default"/>.
          </t>
          <t>
        Two network addresses connected to the same server such that
    	those addresses can be used to support a single common session
        are referred to as session-trunkable.  Note that two addresses
    	may be server-trunkable without being session-trunkable and that
	when two connections of different connection types are made
	to the same network address and are based on a single file
	system location entry
	they are always
	session-trunkable, independent of the connection type, as
	specified by <xref target="Trunking" format="default"/>, since their derivation from
	the same file system location entry together with the identity of
	their network addresses assures that both
	connections are to the
	same server and will return server-owner information allowing
	session trunking to be used.
          </t>
        </section>
        <section anchor="SEC11-TERM-loc" numbered="true" toc="default">
          <name>Terminology Related to File System Location</name>
          <t>
      Regarding terminology relating to the construction of multi-server
      namespaces out of a set of local per-server namespaces:
          </t>
          <ul spacing="normal">
            <li>
	Each server has a set of exported file systems which may be accessed
	by NFSv4 clients.  Typically, this is done by assigning each
	file system a name within the pseudo-fs associated with the
	server, although the pseudo-fs may be dispensed with if there
	is only a single exported file system.  Each such file system
	is part of the server's local namespace, and can be considered
	as a file system instance within a larger multi-server
	namespace.
      </li>
            <li>
	The set of all exported file systems for a given server
	constitutes that server's local namespace.
      </li>
            <li>
	In some cases, a server will have a namespace more extensive
	than its local namespace by using features associated with
	attributes that provide file system location information.
	These features,
	which allow construction of a multi-server namespace,
	are all described in individual sections below and include
	referrals (described in <xref target="SEC11-USES-ref" format="default"/>),
	migration (described in <xref target="SEC11-USES-migr" format="default"/>), and
        replication (described in <xref target="SEC11-USES-repl" format="default"/>).
      </li>
            <li>
	A file system present in a server's pseudo-fs may have multiple
	file system instances on different servers associated with it.
	All such instances are considered replicas of one another.
	Whether such replicas can be used simultaneously is discussed in
	<xref target="SEC11-EFF-simul" format="default"/>, while the level of
	co-ordination between them (important when switching
	between them) is discussed in Sections
	<xref target="SEC11-EFF-fh" format="counter"/>
        through <xref target="SEC11-EFF-data" format="counter"/>
	below.

      </li>
            <li>
	When a file system is present in a server's pseudo-fs, but
	there is no corresponding local file system, it is said to
	be "absent".  In such cases, all associated instances will
	be accessed on other servers.
      </li>
          </ul>
          <t>
      Regarding terminology relating to attributes used in trunking
      discovery and other multi-server namespace features:
          </t>
          <ul spacing="normal">
            <li>
        File system location attributes include the fs_locations and
        fs_locations_info attributes.
      </li>
            <li>
              <t>
        File system location entries provide the individual
	file system locations
        within the file system location attributes.
	Each such entry specifies a
        server, in the form of a host name or an address, and an fs name,
	which designates the location of the file system within
	the server's local namespace.  A file system
	location entry designates a set
	of server endpoints to which the client may establish connections.
	There may be multiple endpoints because a host name may map to
	multiple network addresses and because multiple connection types
	may be
	used to communicate with a single network address.  However,
	except where an explicit port numbers are used to designate a set
	of server within a single server node, all
	such endpoints MUST designate a way of connecting to a single server.
        The exact form of the location entry varies with the
        particular file system location attribute used, as described in
        <xref target="SEC11-loc-attr" format="default"/>.
              </t>
              <t>
        The network addresses used in file system location entries
        typically appear without port number indications and are
        used to designate a server at one of the standard ports for NFS access,
        e.g., 2049 for TCP, or 20049 for use with RPC-over-RDMA.  Port
	numbers may be used
        in file system location entries to designate servers (typically
        user-level ones) accessed using other port numbers.   In the case where
	network addresses indicate trunking relationships, use of an explicit
	port number is inappropriate since trunking is a relationship between
	network addresses.  See <xref target="SEC11-USES-trunk" format="default"/> for
	details.
              </t>
            </li>
            <li>
        File system location elements are derived from
	location entries and each
        describes a particular network access path, consisting of a network
	address and a location within the server's local namespace.
	Such location elements need not appear
        within a file system location attribute, but the
        existence of each location element derives from a corresponding
        location entry.  When a
        location entry specifies an IP address there is only a single
        corresponding location element.  File system location entries that
        contain a host name are resolved using DNS, and may result
        in one or more location elements.  All  location elements
        consist of a location address which includes the IP address of
        an interface to a server and an fs  name which is the location
        of the file system within the server's local namespace.  The fs name
        can be empty if the server has no pseudo-fs and only a single exported
	file system at the root filehandle.
      </li>
            <li>
        Two file system location elements are said to be
	server-trunkable if they
        specify the same fs name and the location addresses are such
        that the location addresses are server-trunkable.  When the
	corresponding network paths are used, the client will always be
	able to use client ID trunking, but will only be able to use
	session trunking if the paths are also session-trunkable.

      </li>
            <li>
        Two file system location elements are said to be session-trunkable
        if they
        specify the same fs name and the location addresses are such
        that the location addresses are session-trunkable.  When the
	corresponding network paths are used, the client will be able to
	able to use either client ID trunking or session trunking.
      </li>
          </ul>
          <t>
      Discussion of the term "replica" is complicated by the fact that
      the term was used in RFC5661 <xref target="RFC5661" format="default"/>, with a meaning
      different from that in this document.  In short,
      in <xref target="RFC5661" format="default"/> each replica is identified by a
      single network access path while, in the current document a set
      of network access paths which have server-trunkable network
      addresses and the same root-relative file system pathname is
      considered to be a single replica with multiple network access
      paths.
          </t>
          <t>
      Each set of server-trunkable location elements defines a set of
      available network access paths to a particular file system.
      When there
      are multiple such file systems, each of  which contains the
      same data, these file systems are considered replicas
      of one another.  Logically, such replication
      is symmetric, since the fs currently in use and an alternate fs
      are replicas of each other.  Often, in other documents, the term
      "replica" is not applied to the fs currently in use, despite the
      fact that the replication relation is inherently symmetric.
          </t>
        </section>
      </section>
      <section anchor="SEC11-loc-attr" numbered="true" toc="default">
        <name>File System Location Attributes</name>
        <t>
      NFSv4.1 contains attributes that provide information
      about how (i.e., at what network address and namespace position)
      a given file system may be accessed.  As a result, file systems
      in the namespace of one server can be
      associated with one or more instances of that
      file system on other servers.  These attributes contain file
      system location
      entries specifying a server address
      target (either as a DNS name representing one or more IP
      addresses or as a specific IP address) together with the pathname
      of that file system within the associated single-server namespace.
        </t>
        <t>
      The fs_locations_info RECOMMENDED attribute
      allows specification of one or more file system instance locations
      where the data corresponding to a given file
      system may be found.  This attribute provides to the client,
      in addition to specification of file system instance locations,
      other helpful
      information such as:
        </t>
        <ul spacing="normal">
          <li>
	Information guiding choices among the various file system instances
	provided (e.g., priority for use, writability, currency, etc.).
      </li>
          <li>

	Information to help the client efficiently effect as seamless
	a transition
        as possible among multiple file system instances, when and if
        that should be necessary.
      </li>
          <li>
	Information helping to guide the selection of the appropriate
	connection type to be used when establishing a connection.
      </li>
        </ul>
        <t>
      Within the fs_locations_info attribute, each
      fs_locations_server4 entry corresponds to a file system
      location entry with the
      fls_server field designating the server, with the location pathname
      within
      the server's pseudo-fs given by the fl_rootpath field of the
      encompassing fs_locations_item4.
        </t>
        <t>
      The fs_locations attribute defined in NFSv4.0 is also a part of
      NFSv4.1.  This attribute
      only allows specification
      of the file system
      locations where the data corresponding to a given file
      system may be found.  Servers SHOULD  make this attribute available
      whenever fs_locations_info is supported, but client use of
      fs_locations_info is preferable, as it provides more information.
        </t>
        <t>
      Within the fs_location attribute, each fs_location4 contains a
      file system location entry with the server field designating
      the server and
      the rootpath field giving the location pathname within the server's
      pseudo-fs.
        </t>
      </section>
      <section anchor="presence_or_absence" numbered="true" toc="default">
        <name>File System Presence or Absence</name>
        <t>
      A given location in an NFSv4.1 namespace (typically but not necessarily
      a multi-server namespace) can have a number of file system instance
      locations
      associated with it (via the fs_locations or fs_locations_info
      attribute).  There may also be an actual current file system at
      that location, accessible via normal namespace operations (e.g.,
      LOOKUP).  In this case, the file system is said to be
      "present" at that position in the namespace, and clients will
      typically use it, reserving use of additional locations
      specified via the location-related attributes to situations in
      which the principal location is no longer available.
        </t>
        <t>
      When there is no actual file system at the namespace location
      in question, the file system is said to be "absent".  An absent
      file system contains no files or directories other than the
      root.  Any reference to it, except
      to access a small set of attributes useful in determining
      alternate locations, will result in an error, NFS4ERR_MOVED.
      Note that if the server ever returns the error NFS4ERR_MOVED,
      it MUST support the fs_locations
      attribute and SHOULD support the fs_locations_info and fs_status
      attributes.
        </t>
        <t>
      While the error name suggests that we have a case of a file system
      that once was present, and has only become absent later, this is
      only one possibility.  A position in the namespace may be permanently
      absent with the set of file system(s) designated by the location
      attributes being the only realization.
      The name NFS4ERR_MOVED reflects an earlier,
      more limited conception of its function, but this error will be
      returned whenever the referenced file system is absent, whether it
      has moved or not.
        </t>
        <t>
      Except in the case of GETATTR-type operations (to be discussed
      later), when the
      current filehandle at the start of an operation is within an
      absent file system, that operation is not performed and the error
      NFS4ERR_MOVED is returned, to indicate that the file system is
      absent on the current server.
        </t>
        <t>
      Because a GETFH cannot succeed if the current filehandle is
      within an absent file system, filehandles within an absent
      file system cannot be transferred to the client.  When a
      client does have filehandles within an absent file system, it
      is the result of obtaining them when the file system was
      present, and having the file system become
      absent subsequently.
        </t>
        <t>
      It should be noted that because the check for the current
      filehandle being within an absent file system happens at the
      start of every operation, operations that change the current
      filehandle so that it is within an absent file system will not
      result in an error.  This allows such combinations as
      PUTFH-GETATTR and LOOKUP-GETATTR to be used to get attribute
      information, particularly location attribute information,
      as discussed below.
        </t>
        <t>
      The RECOMMENDED file system attribute fs_status
      can be used to interrogate the present/absent status of a
      given file system.
        </t>
      </section>
      <section anchor="absent_fs_attributes" numbered="true" toc="default">
        <name>Getting Attributes for an Absent File System</name>
        <t>
      When a file system is absent, most attributes are not available,
      but it is necessary to allow the client access to the small
      set of attributes that are available, and most particularly
      those that give information about the correct current locations
      for this file system: fs_locations and fs_locations_info.
        </t>
        <section anchor="absent_getattr" numbered="true" toc="default">
          <name>GETATTR within an Absent File System</name>
          <t>
        As mentioned above, an exception is made for GETATTR in that
        attributes may be obtained for a filehandle within an absent
        file system.  This exception only applies if the attribute
        mask contains at least one attribute bit that indicates the
        client is interested in a result regarding an absent file
        system: fs_locations, fs_locations_info, or fs_status.
        If none of these attributes
        is requested, GETATTR will result in an NFS4ERR_MOVED error.
          </t>
          <t>
        When a GETATTR is done on an absent file system, the set of
        supported attributes is very limited.  Many attributes, including
        those that are normally REQUIRED, will not be available on an
        absent file system.  In addition to the attributes mentioned
        above (fs_locations, fs_locations_info, fs_status), the following
        attributes SHOULD be available on absent file systems.  In the
        case of RECOMMENDED attributes, they should be available at
        least to the same degree that they are available on present file systems.
          </t>
          <dl newline="false" spacing="normal">
            <dt>change_policy:</dt>
            <dd>
          This attribute is useful for absent file systems
          and can be helpful in summarizing to the client when any
          of the location-related attributes change.
        </dd>
            <dt>fsid:</dt>
            <dd>
          This attribute should be provided so that the client
          can determine file system boundaries, including, in
          particular, the boundary between present and absent file
          systems.  This value must be different from any other fsid
          on the current server and need have no particular relationship
          to fsids on any particular destination to which the client
          might be directed.
        </dd>
            <dt>mounted_on_fileid:</dt>
            <dd>
          For objects at the top of an absent
          file system, this attribute needs to be available.  Since
          the fileid is within the present parent file
          system, there should be no need to reference the absent file
          system to provide this information.
        </dd>
          </dl>
          <t>
        Other attributes SHOULD NOT be made available for absent file
        systems, even when it is possible to provide them.  The server
        should not assume that more information is always better and
        should avoid gratuitously providing additional information.
          </t>
          <t>
        When a GETATTR operation includes a bit mask for one of the
        attributes fs_locations, fs_locations_info, or fs_status, but
        where the bit mask includes attributes that are not supported,
        GETATTR will not return an error, but will return the mask
        of the actual attributes supported with the results.
          </t>
          <t>
        Handling of VERIFY/NVERIFY is similar to GETATTR in that if
        the attribute mask does not include fs_locations, fs_locations_info,
        or fs_status, the error NFS4ERR_MOVED will result.  It differs in
        that any appearance in the attribute mask of an attribute not
        supported for an absent file system (and note that this will
        include some normally REQUIRED attributes) will also cause
        an NFS4ERR_MOVED result.
          </t>
        </section>
        <section anchor="absent_readdir" numbered="true" toc="default">
          <name>READDIR and Absent File Systems</name>
          <t>
        A READDIR performed when the current filehandle is within an
        absent file system will result in an NFS4ERR_MOVED error,
        since, unlike the case of GETATTR, no such exception is
        made for READDIR.
          </t>
          <t>
        Attributes for an absent file system may be fetched via a
        READDIR for a directory in a present file system, when that
        directory contains the root directories of one or more absent
        file systems.  In this case, the handling is as follows:
          </t>
          <ul spacing="normal">
            <li>
          If the attribute set requested includes one of the attributes
          fs_locations, fs_locations_info, or fs_status, then fetching of
          attributes proceeds normally and no NFS4ERR_MOVED indication
          is returned, even when the rdattr_error attribute is
          requested.
        </li>
            <li>
          If the attribute set requested does not include one of the
          attributes
          fs_locations, fs_locations_info, or fs_status, then if the
          rdattr_error attribute is requested, each directory entry for
          the root of an absent file system will report
          NFS4ERR_MOVED as the value of the rdattr_error attribute.
        </li>
            <li>
          If the attribute set requested does not include any of the
          attributes fs_locations, fs_locations_info, fs_status, or
          rdattr_error, then the occurrence of the root of an absent
          file system within the directory will result in the
          READDIR failing with an NFS4ERR_MOVED error.
        </li>
            <li>
          The unavailability of an attribute because of a file system's
          absence, even one that is ordinarily REQUIRED, does not result
          in any error indication.  The set of attributes returned for
          the root directory of the absent file system in that case is
          simply restricted to those actually available.
        </li>
          </ul>
        </section>
      </section>
      <section anchor="SEC11-USES" numbered="true" toc="default">
        <name>Uses of File System Location Information</name>
        <t>
      The file system location attributes
      (i.e. fs_locations and fs_locations_info),
      together with the possibility of absent file systems, provide
      a number of important facilities in providing reliable, manageable,
      and scalable data access.
        </t>
        <t>
      When a file system is present, these attributes can provide
        </t>
        <ul spacing="normal">
          <li>
        The locations of alternative replicas, to be used to access the
        same data in the event of server failures,
        communications problems,
        or other difficulties that make continued access to the current
        replica impossible or otherwise impractical.  Provision and
        use of
        such alternate replicas is referred to as "replication"
        and is discussed in
        <xref target="SEC11-USES-repl" format="default"/> below.
      </li>
          <li>
        The network address(es) to be used to access the current file
	system instance or replicas of it.
        Client use of this information is
        discussed in
        <xref target="SEC11-USES-trunk" format="default"/> below.
      </li>
        </ul>
        <t>
      Under some circumstances, multiple replicas
      may be used simultaneously to provide higher-performance
      access to the file system in question, although the lack of state
      sharing between servers may be an impediment to such use.
        </t>
        <t>
      When a file system is present and becomes absent, clients can be
      given the opportunity to have continued access to their data,
      using a different replica.  In this case, a continued attempt
      to use the data in the now-absent file system will result
      in an NFS4ERR_MOVED error and, at that point, the successor
      replica or set of possible replica choices
      can be fetched and used to continue access.  Transfer of access
      to the new replica location is referred to as
      "migration", and is discussed in
      <xref target="SEC11-USES-repl" format="default"/> below.

        </t>
        <t>
      Where a file system is currently absent, specification
      of file system location provides a means by which file systems
      located on one server can be associated with a namespace
      defined by another server, thus allowing a general multi-server
      namespace facility.  A designation of such a remote instance, in
      place of a file system not previously present, is called
      a "pure referral" and is discussed in
      <xref target="SEC11-USES-ref" format="default"/> below.
        </t>
        <t>
      Because client support for attributes related to file
      system location is
      OPTIONAL, a server may choose to take action
      to hide migration and referral events from such clients, by
      acting as a proxy, for example.  The server can determine
      the presence of client support from the arguments of the
      EXCHANGE_ID operation (see
      <xref target="OP_EXCHANGE_ID_DESCRIPTION" format="default"/>).
        </t>
        <section anchor="SEC11-USES-mult" numbered="true" toc="default">
          <name>Combining Multiple Uses in a Single Attribute</name>
          <t>
        A file system location attribute will sometimes contain information
        relating to the location of multiple replicas which may
        be used in different ways.
          </t>
          <ul spacing="normal">
            <li>
          File system location entries that relate to the file system instance
	  currently in
          use provide trunking information, allowing the client to
          find additional network addresses by which the instance may be
          accessed.
        </li>
            <li>
          File system location entries that provide information about
          replicas to which access is to
          be transferred.
        </li>
            <li>
          Other file system location entries that relate to replicas
	  that are available to
          use in the event that access to the current replica becomes
          unsatisfactory.
        </li>
          </ul>
          <t>
        In order to simplify client handling and allow the best choice
        of replicas to access, the server should adhere to the following
        guidelines.
          </t>
          <ul spacing="normal">
            <li>
          All file system location entries that relate to a
	  single file system instance
	  should be
          adjacent.
        </li>
            <li>
          File system location entries that relate to the instance
	  currently in use
          should appear first.
        </li>
            <li>
          File system location entries that relate to replica(s)
	  to which migration
          is occurring should appear before replicas which are available
          for later use if the current replica should become inaccessible.

        </li>
          </ul>
        </section>
        <section anchor="SEC11-USES-trunk" numbered="true" toc="default">
          <name>File System Location Attributes and Trunking</name>
          <t>
        Trunking is the use of multiple connections between a client and
        server in order to increase the speed of data transfer.
        A client may determine the set of network addresses to use to
        access a given file system in a number of ways:
          </t>
          <ul spacing="normal">
            <li>
	  When the name of the server is known to the client, it may use
	  DNS to obtain a set of network addresses to use in
	  accessing the server.
        </li>
            <li>
	  The client
	  may fetch the file system location attribute for the
	  file system. This will
	  provide either the name of the server (which can be turned
	  into a set of network addresses using DNS), or
	  a set of server-trunkable location entries.  Using the latter
	  alternative, the server can
	  provide addresses it regards as desirable to use
	  to access the file system in question.  Although these entries can
	  contain port numbers, these port numbers are not used in determining
	  trunking relationships.  Once the candidate addresses have been
	  determined and EXCHANGE_ID done to the proper server, only the value
	  of the so_major field returned by the servers in question determines
	  whether a trunking relationship actually exists.
        </li>
          </ul>
          <t>
	It should be noted that the client, when it fetches a location
	attribute for a file system, may encounter multiple entries for
	a number of reasons, so that, when determining trunking information,
	it may have to bypass addresses not trunkable with one already
	known.
          </t>
          <t>
        The server can provide location entries that include either
        names or network addresses.  It might use the latter form
        because of DNS-related security concerns or because the set
        of addresses
        to be used might require active management by the server.
          </t>
          <t>
        Location entries used to discover candidate addresses for
        use in trunking
        are subject to change, as discussed in
        <xref target="SEC11-USES-changes" format="default"/> below.
        The client may respond to
        such changes by using additional addresses once they are
        verified or by ceasing to use
        existing ones.  The server can force the client to cease using
        an address by returning NFS4ERR_MOVED when that address is used to
        access a file system.  This allows a transfer of client access
	which is similar to
        migration, although the same file system instance
	is accessed throughout.
          </t>
        </section>
        <section anchor="SEC11-USES-types" numbered="true" toc="default">
          <name>File System Location Attributes and Connection Type Selection</name>
          <t>
	Because of the need to support multiple types of connections,
	clients face
	the issue of determining the proper connection type to use
	when establishing
	a connection to a given server network address.  In some cases,
	this issue can be addressed through the use of the connection
	"step-up" facility described in
	<xref target="OP_CREATE_SESSION" format="default"/>.  However,
	because there are cases is which that facility is not available,
	the client may have to choose a connection type with no
	possibility of changing it within the scope of a single connection.
          </t>
          <t>
	The two file system location attributes differ as to the
	information made
	available in this regard.   Fs_locations provides no information
	to support connection type selection.  As a result, clients
	supporting multiple connection types would need to attempt to
	establish connections using multiple connection types until
	the one preferred
	by the client is successfully established.
          </t>
          <t>
	Fs_locations_info includes a flag, FSLI4TF_RDMA, which, when
	set indicates that RPC-over-RDMA support is available using
	the specified location entry, by "stepping up" an existing
	TCP connection to include support for RDMA operation. This flag
	makes it convenient for a client wishing to use RDMA.  When
	this flag is set, it can
	establish a TCP connection and then convert that connection
	to use RDMA by using the step-up facility.

          </t>
          <t>
	Irrespective of the particular attribute used, when there is
	no indication that a step-up operation can be performed,
	a client supporting RDMA operation can establish a new RDMA
	connection and it can be bound to
	the session already established by the
	TCP connection, allowing the TCP connection to be dropped
	and the session converted to further use in RDMA mode, if
	the server supports that.
          </t>
        </section>
        <section anchor="SEC11-USES-repl" numbered="true" toc="default">
          <name>File System Replication</name>
          <t>
        The fs_locations and fs_locations_info attributes provide
        alternative file system locations, to be used to access data in place
        of or in addition to
        the current file system instance.  On first access to a
        file system, the client should obtain the set
        of alternate locations by interrogating the fs_locations or
        fs_locations_info attribute, with the latter being preferred.
          </t>
          <t>
        In the event that the occurrence of server failures, communications
	problems,
        or other difficulties make continued access to the current
        file system impossible or otherwise impractical, the client
        can use the alternate locations as a way to get continued
        access to its data.
          </t>
          <t>
        The alternate locations may be physical replicas of the
        (typically read-only) file system data supplemented by
	possible asynchronous propagation of updates.  Alternatively,
	they may
        provide
        for the use of various forms of server
        clustering in which multiple servers provide alternate
        ways of accessing the same physical file system.  How the
        difference between replicas affects file system transitions
	can be represented
        within the fs_locations and fs_locations_info attributes
        and how the client deals with
        file system transition issues will be discussed in detail in
	later sections.
          </t>
          <t>
	Although the location attributes provide some information about
	the nature of the inter-replica transition, many aspects of the
	semantics of possible asynchronous updates are not currently described
	by the protocol, making it necessary that clients using replication
	to switch among replicas undergoing change familiarize themselves
	with the semantics of the update approach used.   Because of this
	lack of specificity, many applications may find use of migration more
	appropriate, since, in that case, the server, when effecting the
	transition, has established a point in time such that all updates made
	before that can propagated to the new replica as part of the migration
	event.
          </t>
          <section anchor="SEC11-USES-repl-trunk" numbered="true" toc="default">
            <name>File System Trunking Presented as Replication</name>
            <t>
	  In some situations, a file system location entry may indicate
	  a file system access path to be used as an alternate location,
	  where trunking, rather than replication, is to be used.   The
	  situations in which this is appropriate are limited to those
	  in which both of the following are true.
            </t>
            <ul spacing="normal">
              <li>
	    The two file system locations (i.e., the one on which the
	    location attribute is obtained and the one specified in the
	    file system location entry) designate the same locations within
	    their respective single-server namespaces.
	  </li>
              <li>
	    The two server network addresses (i.e., the one being used to
	    obtain
	    the location attribute and the one specified in the file system
	    location entry) designate the same server (as indicated by the
	    same value of the so_major_id field of the eir_server_owner field
	    returned in response to EXCHANGE_ID).
	  </li>
            </ul>
            <t>
	  When these conditions hold, operations using both access paths are
	  generally trunked, although, when the attribute fs_locations_info
	  is used, trunking may be disallowed:
            </t>
            <ul spacing="normal">
              <li>
                <t>
	    When the fs_locations_info attribute shows the two entries
	    as not having the same simultaneous-use class, trunking is
	    inhibited and the two access paths cannot be used together.
                </t>
                <t>
	    In this case the two paths can be used serially with
            no transition activity required on the part of the client.  In
	    this case, any transition between access paths is transparent,
	    and the client, in transferring access from one to the other, is
	    acting as it would in the event that communication is interrupted,
	    with a new connection and possibly a new session being established
	    to continue access to the same file system.
                </t>
              </li>
              <li>
	    Note that for two such location entries, any information within
	    the fs_locations_info attribute that indicates the need for special
            transition activity, i.e., the appearance of the two file system
            location entries with different handle, fileid, write-verifier,
	    change, and readdir classes, indicates a serious problem.  The
	    client, if it allows transition to the file system instance at
	    all, must not treat any transition as a transparent one.
            The server SHOULD NOT indicate that these two entries (for the
	    same file system on the same server) belong to
	    different handle, fileid, write-verifier, change, and readdir
	    classes, whether or not the two entries are shown belonging to
	    the same simultaneous-use class.
	  </li>
            </ul>
            <t>
          These situations were recognized by <xref target="RFC5661" format="default"/>,
	  even though
          that document made no explicit mention of trunking.
            </t>
            <ul spacing="normal">
              <li>
            It treated the situation that we describe as trunking as one
	    of simultaneous use of two distinct file system instances,
	    even though, in the explanatory framework now used to
            describe the situation, the case is one in which a single file
            system is accessed by two different trunked addresses.
	  </li>
              <li>
	    It treated the situation in which two paths are to be used
	    serially as a special sort of "transparent transition". however,
	    in the descriptive framework now used to categorize transition
	    situations, this is considered a case of a "network endpoint
	    transition"
	    (see <xref target="SEC11-trans-oview" format="default"/>).
	  </li>
            </ul>
          </section>
        </section>
        <section anchor="SEC11-USES-migr" numbered="true" toc="default">
          <name>File System Migration</name>
          <t>
        When a file system is present and becomes inaccessible using the
	current access path, the NFSv4.1
	protocol provides a means by which clients can be given the
	opportunity to have continued access to their data.   This may
	involve use of a different access path to the existing replica or
	by providing a path to a different replica.  The new access path or
	the location of the new replica is specified
	by a file system
	location attribute.
        The ensuing migration of access includes
	the ability
        to retain locks across the transition.  Depending on circumstances,
	this can involve:
          </t>
          <ul spacing="normal">
            <li>
	    The continued use of the existing clientid when accessing
	    the current
	    replica using a new access path.
	  </li>
            <li>
	    Use of lock reclaim, taking advantage of a per-fs grace period.
	  </li>
            <li>
	    Use of Transparent State Migration.
	  </li>
          </ul>
          <t>
        Typically, a client will be
        accessing the file system in question, get an NFS4ERR_MOVED
        error, and then use a file system location attribute
        to determine the new access path for the data.  When
        fs_locations_info is used, additional information will be
        available that will define the nature of the client's
        handling of the transition to a new server.
          </t>
          <t>
        In most instances, servers will choose to migrate all clients using
	a particular file system to a successor replica at the same time
	to avoid cases in which different clients are updating different
	replicas.  However migration of individual client can be helpful
	in providing load balancing, as long as the replicas in question
	are such that they represent the same data as described in
	<xref target="SEC11-EFF-data" format="default"/>.
          </t>
          <ul spacing="normal">
            <li>
	  In the case in which there is no transition between replicas (i.e.,
	  only a change in access path), there are no special
	  difficulties in using of this
	  mechanism to effect load balancing.
        </li>
            <li>
	  In the case in which the two replicas are sufficiently co-ordinated
	  as to allow coherent simultaneous access to both by a single client,
	  there is, in general, no obstacle to use of migration of particular
	  clients to effect load balancing.  Generally, such simultaneous use
	  involves co-operation between servers to ensure that locks granted
	  on two co-ordinated replicas cannot conflict and can remain effective
	  when transferred to a common replica.
        </li>
            <li>
	  In the case in which a large set of clients are accessing a
	  file system in a read-only fashion, in can be helpful to migrate
	  all clients with writable access simultaneously, while using
	  load balancing on the set of read-only copies, as long as the
	  rules appearing in <xref target="SEC11-EFF-data" format="default"/>, designed to
	  prevent data reversion are adhered to.
        </li>
          </ul>
          <t>
	In other cases, the client might not have sufficient guarantees
	of data similarity/coherence to function properly (e.g. the data
	in the two replicas is similar but not identical), and the
	possibility that different clients are updating different replicas
	can exacerbate the difficulties, making use of load balancing in
	such situations a perilous enterprise.
          </t>
          <t>
	The protocol
        does not specify how the file system will be moved between
        servers or how updates to multiple replicas will be co-ordinated.
	It is anticipated that a number of different
        server-to-server co-ordination mechanisms might be used with the
        choice left to the server implementer.  The NFSv4.1 protocol
        specifies the method used to communicate the migration
        event between client and server.
          </t>
          <t>
        The new location may be, in the case of
        various forms of server
        clustering, another server providing
        access to the same physical file system.  The client's
        responsibilities in dealing with this transition will depend
        on whether a switch between replicas has occurred and
	the means the server
        has chosen to provide continuity of locking state.
        These issues will be discussed in
        detail below.
          </t>
          <t>
        Although a single successor location is typical, multiple
        locations may be provided.  When multiple locations are
        provided, the client will typically use the first one provided.
	If that is
        inaccessible for some reason, later ones can be used.  In such
        cases the client might consider the transition to the new
        replica to be a migration event, even though some of the servers
	involved might not be aware of the use of the server which was
	inaccessible.  In such a case, a client might lose access to
        locking state as a result of the access transfer.
          </t>
          <t>
        When an alternate location is designated as the target for
        migration, it must designate the same data
        (with metadata being the same to the degree indicated by the
        fs_locations_info attribute).  Where file systems are writable,
        a change made on the original file system must be visible on
        all migration targets. Where a file system is not writable
        but represents a read-only copy (possibly periodically
        updated) of
        a writable file system, similar requirements apply to the
        propagation of updates.  Any change visible in the original
        file system must already be effected on all migration targets,
        to avoid any possibility that a client,
        in effecting a transition to
        the migration target, will see any reversion
        in file system state.
          </t>
        </section>
        <section anchor="SEC11-USES-ref" numbered="true" toc="default">
          <name>Referrals</name>
          <t>
        Referrals allow the server to associate a file system namespace
	entry located on
        one server with a file system located on another server.
        When this includes
        the use of pure referrals, servers are provided a way of
        placing a file system in a location
        within the namespace
        essentially without respect to its physical location on a
        particular server.
        This allows a single server or a set of servers
        to present a multi-server namespace that encompasses file systems
        located on a wider range of
        servers.  Some likely uses of this facility include
        establishment of site-wide or organization-wide namespaces,
        with the eventual possibility of combining such
        together into a truly global namespace, such as the one
	provided by AFS (the Andrew File System) <xref target="AFS" format="default"/>.
          </t>
          <t>
        Referrals occur when a client determines, upon first referencing
        a position in the current namespace, that it is part of a new
        file system and that the file system is absent.  When this
        occurs, typically upon receiving the error NFS4ERR_MOVED, the
        actual location or locations of the file system can be
        determined by fetching a locations attribute.
          </t>
          <t>
        The file system location attribute may designate a single
        file system location or multiple file system locations, to
        be selected based on the needs of the client.  The server,
        in the fs_locations_info attribute, may specify priorities to
        be associated with various file system location choices.
        The server may assign different priorities to different
        locations as reported to individual clients, in order to
        adapt to client physical location or to effect load balancing.
        When both read-only and read-write file systems are present,
        some of the read-only locations might not be absolutely up-to-date
        (as they would have to be in the case of replication and
        migration).  Servers may also specify file system locations
        that include client-substituted variables so that different
        clients are referred to different file systems (with different
        data contents) based on client attributes such as CPU
        architecture.
          </t>
          <t>
        When the fs_locations_info attribute is such that that there are
        multiple possible targets listed, the relationships among them
        may be important to the client in selecting which one to use.
        The same rules specified in <xref target="SEC11-USES-migr" format="default"/>
        below regarding multiple migration targets
        apply to these multiple replicas as well.  For example, the
        client might prefer a writable target on a server that has
     	additional writable
        replicas to which it subsequently might switch.  Note that,
        as distinguished from the case of replication, there is no
        need to deal with the case of propagation of updates made by
        the current client, since the current client has not accessed
        the file system in question.
          </t>
          <t>
        Use of multi-server namespaces is enabled by NFSv4.1 but is not
        required.  The use of multi-server namespaces and their scope
        will depend on the applications used and system administration
        preferences.
          </t>
          <t>
        Multi-server namespaces can be established by a single
        server providing a large set of pure referrals to all of the
        included file systems.  Alternatively, a single multi-server
        namespace may be administratively segmented with separate
        referral file systems (on separate servers) for each
        separately administered portion of the namespace. The
        top-level referral file system or any segment may use
        replicated referral file systems for higher availability.
          </t>
          <t>
        Generally, multi-server namespaces are for the most part
        uniform, in that the same data made available to one client
        at a given location in the namespace is made available to
        all clients at that namespace location.  However,
	there are facilities
        provided that allow different clients to be directed to
        different sets of data, for reasons such as enabling
        adaptation to such client
        characteristics as CPU architecture.  These facilities are
	described in
	<xref target="SEC11-fsli-item" format="default"/>.
          </t>
          <t>
	Note that it is possible, when providing a uniform namespace,
	to provide different location entries to different clients, in
	order to provide each client with a copy of the data physically
	closest to it, or otherwise optimize access (e.g. provide load
	balancing).
          </t>
        </section>
        <section anchor="SEC11-USES-changes" numbered="true" toc="default">
          <name>Changes in a File System Location Attribute</name>
          <t>
        Although clients will typically fetch a file system
	location attribute
        when first accessing a file system and when NFS4ERR_MOVED
        is returned, a client can choose to fetch the attribute
        periodically, in which case the value fetched may change over
        time.
          </t>
          <t>
        For clients not prepared to access multiple
        replicas simultaneously (see
        <xref target="SEC11-EFF-simul" format="default"/>),
        the handling of the various cases of location change are as follows:
          </t>
          <ul spacing="normal">
            <li>
	    Changes in the list of replicas or in the network addresses
	    associated with replicas do not require immediate action.
	    The client will typically update its list of replicas to
	    reflect the new information.
          </li>
            <li>
	    Additions to the list of network addresses for the
	    current file system instance need not be acted
	    on promptly.  However, to prepare for the case in which
	    a migration event occurs subsequently, the client can choose
	    to take note of the new address and then use it
	    whenever it needs to switch access to a new
            replica.
          </li>
            <li>
	    Deletions from the list of network addresses for the
	    current file system instance do not need to be acted on
	    immediately
	    by ceasing use of existing access paths although new connections
	    are not to be established on addresses that have been deleted.
	    However, clients can choose to act on such deletions
	    by making preparations for an eventual shift in access which
	    would become unavoidable as soon as the
	    server indicates that a particular network access path is
            not usable to access the current file system,
	    by returning NFS4ERR_MOVED.
          </li>
          </ul>
          <t>
        For clients that are prepared to access several replicas
        simultaneously,
        the following additional cases need to be addressed.  As in
        the cases discussed above, changes in the set of replicas
        need not be acted upon promptly, although the client has
        the option of adjusting its access even in the absence of
        difficulties that would lead to a new replica to be selected.
          </t>
          <ul spacing="normal">
            <li>
          When a new replica is added which may be accessed
          simultaneously with one currently in use, the client is free
          to use the new replica immediately.
        </li>
            <li>
          When a replica currently in use is deleted from the list, the
          client need not cease using it immediately.  However, since
          the server may subsequently force such use to cease (by
          returning NFS4ERR_MOVED), clients might decide to limit the
          need for later state transfer.  For example, new opens might
          be done on other replicas, rather than on one not present in
          the list.
        </li>
          </ul>
        </section>
      </section>
      <section anchor="SEC11-TRUNK" numbered="true" toc="default">
        <name>Trunking without File System Location Information</name>
        <t>
      In situations in which a file system is accessed using two
      server-trunkable addresses (as indicated by the same value of the
      so_major_id field of the eir_server_owner field returned in
      response to EXCHANGE_ID), trunked access is allowed even though
      there might not be any location entries specifically indicating
      the use of trunking for that file system.
        </t>
        <t>
      This situation was recognized by <xref target="RFC5661" format="default"/>, even though
      that document made no explicit mention of trunking and treated the
      situation as one of simultaneous use of two distinct file system
      instances, even though, in the explanatory framework now used to
      describe the situation, the case is one in which a single file
      system is accessed by two different trunked addresses.
        </t>
      </section>
      <section anchor="SEC11-users" numbered="true" toc="default">
        <name>Users and Groups in a Multi-server Namespace</name>
        <t>
      As in the case of a single-server environment (see
      <xref target="owner_owner_group" format="default"/>,
      when an owner or group name of the form "id@domain" is assigned to
      a file, there is an implicit promise to return that same string when
      the corresponding attribute is interrogated subsequently.  In the
      case of a multi-server namespace, that same promise applies even if
      server boundaries have been crossed.   Similarly, when the owner
      attribute of a file is derived from the security principal which created
      the file, that attribute should have the same value even if the
      interrogation occurs on a different server from the file creation.
        </t>
        <t>
      Similarly, the set of security principals recognized by all the
      participating servers needs to be the same, with each such principal
      having the same credentials, regardless of the particular server
      being accessed.
        </t>
        <t>
      In order to meet these requirements, those setting up multi-server
      namespaces will need to limit the servers included so that:
        </t>
        <ul spacing="normal">
          <li>
	In all cases in which more than a single domain is supported,
	the requirements stated in RFC8000 <xref target="RFC8000" format="default"/>
	are to be respected.
      </li>
          <li>
	All servers support a common set of domains which includes all of
	the domains clients use and expect to see returned as the domain
	portion of an owner or group in the form "id@domain".   Note that
	although this set most often consists of a single domain, it is
	possible for multiple domains to be supported.
      </li>
          <li>
	All servers, for each domain that they support, accept the same set
	of user and group ids as valid.
      </li>
          <li>
	All servers recognize the same set of security principals.  For each
	principal, the same credential is required, independent of the
	server being accessed.  In addition, the group membership for each such
	principal is to be the same, independent of the server accessed.
      </li>
        </ul>
        <t>
      Note that there is no requirement in general  that the users
      corresponding to
      particular security principals have the same local
      representation on each server,
      even though it is most often the case that this is so.
        </t>
        <t>
      When AUTH_SYS is used, the following additional requirements must be
      met:
        </t>
        <ul spacing="normal">
          <li>
	Only a single NFSv4 domain can be supported through use of AUTH_SYS.
      </li>
          <li>
	The "local" representation of all owners and groups must be the same
	on all servers.  The word "local" is used here since that is the
	way that numeric user and group ids are described in
	<xref target="owner_owner_group" format="default"/>.   However,
	when AUTH_SYS or stringified numeric owners or
	groups are used, these identifiers are not truly local, since they
	are known to the clients as well as the server.
      </li>
        </ul>
        <t>
      Similarly, when stringified numeric user and group ids are used, the
      "local" representation of all owners and groups must be the same on
      all servers, even when AUTH_SYS is not used.
        </t>
      </section>
      <section anchor="SEC11-csr" numbered="true" toc="default">
        <name>Additional Client-Side Considerations</name>
        <t>
      When clients make use of servers that implement referrals,
      replication, and
      migration, care should be taken that a user who mounts a given
      file system that includes a referral or a relocated file system
      continues to see a coherent picture of that user-side file system
      despite the fact that it contains a number of server-side
      file systems that may be on different servers.
        </t>
        <t>
      One important issue is upward navigation from the root of a
      server-side file system to its parent (specified as ".." in UNIX),
      in the case in which it transitions to that file system as a
      result of referral, migration, or a transition as a result of
      replication.  When the client is at such a point, and it needs to ascend to
      the parent, it must go back to the parent as seen within the
      multi-server namespace rather than sending a LOOKUPP operation to the
      server, which would result in the parent within that server's
      single-server namespace.  In order to do this, the client
      needs to remember the filehandles that represent such
      file system roots and use these instead of sending a
      LOOKUPP operation to the current server.  This will allow the client
      to present to applications a consistent namespace, where
      upward navigation and downward navigation are consistent.
        </t>
        <t>
      Another issue concerns refresh of referral locations.  When
      referrals are used extensively, they may change as server
      configurations change.  It is expected that clients will cache
      information related to traversing referrals so that future
      client-side requests are resolved locally without server
      communication.
      This is usually rooted in client-side name look up caching. Clients
      should periodically purge this data for referral points in order to
      detect changes in location information.  When the change_policy
      attribute changes for directories that hold referral entries
      or for the referral entries themselves, clients should consider
      any associated
      cached referral information to be out of date.
        </t>
      </section>
      <section anchor="SEC11-trans-oview" numbered="true" toc="default">
        <name>Overview of File Access Transitions</name>
        <t>
      File access transitions are of two types:
        </t>
        <ul spacing="normal">
          <li>
        Those that involve a transition from accessing the current
        replica to another one in connection with either
        replication or migration.
        How these are dealt with is discussed in
        <xref target="SEC11-EFF" format="default"/>.
      </li>
          <li>
        Those in which access to the current file system instance
	is retained, while
        the network path used to access that instance is changed.
	This case is
        discussed in <xref target="SEC11-nwa" format="default"/>.
      </li>
        </ul>
      </section>
      <section anchor="SEC11-nwa" numbered="true" toc="default">
        <name>Effecting Network Endpoint Transitions</name>
        <t>
      The endpoints used to access a particular file system instance
      may change in a number of ways, as listed below.  In each of these
      cases, the same fsid, filehandles, stateids, client IDs and
      are
      used to continue access, with a continuity of lock state.   In
      many cases, the same sessions can also be used.
        </t>
        <t>
      The appropriate action depends on the set of replacement addresses
      (i.e. server endpoints which are server-trunkable with one previously
      being used) which are available for use.
        </t>
        <ul spacing="normal">
          <li>
        When use of a particular address is to cease and there is
        also another one
        currently in use which is server-trunkable with it, requests
        that would have been issued on the address whose use is to be
	discontinued can be issued on the remaining address(es).  When an
	address is server-trunkable but not session-trunkable with the
	address whose use is to be discontinued, the request might need
	to be modified to reflect the fact that a different session will
	be used.
      </li>
          <li>
	When use of a particular connection is to cease, as indicated
	by receiving NFS4ERR_MOVED when using that connection but
	that address is
	still indicated as accessible according to the appropriate
	file system location
	entries, it is likely that requests can be issued on a new
	connection of a different connection type, once that connection
	is established. Since any two, non-port-specific
	server endpoints that share a network address are inherently
	session-trunkable, the client can use BIND_CONN_TO_SESSION
        to access the existing session using the new connection and
	proceed to access the file system using the new connection.
      </li>
          <li>
        When there are no potential replacement addresses in use but there
        are valid addresses session-trunkable with the one whose use is
        to be discontinued, the client can use BIND_CONN_TO_SESSION
        to access the existing session using the new address.  Although
        the target session will generally be accessible, there may be
        rare situations in which that session is no longer accessible,
	when an attempt is made to bind the new connection to it.
	In this
        case, the client can create a new session to enable continued
        access to the existing instance using the new connection,
	providing for use of existing
	filehandles, stateids, and client ids while providing continuity
	of locking state.
      </li>
          <li>
        When there is no potential replacement address in use and there
        are no
        valid addresses session-trunkable with the one whose use is
        to be discontinued, other server-trunkable addresses may be
        used to provide continued access.  Although use of CREATE_SESSION
        is available to provide continued access to the existing instance,
        servers have the option of providing continued access to the
        existing session through the new network access
	path in a fashion similar to
        that provided by session migration (see
        <xref target="SEC11-trans-locking" format="default"/>).
        To take advantage of this
        possibility, clients can perform an initial BIND_CONN_TO_SESSION,
        as in the previous case, and use CREATE_SESSION only if that
        fails.
      </li>
        </ul>
      </section>
      <section anchor="SEC11-EFF" numbered="true" toc="default">
        <name>Effecting File System Transitions</name>
        <t>
      There are a range of situations in which there is a change to be
      effected in the set of replicas used to access a particular
      file system.  Some of these may involve an expansion or
      contraction of the set of replicas used as discussed in
      <xref target="SEC11-EFF-simul" format="default"/> below.
        </t>
        <t>
      For reasons explained in that section, most transitions will involve
      a transition from a single replica to a corresponding replacement
      replica.  When effecting replica transition, some types of
      sharing between the replicas may affect handling of the
      transition as described in
      Sections <xref target="SEC11-EFF-fh" format="counter"/>
      through <xref target="SEC11-EFF-data" format="counter"/> below.
      The attribute fs_locations_info provides helpful information
      to allow the client to determine the degree of inter-replica
      sharing.
        </t>
        <t>
      With regard to some types of state,  the degree of continuity
      across the transition
      depends on the occasion prompting the transition, with
      transitions initiated by the servers
      (i.e. migration) offering much more scope for a non-disruptive
      transition than cases in which the client on its own
      shifts its access to
      another replica (i.e. replication).  This issue
      potentially applies to
      locking state and to session state, which are dealt with below as
      follows:
        </t>
        <ul spacing="normal">
          <li>
        An introduction to the possible means of providing continuity in
        these areas appears in <xref target="SEC11-EFF-lock" format="default"/> below.
      </li>
          <li>
        Transparent State Migration is introduced in
        <xref target="SEC11-trans-locking" format="default"/>.
        The possible transfer of
        session state is addressed there as well.
      </li>
          <li>
        The client handling of transitions, including determining how to
        deal with the various means that the server might take to
        supply effective continuity of locking state is discussed in
	<xref target="SEC11-trans-client" format="default"/>.
      </li>
          <li>
        The servers' (source and destination) responsibilities
        in effecting Transparent Migration
        of locking and session state are discussed in
        <xref target="SEC11-trans-server" format="default"/>.
      </li>
        </ul>
        <section anchor="SEC11-EFF-simul" numbered="true" toc="default">
          <name>File System Transitions and Simultaneous Access</name>
          <t>
        The fs_locations_info attribute (described in
	<xref target="SEC11-li-new" format="default"/>)
	may indicate that two replicas
        may be used simultaneously, although some situations in which such
	simultaneous access is permitted are more appropriately described
	as instances of trunking (see <xref target="SEC11-USES-repl-trunk" format="default"/>).
	Although situations
        in which multiple replicas may be accessed simultaneously are
        somewhat similar to those in which a single replica is
        accessed by multiple network addresses, there are important
        differences, since locking state is not shared among multiple
        replicas.
          </t>
          <t>
         Because of this difference in state handling, many clients will
         not have the ability to take advantage of the fact that such
         replicas represent the same data.  Such clients will not be
         prepared to use multiple replicas simultaneously but will access
         each file system using only a single replica, although the
         replica selected might make multiple server-trunkable addresses
         available.
          </t>
          <t>
        Clients who are prepared to use multiple replicas simultaneously
        will divide opens among replicas however they choose.  Once that
        choice is made,
        any subsequent transitions will treat the set of locking
        state associated with each replica as a single entity.
          </t>
          <t>
        For example, if one of the replicas become unavailable,
        access will be
        transferred  to a different replica, also capable of
        simultaneous access with the one still in use.
          </t>
          <t>
        When there is no such replica, the transition may be to the
        replica already in use.  At this point, the client has a
        choice between merging the locking state for the two replicas
        under the aegis of the sole replica in use or treating these
        separately, until another replica capable of simultaneous
        access presents itself.
          </t>
        </section>
        <section anchor="SEC11-EFF-fh" numbered="true" toc="default">
          <name>Filehandles and File System Transitions</name>
          <t>
        There are a number of ways in which filehandles can be handled
        across a file system transition.  These can be divided into
        two broad classes depending upon whether the two file systems
        across which the transition happens share sufficient state to
        effect some sort of continuity of file system handling.
          </t>
          <t>
        When there is no such cooperation in filehandle assignment,
        the two file systems are reported as being in different
        handle classes.  In this case,
        all filehandles are assumed to expire as part of the
        file system transition.  Note that this behavior does not
        depend on the fh_expire_type attribute and supersedes
	the specification
        of the FH4_VOL_MIGRATION bit, which only affects behavior when
        fs_locations_info is not available.
          </t>
          <t>
        When there is cooperation in filehandle assignment,
        the two file systems are reported as being in the same
        handle classes.  In this case,
        persistent filehandles remain valid after the file system
        transition, while volatile filehandles (excluding those
        that are only volatile due to the FH4_VOL_MIGRATION bit) are
        subject to expiration on the target server.
          </t>
        </section>
        <section anchor="SEC11-EFF-fileid" numbered="true" toc="default">
          <name>Fileids and File System Transitions</name>
          <t>
        In NFSv4.0, the issue of continuity of fileids in the event
        of a file system transition was not addressed.  The general
        expectation had been that in situations in
        which the two file system instances are created by a single vendor
        using some sort of file system image copy, fileids would be
        consistent across the transition, while in the analogous
        multi-vendor transitions they would not.  This poses difficulties,
        especially for the client without special knowledge
        of the transition mechanisms adopted by the server.  Note
        that although fileid is not a REQUIRED attribute, many servers
        support fileids and many clients provide APIs that depend on fileids.
          </t>
          <t>
        It is important to note that while clients themselves may have no
        trouble with a fileid changing as a result of a file system
        transition event, applications do typically have access to the
        fileid (e.g., via stat).  The result is that an
        application may work perfectly well if there is no file system
        instance transition or if any such transition is among instances
        created by a single vendor, yet be unable to deal with the
        situation in which a multi-vendor transition occurs at the wrong
        time.
          </t>
          <t>
        Providing the same fileids in a multi-vendor (multiple server
        vendors) environment has generally been held to be quite difficult.
        While there is work to be done, it needs to be pointed out that
        this difficulty is partly self-imposed.  Servers have typically
        identified fileid with inode number, i.e. with a quantity used to
        find the file in question.  This identification poses special
        difficulties for migration of a file system between vendors
        where assigning
        the same index to a given file may not be possible.  Note here that
        a fileid is not required to be useful to find the file in
        question, only that it is unique within the given file system.  Servers
        prepared to accept a fileid as a single piece of metadata and store
        it apart from the value used to index the file information can
        relatively easily maintain a fileid value across a migration event,
        allowing a truly transparent migration event.
          </t>
          <t>
        In any case, where servers can provide continuity of fileids, they
        should, and the client should be able to find out that such
        continuity is available and take appropriate action.  Information
        about the continuity (or lack thereof) of fileids across a file
        system transition is represented by specifying whether the file systems
        in question are of the same fileid class.
          </t>
          <t>
        Note that when consistent fileids do not exist across a
        transition (either because there is no continuity of fileids
        or because fileid is not a supported attribute on one of
        instances involved), and there are
        no reliable filehandles across a transition event (either because
        there is no filehandle continuity or because the filehandles are
        volatile), the client is in a position where it cannot verify
        that files it was accessing before the transition are the
        same objects.  It is forced to assume that no object has been
        renamed, and, unless there are guarantees that provide this
        (e.g., the file system is read-only), problems for applications
        may occur.  Therefore, use of such configurations should be
        limited to situations where the problems that this may cause
        can be tolerated.
          </t>
        </section>
        <section anchor="SEC11-EFF-fsid" numbered="true" toc="default">
          <name>Fsids and File System Transitions</name>
          <t>
        Since fsids are generally only unique on a per-server basis,
        it is likely that they will change during a file system
        transition.
        Clients should not make the fsids received
        from the server visible to applications since they may not be
        globally unique, and because they may change during a file
        system transition event.  Applications are best served if they
        are isolated from such transitions to the extent possible.
          </t>
          <t>
        Although normally a single source file system will transition
        to a single target file system, there is a provision for splitting
        a single source file system into multiple target file systems, by
        specifying the FSLI4F_MULTI_FS flag.
          </t>
          <section anchor="SEC11-EFF-fsid-split" numbered="true" toc="default">
            <name>File System Splitting</name>
            <t>
          When a file system transition is made and the fs_locations_info
          indicates that the file system in question might be split into
          multiple file systems (via the FSLI4F_MULTI_FS flag), the client
          SHOULD do GETATTRs to determine the fsid attribute on all known
          objects within the file system undergoing transition to determine
          the new file system boundaries.
            </t>
            <t>
          Clients might choose to
	  maintain the fsids passed to existing applications
          by mapping all of the fsids for the descendant file systems to
          the common fsid used for the original file system.
            </t>
            <t>
          Splitting a file system can be done on a transition between
          file systems of the same fileid
          class, since the fact that fileids are unique within the
          source file system ensure they will be unique in each of the
          target file systems.
            </t>
          </section>
        </section>
        <section anchor="SEC11-EFF-change" numbered="true" toc="default">
          <name>The Change Attribute and File System Transitions</name>
          <t>
        Since the change attribute is defined as a server-specific one,
        change attributes fetched from one server are normally presumed to
        be invalid on another server.  Such a presumption is troublesome
        since it would invalidate all cached change attributes, requiring
        refetching.  Even more disruptive, the absence of any assured
        continuity for the change attribute means that even if the same
        value is retrieved on refetch, no conclusions can be drawn as to whether
        the object in question has changed.  The identical change
        attribute could be merely an artifact of a modified file with
        a different change attribute construction algorithm, with that
        new algorithm just happening to result in an identical change
        value.
          </t>
          <t>
        When the two file systems have consistent change attribute formats,
        and this fact is communicated to the client by reporting
        in the same change class, the
        client may assume a continuity of change attribute construction
        and handle this situation just as it would be handled without
        any file system transition.
          </t>
        </section>
        <section anchor="SEC11-EFF-wv" numbered="true" toc="default">
          <name>Write Verifiers and File System Transitions</name>
          <t>
        In a file system transition, the two file systems might be
        cooperating in the handling of unstably written data.
        Clients can determine if this is the
        case, by seeing if the two file systems belong to the same
        write-verifier class.   When this is the case,  write
        verifiers returned
        from one system may be compared to those returned  by the
        other and superfluous
        writes avoided.
          </t>
          <t>
        When two file systems belong to different
        write-verifier classes, any verifier
        generated by one must not be compared to one provided by the
        other.  Instead, the two verifiers should be treated as not
        equal even when
        the values are identical.
          </t>
        </section>
        <section anchor="SEC11-EFF-rdc" numbered="true" toc="default">
          <name>Readdir Cookies and Verifiers and File System Transitions</name>
          <t>
        In a file system transition, the two file systems might be
        consistent in their handling of READDIR cookies and verifiers.
        Clients can determine if this is the
        case, by seeing if the two file systems belong to the same
        readdir class.  When this is the
        case, readdir class, READDIR
        cookies and verifiers
        from one system will be recognized by the other and
        READDIR operations started on one server can  be validly
        continued on the other, simply by presenting the
        cookie and verifier returned by a READDIR operation done
        on the first file system to the second.
          </t>
          <t>
        When two file systems belong to different
        readdir classes, any READDIR
        cookie and verifier
        generated by one is not valid on the second, and must not
        be presented to that server by the client.  The client
        should act as if the verifier were rejected.
          </t>
        </section>
        <section anchor="SEC11-EFF-data" numbered="true" toc="default">
          <name>File System Data and File System Transitions</name>
          <t>
        When multiple replicas exist and are used simultaneously or in
        succession by a client, applications using them will
        normally expect
        that they contain either the same data or data that is
        consistent with
        the normal sorts of changes that are made by other clients
        updating the data of the file system
        (with metadata being the same to the degree indicated by the
        fs_locations_info attribute).  However, when
        multiple file systems are
        presented as replicas of one another, the precise relationship
        between the data of one and the data of another is not, as a
        general matter, specified by the NFSv4.1 protocol.  It is quite
        possible to present as replicas file systems where the data of
        those file systems is sufficiently different that some applications
        have problems dealing with the transition between replicas.  The
        namespace will typically be constructed so that applications can
        choose an appropriate level of support, so that in one position in
        the namespace a varied set of replicas might be listed, while in
        another only those that are up-to-date would be considered replicas.
        The protocol does define three special cases
        of the relationship among
        replicas to be specified by the server and relied upon by clients:

          </t>
          <ul spacing="normal">
            <li>
          When multiple replicas exist and are used simultaneously
          by a client (see the FSLIB4_CLSIMUL definition within
          fs_locations_info), they must designate the same
          data. Where file systems are writable, a change made on
          one instance must be visible on all instances at the same
	  time, regardless of whether the interrogated instance is the
	  one on which the modification was done.
          This allows a client to use these replicas
          simultaneously without any special adaptation to the fact
          that there are multiple replicas, beyond adapting to the fact
          that locks obtained on one replica are maintained separately
          (i.e. under a different client ID).
          In this case, locks (whether share reservations or
          byte-range locks) and delegations obtained on one
          replica are immediately reflected on all replicas, in the
          sense that access from all other servers is prevented
	  regardless of
          the replica used.  However, because the servers are
          not required
          to treat two associated client IDs as
          representing the same client, it is best to
          access each file using
          only a single client ID.
        </li>
            <li>
          When one replica is designated as the
          successor instance to another
          existing instance after return NFS4ERR_MOVED
          (i.e., the case of
          migration), the client may depend on the fact that all changes
          written to stable storage on the original instance
          are written to stable storage of the successor (uncommitted
          writes are dealt with in
          <xref target="SEC11-EFF-wv" format="default"/> above).
        </li>
            <li>
          Where a file system is not writable but represents a read-only
          copy (possibly periodically updated) of a writable file system,
          clients have similar requirements with regard
          to the propagation
          of updates.  They may need a guarantee that
          any change visible on
          the original file system instance must
          be immediately visible on
          any replica before the client
          transitions access to that replica,
          in order to
          avoid any possibility that a client,
          in effecting a transition to a
          replica, will see any reversion in file system state.
          The specific
          means of this guarantee varies based on the value of
          the fss_type field that is
          reported as part of the fs_status attribute
          (see <xref target="fs_status" format="default"/>).
          Since these file systems are presumed
          to be unsuitable for simultaneous use,
          there is no specification of how
          locking is handled; in general, locks obtained on one file
          system will be separate from those on others.
          Since these are expected to be read-only file systems,
          this is not
          likely to pose an issue for clients or applications.
        </li>
          </ul>
          <t>
      When none of these special situations apply, there is no basis,
      within the protocol for the client to make assumptions about the
      contents of a replica file system or its relationship to previous
      file system instances.   Thus switching between nominally
      identical read-write file
      systems would not be possible, because either the
      client does not use or the server does not support the fs_locations_info
      attribute.
          </t>
        </section>
        <section anchor="SEC11-EFF-lock" numbered="true" toc="default">
          <name>Lock State and File System Transitions</name>
          <t>
      While accessing a file system, clients obtain locks enforced
      by the server which may prevent actions by other clients
      that are inconsistent with those locks.
          </t>
          <t>
      When access is transferred between replicas, clients need to
      be assured that the actions disallowed by holding these locks
      cannot have occurred during the transition.  This can be ensured
      by the methods below.  Unless at least one of these is implemented,
      clients will not be assured of continuity of lock
      possession across a migration event.
          </t>
          <ul spacing="normal">
            <li>
              <t>
        Providing the client an opportunity to re-obtain his
        locks via a per-fs grace
        period on the destination server, denying all clients using the
	destination file system the
	opportunity to obtain new locks that conflict which those held
	by the transferred client as long as that client
	has not completed its per-fs grace period.  Because the lock reclaim
        mechanism was originally defined to support server reboot, it
        implicitly assumes that file handles will, upon reclaim, will
        be the same as those at open.  In the case of migration, this
        requires that source and destination servers use the same
        filehandles, as evidenced by using the same server scope
        (see <xref target="Server_Scope" format="default"/>)
        or by showing this
        agreement using fs_locations_info
        (see <xref target="SEC11-EFF-fh" format="default"/> above).
              </t>
              <t>
        Note that such a grace period can be implemented without
        interfering with the ability of non-transferred clients to
	obtain new locks while it is going on.   As long as the destination
	server is aware of the transferred locks, it can distinguish requests
	to obtain new locks that contrast with existing locks
	from those that do not, allowing it to treat such client requests
	without reference to the ongoing grace period.
              </t>
            </li>
            <li>
        Locking state can be transferred as part of the transition
	by providing Transparent State Migration as
        described in <xref target="SEC11-trans-locking" format="default"/>.
      </li>
          </ul>
          <t>
      Of these, Transparent State Migration provides the smoother
      experience for clients in that there is no need to go through a
      reclaim process
      before new locks can be obtained.  However, it requires
      a greater degree of inter-server co-ordination.  In general, the
      servers taking part in migration are free to provide either
      facility.  However, when the filehandles can differ across the
      migration event, Transparent State Migration is the only
      available means of providing the needed functionality.
          </t>
          <t>
      It should be noted that these two methods are not mutually
      exclusive and that a server might well provide both.  In
      particular, if there is some circumstance preventing a
      specific lock
      from being transferred transparently,
      the destination server can allow it to be reclaimed, by
      implementing a
      per-fs grace period for the migrated file system.
          </t>
          <section anchor="SEC11-EFF-lock-sc" numbered="true" toc="default">
            <name>Security Consideration Related to Reclaiming Lock State after File System Transitions</name>
            <t>
	  Although it is possible for a client reclaiming state to misrepresent
	  its state, in the same fashion as described in
	  <xref target="reclaim_security_considerations" format="default"/>, most
	  implementations providing for such reclamation in the case of
	  file system transitions
	  will have the ability to detect such misrepresentations.  This limits
	  the ability of unauthenticated clients to execute denial-of-service
	  attacks in these circumstances.   Nevertheless, the rules stated in
	  <xref target="reclaim_security_considerations" format="default"/>, regarding principal
	  verification for reclaim requests, apply in this situation as well.
            </t>
            <t>
	  Typically, implementations that support file system transitions
	  will have
	  extensive information about the locks
	  to be transferred.   This is because:
            </t>
            <ul spacing="normal">
              <li>
	    Since failure is not involved, there is no need store to locking
	    information in persistent storage.
	  </li>
              <li>
	    There is no need, as there is in the failure case, to update
	    multiple repositories containing locking state to keep them in
	    sync.   Instead, there is a one-time communication of locking
	    state from the source to the destination server.
	  </li>
              <li>
	    Providing this information avoids potential interference with
	    existing clients using the destination file system, by denying
	    them the ability to obtain new locks during the grace period.
	  </li>
            </ul>
            <t>
	  When such detailed locking information, not necessarily including
	  the associated stateids, is available:
            </t>
            <ul spacing="normal">
              <li>
	    It is possible to detect reclaim requests that attempt to
	    reclaim locks that did not exist before the transfer, rejecting
	    them with NFS4ERR_RECLAIM_BAD (<xref target="err_RECLAIM_BAD" format="default"/>).
	  </li>
              <li>
	    It is possible when dealing with non-reclaim requests, to determine
	    whether they conflict with existing locks, eliminating the need
	    to return NFS4ERR_GRACE ((<xref target="err_GRACE" format="default"/>) on
	    non-reclaim requests.
	  </li>
            </ul>
            <t>
	  It is possible for implementations of grace periods in connection
	  with file system transitions not to have detailed locking
	  information available at the destination server, in which case
	  the security situation is exactly as described in
	  <xref target="reclaim_security_considerations" format="default"/>.
            </t>
          </section>
          <section anchor="transferred_lease" numbered="true" toc="default">
            <name>Leases and File System Transitions</name>
            <t>
          In the case of lease renewal, the client may not be
          submitting requests for a file system that has been transferred
          to another server.  This can occur
          because of the lease renewal mechanism.  The
          client renews the lease associated with all file systems
          when submitting
          a request on an associated session, regardless of the
          specific file system being referenced.
            </t>
            <t>
          In order for the client to schedule renewal of its lease
          where there is locking state that may have been relocated
          to the new server, the client
          must find out about lease relocation before that lease
          expire.  To accomplish this, the SEQUENCE operation will
          return the status bit SEQ4_STATUS_LEASE_MOVED
          if responsibility for any of the renewed locking state
          has been transferred to a new server.  This
          will continue until the client receives an
          NFS4ERR_MOVED error for each of the file systems for which
          there has been locking state relocation.
            </t>
            <t>
          When a client receives an SEQ4_STATUS_LEASE_MOVED indication from
          a server, for each file system of the server for which the client
          has locking state, the client should perform an operation.
          For simplicity, the client may choose to reference
          all file systems, but what is important
          is that it must reference all file systems for which there was
          locking state where that state has moved.  Once the client
          receives an NFS4ERR_MOVED error for each such file system,
          the server will clear the SEQ4_STATUS_LEASE_MOVED indication.
          The client can terminate the process of checking file systems
          once this indication is cleared (but only if the client
          has received a reply for all outstanding SEQUENCE requests
          on all sessions it has with the server), since there are no others
          for which locking state has moved.
            </t>
            <t>
          A client may use GETATTR of the fs_status
          (or fs_locations_info) attribute on all of the file systems
          to get absence indications in a single (or a few) request(s),
          since absent file systems will not cause an error in this
          context.  However, it still must do an operation that
          receives NFS4ERR_MOVED on each file system, in order to clear
          the SEQ4_STATUS_LEASE_MOVED indication.
            </t>
            <t>
          Once the set of file systems with transferred locking state
          has been determined, the client can follow the normal process
          to obtain the new server information (through the
          fs_locations and fs_locations_info attributes) and perform renewal
          of that lease on the new server, unless information in the
          fs_locations_info attribute shows that no state could have
          been transferred.  If the server has not
          had state transferred to it transparently, the client
          will receive NFS4ERR_STALE_CLIENTID
          from the new server,
          as described above, and the client can then reclaim
          locks
          as is done in the event of server failure.
            </t>
          </section>
          <section anchor="transition_lease_time" numbered="true" toc="default">
            <name>Transitions and the Lease_time Attribute</name>
            <t>
          In order that the client may appropriately manage its lease
          in the case of a file system transition, the destination server must
          establish proper values for the lease_time attribute.
            </t>
            <t>
          When state is transferred transparently, that state
          should include the correct value of the lease_time
          attribute.  The lease_time attribute on the destination
          server must never be less than that on the source, since
          this would result in premature expiration of a lease
          granted by the source server.  Upon transitions in which
          state is transferred transparently, the client is under
          no obligation to refetch the lease_time attribute and
          may continue to use the value
          previously fetched (on the source server).
            </t>
            <t>
          If state has not been transferred transparently, either
          because the associated servers are shown as having different
          eir_server_scope strings or because the client ID
          is rejected when presented to the new server,
          the client should fetch the value
          of lease_time on the new (i.e., destination) server, and
          use it for subsequent locking requests.  However, the server
          must respect a grace
          period of at least as long as the lease_time on the source
          server, in order to ensure that clients have ample time to
          reclaim their lock before potentially conflicting
          non-reclaimed locks are granted.
            </t>
          </section>
        </section>
      </section>
      <section anchor="SEC11-trans-locking" numbered="true" toc="default">
        <name>Transferring State upon Migration</name>
        <t>
      When the transition is a result of a server-initiated decision
      to transition access and the source and destination servers have
      implemented appropriate co-operation, it is possible to:
        </t>
        <ul spacing="normal">
          <li>
        Transfer locking state from the source to the destination
        server, in a fashion similar to that provided by Transparent State
        Migration in NFSv4.0, as described in <xref target="RFC7931" format="default"/>.
        Server responsibilities are described in
        <xref target="SEC11-XS-lock" format="default"/>.

      </li>
          <li>
        Transfer session state from the source to the destination
        server.  Server responsibilities in effecting such a
        transfer are described in
        <xref target="SEC11-XS-session" format="default"/>.
      </li>
        </ul>
        <t>
      The means by which the client determines which of these transfer
      events has occurred are described in
      <xref target="SEC11-trans-client" format="default"/>.
        </t>
        <section anchor="V41p-pnfs" numbered="true" toc="default">
          <name>Transparent State Migration and pNFS</name>
          <t>
        When pNFS is involved, the protocol is capable of supporting:
          </t>
          <ul spacing="normal">
            <li>
          Migration of the Metadata Server (MDS), leaving the Data
          Servers (DS's) in place.
        </li>
            <li>
          Migration of the file system as a whole, including the MDS
          and associated DS's.
        </li>
            <li>
          Replacement of one DS by another.
        </li>
            <li>
          Migration of a pNFS file system to one in which
          pNFS is not used.
        </li>
            <li>
          Migration of a file system not using pNFS to one in which
          layouts are available.
        </li>
          </ul>
          <t>
	Note that migration per se is only involved in the transfer of
	the MDS function.   Although the servicing of a layout may be
	transferred from one data server to another, this not done using
	the file system location attributes.  The MDS can effect such
	transfers by recalling/revoking existing layouts and granting new
	ones on a different data server.
          </t>
          <t>
        Migration of the MDS function is directly supported by
        Transparent State Migration. Layout state will normally be
        transparently transferred, just as other state is.
        As a result, Transparent State Migration provides a framework in
        which, given appropriate inter-MDS data transfer, one MDS can
        be substituted for another.
          </t>
          <t>
        Migration of the file system function as a whole
        can be accomplished by
        recalling all layouts as part of the initial phase of the
        migration process.  As a result, IO will be done through the
        MDS during the migration process, and new layouts can be granted
        once the client is interacting with the new MDS.  An MDS can
        also effect this sort of transition by revoking all layouts
        as part of Transparent State Migration, as long as the client is
        notified about the loss of locking state.
          </t>
          <t>
        In order to allow migration to a file system on which pNFS is
        not supported, clients need to be prepared for a situation in
        which layouts are not available or
	supported on the destination file
        system and so direct IO requests to the destination
        server, rather than depending on layouts being available.
          </t>
          <t>
        Replacement of one DS by another is not addressed by migration as
        such but can be effected by an MDS recalling layouts for the DS
        to be replaced and issuing new ones to be served by the
        successor DS.
          </t>
          <t>
        Migration may transfer a file system from a server which does
        not support pNFS to one which does.  In order to properly adapt
        to this situation, clients which support pNFS, but function
        adequately in its absence should check for pNFS support when
        a file system is migrated and be prepared to use pNFS when
        support is available on the destination.
          </t>
        </section>
      </section>
      <section anchor="SEC11-trans-client" numbered="true" toc="default">
        <name>Client Responsibilities when Access is Transitioned</name>
        <t>
      For a client to respond to an access transition, it must become
      aware of it.  The ways in which this can happen are discussed
      in <xref target="V41c-clrecov" format="default"/> which discusses indications
      that a specific file system access path has transitioned as well as
      situations in which additional activity is necessary to
      determine the set of file systems that have been migrated.
      <xref target="V41c-migrdisc" format="default"/> goes on to complete the discussion
      of how the set of migrated file systems might be determined.
      Sections <xref target="V41c-omoved" format="counter"/> through
      <xref target="V41c-ssnwas" format="counter"/>
      discuss how the client should deal with
      each transition it becomes aware of, either directly or as a
      result of migration discovery.
        </t>
        <t>
      The following terms are used to describe client activities:
        </t>
        <ul spacing="normal">
          <li>
	"Transition recovery" refers to the process of restoring access
	to a file system on which NFS4ERR_MOVED was received.
      </li>
          <li>
	"Migration recovery" to that subset of transition recovery
	which applies when the file system has migrated to a different
	replica.
      </li>
          <li>
	"Migration discovery" refers to the process of determining which
	file system(s) have been migrated.  It is necessary to
	avoid a situation in
	which leases could expire when a file system is not accessed for
	a long period of time, since a client unaware of the migration
	might be referencing an unmigrated file system and not renewing
	the lease associated with the migrated file system.
      </li>
        </ul>
        <section anchor="V41c-clrecov" numbered="true" toc="default">
          <name>Client Transition Notifications</name>
          <t>
        When there is a change in the network access
	path which a client is to use to access a file
        system, there
        are a number of
        related status indications with which clients
        need to deal:
          </t>
          <ul spacing="normal">
            <li>
              <t>
          If an attempt is made to use or return a filehandle
          within a file system that is no longer accessible at the
          address previously used to access it, the
          error NFS4ERR_MOVED is returned.
              </t>
              <t>
          Exceptions are made to allow such file handles to be used
          when interrogating a file system location attribute.
	  This enables a
          client to determine
          a new replica's location or a new network access path.
              </t>
              <t>
          This condition continues on subsequent attempts to access
          the file system in question.  The only way the client
          can avoid the error is to cease accessing the file system in
          question at its old server location and access it instead
          using a different address at which it is now available.
              </t>
            </li>
            <li>
              <t>
          Whenever a SEQUENCE operation is sent by a client to
          a server which generated state held on that client which
          is associated with a file system that is no longer accessible
          on the server at which it was previously available, the response
	  will contain a
	  lease-migrated indication, with the
          SEQ4_STATUS_LEASE_MOVED status bit being set.
              </t>
              <t>
          This condition continues until the client acknowledges
          the notification by fetching a file system
	  location attribute for the
          file system whose network access path is being changed.
	  When there are multiple such file systems, a location attribute
          for each such file system needs to be fetched. The location
	  attribute for all migrated file system needs to be fetched
	  in order to clear the condition.
          Even after the condition is cleared, the
          client needs to respond by using the location information
          to access the file system at its new location
          to ensure that leases are
          not needlessly expired.
              </t>
            </li>
          </ul>
          <t>
        Unlike the case of NFSv4.0, in which the corresponding
        conditions are both errors and thus mutually exclusive,
        in NFSv4.1 the client can,
        and often will, receive both indications on the same
        request.  As a result, implementations need to address the
        question of how to co-ordinate
        the necessary recovery actions when both indications
        arrive in the response to the same request.  It should be noted
	that when processing an NFSv4 COMPOUND, the server
	will normally decide
	whether SEQ4_STATUS_LEASE_MOVED is to be set before
        it determines which file system will be referenced or whether
        NFS4ERR_MOVED is to be returned.
          </t>
          <t>
        Since these indications are not mutually exclusive in NFSv4.1,
        the following combinations are possible results when a COMPOUND
        is issued:
          </t>
          <ul spacing="normal">
            <li>
              <t>
          The COMPOUND status
          is NFS4ERR_MOVED and SEQ4_STATUS_LEASE_MOVED is asserted.
              </t>
              <t>
          In this case, transition recovery is required.  While it is
          possible that migration discovery is needed in addition, it
          is likely that only the accessed file system has transitioned.
          In any case, because addressing NFS4ERR_MOVED is necessary to
          allow the rejected requests to be processed on the target,
          dealing with it will typically have priority over
          migration discovery.

              </t>
            </li>
            <li>
              <t>
          The COMPOUND status
          is NFS4ERR_MOVED and SEQ4_STATUS_LEASE_MOVED is clear.
              </t>
              <t>
          In this case, transition recovery is also required. It is
          clear that migration discovery is not needed to find
          file systems that have been migrated other that the one
          returning NFS4ERR_MOVED.  Cases in which this
          result can arise include a referral or a migration for which
          there is no associated locking state.  This can also arise in
          cases in which an access path transition
          other than migration occurs within the same server.  In such a
          case, there is no need to set SEQ4_STATUS_LEASE_MOVED, since
          the lease remains associated with the current server even though
          the access path has changed.
              </t>
            </li>
            <li>
              <t>
          The COMPOUND status
          is not NFS4ERR_MOVED and SEQ4_STATUS_LEASE_MOVED is asserted.
              </t>
              <t>
          In this case, no transition recovery activity is required on
          the file system(s) accessed by the request.
          However, to prevent avoidable
          lease expiration, migration discovery needs to be done
              </t>
            </li>
            <li>
              <t>
          The COMPOUND status
          is not NFS4ERR_MOVED and SEQ4_STATUS_LEASE_MOVED is clear.
              </t>
              <t>
          In this case, neither transition-related activity nor migration
          discovery is required.
              </t>
            </li>
          </ul>
          <t>
        Note that the specified actions only need to be taken if they are
        not already going on.  For example, when NFS4ERR_MOVED is received
	when accessing a file system
        for which transition recovery already going on, the client
	merely waits for
        that recovery to be completed while the receipt of
	SEQ4_STATUS_LEASE_MOVED indication only
        needs to initiate migration discovery for a server if such
	discovery is not already underway for that server.
          </t>
          <t>
        The fact that a lease-migrated condition does not result in
        an error in NFSv4.1 has a number of important consequences.
        In addition to the fact, discussed above, that the two
        indications are not mutually exclusive, there are number of
        issues that are important in considering implementation of
        migration discovery, as discussed in
        <xref target="V41c-migrdisc" format="default"/>.
          </t>
          <t>
        Because SEQ4_STATUS_LEASE_MOVED is not an
        error condition",  it is possible
	for file systems whose access paths have not changed to be
	successfully accessed on a given server even though recovery
        is necessary for other file systems on the same server.  As
        a result, access can go on while,
          </t>
          <ul spacing="normal">
            <li>
	  The migration discovery process is going on for that server.
	</li>
            <li>
	  The transition recovery process is going on for other
	  file systems connected to that server.
	</li>
          </ul>
        </section>
        <section anchor="V41c-migrdisc" numbered="true" toc="default">
          <name>Performing Migration Discovery</name>
          <t>
        Migration discovery can be performed in the same context as
        transition recovery, allowing recovery for  each migrated file
        system to be invoked as it is discovered.  Alternatively, it may
        be done in a separate migration discovery thread,  allowing
        migration discovery to be done in parallel with
	one or more instances
        of transition recovery.
          </t>
          <t>
        In either case, because the lease-migrated indication
        does not result in an error. other access to file systems on the
        server can proceed normally, with the possibility that further
        such indications will be received, raising the issue of how
        such indications are to be dealt with.  In general,
          </t>
          <ul spacing="normal">
            <li>
          No action needs to be taken for such indications received by any
          threads performing migration discovery, since continuation of that
          work will address the issue.
        </li>
            <li>
          In other cases in which migration discovery is currently
          being performed,
          nothing further needs to be done to respond to such lease
          migration indications, as long as one can be
	  certain that the migration
	  discovery process would deal with those indications.  See below
	  for details.
        </li>
            <li>
          For such indications received in all other contexts, the
          appropriate response is to initiate or
          otherwise provide for the
          execution of migration discovery for file systems
          associated with the server IP address returning the indication.
        </li>
          </ul>
          <t>
        This leaves a potential difficulty in situations in which the
        migration discovery process is near to completion but is still
        operating.  One should not ignore a LEASE_MOVED indication if
        the migration discovery process is not able to respond to
        the discovery of additional
        migrating file
        systems without additional aid.  A further complexity relevant in
        addressing such situations is that a lease-migrated indication may
        reflect the server's state at the time the SEQUENCE operation
        was processed, which may be different from that in effect at the
        time the response is received.  Because new migration events
	may occur
	at any time, and because a LEASE_MOVED indication may reflect
	the situation in effect a considerable time before the indication
	is received,
	special care needs to be taken to ensure that LEASE_MOVED
	indications are not inappropriately ignored.
          </t>
          <t>
        A useful approach to this issue involves the use of separate
        externally-visible migration discovery states for each server.
	Separate values could represent the various possible states for
        the migration discovery process for a server:
          </t>
          <ul spacing="normal">
            <li>
          non-operation, in which migration discovery is not being
	  performed
	</li>
            <li>
	  normal operation, in which there is an ongoing scan for
	  migrated file systems.
	</li>
            <li>
	  completion/verification of migration discovery processing,
	  in which the possible completion of migration discovery
	  processing needs to be verified.
	</li>
          </ul>
          <t>
        Given that framework, migration discovery processing would proceed
        as follows.
          </t>
          <ul spacing="normal">
            <li>
          While in the normal-operation state, the thread performing
	  discovery would fetch, for
          successive file systems known to the client on the server being
          worked on, a file system location
          attribute plus the fs_status attribute.
        </li>
            <li>
          If the fs_status attribute indicates that the file system
	  is a migrated one (i.e. fss_absent is true and
	  fss_type != STATUS4_REFERRAL) then a migrated file system has
	  been found.   In this situation, it is likely
	  that the fetch of the file system location attribute has
          cleared one the file systems contributing to the
	  lease-migrated indication.
        </li>
            <li>
	  In cases in which that happened, the thread cannot know whether
	  the lease-migrated indication has been cleared
	  and so it enters the
	  completion/verification state and proceeds to issue a COMPOUND
	  to see if the LEASE_MOVED indication has been cleared.
	</li>
            <li>
	  When the discovery process is in the
          completion/verification state,
	  if other requests get a lease-migrated indication
          they note that it was received.  Later, the existence of such
	  indications is used when the request completes, as
          described below.
	</li>
          </ul>
          <t>
	When the request used in the completion/verification state
        completes:
          </t>
          <ul spacing="normal">
            <li>
	  If a lease-migrated indication is returned, the discovery
          continues normally.  Note that this is so
          even if all file systems
	  have traversed, since new migrations could have  occurred
          while the process
	  was going on.
	</li>
            <li>
	  Otherwise, if there is any record that other requests saw a
          lease-migrated indication while the request was going on,
	  that record is cleared and the
          verification request retried.  The discovery
	  process remains in completion/verification state.
	</li>
            <li>
	  If there have been no lease-migrated indications, the work of
	  migration discovery is considered completed and it enters the
	  non-operating state.  Once it enters this state, subsequent
          lease-migrated indication will trigger a new migration discovery
          process.
	</li>
          </ul>
          <t>
	It should be noted that the process described above is not
	guaranteed to terminate, as a long series of new migration
	events might continually delay the clearing of the LEASE_MOVED
	indication.  To prevent unnecessary lease expiration, it is
	appropriate for clients
	to use the discovery of migrations to effect lease
	renewal immediately, rather than waiting for clearing of the
	LEASE_MOVED indication when the complete set of migrations is
	available.
          </t>
          <t>
        Lease discovery needs to be provided as described above. This
	ensures that the client discovers file system migrations soon
	enough to renew its leases on each destination server before they
	expire.	  Non-renewal of leases can lead to loss of locking state.
	While the consequences of such
	loss can be ameliorated through implementations of courtesy locks,
	servers are under no obligation to do so, and a conflicting lock request
	may mean that a lock is revoked unexpectedly.  Clients should be aware
	of this possibility.
          </t>
        </section>
        <section anchor="V41c-omoved" numbered="true" toc="default">
          <name>Overview of Client Response to NFS4ERR_MOVED</name>
          <t>
        This section outlines a way in which a client that receives
        NFS4ERR_MOVED can effect transition recovery by using a new
	server or server endpoint
        if one is available.  As part of that process, it will
        determine:
          </t>
          <ul spacing="normal">
            <li>
          Whether the NFS4ERR_MOVED indicates migration has occurred,
          or whether it indicates another sort of file system
          access transition as discussed
          in <xref target="SEC11-nwa" format="default"/> above.
        </li>
            <li>
          In the case of migration, whether Transparent State
          Migration has occurred.
        </li>
            <li>
          Whether any state has been lost during the process of
          Transparent State Migration.
        </li>
            <li>
          Whether sessions have been transferred as part of Transparent
          State Migration.
        </li>
          </ul>
          <t>
        During the first phase of this process, the client proceeds to
	examine file system location entries to find the initial
	network address
        it will use to continue access
        to the file system or its replacement.
	For each location entry that the client examines, the process
        consists of five steps:
          </t>
          <ol spacing="normal" type="1">
            <li>
          Performing an EXCHANGE_ID
          directed at the location address.  This operation is used to
          register the client owner (in the form of a client_owner4)
	  with the server, to obtain a client ID
          to be use subsequently to communicate with it, to obtain that
          client ID's confirmation status, and to determine server_owner
          and scope for the purpose of determining if the entry
          is trunkable with that
          previously being used to access the file system (i.e. that
          it represents another network access path to the same
	  file system and can share
          locking state with it).
        </li>
            <li>
	  Making an initial determination of whether migration has
	  occurred.  The initial determination will be based
	  on whether the EXCHANGE_ID results indicate that the
	  current location element is server-trunkable with that
          used to access the file system when access
          was terminated by receiving NFS4ERR_MOVED.
	  If it is, then migration has not occurred.  In that case, the
	  transition is
	  dealt with, at least initially, as one involving continued
	  access to the same file system on the same server through
	  a new network address.
        </li>
            <li>
          Obtaining access to existing session state or creating new
          sessions.  How this is done depends on the initial
          determination of whether migration has occurred and
          can be done as described in <xref target="V41c-ssmig" format="default"/> below
          in the case of migration or as described in
          <xref target="V41c-ssnwas" format="default"/> below
	  in the case of a network
          address transfer without migration.
        </li>
            <li>
          Verification of the trunking relationship assumed in step
          2 as discussed in <xref target="PREP-trunk-verify" format="default"/>.
          Although this step will generally confirm the initial
          determination, it is possible for verification to fail with
          the result that an initial determination that a network address
          shift (without migration) has occurred may be invalidated and
          migration determined to have occurred.  There is no need to redo
	  step 3 above, since it will be possible to continue use of the
	  session established already.
        </li>
            <li>
          Obtaining access to existing locking state and/or
          reobtaining it.  How this is done depends on the final
          determination of whether migration has occurred and
          can be done as described below in <xref target="V41c-ssmig" format="default"/>
          in the case of migration or as described in
          <xref target="V41c-ssnwas" format="default"/>
	  in the case of a network
          address transfer without migration.

        </li>
          </ol>
          <t>
	Once the initial address has been determined, clients are free
	to apply an abbreviated process to find additional addresses
	trunkable with it (clients may seek session-trunkable or
	server-trunkable addresses depending on whether they support
	clientid trunking).  During this later phase of the process,
	further location entries are examined using the abbreviated
        procedure specified below:
          </t>
          <ol spacing="normal" type="%C:">
            <li>
	  Before the EXCHANGE_ID, the fs name of the location
	  entry is examined and if it
	  does not match that currently being used, the entry is ignored.
	  otherwise, one proceeds as specified by step 1 above.
        </li>
            <li>
	  In the case that the network address is session-trunkable with one
          used previously a BIND_CONN_TO_SESSION is used to access that
          session using the new network address.  Otherwise, or if the bind
          operation fails, a CREATE_SESSION is done.
        </li>
            <li>
	  The verification procedure referred to in step 4 above is
	  used.  However, if it fails, the entry is ignored and the next
	  available entry is used.
        </li>
          </ol>
        </section>
        <section anchor="V41c-ssmig" numbered="true" toc="default">
          <name>Obtaining Access to Sessions and State after Migration</name>
          <t>
        In the event that migration has occurred, migration recovery
	will involve determining
	whether Transparent State Migration has
        occurred. This decision is made based on the client ID returned
	by the EXCHANGE_ID
	and the reported
        confirmation status.
          </t>
          <ul spacing="normal">
            <li>
          If the client ID is an unconfirmed client ID not previously known
          to the  client, then Transparent State
          Migration has not occurred.
        </li>
            <li>
          If the client ID is a confirmed client ID previously known
          to the  client, then any transferred state would have been
          merged with an existing client ID representing the client to the
          destination server. In this state merger case, Transparent
          State Migration might
          or might not have occurred and a determination as to whether
	  it has occurred is deferred until sessions are established
	  and the client is ready to begin state recovery.
        </li>
            <li>
          If the client ID is a confirmed client ID  not previously known
          to the  client, then the client can conclude that the
          client ID was transferred as part of Transparent State Migration.
          In this transferred client ID case, Transparent State Migration
          has occurred although some state might have been lost.
        </li>
          </ul>
          <t>
	Once the client ID has been obtained, it is necessary to
	obtain access to sessions to continue communication with the
	new server.
        In any of the cases in which Transparent State Migration
        has occurred, it is possible that a session was transferred
        as well.  To deal with that possibility, clients can, after
        doing the EXCHANGE_ID, issue a BIND_CONN_TO_SESSION to
        connect the transferred session to a connection to the new
        server.  If that fails,  it is an indication that the session
        was not transferred and that a new session needs to be created to
        take its place.
          </t>
          <t>
        In some situations, it is possible for a BIND_CONN_TO_SESSION
        to succeed without session migration having occurred.  If
        state merger has taken place then the associated client ID
        may have already had a set of existing sessions, with it
        being possible that the sessionid of a given session is the
        same as one that might have been migrated.  In that event,
        a BIND_CONN_TO_SESSION might succeed, even though there
        could have been no migration of the session with that sessionid.
	In such cases, the client will receive sequence errors when the
	slot sequence values used are not appropriate on the new
	session.  When this occurs, the client can create a new a
	session and cease using the existing one.
          </t>
          <t>
        Once the client has determined the initial migration status,
        and determined that there was a shift to a new server, it
        needs to re-establish its locking state, if possible.  To enable
        this to happen without loss of the guarantees normally provided by
        locking, the destination server needs to implement a per-fs grace
        period in all cases in which lock state was lost, including
        those in which Transparent State Migration was not
        implemented.  Each client for which there was a transfer of locking
	state to the new server will have the duration of the grace period
	to reclaim its locks, from the time its locks were transferred.
          </t>
          <t>
        Clients need to deal with the following cases:
          </t>
          <ul spacing="normal">
            <li>
          In the state merger case, it is possible that the server
          has not attempted Transparent State Migration,
          in which case state may have been
          lost without it being reflected in the  SEQ4_STATUS bits.
          To determine whether this has happened, the client can use
          TEST_STATEID to check whether the stateids created on the
          source server are still accessible on the destination server.
          Once a single stateid is found to have been successfully
          transferred, the client can conclude that Transparent State
          Migration was begun and any failure to transport all of the
          stateids will be reflected in the SEQ4_STATUS bits.  Otherwise,
	  Transparent State Migration has not occurred.
        </li>
            <li>
          In a case in which Transparent State Migration has not
          occurred, the client can use the per-fs grace period provided
          by the destination server to reclaim locks that were held on
          the source server.
        </li>
            <li>
          In a case in which Transparent State Migration has
          occurred, and no lock state was lost (as shown by SEQ4_STATUS
          flags), no lock reclaim is necessary.
        </li>
            <li>
          In a case in which Transparent State Migration has
          occurred, and some lock state was lost (as shown by SEQ4_STATUS
          flags), existing stateids need to be checked for validity
          using TEST_STATEID, and reclaim used to re-establish any that
          were not transferred.

        </li>
          </ul>
          <t>
        For all of the cases above, RECLAIM_COMPLETE with an rca_one_fs
	value of TRUE needs to be done before
        normal use of the file system including obtaining new locks for the
        file system.  This applies even if no locks were lost and there
        was no need for any to be reclaimed.
          </t>
        </section>
        <section anchor="V41c-ssnwas" numbered="true" toc="default">
          <name>Obtaining Access to Sessions and State after Network Address Transfer</name>
          <t>
        The case in which there is a transfer to a new network
        address without migration is similar to that described
        in <xref target="V41c-ssmig" format="default"/> above in that there is a need to
        obtain access to needed sessions and locking state.  However,
        the details are simpler and will vary depending on the
        type of trunking between the address receiving
        NFS4ERR_MOVED and that to which the transfer is to be made
          </t>
          <t>
        To make a session available for use, a BIND_CONN_TO_SESSION
        should be used to obtain access to the session previously
        in use.  Only if this fails, should a CREATE_SESSION be done.
        While this procedure mirrors that in <xref target="V41c-ssmig" format="default"/>
        above,
        there is an important difference in that preservation of the
        session is not purely optional but depends on the type of
        trunking.
          </t>
          <t>
        Access to appropriate locking state will generally need no actions
	beyond
	access to the session.  However, the SEQ4_STATUS bits need to be
	checked for lost locking state, including the need to reclaim
	locks after a server reboot, since there is always a possibility
	of locking state being lost.
          </t>
        </section>
      </section>
      <section anchor="SEC11-trans-server" numbered="true" toc="default">
        <name>Server Responsibilities Upon Migration</name>
        <t>
      In the event of file system migration, when the client connects
      to the destination server, that server needs to be able to provide the
      client continued to access
      the files it had open on the source server.  There are two ways
      to provide this:
        </t>
        <ul spacing="normal">
          <li>
	By provision of an fs-specific grace period, allowing the client the
	ability to reclaim its locks, in a fashion similar to what would
	have been done in the
	case of recovery from a server restart.  See
	<xref target="SEC11-XS-reclaim" format="default"/> for a more complete
	discussion.
      </li>
          <li>
            <t>
	By implementing Transparent State Migration possibly in
	connection with session migration, the server can provide
	the client immediate access to the state built up on the
	source server, on the destination.
            </t>
            <t>
        These features are discussed separately in Sections
        <xref target="SEC11-XS-lock" format="counter"/> and
        <xref target="SEC11-XS-session" format="counter"/>,
	which discuss Transparent State Migration and session
	migration respectively.
            </t>
          </li>
        </ul>
        <t>
      All the features described above can involve transfer of
      lock-related information between source and destination
      servers.   In some cases, this transfer is a necessary part
      of the implementation while in other cases it is a helpful
      implementation aid which servers might or might not use.
      The sub-sections below discuss the information which would be
      transferred but do not define the specifics of the transfer
      protocol.  This is left as an implementation choice although
      standards in this area could be developed at a later time.
        </t>
        <section anchor="SEC11-XS-reclaim" numbered="true" toc="default">
          <name>Server Responsibilities in Effecting State Reclaim after Migration</name>
          <t>
	In this case, the destination server needs no knowledge of
	the locks held
	on the source server.  It relies on the clients to accurately report
	(via reclaim operations) the locks previously held, and does not allow
	new locks to be granted on migrated file systems until the grace
	period expires.   Disallowing of new locks applies to
	all clients accessing these file system, while grace period
	expiration occurs for each migrated client independently.
          </t>
          <t>
	During this grace period clients have the opportunity to use
	reclaim operations to obtain locks for file system objects within
	the migrated file system, in the same way that they do when
	recovering from server restart, and the servers typically
	rely on clients to accurately report their locks, although they
	have the option of subjecting these requests to verification.
	If the clients only reclaim locks held on the source server, no
	conflict can arise.  Once the client has reclaimed its locks,
	it indicates the completion of lock reclamation by performing a
	RECLAIM_COMPLETE specifying rca_one_fs as TRUE.

          </t>
          <t>
	While it is not necessary for source and destination servers
	to co-operate to transfer information about locks, implementations
	are well-advised to consider transferring the following
	useful information:
          </t>
          <ul spacing="normal">
            <li>
	  If information about the set of clients that have
	  locking state for the transferred file system is made available,
	  the destination
	  server will be able to terminate the grace period once all
	  such clients have reclaimed their locks, allowing normal
	  locking activity to resume earlier than it would have otherwise.
	</li>
            <li>
	  Locking summary information for individual clients (at various
	  possible levels of detail) can detect
	  some instances in which clients do not accurately represent the
	  locks held on the source server.
	</li>
          </ul>
        </section>
        <section anchor="SEC11-XS-lock" numbered="true" toc="default">
          <name>Server Responsibilities in Effecting Transparent State Migration</name>
          <t>
	The basic responsibility of the source server in effecting
	Transparent State Migration is to make available to the
	destination server a description of each piece of locking state
	associated with the file system being migrated.  In addition to
        client id string and verifier, the source server needs to provide,
        for each stateid:
          </t>
          <ul spacing="normal">
            <li>
	  The stateid including the current sequence value.
        </li>
            <li>
	  The associated client ID.
        </li>
            <li>
	  The handle of the associated file.
        </li>
            <li>
	  The type of the lock, such as open, byte-range lock, delegation,
	  or layout.
        </li>
            <li>
	  For locks such as opens and byte-range locks, there will be
	  information about the owner(s) of the lock.
        </li>
            <li>
	  For recallable/revocable lock types, the current recall status
	  needs to be included.
        </li>
            <li>
	  For each lock type, there will be type-specific information, such
	  as share and deny modes for opens and type and byte ranges for
	  byte-range locks and layouts.
        </li>
          </ul>
          <t>
	Such information will most probably be organized by client id string
	on the destination server
	so that it can be used to provide appropriate context to each client
	when it makes itself known to the client.  Issues connected with a
	client impersonating another by presenting another client's client
	id string can be addressed using NFSv4.1 state protection features,
	as described in <xref target="SECCON" format="default"/>.

          </t>
          <t>
	A further server responsibility concerns locks that are revoked
	or otherwise lost during the process of file system migration.
	Because locks that appear to be lost during the process of
	migration will be reclaimed by the client, the servers have to
	take steps to ensure that locks revoked soon before or soon
	after migration are not inadvertently allowed to be reclaimed
	in situations in which the continuity of lock possession
	cannot be assured.
          </t>
          <ul spacing="normal">
            <li>
	  For locks lost on the source but whose loss has not yet been
	  acknowledged by the client (by using FREE_STATEID), the
	  destination must be aware of this loss so that it can deny
	  a request to reclaim them.
	</li>
            <li>
	  For locks lost on the destination after the state transfer
	  but before the client's RECLAIM_COMPLTE is done, the
	  destination server should note these and not allow them to
	  be reclaimed.
	</li>
          </ul>
          <t>
	An additional responsibility of the cooperating
	servers concerns situations
	in which a stateid cannot be transferred transparently because it
	conflicts with an existing stateid held by the client and
	associated with a different file system.  In this case there
	are two valid choices:
          </t>
          <ul spacing="normal">
            <li>
	  Treat the transfer, as in NFSv4.0, as one without Transparent
	  State Migration.  In this case, conflicting locks cannot be
	  granted until the client does a RECLAIM_COMPLETE, after
	  reclaiming the locks it had, with the exception of reclaims
	  denied because they were attempts to reclaim locks that had
	  been lost.
	</li>
            <li>
	  Implement Transparent State Migration, except for the lock
	  with the conflicting stateid.  In this case, the client will
	  be aware of a lost lock (through the SEQ4_STATUS flags) and be
	  allowed to reclaim it.
	</li>
          </ul>
          <t>
        When transferring state between the source and destination, the
        issues discussed in Section 7.2 of <xref target="RFC7931" format="default"/>
        must still be attended to.  In this case,
        the use of NFS4ERR_DELAY may still
        necessary in NFSv4.1, as it was in NFSv4.0, to prevent locking
        state changing while it is being transferred.  See
	<xref target="err_DELAY" format="default"/> for information about
	appropriate client retry approaches in the event that NFS4ERR_DELAY
	is returned.
          </t>
          <t>
        There are a number of important differences in the NFS4.1
        context:
          </t>
          <ul spacing="normal">
            <li>
          The absence of RELEASE_LOCKOWNER means that the one case
          in which an operation could not be deferred by use of
          NFS4ERR_DELAY no longer exists.
        </li>
            <li>
          Sequencing of operations is no longer done using owner-based
          operation sequences numbers.  Instead, sequencing is session-
          based
        </li>
          </ul>
          <t>
        As a result, when sessions are not transferred, the techniques
        discussed in Section 7.2 of <xref target="RFC7931" format="default"/>
        are adequate and will not
        be further discussed.
          </t>
        </section>
        <section anchor="SEC11-XS-session" numbered="true" toc="default">
          <name>Server Responsibilities in Effecting Session Transfer</name>
          <t>
	The basic responsibility of the source server in effecting
	session transfer is to make available to the
	destination server a description of the current state of each
	slot with the session, including:
          </t>
          <ul spacing="normal">
            <li>
	  The last sequence value received for that slot.
        </li>
            <li>
	  Whether there is cached reply data for the last request
	  executed and, if so, the cached reply.
        </li>
          </ul>
          <t>
        When sessions are transferred, there are a number of issues that
        pose challenges in terms of making the transferred state
	unmodifiable during the period it is gathered up and
	transferred to the destination server.
          </t>
          <ul spacing="normal">
            <li>
          A single session may be used to access multiple file systems,
          not all of which are being transferred.
        </li>
            <li>
          Requests made on a session may, even if rejected, affect
          the state of the session by advancing the sequence number
          associated with the slot used.
        </li>
          </ul>
          <t>
        As a result, when the file system state might otherwise be
        considered unmodifiable, the client might have any number of
        in-flight requests, each of which is capable of changing session
        state, which may be of a number of types:
          </t>
          <ol spacing="normal" type="1">
            <li>
          Those requests that were processed on the migrating file system,
          before migration began.
        </li>
            <li>
          Those requests which got the error NFS4ERR_DELAY because the
          file system being accessed was in the process of being
          migrated.
        </li>
            <li>
          Those requests which got the error NFS4ERR_MOVED because the
          file system being accessed had been migrated.
        </li>
            <li>
          Those requests that accessed the migrating file system,
          in order to obtain location or status information.
        </li>
            <li>
          Those requests that did not reference the migrating file system.
        </li>
          </ol>
          <t>
	It should be noted that the history of any
        particular slot is likely
	to include a number of these request classes.  In the case in which
	a session which is migrated is used by file systems other than the
	one migrated, requests of class 5 may be common and be the last
	request processed, for many slots.
          </t>
          <t>
	Since session state can change even after the locking
	state has been fixed as part of the migration process,
	the session state known to the client could
	be different from that on
	the destination server, which necessarily reflects the session
	state on the source server, at an earlier time.
        In deciding how to deal with this situation, it is helpful to
        distinguish between two sorts of behavioral consequences of
        the choice of initial sequence ID values.
          </t>
          <ul spacing="normal">
            <li>
              <t>
          The error NFS4ERR_SEQ_MISORDERED is returned when the sequence ID
          in a request is neither equal to the last one seen for the
          current slot nor the next greater one.
              </t>
              <t>
          In view of the difficulty of arriving at a mutually acceptable
          value for the correct last sequence value
	  at the point of migration,
          it may be necessary for the server to show some degree of
          forbearance, when the sequence ID is one that would be
          considered unacceptable if session migration were not
          involved.
              </t>
            </li>
            <li>
              <t>
          Returning the cached reply for a previously executed
          request when the sequence ID
          in the request matches the last value recorded for the slot.
              </t>
              <t>
          In the cases in which an error is returned and there is no
          possibility of any non-idempotent operation having been executed,
          it may not be necessary to adhere to this as strictly as might
          be proper if session migration were not
          involved.   For example, the fact that the error NFS4ERR_DELAY
          was returned may not assist the client in any material way, while
          the fact that NFS4ERR_MOVED was returned by the source server
          may not be relevant when the request was reissued, directed
          to the
          destination server.
              </t>
            </li>
          </ul>
          <t>
        An important issue is that the specification needs to take note of
        all potential COMPOUNDs, even if they might be unlikely
        in practice.  For example, a COMPOUND is allowed to access
        multiple file systems and might perform non-idempotent operations
        in some of them before accessing a file system being migrated.
        Also, a COMPOUND may return considerable data in the response,
        before
        being rejected with NFS4ERR_DELAY or NFS4ERR_MOVED, and  may
        in addition be marked as sa_cachethis.  However, note that
	if the client and server adhere to rules in
	<xref target="err_DELAY" format="default"/>, there is no possibility of
	non-idempotent operations being spuriously reissued after receiving
	NFS4ERR_DELAY response.
          </t>
          <t>
        To address these issues,  a destination server MAY do any of
        the following when implementing session transfer.
          </t>
          <ul spacing="normal">
            <li>
          Avoid enforcing any sequencing semantics for a particular slot
          until the client has established the starting sequence for that
          slot on the destination server.
        </li>
            <li>
          For each slot, avoid
          returning a cached reply returning NFS4ERR_DELAY or NFS4ERR_MOVED
          until the client has established the starting sequence for that
          slot on the destination server.
        </li>
            <li>
          Until the client has established the starting sequence for a
          particular slot on the destination server,
          avoid reporting NFS4ERR_SEQ_MISORDERED or
          returning  a cached reply returning NFS4ERR_DELAY or NFS4ERR_MOVED,
          where the reply consists solely of a series of operations
          where the response is NFS4_OK until the final error.
        </li>
          </ul>
          <t>
	Because of the considerations mentioned above including the rules
	for the handling of NFS4ERR_DELAY included in
	<xref target="err_DELAY" format="default"/>, the destination
	server can respond appropriately to SEQUENCE operations received
	from the client by adopting the three policies listed below:
          </t>
          <ul spacing="normal">
            <li>
          Not responding with NFS4ERR_SEQ_MISORDERED for the initial
	  request on a slot within a transferred session, since the
	  destination server cannot be aware of requests made by the
	  client after the server handoff but before the client became
	  aware of the shift.  In cases in which NFS4ERR_SEQ_MISORDERED
	  would normally have been reported, the request is to be processed
	  normally, as a new request.
	</li>
            <li>
          Replying as it would for a retry whenever the sequence matches
	  that transferred by the source server, even though this would
	  not provide retry handling for requests issued after the server
	  handoff, under the assumption that when such requests are issued
	  they will never be responded to in a state-changing fashion,
	  making retry support for them unnecessary.
	</li>
            <li>
          Once a non-retry SEQUENCE is received for a given slot, using
	  that as the basis for further sequence checking, with no further
	  reference to the sequence value transferred by the source.
	  server.
	</li>
          </ul>
        </section>
      </section>
      <section anchor="effecting_referrals" numbered="true" toc="default">
        <name>Effecting File System Referrals</name>
        <t>
      Referrals are effected when an absent file system is encountered
      and one or more alternate locations are made available by the
      fs_locations or fs_locations_info attributes.  The client will
      typically get an NFS4ERR_MOVED error, fetch the appropriate
      location information, and proceed to access the file system on
      a different server, even though it retains its logical position
      within the original namespace.  Referrals differ from migration
      events in that they happen only when the client has not
      previously referenced the file system in question (so there
      is nothing to transition).  Referrals can only come into
      effect when an absent file system is encountered at its
      root.
        </t>
        <t>
      The examples given in the sections below are somewhat artificial in
      that an actual client will not typically do a multi-component
      look up, but will have cached information regarding the upper levels
      of the name hierarchy.  However, these examples are chosen to make
      the required behavior clear and easy to put within the scope of a
      small number of requests, without getting a discussion of the details of
      how specific clients might choose to cache things.
        </t>
        <section anchor="referrals_lookup" numbered="true" toc="default">
          <name>Referral Example (LOOKUP)</name>
          <t>
        Let us suppose that the following COMPOUND is sent in an
        environment in which /this/is/the/path is absent from the
        target server.  This may be for a number of reasons.  It may
        be that the file system has moved, or it may be that
        the target server is functioning mainly, or solely, to refer
        clients to the servers on which various file systems are located.
          </t>
          <ul spacing="normal">
            <li>
          PUTROOTFH
        </li>
            <li>
          LOOKUP "this"
        </li>
            <li>
          LOOKUP "is"
        </li>
            <li>
          LOOKUP "the"
        </li>
            <li>
          LOOKUP "path"
        </li>
            <li>
          GETFH
        </li>
            <li>
          GETATTR (fsid, fileid, size, time_modify)
        </li>
          </ul>
          <t>
        Under the given circumstances, the following will be the result.
          </t>
          <ul spacing="normal">
            <li>
          PUTROOTFH  --&gt; NFS_OK.  The current fh is now the root of
          the pseudo-fs.
        </li>
            <li>
          LOOKUP "this" --&gt; NFS_OK.  The current fh is for /this and is
          within the pseudo-fs.
        </li>
            <li>
          LOOKUP "is" --&gt; NFS_OK.  The current fh is for /this/is
          and is within the pseudo-fs.
        </li>
            <li>
          LOOKUP "the" --&gt; NFS_OK.  The current fh is for /this/is/the
          and is within the pseudo-fs.
        </li>
            <li>
          LOOKUP "path" --&gt; NFS_OK.  The current fh is for
          /this/is/the/path and is within a new, absent file system, but ...
          the client will never see the value of that fh.
        </li>
            <li>
          GETFH --&gt; NFS4ERR_MOVED.
          Fails because current fh is in an absent file system at the start of
          the operation, and the specification makes no exception for GETFH.
        </li>
            <li>
          GETATTR (fsid, fileid, size, time_modify).
          Not executed because the failure of the GETFH stops processing
          of the COMPOUND.
        </li>
          </ul>
          <t>
        Given the failure of the GETFH, the client has the job of
        determining the root of the absent file system and where to find
        that file system, i.e., the server and path relative to that
        server's root fh.  Note that in this example, the client did
        not obtain filehandles and attribute information (e.g., fsid) for
        the intermediate directories, so that it would not be sure where
        the absent file system starts.  It could be the case, for example,
        that /this/is/the is the root of the moved file system and that
        the reason that the look up of "path" succeeded is that the
        file system was not absent on that operation but was moved between the last
        LOOKUP and the GETFH (since COMPOUND is not atomic).  Even if we
        had the fsids for all of the intermediate directories, we could
        have no way of knowing that /this/is/the/path was the root of a
        new file system, since we don't yet have its fsid.
          </t>
          <t>
        In order to get the necessary information, let us re-send the
        chain of LOOKUPs with GETFHs and GETATTRs to at least get the
        fsids so we can be sure where the appropriate file system boundaries are.
        The client could choose to get fs_locations_info
        at the same time but in
        most cases the client will have a good guess as to where file system
        boundaries are (because of where NFS4ERR_MOVED was, and was not,
        received) making fetching of fs_locations_info unnecessary.
          </t>
          <dl newline="false" spacing="normal">
            <dt>OP01:</dt>
            <dd>
          PUTROOTFH  --&gt; NFS_OK
        </dd>
            <dt>- </dt>
            <dd>
          Current fh is root of pseudo-fs.
        </dd>
            <dt>OP02:</dt>
            <dd>
          GETATTR(fsid)  --&gt; NFS_OK
        </dd>
            <dt>- </dt>
            <dd>
          Just for completeness.  Normally, clients will know the fsid
          of the pseudo-fs as soon as they establish communication with
          a server.
        </dd>
            <dt>OP03:</dt>
            <dd>
          LOOKUP "this" --&gt; NFS_OK
        </dd>
            <dt>OP04:</dt>
            <dd>
          GETATTR(fsid)  --&gt; NFS_OK
        </dd>
            <dt>- </dt>
            <dd>
          Get current fsid to see where file system boundaries are.  The fsid
          will be that for the pseudo-fs in this example, so no
          boundary.
        </dd>
            <dt>OP05:</dt>
            <dd>
          GETFH --&gt; NFS_OK
        </dd>
            <dt>- </dt>
            <dd>
          Current fh is for /this and is within pseudo-fs.
        </dd>
            <dt>OP06:</dt>
            <dd>
          LOOKUP "is" --&gt; NFS_OK
        </dd>
            <dt>- </dt>
            <dd>
          Current fh is for /this/is and is within pseudo-fs.
        </dd>
            <dt>OP07:</dt>
            <dd>
          GETATTR(fsid)  --&gt; NFS_OK
        </dd>
            <dt>- </dt>
            <dd>
          Get current fsid to see where file system boundaries are.  The fsid
          will be that for the pseudo-fs in this example, so no
          boundary.
        </dd>
            <dt>OP08:</dt>
            <dd>
          GETFH --&gt; NFS_OK
        </dd>
            <dt>- </dt>
            <dd>
          Current fh is for /this/is and is within pseudo-fs.
        </dd>
            <dt>OP09:</dt>
            <dd>
          LOOKUP "the" --&gt; NFS_OK
        </dd>
            <dt>- </dt>
            <dd>
          Current fh is for /this/is/the and is within pseudo-fs.
        </dd>
            <dt>OP10:</dt>
            <dd>
          GETATTR(fsid)  --&gt; NFS_OK
        </dd>
            <dt>- </dt>
            <dd>
          Get current fsid to see where file system boundaries are.  The fsid
          will be that for the pseudo-fs in this example, so no
          boundary.
        </dd>
            <dt>OP11:</dt>
            <dd>
          GETFH --&gt; NFS_OK
        </dd>
            <dt>- </dt>
            <dd>
          Current fh is for /this/is/the and is within pseudo-fs.
        </dd>
            <dt>OP12:</dt>
            <dd>
          LOOKUP "path" --&gt; NFS_OK
        </dd>
            <dt>- </dt>
            <dd>
          Current fh is for /this/is/the/path and is within a new,
          absent file system, but ...
        </dd>
            <dt>- </dt>
            <dd>
          The client will never see the value of that fh.
        </dd>
            <dt>OP13:</dt>
            <dd>
          GETATTR(fsid, fs_locations_info)  --&gt; NFS_OK
        </dd>
            <dt>- </dt>
            <dd>
          We are getting the fsid to know where the file system boundaries are.
          In this operation, the fsid will be different than that of the
          parent directory (which in turn was retrieved in OP10).
          Note that the fsid we are given will not necessarily be preserved at the new
          location.  That fsid might be different, and in fact the fsid
          we have for this file system might be a valid fsid of a different
          file system on that new server.
        </dd>
            <dt>- </dt>
            <dd>
          In this particular case, we are pretty sure anyway that what
          has moved is /this/is/the/path rather than /this/is/the
          since we have the fsid of the latter and it is that of the
          pseudo-fs, which presumably cannot move.  However, in other
          examples, we might not have this kind of information to rely
          on (e.g., /this/is/the might be a non-pseudo file system
          separate from /this/is/the/path), so we need to have
          other reliable source information on the boundary of the file system
          that is moved.  If, for example, the file system /this/is
          had moved, we would have a case of migration rather than
          referral, and once the boundaries of the migrated file system
          was clear we could fetch fs_locations_info.
        </dd>
            <dt>- </dt>
            <dd>
          We are fetching fs_locations_info because the fact that we got an
          NFS4ERR_MOVED at this point means that it is most likely that
          this is a referral and we need the destination.  Even if it is
          the case that /this/is/the is a file system that has
          migrated, we will still need the location information for that
          file system.
        </dd>
            <dt>OP14:</dt>
            <dd>
          GETFH --&gt; NFS4ERR_MOVED
        </dd>
            <dt>- </dt>
            <dd>
          Fails because current fh is in an absent file system at the start of
          the operation, and the specification makes no exception for GETFH.  Note
          that this means the server will never send the client a
          filehandle from within an absent file system.
        </dd>
          </dl>
          <t>
        Given the above, the client knows where the root of the absent file
        system is (/this/is/the/path) by noting where the change of
        fsid occurred (between "the" and "path").  The
        fs_locations_info attribute also gives the client the
        actual location of
        the absent file system, so that the referral can proceed.  The
        server gives the client the bare minimum of information about the
        absent file system so that there will be very little scope for
        problems of conflict between information sent by the referring
        server and information of the file system's home.  No filehandles
        and very few attributes are present on the referring server, and the
        client can treat those it receives as transient
        information with the function of enabling the referral.
          </t>
        </section>
        <section anchor="referrals_readdir" numbered="true" toc="default">
          <name>Referral Example (READDIR)</name>
          <t>
        Another context in which a client may encounter referrals is when
        it does a READDIR on a directory in which some of the sub-directories
        are the roots of absent file systems.
          </t>
          <t>
        Suppose such a directory is read as follows:
          </t>
          <ul spacing="normal">
            <li>
          PUTROOTFH
        </li>
            <li>
          LOOKUP "this"
        </li>
            <li>
          LOOKUP "is"
        </li>
            <li>
          LOOKUP "the"
        </li>
            <li>
          READDIR (fsid, size, time_modify, mounted_on_fileid)
        </li>
          </ul>
          <t>
        In this case, because rdattr_error is not requested,
        fs_locations_info
        is not requested, and some of the attributes cannot be provided, the
        result will be an NFS4ERR_MOVED error on the READDIR, with the
        detailed results as follows:
          </t>
          <ul spacing="normal">
            <li>
          PUTROOTFH  --&gt; NFS_OK.  The current fh is at the root of the
          pseudo-fs.
        </li>
            <li>
          LOOKUP "this" --&gt; NFS_OK. The current fh is for /this and is
          within the pseudo-fs.
        </li>
            <li>
          LOOKUP "is" --&gt; NFS_OK.  The current fh is for /this/is
          and is within the pseudo-fs.
        </li>
            <li>
          LOOKUP "the" --&gt; NFS_OK.  The current fh is for /this/is/the
          and is within the pseudo-fs.
        </li>
            <li>
          READDIR (fsid, size, time_modify, mounted_on_fileid) --&gt;
          NFS4ERR_MOVED.  Note that the same error would have been
          returned if /this/is/the had migrated, but it is returned because the
          directory contains the root of an absent file system.
        </li>
          </ul>
          <t>
        So now suppose that we re-send with rdattr_error:
          </t>
          <ul spacing="normal">
            <li>
          PUTROOTFH
        </li>
            <li>
          LOOKUP "this"
        </li>
            <li>
          LOOKUP "is"
        </li>
            <li>
          LOOKUP "the"
        </li>
            <li>
          READDIR (rdattr_error, fsid, size, time_modify, mounted_on_fileid)
        </li>
          </ul>
          <t>
        The results will be:
          </t>
          <ul spacing="normal">
            <li>
          PUTROOTFH  --&gt; NFS_OK.  The current fh is at the root of the
          pseudo-fs.
        </li>
            <li>
          LOOKUP "this" --&gt; NFS_OK. The current fh is for /this and is
          within the pseudo-fs.
        </li>
            <li>
          LOOKUP "is" --&gt; NFS_OK.  The current fh is for /this/is
          and is within the pseudo-fs.
        </li>
            <li>
          LOOKUP "the" --&gt; NFS_OK.  The current fh is for /this/is/the
          and is within the pseudo-fs.
        </li>
            <li>
          READDIR (rdattr_error, fsid, size, time_modify, mounted_on_fileid)
          --&gt; NFS_OK.  The attributes for directory entry with the
          component named "path" will only contain
          rdattr_error
          with the value NFS4ERR_MOVED, together with an fsid
          value and a value for mounted_on_fileid.
        </li>
          </ul>
          <t>
        Suppose we do another READDIR to get fs_locations_info (although
        we could have used a GETATTR directly, as in
        <xref target="referrals_lookup" format="default"/>).
          </t>
          <ul spacing="normal">
            <li>
          PUTROOTFH
        </li>
            <li>
          LOOKUP "this"
        </li>
            <li>
          LOOKUP "is"
        </li>
            <li>
          LOOKUP "the"
        </li>
            <li>
          READDIR (rdattr_error, fs_locations_info, mounted_on_fileid, fsid,
          size, time_modify)
        </li>
          </ul>
          <t>
        The results would be:
          </t>
          <ul spacing="normal">
            <li>
          PUTROOTFH  --&gt; NFS_OK.  The current fh is at the root of the
          pseudo-fs.
        </li>
            <li>
          LOOKUP "this" --&gt; NFS_OK. The current fh is for /this and is
          within the pseudo-fs.
        </li>
            <li>
          LOOKUP "is" --&gt; NFS_OK.  The current fh is for /this/is
          and is within the pseudo-fs.
        </li>
            <li>
          LOOKUP "the" --&gt; NFS_OK.  The current fh is for /this/is/the
          and is within the pseudo-fs.
        </li>
            <li>
          READDIR (rdattr_error, fs_locations_info, mounted_on_fileid, fsid,
          size, time_modify) --&gt; NFS_OK.  The attributes will be as shown below.
        </li>
          </ul>
          <t>
         The attributes for the directory entry with the
         component named "path" will only contain:
          </t>
          <ul spacing="normal">
            <li>
          rdattr_error (value: NFS_OK)
        </li>
            <li>
          fs_locations_info
        </li>
            <li>
          mounted_on_fileid (value: unique fileid within referring file system)
        </li>
            <li>
          fsid (value: unique value within referring server)
        </li>
          </ul>
          <t>
        The attributes for entry "path" will not contain size or
        time_modify because these attributes are not available within an
        absent file system.
          </t>
        </section>
      </section>
      <section anchor="fs_locations" numbered="true" toc="default">
        <name>The Attribute fs_locations</name>
        <t>
      The fs_locations attribute is structured in the following way:
        </t>
        <artwork name="" type="" align="left" alt=""><![CDATA[
struct fs_location4 {
        utf8str_cis     server<>;
        pathname4       rootpath;
};
 ]]></artwork>
        <artwork name="" type="" align="left" alt=""><![CDATA[
struct fs_locations4 {
        pathname4       fs_root;
        fs_location4    locations<>;
};
 ]]></artwork>
        <t>
      The fs_location4 data type is used to represent the location of a
      file system by providing a server name and the path to the root
      of the file system within that server's namespace.
      When a set of servers have corresponding file systems at the
      same path within their namespaces, an array of server names may
      be provided.  An
      entry in the server array is a UTF-8 string and represents one
      of a
      traditional DNS host name, IPv4 address, IPv6 address, or a
      zero-length string.
      An IPv4 or IPv6 address is represented as a universal
      address (see <xref target="netaddr4" format="default"/> and <xref target="RFC5665" format="default"/>), minus the netid, and either with
      or without the trailing ".p1.p2" suffix that
      represents the port number. If the suffix is omitted,
      then the default port, 2049, SHOULD be assumed.

      A zero-length string SHOULD be used to indicate the current address
      being used for the RPC call. It is not
      a requirement that all servers that share the same rootpath
      be listed
      in one fs_location4 instance.  The array of server names is provided for
      convenience.  Servers that share the same rootpath may also be listed
      in separate fs_location4 entries in the fs_locations attribute.
        </t>
        <t>
     The fs_locations4 data type and the fs_locations attribute each
     contain an array of
     such locations.  Since the namespace of each server may be
     constructed differently, the "fs_root" field is provided.  The
     path represented
     by fs_root represents the location of the file system in the
     current server's namespace, i.e., that of the
     server from which the fs_locations attribute was obtained.  The
     fs_root path is meant to aid the client by clearly referencing
     the root of the file system whose locations are being reported,
     no matter what object within the current file system the
     current filehandle designates.  The fs_root is simply the
     pathname the client used to reach the object on the current server
     (i.e., the object to which the fs_locations attribute applies).
        </t>
        <t>
     When the fs_locations attribute
     is interrogated and there are no alternate file system locations,
     the server SHOULD return a zero-length array of fs_location4
     structures, together with a valid fs_root.
        </t>
        <t>
     As an example, suppose there is a replicated file system located
     at two
     servers (servA and servB).  At servA, the file system is located at
     path /a/b/c.  At, servB the file system is located at path /x/y/z.
     If the client were to obtain the fs_locations value for the
     directory at /a/b/c/d, it might not necessarily know
     that the file system's root is located in servA's namespace
     at /a/b/c.  When the client switches to servB, it will need
     to determine that the directory it first referenced at servA is now
     represented by the path /x/y/z/d on servB.  To facilitate this, the
     fs_locations attribute provided by servA would have an fs_root value
     of /a/b/c and two entries in fs_locations.  One entry in fs_locations
     will be for itself (servA) and the other will be for servB with a
     path of /x/y/z.  With this information, the client is able to
     substitute /x/y/z for the /a/b/c at the beginning of its access
     path and construct /x/y/z/d to use for the new server.
        </t>
        <t>
     Note that there is no requirement that the number
     of components in each rootpath be the same; there
     is no relation between the number of components in
     rootpath or fs_root, and none of the components
     in a rootpath and fs_root have to be the same. In
     the above example, we could have had a third element
     in the locations array, with server equal to "servC"
     and rootpath equal to "/I/II", and a fourth element in
     locations with server equal to "servD" and rootpath
     equal to "/aleph/beth/gimel/daleth/he".

        </t>
        <t>
     The relationship between fs_root to a rootpath is
     that the client replaces the pathname indicated in
     fs_root for the current server for the substitute
     indicated in rootpath for the new server.

        </t>
        <t>
     For an example of a referred or migrated file
     system, suppose there is a file system located
     at serv1. At serv1, the file system is located at
     /az/buky/vedi/glagoli. The client finds that object
     at glagoli has migrated (or is a referral).  The
     client gets the fs_locations attribute, which contains
     an fs_root of /az/buky/vedi/glagoli, and one element
     in the locations array, with server equal to serv2,
     and rootpath equal to /izhitsa/fita. The client
     replaces /az/buky/vedi/glagoli with /izhitsa/fita,
     and uses the latter pathname on serv2.

        </t>
        <t>
     Thus, the server MUST return an fs_root that is equal
     to the path the client used to reach the object to which the
     fs_locations attribute applies. Otherwise, the
     client cannot determine the new path to use on the new server.

        </t>
        <t>
     Since the fs_locations attribute lacks information defining various
     attributes of the various file system choices presented, it SHOULD
     only be interrogated and used when fs_locations_info is not available.
     When fs_locations is used, information about the
     specific locations should be assumed based on the following rules.
        </t>
        <t>
     The following rules are general and apply irrespective of the
     context.
        </t>
        <ul spacing="normal">
          <li>
       All listed
       file system instances should be considered as of the
       same handle class, if and only if, the
       current fh_expire_type attribute does not include the
       FH4_VOL_MIGRATION
       bit.  Note that in the case of referral, filehandle issues do
       not apply since there can be no filehandles known within the
       current file system, nor is there any access to the fh_expire_type
       attribute on the referring (absent) file system.
     </li>
          <li>
       All listed file system instances should be considered as of the
       same fileid class if and only if the
       fh_expire_type attribute indicates persistent filehandles and
       does not include the FH4_VOL_MIGRATION
       bit.  Note that in the case of referral, fileid issues do
       not apply since there can be no fileids known within the
       referring (absent) file system, nor is there any access to
       the fh_expire_type attribute.
     </li>
          <li>
       All file system instances
       servers should be considered as of different
       change classes.
     </li>
        </ul>
        <t>
     For other class assignments, handling of file system
     transitions depends on the reasons for the transition:
        </t>
        <ul spacing="normal">
          <li>
       When the transition is due to migration, that is, the client was
       directed to a new file system after receiving an NFS4ERR_MOVED error,
       the target should be
       treated as being of the same
       write-verifier class as the source.
     </li>
          <li>
       When the transition is due to failover to another replica,
       that is, the client selected another replica without
       receiving an NFS4ERR_MOVED error, the target should be
       treated as being of a different
       write-verifier class from the source.
     </li>
        </ul>
        <t>
     The specific choices reflect typical implementation patterns for
     failover and controlled migration, respectively.  Since other
     choices are possible and useful, this information is better
     obtained by using fs_locations_info.  When a server implementation
     needs to communicate other choices, it MUST support the
     fs_locations_info attribute.
        </t>
        <t>
     See <xref target="SECCON" format="default"/> for a
     discussion on the recommendations for the security
     flavor to be used by any GETATTR operation that
     requests the "fs_locations" attribute.

        </t>
      </section>
      <section anchor="SEC11-li-new" numbered="true" toc="default">
        <name>The Attribute fs_locations_info</name>
        <t>
      The fs_locations_info attribute is intended as a more functional
      replacement for the fs_locations attribute which will continue to exist
      and be
      supported.  Clients can use it to get a more complete set of
      data about alternative file system locations, including additional
      network paths to access replicas in use and additional replicas.
      When the server does not support
      fs_locations_info, fs_locations can be used to get a subset of the
      data.  A server that supports fs_locations_info MUST support
      fs_locations as well.
        </t>
        <t>
      There is additional data present in
      fs_locations_info, that is not available in fs_locations:
        </t>
        <ul spacing="normal">
          <li>
        Attribute continuity information. This information
        will allow a client to select a
        replica that meets the transparency requirements of the
        applications accessing the data and to leverage
        optimizations due to the server guarantees of attribute
        continuity (e.g., if the
        change attribute of a file of the file system is continuous
	between multiple replicas,
        the client does not have to invalidate the file's cache
	when switching to a different replica).
      </li>
          <li>
            <t>
        File system identity information that indicates when multiple
        replicas, from the client's point of view, correspond to the
        same target file system, allowing them to be used
        interchangeably, without disruption, as distinct synchronized
	replicas of the same file data.
            </t>
            <t>
        Note that having two replicas with common identity information is
        distinct from the case of two (trunked) paths to the same
	replica.
            </t>
          </li>
          <li>
        Information that will bear on the suitability of various
        replicas, depending on the use that the client intends.  For
        example, many applications need an absolutely up-to-date copy
        (e.g., those that write), while others may only need access to
        the most up-to-date copy reasonably available.
      </li>
          <li>
        Server-derived preference information for replicas, which can
        be used to implement load-balancing while giving the client
        the entire file system list to be used in case the primary fails.
      </li>
        </ul>
        <t>
      The fs_locations_info attribute is structured similarly to the
      fs_locations attribute.  A top-level structure
      (fs_locations_info4) contains the entire attribute including the root
      pathname of the file system and an array of lower-level structures that
      define replicas that share a common rootpath on their respective
      servers.  The lower-level structure in turn
      (fs_locations_item4) contains a specific pathname and information on one
      or more individual network access paths.  For that last lowest level,
      fs_locations_info has an fs_locations_server4
      structure that contains per-server-replica information in addition
      to the file system
      location entry.  This per-server-replica information includes a
      nominally opaque array, fls_info, within which specific pieces
      of information
      are located at the specific indices listed below.
        </t>
        <t>
      Two fs_location_server4 entries that are within different
      fs_location_item4 structures are never trunkable, while two entries
      within in the same fs_location_item4 structure might or might not be
      trunkable.  Two entries that are trunkable will have identical
      identity information, although, as noted above, the converse is
      not the case.
        </t>
        <t>
      The attribute will always contain at least a single fs_locations_server
      entry.  Typically, there  will be an entry with the FS4LIGF_CUR_REQ
      flag set, although in the case of a referral there will be no
      entry with that flag set.
        </t>
        <t>
      It should be noted that fs_locations_info attributes returned by
      servers for various replicas may differ for various reasons.
      One server may know about a set of replicas that are not known to
      other servers.  Further, compatibility attributes may differ.
      Filehandles might be of the same class going from replica A to
      replica B but not going in the reverse direction.  This might happen
      because the filehandles are the same, but
      replica B's server implementation might not have provision to note
      and report that equivalence.
        </t>
        <t>
      The fs_locations_info attribute consists of a root
      pathname (fli_fs_root, just like fs_root in the
      fs_locations attribute), together with an array of
      fs_location_item4 structures.  The fs_location_item4
      structures in turn consist of a root pathname
      (fli_rootpath) together with an array (fli_entries)
      of elements of data type fs_locations_server4,
      all defined as follows.

        </t>
        <artwork name="" type="" align="left" alt=""><![CDATA[

/*
 * Defines an individual server access path
 */
struct  fs_locations_server4 {
        int32_t         fls_currency;
        opaque          fls_info<>;
        utf8str_cis     fls_server;
};

/*
 * Byte indices of items within
 * fls_info: flag fields, class numbers,
 * bytes indicating ranks and orders.
 */
const FSLI4BX_GFLAGS            = 0;
const FSLI4BX_TFLAGS            = 1;

const FSLI4BX_CLSIMUL           = 2;
const FSLI4BX_CLHANDLE          = 3;
const FSLI4BX_CLFILEID          = 4;
const FSLI4BX_CLWRITEVER        = 5;
const FSLI4BX_CLCHANGE          = 6;
const FSLI4BX_CLREADDIR         = 7;

const FSLI4BX_READRANK          = 8;
const FSLI4BX_WRITERANK         = 9;
const FSLI4BX_READORDER         = 10;
const FSLI4BX_WRITEORDER        = 11;

/*
 * Bits defined within the general flag byte.
 */
const FSLI4GF_WRITABLE          = 0x01;
const FSLI4GF_CUR_REQ           = 0x02;
const FSLI4GF_ABSENT            = 0x04;
const FSLI4GF_GOING             = 0x08;
const FSLI4GF_SPLIT             = 0x10;

/*
 * Bits defined within the transport flag byte.
 */
const FSLI4TF_RDMA              = 0x01;

/*
 * Defines a set of replicas sharing
 * a common value of the rootpath
 * within the corresponding
 * single-server namespaces.
 */
struct  fs_locations_item4 {
        fs_locations_server4    fli_entries<>;
        pathname4               fli_rootpath;
};

/*
 * Defines the overall structure of
 * the fs_locations_info attribute.
 */
struct  fs_locations_info4 {
        uint32_t                fli_flags;
        int32_t                 fli_valid_for;
        pathname4               fli_fs_root;
        fs_locations_item4      fli_items<>;
};

/*
 * Flag bits in fli_flags.
 */
const FSLI4IF_VAR_SUB           = 0x00000001;

typedef fs_locations_info4 fattr4_fs_locations_info;

 ]]></artwork>
        <t>
      As noted above, the fs_locations_info attribute, when supported, may
      be requested of absent file systems without causing NFS4ERR_MOVED to
      be returned.  It is generally expected that it will be available for
      both present and absent file systems even if only a single
      fs_locations_server4 entry is present, designating the current (present)
      file system, or two fs_locations_server4 entries designating the
      previous location of an absent file system (the one just referenced) and its
      successor location.  Servers are strongly urged to support this
      attribute on all file systems if they support it on any file system.
        </t>
        <t>
      The data presented in the fs_locations_info attribute may be obtained
      by the server in any number of ways, including specification by
      the administrator or by current protocols for transferring data
      among replicas and protocols not yet developed.  NFSv4.1 only defines
      how this information is presented by the server to
      the client.
        </t>
        <section anchor="SEC11-fsli-server" numbered="true" toc="default">
          <name>The fs_locations_server4 Structure</name>
          <t>
        The fs_locations_server4 structure consists of the following items
	in addition to the fls_server field which specifies a network
	address or set of addresses to be used to access the specified file
	system.  Note that both of these items (i.e., fls_currency and flinfo)
	specify attributes of the
	file system replica and should not be different when there are
	multiple fs_locations_server4 structures for the same replica, each
	specifying a network path to the chosen replica.
          </t>
          <t>
	When these values are different in two fs_locations_server4 structures,
	a client has no basis for choosing one over the other and is best off
	simply ignoring both entries, whether these entries apply to migration
	replication or referral.  When there are more than two such entries,
	majority voting can be used to exclude a single erroneous entry from
	consideration.  In the case in which trunking information is provided
	for a replica currently being accessed, the additional trunked addresses
	can be ignored while access continues on the address currently being
	used, even if the entry corresponding to that path might be considered
	invalid.
          </t>
          <ul spacing="normal">
            <li>
          An indication of how up-to-date the file system is (fls_currency) in
          seconds.  This value
          is relative to the master copy.  A negative
          value indicates that the server is unable to give any
          reasonably useful value here.  A value of zero indicates that the
          file system is the actual writable data or a reliably coherent
          and fully up-to-date copy.  Positive values indicate how
          out-of-date this copy can normally be before it is considered for
          update.  Such a value is not a guarantee that such updates
          will always be performed on the required schedule but instead
          serves as a hint about how far the copy of the data would be
          expected to be behind the most up-to-date copy.
        </li>
            <li>
          A counted array of one-byte values (fls_info) containing
          information about the particular file system instance.  This
          data includes general flags, transport capability flags,
          file system equivalence class information, and selection
          priority information.  The encoding will be discussed below.
        </li>
            <li>
          The server string (fls_server).  For the case of the
          replica currently
          being accessed (via GETATTR), a zero-length string MAY be used to
          indicate the current address being used for the RPC call.
          The fls_server field can also be an IPv4 or IPv6 address,
          formatted the same way as an IPv4 or IPv6 address in the "server"
          field of the fs_location4 data type (see
	  <xref target="fs_locations" format="default"/>).
        </li>
          </ul>
          <t>
	With the exception of the transport-flag field (at offset
	FSLI4BX_TFLAGS with the fls_info array), all of this data defined
	in this specification applies
	to the replica specified by the entry, rather that the specific
	network path used to access it.  The classification of data in
	extensions to this data is discussed below.
          </t>
          <t>
        Data within the fls_info array is in the form of 8-bit data items
        with constants giving the offsets within the array of various
        values describing this particular file system instance.
        This style of
        definition was chosen, in preference to explicit XDR
        structure definitions for these values, for a number of
        reasons.
          </t>
          <ul spacing="normal">
            <li>
          The kinds of data in the fls_info array, representing flags,
          file system classes, and priorities among sets of file systems
          representing the same data, are such that 8 bits provide
          a quite acceptable range of values.  Even where there might
          be more than 256 such file system instances, having more than
          256 distinct classes or priorities is unlikely.
        </li>
            <li>
          Explicit definition of the various specific data items within
          XDR would limit expandability in that any extension within
          would require yet another attribute,
          leading to specification and implementation clumsiness.
	  In the context of the NFSv4 extension model in effect at the time
	  fs_locations_info was designed (i.e. that described in
	  RFC5661 <xref target="RFC5661" format="default"/>), this would necessitate a new minor
	  version
	  to effect any Standards Track extension to the data in in
	  fls_info.
        </li>
          </ul>
          <t>
        The set of fls_info data is subject to expansion in a future minor
        version, or in a Standards Track RFC, within the context of a single
        minor version.  The server SHOULD NOT send and the client MUST NOT
        use indices within the fls_info array or flag bits that are not
	defined in
        Standards Track RFCs.
          </t>
          <t>
	In light of the new extension model defined in RFC8178
	<xref target="RFC8178" format="default"/>
	and the fact that the individual items within fls_info are not
	explicitly referenced in the XDR, the following practices should be
	followed when extending or otherwise changing the structure of
	the data returned in fls_info within the scope of a single minor
	version.
          </t>
          <ul spacing="normal">
            <li>
	  All extensions need to be described by Standards Track documents.
	  There
	  is no need for such documents to be marked as updating
	  RFC5661 <xref target="RFC5661" format="default"/> or this document.
        </li>
            <li>
	  It needs to be made clear whether the information in any added data
	  items applies to the replica specified by the entry or to the specific
	  network paths specified in the entry.
	</li>
            <li>
	  There needs to be a reliable way defined to determine whether the
	  server is aware of the extension.  This may be based on the
	  length field of the fls_info array, but it is more flexible to
	  provide fs-scope or server-scope attributes to indicate what
	  extensions are provided.
        </li>
          </ul>
          <t>
        This encoding scheme can be adapted to the specification of
        multi-byte numeric values, even though none are currently
        defined.  If extensions are made via Standards Track RFCs,
        multi-byte quantities will be encoded as a range of bytes
        with a range of indices, with the byte interpreted in big-endian
        byte order.  Further, any such index assignments will be constrained
        by the need for the relevant quantities not to
	cross XDR word boundaries.
          </t>
          <t>
        The fls_info array currently contains:
          </t>
          <ul spacing="normal">
            <li>
           Two 8-bit flag fields, one devoted to general file-system
           characteristics and a second reserved for transport-related
           capabilities.
         </li>
            <li>
           Six 8-bit class values that define various file system
           equivalence classes as explained below.
         </li>
            <li>
           Four 8-bit priority values that govern file system selection
           as explained below.
         </li>
          </ul>
          <t>
        The general file system characteristics flag (at byte index
        FSLI4BX_GFLAGS) has the following
        bits defined within it:
          </t>
          <ul spacing="normal">
            <li>
          FSLI4GF_WRITABLE indicates that this file system target is writable,
          allowing it to be selected by clients that may need to write
          on this file system.  When the current file system instance
          is writable and is defined as of the same simultaneous use
          class (as specified by the value at index FSLI4BX_CLSIMUL)
          to which the client was previously writing, then it must
          incorporate within its data any committed
          write made on the source file system instance.  See
          <xref target="SEC11-EFF-wv" format="default"/>, which discusses
          the write-verifier class.  While there is no harm in not setting
          this flag for a file system that turns out to be writable,
          turning the flag on for a read-only file system can cause
          problems for clients that select a migration or replication
          target based on the flag and then find themselves unable to write.
        </li>
            <li>
          FSLI4GF_CUR_REQ indicates that this replica is the one on which
          the request is being made.  Only a single server entry may
          have this flag set and, in the case of a referral, no entry
          will have it set.  Note that this flag might be set even if the
	  request was made on a network access path different from any of
	  those specified in the current entry.
        </li>
            <li>
          FSLI4GF_ABSENT indicates that this entry corresponds to an absent
          file system replica.  It can only be set if FSLI4GF_CUR_REQ is set.
          When both such bits are set, it indicates that a file system
          instance is not usable but that the information in the entry
          can be used to determine the sorts of continuity available
          when switching from this replica to other possible replicas.
          Since this bit can only be true if FSLI4GF_CUR_REQ is true, the
          value could be determined using the fs_status attribute, but
          the information is also made available here for the
          convenience of the client.  An entry with this bit, since it
          represents a true file system (albeit absent), does not appear
          in the event of a referral, but only when a file system has
          been accessed at this location and has subsequently been migrated.
        </li>
            <li>
              <t>
          FSLI4GF_GOING indicates that a replica, while still available,
          should not be used further.  The client, if using it, should
          make an orderly transfer to another file system instance as
          expeditiously as possible.  It is expected that file systems
          going out of service will be announced as FSLI4GF_GOING some time
          before the actual loss of service. It is also expected that the
	  fli_valid_for value
          will be sufficiently small to allow clients to detect and act
          on scheduled events, while large enough that the cost of the
          requests to fetch the fs_locations_info values will not be
          excessive.  Values on the order of ten minutes seem
          reasonable.
              </t>
              <t>
          When this flag is seen as part of a transition into a new
          file system, a client might choose to transfer immediately
          to another replica, or it may reference the current file system
          and only transition when a migration event occurs.  Similarly,
          when this flag appears as a replica in the referral, clients
          would likely avoid being referred to this instance whenever
          there is another choice.
              </t>
              <t>
	  This flag, like the other items within fls_info applies to the
	  replica, rather than to a particular path to that replica.  When
	  it appears, a transition to a new replica rather than to a
	  different path to the same replica, is indicated.
              </t>
            </li>
            <li>
              <t>
          FSLI4GF_SPLIT indicates that when a transition occurs from
          the current file system instance to this one, the replacement
          may consist of multiple file systems.  In this case, the
          client has to be prepared for the possibility that objects
          on the same file system before migration will be on different ones
          after.  Note that FSLI4GF_SPLIT is not incompatible with the
          file systems belonging to the same fileid
          class
          since, if one has a set of fileids that are unique within
          a file system, each subset assigned to a smaller file system after migration
          would not have any conflicts internal to that file system.
              </t>
              <t>
          A client, in the case of a split file system, will interrogate
          existing files with which it has continuing connection (it
          is free to simply forget cached filehandles).  If the client
          remembers the directory filehandle associated with each open
          file, it may proceed upward using LOOKUPP to find the new file system
          boundaries.  Note that in the event of a referral, there will
          not be any such files and so these actions will not be performed.
	  Instead, a reference to a portion of the original
	  file system now split off into other file systems
	  will encounter an fsid change and possibly a
	  further referral.

              </t>
              <t>
          Once the client recognizes that one file system has been split
          into two, it can prevent the disruption of running applications
          by presenting the two file systems as a single
          one until a convenient point to recognize the transition,
          such as a restart.  This would require a mapping
          from the server's fsids to fsids as seen by the client, but
          this is already necessary for other reasons.  As noted
          above, existing fileids within the two descendant file systems
          will not conflict.  Providing non-conflicting fileids for
          newly created files on the split file systems
          is the responsibility of the server (or servers working in
          concert).  The server can encode filehandles such
          that filehandles generated before the split event can be discerned
          from those generated after the split,
          allowing the server to determine when the need
          for emulating two file systems as one is over.
              </t>
              <t>
          Although it is possible for this flag to be present in the
          event of referral, it would generally be of little interest
          to the client, since the client is not expected to have
          information regarding the current contents of the absent
          file system.
              </t>
            </li>
          </ul>
          <t>
        The transport-flag field (at byte index FSLI4BX_TFLAGS) contains
        the following bits related to the transport
        capabilities of the specific network path(s) specified by the
	entry.
          </t>
          <ul spacing="normal">
            <li>
          FSLI4TF_RDMA indicates that any specified network paths
	  provide NFSv4.1 clients
          access using an RDMA-capable transport.
        </li>
          </ul>
          <t>
        Attribute continuity and file system identity information are
        expressed by defining equivalence relations on the sets of
        file systems presented to the client.  Each such relation
        is expressed as a set of file system equivalence classes.
        For each relation, a file system has an 8-bit class number.
        Two file systems belong to the same class if both have
        identical non-zero class numbers.  Zero is treated as
        non-matching.  Most often,
        the relevant question for the client will be whether a
        given replica is identical to / continuous with the current one in a
        given respect, but the information should be available also as to
        whether two other replicas match in that respect as well.
          </t>
          <t>
        The following fields specify the file system's class numbers
        for the equivalence relations used in determining the nature of
        file system transitions.  See Sections
	<xref target="SEC11-trans-oview" format="counter"/>
	through <xref target="SEC11-trans-server" format="counter"/>
	and their various subsections
        for details about how
        this information is to be used.  Servers may assign these values
        as they wish, so long as file system instances that share the
        same value have the specified relationship to one another;
        conversely, file systems that have the specified relationship
        to one another share a common class value. As each instance
        entry is added, the relationships of this instance to previously
        entered instances can be consulted, and if one is found that
        bears the specified relationship, that entry's class value can
        be copied to the new entry.  When no such previous entry exists,
        a new value for that byte index (not previously used) can be
        selected, most likely by incrementing the value of the last class
        value assigned for that index.
          </t>
          <ul spacing="normal">
            <li>
          The field with byte index FSLI4BX_CLSIMUL defines the
          simultaneous-use class for the file system.
        </li>
            <li>
          The field with byte index FSLI4BX_CLHANDLE defines the handle
          class for the file system.
        </li>
            <li>
          The field with byte index FSLI4BX_CLFILEID defines the fileid
          class for the file system.
        </li>
            <li>
          The field with byte index FSLI4BX_CLWRITEVER defines the
          write-verifier class for the file system.
        </li>
            <li>
          The field with byte index FSLI4BX_CLCHANGE defines the change
          class for the file system.
        </li>
            <li>
          The field with byte index FSLI4BX_CLREADDIR defines the readdir
          class for the file system.
        </li>
          </ul>
          <t>
        Server-specified preference information is also provided via
        8-bit values within the fls_info array.  The values provide a
        rank and an order (see below) to be used with separate values
        specifiable for the cases of read-only and writable file
        systems.
        These values are compared
        for different file systems to establish the server-specified
        preference, with lower values indicating "more preferred".
          </t>
          <t>
        Rank is used to express a strict server-imposed ordering on
        clients, with lower values indicating "more preferred".  Clients
        should attempt to use all replicas with a given rank before they
        use one with a higher rank.  Only if all of those file systems are
        unavailable should the client proceed to those of a higher rank.
        Because specifying a rank will override client preferences, servers
        should be conservative about using this mechanism, particularly
        when the environment is one in which client communication characteristics
        are neither tightly controlled nor visible to the server.
          </t>
          <t>
        Within a rank, the order value is used to specify the server's
        preference to guide the client's selection when the client's own
        preferences are not controlling, with lower values of order
        indicating "more preferred".  If replicas are approximately equal
        in all respects, clients should defer to the order specified by the
        server.  When clients look at server latency as part of their
        selection, they are free to use this criterion, but it is suggested
        that when latency differences are not significant, the
        server-specified order should guide selection.

          </t>
          <ul spacing="normal">
            <li>
          The field at byte index FSLI4BX_READRANK gives the rank value to
          be used for read-only access.
        </li>
            <li>
          The field at byte index FSLI4BX_READORDER gives the order value to
          be used for read-only access.
        </li>
            <li>
          The field at byte index FSLI4BX_WRITERANK gives the rank value to
          be used for writable access.
        </li>
            <li>
          The field at byte index FSLI4BX_WRITEORDER gives the order value to
          be used for writable access.
        </li>
          </ul>
          <t>
        Depending on the potential need for write access by a given client,
        one of the pairs of rank and order values is used.
        The read rank and order should only be used
        if the client knows that only reading will ever be done or if it is
        prepared to switch to a different replica in the event that any
        write access capability is required in the future.
          </t>
        </section>
        <section anchor="SEC11-fsli-info" numbered="true" toc="default">
          <name>The fs_locations_info4 Structure</name>
          <t>
        The fs_locations_info4 structure, encoding the fs_locations_info
        attribute, contains the following:
          </t>
          <ul spacing="normal">
            <li>
          The fli_flags field, which contains general flags that affect
          the interpretation of this fs_locations_info4 structure and
          all fs_locations_item4 structures within it.  The only flag
          currently defined is FSLI4IF_VAR_SUB.  All bits in the
	  fli_flags field that are not defined should always be returned as zero.
        </li>
            <li>
          The fli_fs_root field, which contains the pathname of the root of
          the current file system on the current server, just as it does
          in the fs_locations4 structure.
        </li>
            <li>
          An array called fli_items of fs_locations4_item structures, which contain
          information about replicas of the current file system.  Where
          the current file system is actually present, or has been
          present, i.e., this is not a referral situation, one of the
          fs_locations_item4 structures will contain an fs_locations_server4 for
          the current server.  This structure will have FSLI4GF_ABSENT set
          if the current file system is absent, i.e., normal access to it
          will return NFS4ERR_MOVED.
        </li>
            <li>
          The fli_valid_for field specifies a time in seconds
          for which it is reasonable for a client to use the fs_locations_info attribute
          without refetch.  The fli_valid_for value does not provide a
          guarantee of validity since servers can unexpectedly go out of
          service or become inaccessible for any number of reasons.
          Clients are well-advised to refetch this information for an
          actively accessed file system at every fli_valid_for seconds.  This
          is particularly important when file system replicas may go out
          of service in a controlled way using the FSLI4GF_GOING flag to
          communicate an ongoing change.  The server should set
          fli_valid_for to a value that allows well-behaved clients to
          notice the FSLI4GF_GOING flag and make an orderly switch before
          the loss of service becomes effective.  If this value is zero,
          then no refetch interval is appropriate and the client need
          not refetch this data on any particular schedule.
          In the event of a transition to a new file system instance, a
          new value of the fs_locations_info attribute will be fetched at
          the destination.  It is to be expected that this may have a
          different fli_valid_for value, which the client should then use
          in the same fashion as the previous value.   Because a refetch
	  of the attribute causes information from all component entries to
	  be refetched, the server will typically provide a low value for
	  this field if any of the replicas are likely to go out of service
	  in a short time frame.   Note that, because of the ability of the
	  server to return NFS4ERR_MOVED to trigger the use of different paths,
	  when alternate trunked paths are available, there is generally no
	  need to use low values of fli_valid_for in connection with the
	  management of alternate paths to the same replica.
        </li>
          </ul>
          <t>
        The FSLI4IF_VAR_SUB flag within fli_flags controls whether variable
        substitution is to be enabled.  See <xref target="SEC11-fsli-item" format="default"/>
        for an explanation of variable substitution.
          </t>
        </section>
        <section anchor="SEC11-fsli-item" numbered="true" toc="default">
          <name>The fs_locations_item4 Structure</name>
          <t>
        The fs_locations_item4 structure contains a pathname
        (in the field fli_rootpath) that encodes
        the path of the target file system replicas on the set of
        servers designated by the included fs_locations_server4 entries.
        The precise manner in which this target location
        is specified depends on the value of the FSLI4IF_VAR_SUB
        flag within the associated fs_locations_info4 structure.
          </t>
          <t>
        If this flag is not set, then fli_rootpath simply designates
        the location of the target file system within each server's
        single-server namespace just as it does for the rootpath
        within the fs_location4 structure.  When this bit is set,
        however, component entries of a certain form are subject
        to client-specific variable substitution so as to allow
        a degree of namespace non-uniformity in order to accommodate
        the selection of client-specific file system targets to
        adapt to different client architectures or other
        characteristics.
          </t>
          <t>
        When such substitution is in effect, a variable beginning
        with the string "${" and ending with the string "}"
        and containing a colon is to be
        replaced by the client-specific value associated with
        that variable.  The string "unknown" should be used
        by the client when it has no value for such a variable.
        The pathname resulting from such
        substitutions is used to designate the target file system,
        so that different clients may have different file systems,
        corresponding to that location in the multi-server namespace.
          </t>
          <t>
        As mentioned above, such substituted pathname variables
        contain a colon.  The part before the colon is to be a
        DNS domain name, and the part after is to be a case-insensitive
        alphanumeric string.
          </t>
          <t>
        Where the domain is "ietf.org", only variable names defined
        in this document or subsequent Standards Track RFCs
        are subject to such substitution.  Organizations are
        free to use their domain names to create their own sets
        of client-specific variables, to be subject to such
        substitution.  In cases where such variables are intended
        to be used more broadly than a single organization,
        publication of an Informational RFC defining such variables
        is RECOMMENDED.
          </t>
          <t>
        The variable ${ietf.org:CPU_ARCH} is used to denote that the
        CPU architecture object files are compiled.  This specification
        does not limit the acceptable values (except that they must be
        valid UTF-8 strings), but such values as "x86", "x86_64", and "sparc"
        would be expected to be used in line with industry practice.
          </t>
          <t>
        The variable ${ietf.org:OS_TYPE} is used to denote the
        operating system, and thus the kernel and library APIs,
        for which code might be compiled.  This specification does
        not limit the acceptable values (except that they must be
        valid UTF-8 strings), but such values as "linux" and "freebsd"
        would be expected to be used in line with industry practice.
          </t>
          <t>
        The variable ${ietf.org:OS_VERSION} is used to denote the
        operating system version, and thus the specific details
        of versioned interfaces,
        for which code might be compiled.  This specification does
        not limit the acceptable values (except that they must be
        valid UTF-8 strings). However, combinations of numbers and
        letters with interspersed dots would be expected to be used
        in line with industry practice, with the details of the
        version format depending on the specific value of
        the variable ${ietf.org:OS_TYPE} with which
        it is used.
          </t>
          <t>
        Use of these variables could result in the direction of different
        clients to different file systems on the same server, as
        appropriate to particular clients.  In cases in which the
        target file systems are located on different servers, a single
        server could serve as a referral point so that each valid
        combination of variable values would designate a referral
        hosted on a single server, with the targets of those referrals on
        a number of different servers.
          </t>
          <t>
        Because namespace administration is affected by the values
        selected to substitute for various variables, clients should
        provide convenient means of determining what variable
        substitutions a client will implement, as well as, where
        appropriate, providing means to control the substitutions to
        be used.  The exact means by which this will be done is
        outside the scope of this specification.
          </t>
          <t>
        Although variable substitution is most suitable for use
        in the context of referrals, it may be used in the context
        of replication and migration.  If it is used in these contexts,
        the server must ensure that no matter what values the
        client presents for the substituted variables, the result
        is always a valid successor file system instance to that
        from which a transition is occurring, i.e., that the data is
        identical or represents a later image of a writable file
        system.
          </t>
          <t>
        Note that when fli_rootpath is a null pathname (that is, one
        with zero components), the file system designated is at the
        root of the specified server, whether or not the FSLI4IF_VAR_SUB
        flag within the associated fs_locations_info4 structure is
        set.
          </t>
        </section>
      </section>
      <section anchor="fs_status" numbered="true" toc="default">
        <name>The Attribute fs_status</name>
        <t>
       In an environment in which multiple copies of the same basic set of
       data are available, information regarding the particular source of
       such data and the relationships among different copies can be very
       helpful in providing consistent data to applications.
        </t>
        <artwork name="" type="" align="left" alt=""><![CDATA[
enum fs4_status_type {
        STATUS4_FIXED = 1,
        STATUS4_UPDATED = 2,
        STATUS4_VERSIONED = 3,
        STATUS4_WRITABLE = 4,
        STATUS4_REFERRAL = 5
};

struct fs4_status {
        bool            fss_absent;
        fs4_status_type fss_type;
        utf8str_cs      fss_source;
        utf8str_cs      fss_current;
        int32_t         fss_age;
        nfstime4        fss_version;
};
 ]]></artwork>
        <t>
      The boolean fss_absent indicates whether the file system is
      currently absent.  This value will be set if the file system was
      previously present and becomes absent, or if the file system has
      never been present and the type is STATUS4_REFERRAL.  When this
      boolean is set and the type is not STATUS4_REFERRAL, the
      remaining information in the fs4_status reflects that last valid
      when the file system was present.
        </t>
        <t>
      The fss_type field indicates the kind of file system image represented.
      This is of particular importance when using the version values to
      determine appropriate succession of file system images.
      When fss_absent is set, and the file system was previously
      present, the value of fss_type reflected is that when the file was last present.
      Five values are distinguished:
        </t>
        <ul spacing="normal">
          <li>
        STATUS4_FIXED, which indicates a read-only image in the sense
        that it will never change.  The possibility is allowed that, as
        a result of migration or switch to a different image, changed
        data can be accessed, but within the confines of this instance,
        no change is allowed.  The client can use this fact to
        cache aggressively.
      </li>
          <li>
        STATUS4_VERSIONED, which indicates that the image, like the
        STATUS4_UPDATED case, is updated externally, but it provides
        a guarantee that the server will carefully update an
        associated version value so that the client can
        protect itself from a situation in which it reads
        data from one version of the file system and then later reads
        data from an earlier version of the same file system.  See
        below for a discussion of how this can be done.
      </li>
          <li>
        STATUS4_UPDATED, which indicates an image that cannot be
        updated by the user writing to it but that may be changed
        externally, typically because it is a periodically updated
        copy of another writable file system somewhere else.  In
        this case, version information is not provided, and the
        client does not have the responsibility of making sure
        that this version only advances upon a file system instance
        transition.  In this case, it is the responsibility of the
        server to make sure that the data presented after a file
        system instance transition is a proper successor image and
        includes all changes seen by the client and any change made
        before all such changes.
      </li>
          <li>
        STATUS4_WRITABLE, which indicates that the file system is an
        actual writable one.  The client need not, of course, actually
        write to the file system, but once it does, it should not
        accept a transition to anything other than a writable instance
        of that same file system.
      </li>
          <li>
        STATUS4_REFERRAL, which indicates that the file system in
        question is absent and has never been present on this
        server.
      </li>
        </ul>
        <t>
      Note that in the STATUS4_UPDATED and STATUS4_VERSIONED cases, the
      server is responsible for the appropriate handling of locks that
      are inconsistent with external changes to delegations.
      If a server gives out delegations, they SHOULD be recalled
      before an inconsistent change is made to the data, and MUST
      be revoked if this is not possible.  Similarly, if an OPEN is
      inconsistent with data that is changed (the OPEN has
      OPEN4_SHARE_DENY_WRITE/OPEN4_SHARE_DENY_BOTH
      and the data is changed), that OPEN SHOULD be considered
      administratively revoked.
        </t>
        <t>
      The opaque strings fss_source and fss_current provide a way of presenting
      information about the source of the file system image being present.
      It is not intended that the client do anything with this information
      other than make it available to administrative tools.  It is
      intended that this information be helpful when researching possible
      problems with a file system image that might arise when it is
      unclear if the correct image is being accessed and, if not, how that
      image came to be made.  This kind of diagnostic information will be
      helpful, if, as seems likely, copies of file systems are made in
      many different ways (e.g., simple user-level copies,
      file-system-level point-in-time copies,
      clones of the underlying storage),
      under a variety of administrative arrangements.  In such
      environments, determining how a given set of data was constructed
      can be very helpful in resolving problems.
        </t>
        <t>
      The opaque string fss_source is used to indicate the source of a
      given file system with the expectation that tools capable of
      creating a file system image propagate this information, when
      possible.  It is understood that this may not always be possible
      since a user-level copy may be thought of as creating a new data
      set and the tools used may have no mechanism to propagate this
      data.  When a file system is initially created, it is desirable
      to associate with it
      data regarding how the file system was created, where it was
      created, who created it, etc. Making this information available
      in this attribute in a human-readable
      string will be helpful for applications and
      system administrators and will also serve to make it available when
      the original file system is used to make subsequent copies.
        </t>
        <t>
      The opaque string fss_current should provide whatever information is
      available about the source of the current copy.  Such
      information includes
      the tool creating it, any relevant parameters to that tool, the
      time at which the copy was done, the user making the change, the
      server on which the change was made, etc.  All information should be
      in a human-readable string.
        </t>
        <t>
      The field fss_age provides an indication of how out-of-date the file system
      currently is with respect to its ultimate data source (in case of
      cascading data updates).  This complements the fls_currency field of
      fs_locations_server4 (see <xref target="SEC11-li-new" format="default"/>) in the
      following way: the information in fls_currency
      gives a bound for how out of date the data in a file system might
      typically get, while the value in fss_age gives a bound on how out-of-date that
      data actually is.  Negative values imply that no information is
      available.  A zero means that this data is known to be current.
      A positive value means that this data is known to be no older than
      that number of seconds with respect to the ultimate data source.
      Using this value, the client may be able to decide that a data copy
      is too old, so that it may search for a newer version to use.
        </t>
        <t>
      The fss_version field provides a version identification, in the form of
      a time value, such that successive versions always have later time
      values.  When the fs_type is anything other than
      STATUS4_VERSIONED, the server may provide such a value, but there is
      no guarantee as to its validity and clients will not use it except
      to provide additional information to add to fss_source and fss_current.
        </t>
        <t>
      When fss_type is STATUS4_VERSIONED, servers SHOULD provide a value
      of fss_version that progresses monotonically whenever any new version
      of the data is established.  This allows the client, if reliable
      image progression is important to it, to fetch this attribute as
      part of each COMPOUND where data or metadata from the file system is
      used.
        </t>
        <t>
      When it is important to the client to make sure that only valid
      successor images are accepted, it must make sure that it does not
      read data or metadata from the file system without updating its
      sense of the current state of the image. This is to avoid the possibility
      that the fs_status that the client holds will be one for an
      earlier image, which would cause the client to accept a new file
      system instance that is later than that but still earlier than
      the updated data read by the client.
        </t>
        <t>
      In order to accept valid images reliably, the client must do a GETATTR of the fs_status
      attribute that follows any interrogation of data or metadata within the
      file system in question.  Often this is most conveniently done by
      appending such a GETATTR after all other operations that reference
      a given file system.  When errors occur between reading file system
      data and performing such a GETATTR, care must be exercised to make
      sure that the data in question is not used before obtaining the
      proper fs_status value.  In this connection, when an OPEN is done
      within such a versioned file system and the associated GETATTR of
      fs_status is not successfully completed, the open file in question
      must not be accessed until that fs_status is fetched.
        </t>
        <t>
      The procedure above will ensure that before using any data from the
      file system the client has in hand a newly-fetched current version
      of the file system image.  Multiple values for multiple requests in
      flight can be resolved by assembling them into the required partial
      order (and the elements should form a total order within the
      partial order) and
      using the last.
The client may then, when switching among
      file system instances, decline to use an instance that does not have
      an fss_type of STATUS4_VERSIONED or whose fss_version field is earlier than the
      last one obtained from the predecessor file system instance.
        </t>
      </section>
    </section>
    <!-- $Id: 2009-12-20-TO-rfc5661.xml,v 1.2 2009/12/21 05:59:32 shepler.mre Exp $ -->
<!-- Copyright (C) The IETF Trust (2007-2008) -->
<!-- Copyright (C) The Internet Society (2006) -->
<section anchor="pnfs" numbered="true" toc="default">
      <name>Parallel NFS (pNFS)</name>
      <section anchor="pnfs_intro" numbered="true" toc="default">
        <name>Introduction</name>
        <t>
  pNFS is an OPTIONAL feature within NFSv4.1; the pNFS feature
  set allows direct client access to the storage devices containing
  file data.  When file data for a single NFSv4 server is stored on
  multiple and/or higher-throughput storage devices (by comparison to
  the server's throughput capability), the result can be significantly
  better file access performance.  The relationship among multiple
  clients, a single server, and multiple storage devices for pNFS
  (server and clients have access to all storage devices) is shown in
  <xref target="fig_system" format="default"/>.
</t>
        <figure anchor="fig_system">
          <artwork name="" type="" align="left" alt=""><![CDATA[
    +-----------+
    |+-----------+                                 +-----------+
    ||+-----------+                                |           |
    |||           |        NFSv4.1 + pNFS          |           |
    +||  Clients  |<------------------------------>|   Server  |
     +|           |                                |           |
      +-----------+                                |           |
           |||                                     +-----------+
           |||                                           |
           |||                                           |
           ||| Storage        +-----------+              |
           ||| Protocol       |+-----------+             |
           ||+----------------||+-----------+  Control   |
           |+-----------------|||           |    Protocol|
           +------------------+||  Storage  |------------+
                               +|  Devices  |
                                +-----------+
]]></artwork>
        </figure>
        <t>
  In this model, the clients, server, and storage devices are
  responsible for managing file access.  This is in contrast to NFSv4
  without pNFS, where it is primarily the server's responsibility; some
  of this responsibility may be delegated to the client under strictly
  specified conditions. See <xref target="storage_protocol" format="default"/>
  for a discussion of the Storage Protocol. See <xref target="control_protocol" format="default"/> for a
  discussion of the Control Protocol.
</t>
        <t>
  pNFS takes the form of OPTIONAL operations that manage protocol
  objects called 'layouts' (<xref target="layout_types" format="default"/>) that
  contain a byte-range and storage location information.  The layout
  is managed in a similar fashion
  as NFSv4.1 data delegations.  For example, the layout is leased,
  recallable, and revocable.  However, layouts are distinct abstractions
  and are manipulated with new operations.  When a client holds a
  layout, it is granted the ability to directly access the byte-range
  at the storage location specified in the layout.

</t>
        <t>
  There are interactions between layouts and other NFSv4.1
  abstractions such as data delegations and byte-range locking.
  Delegation issues are discussed in <xref target="recalling_layout" format="default"/>.  Byte-range locking issues are
  discussed in Sections <xref target="layout_iomode" format="counter"/> and <xref target="layout_semantics" format="counter"/>.
</t>
      </section>
      <section numbered="true" toc="default">
        <name>pNFS Definitions</name>
        <t>
  NFSv4.1's pNFS feature provides parallel data access to a
  file system that stripes its content across multiple
  storage servers.  The first instantiation of pNFS, as
  part of NFSv4.1, separates the file system protocol
  processing into two parts: metadata processing and data
  processing.  Data consist of the contents of regular
  files that are striped across storage servers. Data
  striping occurs in at least two ways:  on a file-by-file
  basis and, within sufficiently large files, on a
  block-by-block basis. In contrast, striped access to
  metadata by pNFS clients is not provided in NFSv4.1, even
  though the file system back end of a pNFS server might
  stripe metadata.  Metadata consist of everything else,
  including the contents of non-regular files (e.g.,
  directories); see <xref target="metadata" format="default"/>.  The
  metadata functionality is implemented by an NFSv4.1
  server that supports pNFS and the operations described in
  <xref target="nfsv41operations" format="default"/>; such a server is
  called a metadata server (<xref target="mds" format="default"/>).

</t>
        <t>
  The data functionality is implemented by one or more storage devices, each of which
  are accessed by the client via a storage protocol.  A subset (defined in <xref target="ds_ops" format="default"/>) of NFSv4.1 is one such storage protocol.  New terms are
  introduced to the NFSv4.1 nomenclature and existing terms are
  clarified to allow for the description of the pNFS feature.

</t>
        <section anchor="metadata" numbered="true" toc="default">
          <name>Metadata</name>
          <t>
  Information about a file system object, such as its name, location
  within the namespace, owner, ACL, and other attributes.  Metadata may
  also include storage location information, and this will vary based
  on the underlying storage mechanism that is used.
</t>
        </section>
        <section anchor="mds" numbered="true" toc="default">
          <name>Metadata Server</name>
          <t>
  An NFSv4.1 server that supports the pNFS feature.  A variety of
  architectural choices exist for the metadata server and its use of
  file system information held at the server.  Some servers may
  contain metadata only for file objects residing at the
  metadata server, while the file data resides on associated storage
  devices.  Other metadata servers may hold both metadata and a
  varying degree of file data.

</t>
        </section>
        <section numbered="true" toc="default">
          <name>pNFS Client</name>
          <t>
  An NFSv4.1 client that supports pNFS operations and supports at
  least one storage protocol for performing I/O
  to storage devices.
</t>
        </section>
        <section numbered="true" toc="default">
          <name>Storage Device</name>
          <t>
  A storage device stores a regular file's data, but leaves metadata
  management to the metadata server.  A storage device could be
  another NFSv4.1 server, an object-based storage device (OSD),
a block
  device accessed over a System Area Network (SAN, e.g., either
  FiberChannel or iSCSI SAN), or some other entity.
</t>
        </section>
        <section anchor="storage_protocol" numbered="true" toc="default">
          <name>Storage Protocol</name>
          <t>
  As noted in <xref target="fig_system" format="default"/>,
  the storage protocol is the method used by the client to
  store and retrieve data directly from the storage devices.
          </t>
          <t>

  The NFSv4.1 pNFS feature has been structured to allow for a variety
  of storage protocols to be defined and used.

  One example storage protocol is NFSv4.1 itself (as documented in <xref target="file_layout_type" format="default"/>).  Other options for the storage protocol
  are described elsewhere and include:
          </t>
          <ul spacing="normal">
            <li>
      Block/volume protocols such as Internet SCSI (iSCSI) <xref target="RFC3720" format="default"/> and FCP <xref target="FCP-2" format="default"/>.  The block/volume
      protocol support can be independent of the addressing structure
      of the block/volume protocol used, allowing more than one
      protocol to access the same file data and enabling extensibility
      to other block/volume protocols. See
      <xref target="RFC5663" format="default"/> for a layout
      specification that
      allows pNFS to use block/volume storage protocols.
    </li>
            <li>
      Object protocols such as OSD over iSCSI or Fibre Channel <xref target="OSD-T10" format="default"/>. See
      <xref target="RFC5664" format="default"/> for a layout specification
      that allows pNFS to use object storage protocols.
    </li>
          </ul>
          <t>
  It is possible that various storage protocols are available to
  both client and server and it may be possible that a client and
  server do not have a matching storage protocol available to them.
  Because of this, the pNFS server MUST support normal NFSv4.1 access
  to any file accessible by the pNFS feature; this will allow for
  continued interoperability between an NFSv4.1 client and server.
</t>
        </section>
        <section anchor="control_protocol" numbered="true" toc="default">
          <name>Control Protocol</name>
          <t>
  As noted in <xref target="fig_system" format="default"/>,
  the control protocol is used by the exported file system between the
  metadata server and storage devices.  Specification of such
  protocols is outside the scope of the NFSv4.1 protocol.  Such
  control protocols would be used to control activities such as the
  allocation and deallocation of storage, the management of state
  required by the storage devices to perform client access control,
  and, depending on the storage protocol, the enforcement of
  authentication and authorization so that restrictions that
  would be enforced by the metadata server are also enforced by
  the storage device.
</t>
          <t>
  A particular control protocol is not REQUIRED by NFSv4.1 but
  requirements are placed on the control protocol for maintaining
  attributes like modify time, the change attribute, and the end-of-file
  (EOF) position. Note that if pNFS is layered over a clustered, parallel
  file system (e.g., <xref target="PVFS" format="default">PVFS</xref>), the mechanisms that
  enable clustering and parallelism in that file system can be considered
  the control protocol.

</t>
        </section>
        <section anchor="layout_types" numbered="true" toc="default">
          <name>Layout Types</name>
          <t>
  A layout describes the mapping of a file's data to the storage
  devices that hold the data.  A layout is said to belong to a
  specific layout type (data type layouttype4, see <xref target="layouttype4" format="default"/>).  The layout type allows for variants to
  handle different storage protocols, such as those associated with
  block/volume <xref target="RFC5663" format="default"/>, object <xref target="RFC5664" format="default"/>, and file (<xref target="file_layout_type" format="default"/>) layout types.  A metadata server, along with its control
  protocol, MUST support at least one layout type.  A private
  sub-range of the layout type namespace is also defined. Values from
  the private layout type range MAY be used for internal testing or
  experimentation (see <xref target="layouttype4" format="default"/>).
</t>
          <t>
  As an example,  the organization of the file layout type could be
  an array of tuples (e.g., device ID, filehandle), along with a
  definition of how the data is
  stored across the devices (e.g., striping). A block/volume layout
  might be an array of tuples that store &lt;device ID, block number,
  block count&gt;
along with information about block size and the
  associated file offset of the block number.  An object layout might
  be an array of tuples &lt;device ID, object ID&gt; and an additional
  structure (i.e., the aggregation map) that defines how the logical
  byte sequence of the file data is serialized into the different
  objects.  Note that the actual layouts are typically more complex
  than these simple expository examples.
</t>
          <t>
  Requests for pNFS-related operations will often specify a layout
  type.  Examples of such operations are GETDEVICEINFO and LAYOUTGET.
  The response for these operations will include structures such
  as a device_addr4 or a layout4, each of which includes a layout type within
  it.  The layout type sent by the server MUST always be the same
  one requested by the client.  When a server sends a response that
  includes a different layout type, the client SHOULD ignore the
  response and behave as if the server had returned an error response.
</t>
        </section>
        <section anchor="layout" numbered="true" toc="default">
          <name>Layout</name>
          <t>
  A layout defines how a file's data is organized on one or more
  storage devices.  There are many potential layout types; each of the
  layout types are differentiated by the storage protocol used to
  access data and by the aggregation scheme that lays out the file
  data on the underlying storage devices.  A layout is precisely
  identified by the tuple &lt;client ID, filehandle, layout
  type, iomode, range&gt;, where filehandle refers to the filehandle
  of the file on the metadata server.
</t>
          <t>
  It is important to define when layouts overlap and/or conflict with
  each other.  For two layouts with overlapping byte-ranges to
  actually overlap each other, both layouts must be of the same layout
  type, correspond to the same filehandle, and have the same iomode.
  Layouts conflict when they overlap and differ in the content of the
  layout (i.e., the storage device/file mapping parameters differ).
  Note that differing iomodes do not lead to conflicting layouts.  It
  is permissible for layouts with different iomodes, pertaining to the
  same byte-range, to be held by the same client.  An example of this
  would be copy-on-write functionality for a block/volume layout type.
</t>
        </section>
        <section anchor="layout_iomode" numbered="true" toc="default">
          <name>Layout Iomode</name>
          <t>
  The layout iomode (data type layoutiomode4, see <xref target="layoutiomode4" format="default"/>) indicates to the metadata server the
  client's intent to perform either just READ operations
  or a mixture containing READ
  and WRITE operations. For certain layout
  types, it is useful for a client to specify this intent at the time it sends LAYOUTGET
  (<xref target="OP_LAYOUTGET" format="default"/>).  For example, for
  block/volume-based protocols, block allocation could occur when a
  LAYOUTIOMODE4_RW iomode is specified.  A special LAYOUTIOMODE4_ANY iomode is defined
  and can only be used for LAYOUTRETURN and CB_LAYOUTRECALL, not for
  LAYOUTGET.  It specifies that layouts pertaining to both LAYOUTIOMODE4_READ and
  LAYOUTIOMODE4_RW iomodes are being returned or recalled, respectively.
</t>
          <t>
  A storage device may validate I/O with regard to the iomode; this
  is dependent upon storage device implementation and layout type.
  Thus, if the client's layout iomode is inconsistent with the I/O
  being performed, the storage device may reject the client's I/O with
  an error indicating that a new layout with the correct iomode should be
  obtained via LAYOUTGET.  For example, if a client gets a layout with a LAYOUTIOMODE4_READ iomode and
  performs a WRITE to a storage device, the storage device is allowed
  to reject that WRITE.
</t>
          <t>
  The use of the layout iomode does not conflict with OPEN share modes or byte-range LOCK operations;
  open share mode and byte-range lock conflicts are enforced as they are without the
  use of pNFS and are logically separate from the pNFS layout level.
  Open share modes and byte-range locks are the preferred method for
  restricting user access to data files.  For example, an OPEN of
  OPEN4_SHARE_ACCESS_WRITE does not conflict with a LAYOUTGET containing an iomode
  of LAYOUTIOMODE4_RW performed by another client.  Applications that depend
  on writing into the same file concurrently may use byte-range locking to
  serialize their accesses.
</t>
        </section>
        <section anchor="device_ids" numbered="true" toc="default">
          <name>Device IDs</name>
          <t>
    The device ID (data type deviceid4, see
    <xref target="deviceid4" format="default"/>) identifies a group of storage devices. The scope
    of a device ID is the pair &lt;client ID, layout type&gt;. In practice, a
    significant amount of information may be required to fully address
    a storage device.  Rather than embedding all such information in a
    layout, layouts embed device IDs.  The NFSv4.1 operation
    GETDEVICEINFO (<xref target="OP_GETDEVICEINFO" format="default"/>) is used to
    retrieve the complete address information (including
    all device addresses for the device ID) regarding the storage
    device according to its layout type and device ID.  For example,
    the address of an NFSv4.1 data server or of an object-based storage
    device could be an IP address and port.  The address of a block
    storage device could be a volume label.
          </t>
          <t>
    Clients cannot expect the mapping between a device ID and
    its storage device address(es) to persist across metadata server restart.
    See <xref target="mds_recovery" format="default"/> for a description of how
    recovery works in that situation.
          </t>
          <t>
    A device ID lives as long as there is a layout
    referring to the device ID.  If there are no layouts
    referring to the device ID, the server is free to
    delete the device ID any time.
    Once a device ID is deleted by the server, the server MUST NOT
    reuse the device ID for the same layout type and client ID again.
    This requirement is feasible because the device ID is 16 bytes
    long, leaving sufficient room to store a generation number if the
    server's implementation requires most of the rest of the device ID's
    content to be reused. This requirement is necessary because
    otherwise the race conditions between asynchronous notification
    of device ID addition and deletion would be too difficult to
    sort out.

          </t>
          <t>
    Device ID to device address mappings are not leased,
    and can be changed at any time. (Note that while
    device ID to device address mappings are likely
    to change after the metadata server restarts, the
    server is not required to change the mappings.)
    A server has two
    choices for changing mappings.  It can recall all
    layouts referring to the device ID or it can use a
    notification mechanism.

          </t>
          <t>
    The NFSv4.1 protocol has no optimal way to recall
    all layouts that referred to a particular device ID
    (unless the server associates a single device ID with
    a single fsid or a single client ID; in which case,
    CB_LAYOUTRECALL has options for recalling all layouts
    associated with the fsid, client ID pair, or just the
    client ID).

          </t>
          <t>
    Via a notification mechanism
    (see <xref target="OP_CB_NOTIFY_DEVICEID" format="default"/>),
    device ID to device address mappings can change over the duration
    of server operation without recalling or revoking the layouts that
    refer to device ID. The notification mechanism can also delete
    a device ID, but only if the client has no layouts referring
    to the device ID.
    A notification of a change to a device ID to device address
    mapping will immediately or eventually invalidate some or all of
    the device ID's mappings.
    The server MUST support notifications and the client must
    request them before they can be used.  For further information
    about the notification types <xref target="OP_CB_NOTIFY_DEVICEID" format="default"/>.

          </t>
        </section>
      </section>
      <section anchor="pnfs_ops" numbered="true" toc="default">
        <name>pNFS Operations</name>
        <t>
  NFSv4.1 has several operations that are needed for
  pNFS servers, regardless of layout type or storage
  protocol. These operations are all sent to a metadata
  server and summarized here. While pNFS is an OPTIONAL
  feature, if pNFS is implemented, some operations
  are REQUIRED in order to comply with pNFS. See <xref target="operation_mandlist" format="default"/>.
</t>
        <t>
 These are the fore channel pNFS operations:

        </t>
        <dl newline="false" spacing="normal">
          <dt>GETDEVICEINFO</dt>
          <dd>
  (<xref target="OP_GETDEVICEINFO" format="default"/>), as noted previously
  (<xref target="device_ids" format="default"/>), returns the mapping of device ID to
  storage device address.
 </dd>
          <dt>GETDEVICELIST</dt>
          <dd>
  (<xref target="OP_GETDEVICELIST" format="default"/>)
  allows clients to fetch all device IDs
  for a specific file system.
 </dd>
          <dt>LAYOUTGET</dt>
          <dd>
  (<xref target="OP_LAYOUTGET" format="default"/>) is used by a client to get
  a layout for a file.
 </dd>
          <dt>LAYOUTCOMMIT</dt>
          <dd>
  (<xref target="OP_LAYOUTCOMMIT" format="default"/>) is used
  to inform the metadata server of the client's intent to commit data
  that has been written to the storage device (the storage device as
  originally indicated in the return value of LAYOUTGET).
 </dd>
          <dt>LAYOUTRETURN</dt>
          <dd>
  (<xref target="OP_LAYOUTRETURN" format="default"/>) is used
  to return layouts for a file, a file system ID (FSID), or a client ID.
 </dd>
        </dl>
        <t>

  These are the backchannel pNFS operations:

        </t>
        <dl newline="false" spacing="normal">
          <dt>CB_LAYOUTRECALL</dt>
          <dd>
   (<xref target="OP_CB_LAYOUTRECALL" format="default"/>) recalls
   a layout, all layouts belonging to a file system, or all
   layouts belonging to a client ID.
 </dd>
          <dt>CB_RECALL_ANY</dt>
          <dd>
  (<xref target="OP_CB_RECALL_ANY" format="default"/>)
  tells a client that it needs to return some number of recallable
  objects, including layouts, to the metadata server.
 </dd>
          <dt>CB_RECALLABLE_OBJ_AVAIL</dt>
          <dd>
  (<xref target="OP_CB_RECALLABLE_OBJ_AVAIL" format="default"/>) tells a client
  that a recallable object that it was denied (in case of
  pNFS, a layout denied by LAYOUTGET) due to resource exhaustion
  is now available.
 </dd>
          <dt>CB_NOTIFY_DEVICEID</dt>
          <dd>
   (<xref target="OP_CB_NOTIFY_DEVICEID" format="default"/>) notifies the client of
   changes to device IDs.
 </dd>
        </dl>
      </section>
      <section anchor="pnfs_attr" numbered="true" toc="default">
        <name>pNFS Attributes</name>
        <t>
 A number of attributes specific to pNFS are listed and described in
 <xref target="pnfs_attr_full" format="default"/>.
</t>
      </section>
      <section numbered="true" toc="default">
        <name>Layout Semantics</name>
        <section anchor="layout_semantics" numbered="true" toc="default">
          <name>Guarantees Provided by Layouts</name>
          <t>
  Layouts grant to the client the ability to access data located at
  a storage device with the appropriate storage protocol.  The client
  is guaranteed the layout will be recalled when one of two things
  occur: either a conflicting layout is requested or the state
  encapsulated by the layout becomes invalid (this can happen when
  an event directly or indirectly modifies the layout).  When a layout
  is recalled and returned by the client, the client continues with
  the ability to access file data with normal NFSv4.1 operations
  through the metadata server.  Only the ability to access the storage
  devices is affected.
</t>
          <t>
  The requirement of NFSv4.1 that all user access rights MUST be
  obtained through the appropriate OPEN, LOCK, and ACCESS operations
  is not modified with the existence of layouts.  Layouts are provided
  to NFSv4.1 clients, and user access still follows the rules of the
  protocol as if they did not exist.  It is a requirement that for a
  client to access a storage device, a layout must be held by the
  client.  If a storage device receives an I/O request for a byte-range for
  which the client does not hold a layout, the storage device SHOULD
  reject that I/O request.  Note that the act of modifying a file for
  which a layout is held does not necessarily conflict with the
  holding of the layout that describes the file being modified.
  Therefore, it is the requirement of the storage protocol or layout
  type that determines the necessary behavior.  For example,
  block/volume layout types require that the layout's
  iomode agree with the type of I/O being performed.
</t>
          <t>
  Depending upon the layout type and storage protocol in use, storage
  device access permissions may be granted by LAYOUTGET and may be
  encoded within the type-specific layout.  For an example of storage
  device access permissions, see an object-based protocol such as <xref target="OSD-T10" format="default"/>.  If access permissions are encoded within the
  layout, the metadata server SHOULD recall the layout when those
  permissions become invalid for any reason -- for example, when a file
  becomes unwritable or inaccessible to a client.  Note, clients are
  still required to perform the appropriate
  OPEN, LOCK, and ACCESS operations as described above.  The degree to which it is
  possible for the client to circumvent these operations and
  the consequences of doing so must be clearly specified by the
  individual layout type specifications.  In addition, these
  specifications must be clear about the requirements and
  non-requirements for the checking performed by the server.
</t>
          <t>
  In the presence of pNFS functionality, mandatory byte-range locks MUST
  behave as they would without pNFS.  Therefore, if mandatory file
  locks and layouts are provided simultaneously, the storage device
  MUST be able to enforce the mandatory byte-range locks.  For example, if
  one client obtains a mandatory byte-range lock and a second client accesses the
  storage device, the storage device MUST appropriately restrict I/O
  for the range of the mandatory byte-range lock.  If the storage
  device is incapable of providing this check in the presence of
  mandatory byte-range locks, then the metadata server MUST NOT grant
  layouts and mandatory byte-range locks simultaneously.
</t>
        </section>
        <section anchor="obtaining_layout" numbered="true" toc="default">
          <name>Getting a Layout</name>
          <t>
  A client obtains a layout with the
  LAYOUTGET operation.  The metadata server
  will grant layouts of a particular type
  (e.g., block/volume, object, or file).
  The client selects an appropriate layout
  type that the server supports and the client
  is prepared to use.  The layout returned to
  the client might not exactly match the
  requested byte-range as described in <xref target="OP_LAYOUTGET_DESCRIPTION" format="default"/>.  As needed a client
  may send multiple LAYOUTGET operations; these might result
  in multiple overlapping, non-conflicting layouts (see
  <xref target="layout" format="default"/>).

</t>
          <t>
  In order to get a layout, the client must first have opened the file
  via the OPEN operation. When a client has no layout on a file, it
  MUST present an open stateid, a delegation stateid, or
  a byte-range lock stateid in the loga_stateid argument. A successful
  LAYOUTGET result includes a layout stateid. The first successful
  LAYOUTGET processed by the server using a non-layout stateid as an
  argument MUST have the "seqid" field of the layout stateid in the
  response set to one. Thereafter, the client MUST use a layout
  stateid (see <xref target="layout_stateid" format="default"/>) on future invocations
  of LAYOUTGET on the file, and the "seqid" MUST NOT be set to
  zero.  Once the layout has been retrieved, it can be held across
  multiple OPEN and CLOSE sequences.  Therefore, a client may hold a
  layout for a file that is not currently open by any user on the
  client.  This allows for the caching of layouts beyond CLOSE.
</t>
          <t>
  The storage protocol used by the client to access the data on the
  storage device is determined by the layout's type.  The client is
  responsible for matching the layout type with an available method to
  interpret and use the layout.  The method for this layout type
  selection is outside the scope of the pNFS functionality.
</t>
          <t>
  Although the metadata server is in control
  of the layout for a file, the pNFS client
  can provide hints to the server when a file
  is opened or created about the preferred
  layout type and aggregation schemes.
  pNFS introduces a layout_hint attribute (<xref target="attrdef_layout_hint" format="default"/>)
  that the client can set at file creation
  time to provide a hint to the server for new
  files. Setting this attribute separately,
  after the file has been created might make
  it difficult, or impossible, for the server
  implementation to comply.
</t>
          <t>
  Because the EXCLUSIVE4 createmode4 does not allow the
  setting of attributes at file creation time, NFSv4.1
  introduces the EXCLUSIVE4_1 createmode4, which does
  allow attributes to be set at file creation time. In
  addition, if the session is created with persistent
  reply caches, EXCLUSIVE4_1 is neither necessary
  nor allowed. Instead, GUARDED4 both works better and is
  prescribed. <xref target="exclusive_create" format="default"/> in <xref target="OP_OPEN_DESCRIPTION" format="default"/> summarizes how a client
  is allowed to send an exclusive create.

</t>
        </section>
        <section anchor="layout_stateid" numbered="true" toc="default">
          <name>Layout Stateid</name>
          <t>
  As with all other stateids, the layout stateid consists of a "seqid" and
  "other" field. Once a layout stateid is established, the "other" field
  will stay constant unless the stateid is revoked or the client
  returns all layouts on the file and the server disposes of the
  stateid.  The "seqid" field is initially set to one, and is never
  zero on any NFSv4.1 operation that uses layout stateids, whether it
  is a fore channel or backchannel operation. After the layout stateid
  is established, the server increments by one the value of the
  "seqid" in each subsequent LAYOUTGET and LAYOUTRETURN response, and
  in each CB_LAYOUTRECALL request.
</t>
          <t>
  Given the design goal of pNFS to provide parallelism, the layout
  stateid differs from other stateid types in that the client is
  expected to send LAYOUTGET and LAYOUTRETURN operations in parallel.
  The "seqid" value is used by the client to properly sort responses
  to LAYOUTGET and LAYOUTRETURN.  The "seqid" is also used to prevent
  race conditions between LAYOUTGET and CB_LAYOUTRECALL.  Given that the
  processing rules differ from layout stateids and other stateid
  types, only the pNFS sections of this document should be considered
  to determine proper layout stateid handling.
</t>
          <t>
  Once the client receives a layout stateid, it MUST use the correct
  "seqid" for subsequent LAYOUTGET or LAYOUTRETURN operations.  The
  correct "seqid" is defined as the highest "seqid" value from
  responses of fully processed LAYOUTGET or LAYOUTRETURN operations or
  arguments of a fully processed CB_LAYOUTRECALL operation.  Since the
  server is incrementing the "seqid" value on each layout operation,
  the client may determine the order of operation processing by
  inspecting the "seqid" value.  In the case of overlapping layout
  ranges, the ordering information will provide the client the
  knowledge of which layout ranges are held.  Note that overlapping
  layout ranges may occur because of the client's specific requests or
  because the server is allowed to expand the range of a requested
  layout and notify the client in the LAYOUTRETURN results. Additional
  layout stateid sequencing requirements are provided in
  <xref target="pnfs_operation_sequencing" format="default"/>.
</t>
          <t>
  The client's receipt of a "seqid" is not sufficient for subsequent
  use.  The client must fully process the operations before the
  "seqid" can be used.  For LAYOUTGET results, if
  the client is not using the forgetful model
  (<xref target="recall_robustness" format="default"/>), it MUST first update its
  record of what ranges of the file's layout it has before using the
  seqid. For LAYOUTRETURN results, the client MUST delete the range
  from its record of what ranges of the file's layout it had before
  using the seqid. For CB_LAYOUTRECALL arguments, the client MUST send
  a response to the recall before using the seqid.
  The fundamental requirement in client
  processing is that the "seqid" is used to provide the order of
  processing.  LAYOUTGET results may be processed in parallel.
  LAYOUTRETURN results may be processed in parallel.  LAYOUTGET and
  LAYOUTRETURN responses may be processed in parallel as long as the
  ranges do not overlap.  CB_LAYOUTRECALL request processing MUST be
  processed in "seqid" order at all times.
</t>
          <t>
  Once a client has no more layouts on a file, the layout stateid is
  no longer valid and MUST NOT be used. Any attempt to use such a
  layout stateid will result in NFS4ERR_BAD_STATEID.
</t>
        </section>
        <section anchor="committing_layout" numbered="true" toc="default">
          <name>Committing a Layout</name>
          <t>
  Allowing for varying storage protocol capabilities, the pNFS
  protocol does not require the metadata server and storage devices to
  have a consistent view of file attributes and data location
  mappings.  Data location mapping refers to aspects such as which offsets
  store data as opposed to storing holes (see <xref target="sparse_dense" format="default"/> for a discussion).  Related issues arise
  for storage protocols where a layout may hold provisionally
  allocated blocks where the allocation of those blocks does not
  survive a complete restart of both the client and server.  Because
  of this inconsistency, it is necessary to resynchronize the client
  with the metadata server and its storage devices and make any
  potential changes available to other clients.  This is accomplished
  by use of the LAYOUTCOMMIT operation.
</t>
          <t>
  The LAYOUTCOMMIT operation is responsible for committing a modified
  layout to the metadata server.  The data should be written
  and committed to the appropriate storage devices before the
  LAYOUTCOMMIT occurs.  The
  scope of the LAYOUTCOMMIT operation depends on the storage protocol
  in use.  It is important to note that the level of
  synchronization is from the point of view of the client that sent
  the LAYOUTCOMMIT.  The updated state on the metadata server need
  only reflect the state as of the client's last operation previous to
  the LAYOUTCOMMIT.  The metadata server is not REQUIRED to maintain a global view
  that accounts for other clients' I/O that may have occurred within
  the same time frame.
</t>
          <t>
  For block/volume-based layouts, LAYOUTCOMMIT may require
  updating the block list that comprises the file and committing this
  layout to stable storage.  For file-based layouts, synchronization of
  attributes between the metadata and storage devices, primarily the
  size attribute, is required.
</t>
          <t>
  The control protocol is free to synchronize the attributes before
  it receives a LAYOUTCOMMIT; however, upon successful completion of a
  LAYOUTCOMMIT, state that exists on the metadata server that
  describes the file MUST be synchronized with the state that exists on the
  storage devices that comprise that file as of the client's
  last sent operation.  Thus, a client that queries the size of a file
  between a WRITE to a storage device and the LAYOUTCOMMIT might observe
  a size that does not reflect the actual data written.
</t>
          <t>
  The client MUST have a layout in order to send a LAYOUTCOMMIT operation.
</t>
          <section numbered="true" toc="default">
            <name>LAYOUTCOMMIT and change/time_modify</name>
            <t>
  The change and time_modify attributes may be updated
  by the server when the LAYOUTCOMMIT operation is processed.  The
  reason for this is that some layout types do not support the update
  of these attributes when the storage devices process I/O operations.
  If a client has a layout with the LAYOUTIOMODE4_RW iomode on the file,
  the client MAY provide a suggested value to the server for
  time_modify within the arguments to LAYOUTCOMMIT.
  Based on the layout type, the provided value may or may not be used.
  The server should sanity-check the client-provided values
  before they are used.  For example, the server should ensure that
  time does not flow backwards.  The client always has the option to
  set time_modify through an explicit SETATTR operation.
</t>
            <t>
  For some layout protocols, the storage device is able to notify the
  metadata server of the occurrence of an I/O; as a result, the
  change and time_modify attributes may be updated at
  the metadata server.  For a metadata server that is capable of
  monitoring updates to the change and time_modify
  attributes, LAYOUTCOMMIT processing is not required to update the
  change attribute.  In this case, the metadata server must ensure that
  no further update to the data has occurred since the last update of
  the attributes; file-based protocols may have enough information to
  make this determination or may update the change attribute upon each
  file modification.  This also applies for the time_modify
  attribute.  If the server implementation is able to
  determine that the file has not been modified since the last
  time_modify update, the server need not update time_modify at
  LAYOUTCOMMIT.  At LAYOUTCOMMIT completion, the updated attributes
  should be visible if that file was modified since the latest
  previous LAYOUTCOMMIT or LAYOUTGET.
</t>
          </section>
          <section anchor="general_layoutcommit" numbered="true" toc="default">
            <name>LAYOUTCOMMIT and size</name>
            <t>
  The size of a file may be updated when the LAYOUTCOMMIT operation is
  used by the client.  One of the fields in the argument to
  LAYOUTCOMMIT is loca_last_write_offset; this field indicates the
  highest byte offset written but not yet committed with the
  LAYOUTCOMMIT operation.  The data type of loca_last_write_offset is
  newoffset4 and is switched on a boolean value, no_newoffset, that
  indicates if a previous write occurred or not.  If no_newoffset is
  FALSE, an offset is not given.  If the client has a layout with
  LAYOUTIOMODE4_RW iomode on the file, with a byte-range (denoted by the values of lo_offset and lo_length)
  that overlaps loca_last_write_offset, then the client MAY
  set no_newoffset to TRUE and provide an offset that will
  update the file size. Keep in mind that offset is not the same
  as length, though they are related. For example, a loca_last_write_offset
  value of zero means that one byte was written at offset zero, and so
  the length of the file is at least one byte.
</t>
            <t>
  The metadata server may do one of the following:
            </t>
            <ol spacing="normal" type="1">
              <li>
      Update the file's size using the last write offset provided by
      the client as either the true file size or as a hint of the file
      size.  If the metadata server has a method available, any new
      value for file size should be sanity-checked.  For example, the
      file must not be truncated if the client presents a last write
      offset less than the file's current size.
    </li>
              <li>
      Ignore the client-provided last write offset; the metadata
      server must have sufficient knowledge from other sources to
      determine the file's size.  For example, the metadata server
      queries the storage devices with the control protocol.
    </li>
            </ol>
            <t>
  The method chosen to update the file's size will depend on the
  storage device's and/or the control protocol's capabilities.  For
  example, if the storage devices are block devices with no knowledge
  of file size, the metadata server must rely on the client to set the
  last write offset appropriately.
</t>
            <t>
  The results of LAYOUTCOMMIT contain a new size value in the form of
  a newsize4 union data type.  If the file's size is set as a result
  of LAYOUTCOMMIT, the metadata server must reply with the new size;
  otherwise, the new size is not provided.
  If the file size is updated, the metadata server SHOULD update the
  storage devices such that the new file size is reflected when
  LAYOUTCOMMIT processing is complete.  For example, the client should
  be able to read up to the new file size.
</t>
            <t>
  The client can extend the length of a file
  or truncate a file by sending a SETATTR operation to the metadata server
  with the size attribute specified. If the size specified is larger than
  the current size of the file, the file is "zero extended", i.e., zeros are
  implicitly added between the file's previous EOF and the new EOF.
  (In many implementations, the zero-extended byte-range
  of the file consists of unallocated
  holes in the file.) When the client writes past EOF via WRITE,
  the SETATTR operation does not need to be used.

</t>
          </section>
          <section anchor="layoutcommit_update" numbered="true" toc="default">
            <name>LAYOUTCOMMIT and layoutupdate</name>
            <t>
  The LAYOUTCOMMIT argument contains a loca_layoutupdate field (<xref target="OP_LAYOUTCOMMIT_ARGUMENT" format="default"/>) of data type layoutupdate4
  (<xref target="layoutupdate4" format="default"/>).  This argument is a
  layout-type-specific structure.  The structure can be used to pass
  arbitrary layout-type-specific information from the client to the
  metadata server at LAYOUTCOMMIT time.  For example, if using a
  block/volume layout, the client can indicate to the metadata server
  which reserved or allocated blocks the client used or did not use.
  The content of loca_layoutupdate (field lou_body) need not be the
  same layout-type-specific content returned by LAYOUTGET (<xref target="OP_LAYOUTGET_RESULT" format="default"/>) in the loc_body field of the
  lo_content field of the logr_layout field.
The content of
  loca_layoutupdate is defined by the layout type specification and is
  opaque to LAYOUTCOMMIT.
</t>
          </section>
        </section>
        <!-- Layout Semantics -->

<section anchor="recalling_layout" numbered="true" toc="default">
          <name>Recalling a Layout</name>
          <t>
  Since a layout protects a client's access to a file via a direct
  client-storage-device path, a layout need only be recalled when it
  is semantically unable to serve this function.  Typically, this
  occurs when the layout no longer encapsulates the true location of
  the file over the byte-range it represents.  Any operation or
  action, such as server-driven restriping or load balancing, that
  changes the layout will result in a recall of the layout.  A layout
  is recalled by the CB_LAYOUTRECALL callback operation (see <xref target="OP_CB_LAYOUTRECALL" format="default"/>) and returned with LAYOUTRETURN (see <xref target="OP_LAYOUTRETURN" format="default"/>).  The CB_LAYOUTRECALL operation may
  recall a layout identified by a byte-range, all layouts
  associated with a file system ID (FSID), or all layouts associated with
  a client ID.
  <xref target="pnfs_operation_sequencing" format="default"/> discusses sequencing issues
  surrounding the getting, returning, and recalling of layouts.
</t>
          <t>
  An iomode is also specified when recalling a layout.
  Generally, the iomode in the recall request must match the layout
  being returned; for example, a recall with an iomode of
  LAYOUTIOMODE4_RW should cause the client to only return
  LAYOUTIOMODE4_RW layouts and not LAYOUTIOMODE4_READ layouts.
  However, a special LAYOUTIOMODE4_ANY enumeration is
  defined to enable recalling a layout of any iomode; in other words,
  the client must return both LAYOUTIOMODE4_READ and LAYOUTIOMODE4_RW layouts.
</t>
          <t>
  A REMOVE operation SHOULD cause the metadata server to recall the
  layout to prevent the client from accessing a non-existent file and
  to reclaim state stored on the client.  Since a REMOVE may be delayed
  until the last close of the file has occurred, the recall may also
  be delayed until this time.  After the last reference on the file
  has been released and the file has been removed, the client should
  no longer be able to perform I/O using the layout.  In the case of a
  file-based layout, the data server SHOULD return NFS4ERR_STALE in
  response to any operation on the removed file.
</t>
          <t>
  Once a layout has been returned, the client MUST NOT send I/Os to
  the storage devices for the file, byte-range, and iomode
  represented by the returned layout. If a client does send an I/O to
  a storage device for which it does not hold a layout, the storage
  device SHOULD reject the I/O.
</t>
          <t anchor="pnfs_and_delegations">
  Although pNFS does not alter the file data caching capabilities of
  clients, or their semantics, it recognizes that some clients may
  perform more aggressive write-behind caching to optimize the
  benefits provided by pNFS.  However, write-behind caching may
  negatively affect the latency in returning a layout in response to a
  CB_LAYOUTRECALL; this is similar to file delegations and the impact
  that file data caching has on DELEGRETURN.  Client implementations
  SHOULD limit the amount of unwritten data they have outstanding at
  any one time in order to prevent excessively long responses to
  CB_LAYOUTRECALL.  Once a layout is recalled, a server MUST wait one
  lease period before taking further action.  As soon as a lease
  period has passed, the server may choose to fence the client's access
  to the storage devices if the server perceives the client has taken
  too long to return a layout. However, just as in the case of data
  delegation and DELEGRETURN, the server may choose to wait, given that
  the client is showing forward progress on its way to returning the
  layout.  This forward progress can take the form of successful
  interaction with the storage devices or of sub-portions of the layout
  being returned by the client.  The server can also limit exposure to
  these problems by limiting the byte-ranges initially provided in
  the layouts and thus the amount of outstanding modified data.
</t>
          <section anchor="recall_robustness" numbered="true" toc="default">
            <name>Layout Recall Callback Robustness</name>
            <t>
  It has been assumed thus far that pNFS client
  state
  (layout ranges and iomode)
  for a file exactly matches that of the pNFS server for that file.
  This assumption
  leads to the implication that any callback results in a
  LAYOUTRETURN or set of LAYOUTRETURNs that exactly match the range in
  the callback, since both client and server agree about the state
  being maintained.  However, it can be useful if this assumption does
  not always hold.  For example:
</t>
            <ul spacing="normal">
              <li>
  If conflicts that require
  callbacks are very rare, and a server can use a multi-file callback
  to recover per-client resources (e.g., via an FSID recall or a
  multi-file recall within a single CB_COMPOUND), the result may be
  significantly less client-server pNFS traffic.
</li>
              <li>
  It may be useful for servers to maintain information about
  what ranges are held by a client on a coarse-grained basis, leading
  to the server's layout ranges being beyond those actually held by
  the client.
  In the extreme, a server could manage conflicts on
  a per-file basis, only sending whole-file callbacks even though
  clients may request and be granted sub-file ranges.
</li>
              <li>
  It may be useful for clients to "forget" details about
  what layouts and ranges the client actually has, leading
  to the server's layout ranges being beyond those that the
  client "thinks" it has. As long as the client does not
  assume it has layouts that are beyond what the server
  has granted, this is a safe practice.  When a client
  forgets what ranges and layouts it has, and it receives
  a CB_LAYOUTRECALL operation, the client MUST follow up
  with a LAYOUTRETURN for what the server recalled, or
  alternatively return the NFS4ERR_NOMATCHING_LAYOUT error
  if it has no layout to return in the recalled range.

</li>
              <li>
  In order to avoid errors, it is vital that a client not assign
  itself layout permissions beyond what the server has granted, and
  that the server not forget layout permissions that have been granted.
  On the other hand, if a
  server believes that a client holds a layout that the client
  does not know about, it is useful for the client to cleanly indicate
  completion of the requested recall either by sending a LAYOUTRETURN
  operation for the entire requested range or by returning an
  NFS4ERR_NOMATCHING_LAYOUT error to the CB_LAYOUTRECALL.
</li>
            </ul>
            <t>
  Thus, in light of the above, it is useful for a server to be able to
  send callbacks for layout ranges it has not granted to a client,
  and for a client to return ranges it does not hold.  A pNFS client
  MUST always return layouts that comprise the full range
  specified by the recall.  Note, the full recalled layout range need
  not be returned as part of a single operation, but may be returned
  in portions.  This allows the client to stage the flushing of dirty
  data and commits and returns of layouts.
Also, it indicates to the
  metadata server that the client is making progress.
</t>
            <t>
  When a layout is returned, the client MUST NOT have any outstanding
  I/O requests to the storage devices involved in the layout.
  Rephrasing, the client MUST NOT return the layout while it has
  outstanding I/O requests to the storage device.
</t>
            <t>
  Even with this requirement for the client, it is possible that I/O
  requests may be presented to a storage device no longer allowed to
  perform them.  Since the server has no strict control as to when the
  client will return the layout, the server may later decide to
  unilaterally revoke the client's access to the storage devices
  as provided by the layout.  In
  choosing to revoke access, the server must deal with the possibility
  of lingering I/O requests, i.e., I/O requests that are
  still in flight to
  storage devices identified by the revoked layout.

  All layout type specifications MUST define whether unilateral layout revocation by
  the metadata server is supported; if it is, the specification must
  also describe how lingering writes are processed.  For example,
  storage devices identified by the revoked layout could be fenced off
  from the client that held the layout.
</t>
            <t>
  In order to ensure client/server convergence with regard to layout state,
  the final LAYOUTRETURN operation in a sequence of LAYOUTRETURN
  operations for a particular recall MUST specify the entire range
  being recalled, echoing the recalled layout type, iomode,
  recall/return type (FILE, FSID, or ALL), and byte-range, even if
  layouts pertaining to partial ranges were previously
  returned.  In addition, if the client holds no layouts that
  overlap the range being recalled, the client should return the
  NFS4ERR_NOMATCHING_LAYOUT error code to CB_LAYOUTRECALL.  This
  allows the server to update its view of the client's layout state.
</t>
          </section>
          <section anchor="pnfs_operation_sequencing" numbered="true" toc="default">
            <name>Sequencing of Layout Operations</name>
            <t>
  As with other stateful operations, pNFS requires the correct
  sequencing of layout operations. pNFS uses the "seqid" in the
  layout stateid to provide the correct sequencing between regular
  operations and callbacks.  It is the server's responsibility to
  avoid inconsistencies regarding the layouts provided and the
  client's responsibility to properly serialize its layout requests
  and layout returns.
</t>
            <section numbered="true" toc="default">
              <name>Layout Recall and Return Sequencing</name>
              <t>
  One critical issue with regard to layout operations sequencing
  concerns callbacks.  The protocol must defend against
  races between the reply to a LAYOUTGET or LAYOUTRETURN
  operation and a subsequent CB_LAYOUTRECALL.  A client
  MUST NOT process a CB_LAYOUTRECALL that implies one or
  more outstanding LAYOUTGET or LAYOUTRETURN operations to
  which the client has not yet received a reply. The client
  detects such a CB_LAYOUTRECALL by examining the "seqid"
  field of the recall's layout stateid. If the "seqid"
  is not exactly one higher than what the client currently has recorded, and the
  client has at least one LAYOUTGET and/or LAYOUTRETURN operation
  outstanding, the client knows the server sent the CB_LAYOUTRECALL
  after sending a response to an outstanding LAYOUTGET or LAYOUTRETURN.
  The client MUST wait before processing such a CB_LAYOUTRECALL
  until it processes all replies for outstanding LAYOUTGET and
  LAYOUTRETURN operations for the corresponding file
  with seqid less than the seqid given by CB_LAYOUTRECALL
  (lor_stateid; see <xref target="OP_CB_LAYOUTRECALL" format="default"/>.)
</t>
              <t>
  In addition to the seqid-based mechanism,
  <xref target="sessions_callback_races" format="default"/>
  describes the sessions mechanism for allowing the
  client to detect callback race conditions and delay processing such a
  CB_LAYOUTRECALL. The server MAY reference conflicting operations
  in the CB_SEQUENCE that precedes the CB_LAYOUTRECALL.
  Because the server has already sent replies for these operations before
  sending the callback, the replies may race with the CB_LAYOUTRECALL.
  The client MUST wait for all the referenced calls to complete and update
  its view of the layout state before processing the CB_LAYOUTRECALL.
</t>
              <section numbered="true" toc="default">
                <name>Get/Return Sequencing</name>
                <t>
  The protocol allows the client to send concurrent
  LAYOUTGET and LAYOUTRETURN operations to the server. The
  protocol does not provide any means for the server to
  process the requests in the same order in which they
  were created. However, through the use of the "seqid"
  field in the layout stateid, the client can determine
  the order in which parallel outstanding operations were
  processed by the server. Thus, when a layout retrieved
  by an outstanding LAYOUTGET operation intersects with
  a layout returned by an outstanding LAYOUTRETURN on
  the same file, the order in which the two conflicting
  operations are processed determines the final state of
  the overlapping layout. The order is determined by
  the "seqid" returned in each operation: the operation with the
  higher seqid was executed later.

</t>
                <t>
  It is permissible for the client to send multiple parallel
  LAYOUTGET operations for the same file or multiple parallel LAYOUTRETURN
  operations for the same file or a mix of both.
</t>
                <t>
  It is permissible for the client to use the current stateid (see
  <xref target="current_stateid" format="default"/>) for LAYOUTGET operations, for
  example, when compounding LAYOUTGETs or compounding OPEN and
  LAYOUTGETs.  It is also permissible to use the current stateid when
  compounding LAYOUTRETURNs.
</t>
                <t>
  It is permissible for the client to use the current stateid when
  combining LAYOUTRETURN and LAYOUTGET operations for the same file in
  the same COMPOUND request since the server MUST process these in
  order.  However, if a client does send such COMPOUND requests, it
  MUST NOT have more than one outstanding for the same file at the
  same time, and it MUST NOT have other LAYOUTGET or LAYOUTRETURN
  operations outstanding at the same time for that same file.
</t>
              </section>
              <section numbered="true" toc="default">
                <name>Client Considerations</name>
                <t>
  Consider a pNFS client that has sent a LAYOUTGET, and before
  it receives the reply to LAYOUTGET, it receives
  a CB_LAYOUTRECALL for the same file with an overlapping range.  There are two
  possibilities, which the client can distinguish
  via the layout stateid in the recall.

                </t>
                <ol spacing="normal" type="1">
                  <li>
    The server processed the LAYOUTGET before sending the recall, so the
    LAYOUTGET must be waited for because it
    may be carrying layout information that will need to be returned to deal
    with the CB_LAYOUTRECALL.
  </li>
                  <li>
    The
    server sent the callback before receiving the
    LAYOUTGET. The server will not respond to the LAYOUTGET
    until the CB_LAYOUTRECALL is processed.

  </li>
                </ol>
                <t>

  If these possibilities cannot be distinguished, a
  deadlock could result, as the client must wait for the
  LAYOUTGET response before processing the recall in the
  first case, but that response will not arrive until after
  the recall is processed in the second case. Note that
  in the first case, the "seqid" in the layout stateid
  of the recall is two greater than what the client has
  recorded; in the second case, the "seqid" is one greater than
  what the client has recorded.  This allows the client
  to disambiguate between the two cases. The client thus
  knows precisely which possibility applies.

</t>
                <t>

  In case 1, the client knows it needs to wait for
  the LAYOUTGET response before processing the recall
  (or the client can return NFS4ERR_DELAY).
</t>
                <t>
  In case 2, the client will not wait for the LAYOUTGET
  response before processing the recall because waiting
  would cause deadlock.  Therefore, the action at the
  client will only require waiting in the case that the
  client has not yet seen the server's earlier responses
  to the LAYOUTGET operation(s).

</t>
                <t>
  The recall process can be considered completed when
  the final LAYOUTRETURN operation for the recalled range is completed.
  The LAYOUTRETURN uses the layout stateid (with seqid) specified in
  CB_LAYOUTRECALL.  If the client uses multiple LAYOUTRETURNs in
  processing the recall, the first LAYOUTRETURN will use the layout
  stateid as specified in CB_LAYOUTRECALL.  Subsequent LAYOUTRETURNs
  will use the highest seqid as is the usual case.
</t>
              </section>
              <section anchor="layout_server_consider" numbered="true" toc="default">
                <name>Server Considerations</name>
                <t>
  Consider a race from the metadata server's point of
  view.  The metadata server has sent a CB_LAYOUTRECALL and receives
  an overlapping LAYOUTGET for the same file before the
  LAYOUTRETURN(s) that respond to the CB_LAYOUTRECALL. There are
  three cases:

</t>
                <ol spacing="normal" type="1">
                  <li>
  The client sent the LAYOUTGET before processing the CB_LAYOUTRECALL.
  The "seqid" in the layout stateid of the arguments of LAYOUTGET is one less
  than the "seqid" in CB_LAYOUTRECALL. The server returns
  NFS4ERR_RECALLCONFLICT to the client, which indicates to the client
  that there is a pending recall.
</li>
                  <li>
  The client sent the LAYOUTGET after processing the
  CB_LAYOUTRECALL, but the LAYOUTGET arrived before the LAYOUTRETURN and
  the response to CB_LAYOUTRECALL that
  completed that processing.
  The "seqid" in the layout stateid
  of LAYOUTGET is equal to or greater than that of the "seqid" in
  CB_LAYOUTRECALL.
  The server has not received a response to the CB_LAYOUTRECALL,
  so it returns NFS4ERR_RECALLCONFLICT.
</li>
                  <li>
  The client sent the LAYOUTGET after processing the
  CB_LAYOUTRECALL; the server received the CB_LAYOUTRECALL
  response, but the LAYOUTGET arrived before the LAYOUTRETURN that
  completed that processing.
  The "seqid" in the layout stateid
  of LAYOUTGET is equal to that of the "seqid" in
  CB_LAYOUTRECALL.
  The server has received a response to the CB_LAYOUTRECALL,
  so it returns NFS4ERR_RETURNCONFLICT.
</li>
                </ol>
              </section>
              <section numbered="true" toc="default">
                <name>Wraparound and Validation of Seqid</name>
                <t>
  The rules for layout stateid processing differ from other stateids
  in the protocol because the "seqid" value cannot be zero and the
  stateid's "seqid" value changes in a CB_LAYOUTRECALL operation.  The
  non-zero requirement combined with the inherent parallelism of
  layout operations means that a set of LAYOUTGET and LAYOUTRETURN
  operations may contain the same value for "seqid".
  The server uses a slightly modified version of the modulo arithmetic
  as described in
  <xref target="Slot_Identifiers_and_Server_Reply_Cache" format="default"/>
  when incrementing the layout stateid's "seqid".  The difference
  is that zero is not a valid value for "seqid"; when the value
  of a "seqid" is 0xFFFFFFFF, the next valid value will be 0x00000001.
  The modulo arithmetic is also used for the comparisons of
  "seqid" values in the processing of CB_LAYOUTRECALL events as
  described above in <xref target="layout_server_consider" format="default"/>.
</t>
                <t>
  Just as the server validates the "seqid" in the event of
  CB_LAYOUTRECALL usage, as described in
  <xref target="layout_server_consider" format="default"/>, the server also validates
  the "seqid" value to ensure that it is within an appropriate range.
  This range represents the degree of parallelism the server supports
  for layout stateids.  If the client is sending multiple layout
  operations to the server in parallel, by definition, the "seqid"
  value in the supplied stateid will not be the current "seqid" as
  held by the server.  The range of parallelism spans from the highest
  or current "seqid" to a "seqid" value in the past.  To assist in the
  discussion, the server's current "seqid" value for a layout stateid
  is defined as SERVER_CURRENT_SEQID.  The lowest "seqid" value that
  is acceptable to the server is represented by PAST_SEQID.  And the
  value for the range of valid "seqid"s or range of parallelism is
  VALID_SEQID_RANGE.  Therefore, the following holds:
  VALID_SEQID_RANGE = SERVER_CURRENT_SEQID - PAS