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<rfc xmlns:xi="http://www.w3.org/2001/XInclude" category="std"
     consensus="true" docName="draft-ietf-dnssd-push-25" ipr="trust200902">
 <!-- category values: std, bcp, info, exp, and historic
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       or pre5378Trust200902
    you can add the attributes updates="NNNN" and obsoletes="NNNN"
    they will automatically be output with "(if approved)" -->

 <!-- ***** FRONT MATTER ***** --> number="8765"
     ipr="trust200902" obsoletes="" updates="" submissionType="IETF"
     xml:lang="en" tocInclude="true" tocDepth="4" symRefs="true"
     sortRefs="true" version="3">

  <front>
   <!-- The abbreviated title is used in the page header - it is only necessary if the
        full title is longer than 39 characters -->

    <title abbrev="DNS Push Notifications">DNS Push Notifications</title>

   <!-- add 'role="editor"' below for the editors if appropriate -->

   <!-- Another author who claims to be an editor -->
    <seriesInfo name="RFC" value="8765"/>

    <author fullname="Tom Pusateri" initials="T.J." initials="T." surname="Pusateri">
      <organization>Unaffiliated</organization>
      <address>
        <postal>
         <street></street>
         <!-- Reorder these if your country does things differently -->
          <street/>

          <city>Raleigh</city>
          <region>NC</region>
          <code>27608</code>
         <country>USA</country>
          <country>United States of America</country>
        </postal>
        <phone>+1 919 867 1330</phone>
        <email>pusateri@bangj.com</email>
       <!-- uri and facsimile elements may also be added -->

      </address>
    </author>
    <author fullname="Stuart Cheshire" initials="S." surname="Cheshire">
      <organization>Apple Inc.</organization>
      <address>
        <postal>
          <street>One Apple Park Way</street>
         <!-- Reorder these if your country does things differently -->

          <city>Cupertino</city>
          <region>CA</region>
          <code>95014</code>
         <country>USA</country>
          <country>United States of America</country>
        </postal>
        <phone>+1 (408) 996-1010</phone>
        <email>cheshire@apple.com</email>
       <!-- uri and facsimile elements may also be added -->

      </address>
    </author>
    <date year='2019' month='October' day='13'/>

   <!-- If the month and year are both specified and are the current ones, xml2rfc will fill
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   <!-- Meta-data Declarations -->

   <area>DNSSD</area>

   <workgroup>Internet Engineering Task Force</workgroup>

   <!-- WG name at the upper left corner of the doc,
        IETF is fine for individual submissions.
        If this element is not present, the default is "Network Working Group",
        which is used by the RFC Editor as a nod to the history of the IETF. -->

   <keyword>dns update push year="2020" month="April"/>

    <area>INT</area>
    <workgroup>DNSSD</workgroup>

    <keyword>Push notification</keyword>
    <keyword>Asynchronous notification</keyword>

   <!-- Keywords will be incorporated into HTML output
        files in a meta tag but they have no effect on text or nroff
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    <abstract>
      <t>The Domain Name System (DNS) was designed to return matching records
      efficiently for queries for data that are relatively static.  When those
      records change frequently, DNS is still efficient at returning the
      updated results when polled, as long as the polling rate is not too
      high.
     But
      But, there exists no mechanism for a client to be asynchronously
      notified
      when these changes occur.  This document defines a mechanism for a
      client to be notified of such changes to DNS records, called DNS Push
      Notifications.</t>
    </abstract>
  </front>
  <middle>
<?rfc needLines="14" ?>
    <section title="Introduction"> numbered="true" toc="default">
      <name>Introduction</name>
      <t>Domain Name System (DNS) records may be updated using DNS Update
      <xref target="RFC2136">DNS Update</xref>. target="RFC2136" format="default"></xref>.  Other mechanisms such
      as a Discovery Proxy <xref target="DisProx">Discovery Proxy</xref> target="RFC8766" format="default"></xref> can
      also generate changes to a DNS zone.  This document specifies a protocol
      for DNS clients to subscribe to receive asynchronous notifications of
      changes to RRsets of interest. It is immediately relevant in the case of <xref target="RFC6763">DNS
      DNS Service Discovery</xref> Discovery <xref target="RFC6763" format="default"></xref>
      but is not limited to that use case, and case; it provides a general DNS
      mechanism for DNS record change notifications. Familiarity with the DNS
      protocol and DNS packet formats is assumed <xref target="RFC1034"/> target="RFC1034"
      format="default"/> <xref target="RFC1035"/> target="RFC1035" format="default"/> <xref target="RFC6895"/>.</t>

<?rfc needLines="7" ?>
      target="RFC6895" format="default"/>.</t>
      <section title="Requirements Language">
       <t>The numbered="true" toc="default">
        <name>Requirements Language</name>

        <t>
    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
       "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>",
    "<bcp14>REQUIRED</bcp14>", "<bcp14>SHALL</bcp14>", "<bcp14>SHALL
    NOT</bcp14>", "<bcp14>SHOULD</bcp14>", "<bcp14>SHOULD NOT</bcp14>",
    "<bcp14>RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>",
    "<bcp14>MAY</bcp14>", and "OPTIONAL" "<bcp14>OPTIONAL</bcp14>" in this document are
    to be interpreted as
    described in BCP&nbsp;14 <xref target="RFC2119"/> <xref target="RFC8174"/>
    when, and only when, they appear in all capitals, as shown here.
       These words may also appear in this document in lower case as
       plain English words, absent their normative meanings.</t>
        </t>

      </section>
      <section title="Fatal Errors"> numbered="true" toc="default">
        <name>Fatal Errors</name>
        <t>Certain invalid situations are described in this specification,
       like
	such
        as a server sending a Push Notification subscription request to a
        client, or a client sending a Push Notification response to a server.
        These should never occur with a correctly implemented client and
        server, and if they do occur occur, then they indicate a serious
        implementation error.  In these extreme cases cases, there is no reasonable
        expectation of a graceful recovery, and the recipient detecting the
        error should respond by unilaterally aborting the session without
        regard for data loss.  Such cases are addressed by having an engineer
        investigate the cause of the failure and fixing the problem in the
        software.</t>
        <t>Where this specification says "forcibly abort", it means
       sending a TCP RST to terminate the TCP connection, connection
       and the TLS session running over that TCP connection.
       In the BSD Sockets API, this is achieved by setting the
       SO_LINGER option to zero before closing the socket.</t>

<?rfc needLines="40" ?>
      </section>
    </section>
    <section title="Motivation"> numbered="true" toc="default">
      <name>Motivation</name>
      <t>As the domain name system continues to adapt to new uses and changes
      in deployment, polling has the potential to burden DNS servers at many
      levels throughout the network. Other network protocols have successfully
      deployed a publish/subscribe model following the <xref target="obs">Observer Observer design pattern</xref>. pattern
      <xref target="OBS" format="default"></xref>.  Extensible Messaging and
      Presence Protocol (XMPP) Publish-Subscribe
      <xref target="XEP0060">XMPP Publish-Subscribe</xref> target="XEP0060" format="default"></xref> and Atom <xref target="RFC4287">Atom</xref>
      target="RFC4287" format="default"></xref> are examples. While DNS
      servers are generally highly tuned and capable of a high rate of
      query/response traffic, adding a publish/subscribe model for tracking
      changes to DNS records can deliver more timely notification notifications of changes
      with reduced CPU usage and lower network traffic.</t>

     <t><xref target="RFC6762">Multicast DNS</xref>
      <t>The guiding design principle of DNS Push Notifications
      is that clients that choose to use DNS Push Notifications,
      instead of repeated polling with DNS queries,
      will receive results the same as they could
      receive via sufficiently rapid polling, except more efficiently.
      This means that the rules for
      which records match a given DNS Push Notification subscription are the
      same as the already established rules used to determine
      which records match a given DNS query <xref target="RFC1034"
      format="default"/>.
      For example, name comparisons are done in a case-insensitive manner,
      and a record of type CNAME in a zone matches any DNS TYPE in a query or
      subscription.</t>
      <t>Multicast DNS <xref target="RFC6762" format="default"></xref>
      implementations always listen on a well known well-known link-local IP multicast
      group address, and changes are sent to that multicast group address for
      all group members to receive.  Therefore, Multicast DNS already has
      asynchronous change notification capability.  When <xref target="RFC6763">DNS DNS Service Discovery</xref> Discovery
      <xref target="RFC6763" format="default"></xref> is used across a wide
      area network using Unicast DNS (possibly facilitated via a Discovery
      Proxy <xref target="DisProx">Discovery Proxy</xref>) target="RFC8766" format="default"></xref>), it would be
      beneficial to have an equivalent capability for Unicast DNS, DNS in order to
      allow clients to learn about DNS record changes in a timely manner
      without polling.</t>
      <t>The <xref target="LLQ">DNS DNS Long-Lived Queries (LLQ) mechanism</xref> mechanism <xref target="RFC8764"
      format="default"></xref> is an existing deployed solution to provide
      asynchronous change notifications, notifications; it was used by Apple's <xref target="RFC6281">Back Back to
      My Mac</xref> Mac <xref target="RFC6281" format="default"></xref> service
      introduced in Mac OS X 10.5 Leopard in 2007.  Back to My Mac was
      designed in an era when the data center operations staff asserted that
      it was impossible for a server to handle large numbers of mostly-idle
      TCP connections, so LLQ was defined as a UDP-based protocol, effectively
      replicating much of TCP's connection state management logic in user space,
      space and creating its own imitation of existing TCP features like the three-way handshake, flow
      control, reliability, and reliability.</t> the three-way handshake.</t>

      <t>This document builds on experience gained with the LLQ protocol, with
      an improved design.  Instead of using UDP, this specification uses
     <xref target="RFC8490">DNS DNS
      Stateful Operations (DSO)</xref> (DSO) <xref target="RFC8490"
      format="default"></xref> running over TLS over TCP, and therefore
      doesn't need to reinvent existing TCP functionality.  Using TCP also
      gives long-lived low-traffic connections better longevity through NAT
      gateways without depending on the gateway to support
     <xref target="RFC6886">NAT NAT Port Mapping
      Protocol (NAT-PMP)</xref> or (NAT-PMP) <xref target="RFC6887">Port target="RFC6886" format="default"></xref> or
      Port Control Protocol (PCP)</xref>, (PCP) <xref target="RFC6887"
      format="default"></xref>, or resorting to excessive keepalive
      traffic.</t>

<?rfc needLines="9" ?>
    </section>

    <section title="Overview"> numbered="true" toc="default">
      <name>Overview</name>
      <t>A DNS Push Notification client subscribes for Push Notifications for
      a particular RRset by connecting to the appropriate Push Notification
      server for that RRset, RRset and sending DSO message(s) indicating the
      RRset(s) of interest. When the client loses interest in receiving
      further updates to these records, it unsubscribes.</t>
      <t>The DNS Push Notification server for a DNS zone is any server capable
     of generating the correct change notifications for a name.
     It may be a primary, secondary, or stealth name server <xref target="RFC7719"/>.</t>
     target="RFC8499" format="default"/>.</t>
      <t>The <spanx style="verb">_dns&nbhy;push&nbhy;tls._tcp.&lt;zone&gt;</spanx> <tt>_dns&nbhy;push&nbhy;tls._tcp.&lt;zone&gt;</tt> SRV record for
      a zone MAY <bcp14>MAY</bcp14> reference the same target host and port as
      that zone's
     <spanx style="verb">_dns&nbhy;update&nbhy;tls._tcp.&lt;zone&gt;</spanx> <tt>_dns&nbhy;update&nbhy;tls._tcp.&lt;zone&gt;</tt> SRV
      record. When the same target host and port is offered for both DNS
      Updates and DNS Push Notifications, a client MAY <bcp14>MAY</bcp14> use a
      single DSO session to that server for both DNS Updates and DNS Push
      Notification Subscriptions. subscriptions.  DNS Updates and DNS Push Notifications may
      be handled on different ports on the same target host, in which case
      they are not considered to be the "same server" for the purposes of this
      specification, and communications with these two ports are handled
      independently.  Supporting DNS Updates and DNS Push Notifications on the
      same server is OPTIONAL. <bcp14>OPTIONAL</bcp14>. A DNS Push Notification server
      is not required to support DNS Update.</t>
      <t>Standard DNS Queries MAY <bcp14>MAY</bcp14> be sent over a DNS Push
      Notification (i.e., DSO) session.  For any zone for which the server is
      authoritative, it
   MUST <bcp14>MUST</bcp14> respond authoritatively for
      queries for names falling within that zone (e.g., the <spanx style="verb">_dns&nbhy;push&nbhy;tls._tcp.&lt;zone&gt;</spanx>
      <tt>_dns&nbhy;push&nbhy;tls._tcp.&lt;zone&gt;</tt> SRV record) both for
      normal DNS queries and for DNS Push Notification subscriptions.  For
      names for which the server is acting as a recursive resolver (e.g., when
      the server is the local recursive resolver) for any query for which it
      supports DNS Push Notification subscriptions, it MUST <bcp14>MUST</bcp14>
      also support standard queries.</t>
      <t>DNS Push Notifications impose less load on the responding server than
      rapid polling would, but Push Notifications do still have a cost, so
      cost. Therefore, DNS Push Notification clients MUST NOT <bcp14>MUST NOT</bcp14>
      recklessly create an excessive number of Push Notification
      subscriptions. Specifically:</t>

     <t>(a) A
<ol type="(%c)" >
<li>A subscription should only be active when there is a valid reason to need
live data (for example, an on-screen display is currently showing the results
to the user) user), and the subscription SHOULD <bcp14>SHOULD</bcp14> be cancelled canceled as soon
as the need for that data ends (for example, when the user dismisses that
display). In the case of a device like a smartphone which, that, after some period
of inactivity, goes to sleep or otherwise darkens its screen, it should cancel
its subscriptions when darkening the screen (since the user cannot see any
changes on the display anyway) and reinstate its subscriptions when re-awakening
reawakening from display sleep.</t>

     <t>(b) A sleep.
</li>
<li>A DNS Push Notification client SHOULD NOT <bcp14>SHOULD NOT</bcp14> routinely keep a
DNS Push Notification subscription active 24 hours a day, 7 days a week, just
to keep a list in memory up to date so that if the user does choose to bring
up an on-screen display of that data, it can be displayed really fast. DNS
Push Notifications are designed to be fast enough that there is no need to
pre-load a "warm" list in memory just in case it might be needed later.</t> later.
</li>
</ol>

      <t>Generally, as described in the <xref target="RFC8490">DNS DNS Stateful Operations specification</xref>, specification
      <xref target="RFC8490"
      format="default"></xref>, a client
      must not keep a DSO session to a server open indefinitely if it has no
      subscriptions (or other operations) active on that session. A client may close
      should begin closing a DSO session immediately after it becomes idle,
      and then then, if needed in
      the future, open a new session when required. Alternatively, a client
      may speculatively keep an idle DSO session open for some time, subject
      to the constraint that it must not keep a session open that has been
      idle for more than the session's idle timeout (15 seconds by default)
      <xref target="RFC8490"/>.</t> target="RFC8490" format="default"/>.</t>
      <t>Note that a DSO session that has an active DNS Push Notification
      subscription is not considered idle, even if there is no traffic flowing
      for an extended period of time.  In this case case, the DSO inactivity
      timeout does not apply, because the session is not inactive, but the
      keepalive interval does still apply, to ensure the generation of
      sufficient messages to maintain state in middleboxes (such at NAT
      gateways or firewalls) and for the client and server to periodically
      verify that they still have connectivity to each other.  This is
      described in Section 6.2 of the <xref target="RFC8490">DSO target="RFC8490" sectionFormat="of"
      section="6.2">the DSO specification</xref>.</t>

<?rfc needLines="14" ?>
    </section>
    <section title="State Considerations"> numbered="true" toc="default">
      <name>State Considerations</name>
      <t>Each DNS Push Notification server is capable of handling some finite
     number of Push Notification subscriptions. This number will vary from
     server to server and is based on physical machine characteristics,
     network bandwidth, capacity, and operating system resource allocation. After a
     client establishes a session to a DNS server, each subscription is
     individually accepted or rejected. Servers may employ various techniques
     to limit subscriptions to a manageable level. Correspondingly, the client
     is free to establish simultaneous sessions to alternate DNS servers that
     support DNS Push Notifications for the zone and distribute subscriptions
     at the client's discretion. In this way, both clients and servers can
     react to resource constraints.</t>
<?rfc needLines="35" ?>
    </section>
    <section title="Transport"> numbered="true" toc="default">
      <name>Transport</name>
      <t>Other DNS operations like DNS Update <xref target="RFC2136">DNS Update</xref> MAY target="RFC2136"
      format="default"></xref> <bcp14>MAY</bcp14> use either DNS over User
      Datagram
      Protocol (UDP) <xref target="RFC0768">(UDP)</xref> target="RFC0768" format="default"></xref> or
      DNS over Transmission Control Protocol (TCP) <xref target="RFC0793">(TCP)</xref> target="RFC0793"
      format="default"></xref> as the transport protocol, in keeping with provided they follow
      the historical precedent that DNS queries must first be sent using DNS
      over UDP
      and only switch to DNS over TCP if needed <xref target="RFC1123"/>. target="RFC1123"
      format="default"/>.
      This requirement to use prefer UDP
      has subsequently been relaxed <xref target="RFC7766"/>.</t> target="RFC7766"
      format="default"/>.</t>
      <t>In keeping with the more recent precedent, DNS Push Notification is
      defined only for TCP.
     DNS Push Notification clients MUST <bcp14>MUST</bcp14> use
     <xref target="RFC8490">DNS
     DNS Stateful Operations</xref> Operations <xref target="RFC8490" format="default"></xref>
     running over TLS over TCP <xref target="RFC7858"/>.</t>

     <t>Connection target="RFC7858" format="default"/>.</t>

 <t>
Connection setup over TCP ensures return reachability and alleviates concerns
of state overload at the server,
     which is a potential problem with connectionless
protocols, which can be more vulnerable to being exploited by attackers using
spoofed source addresses.

     All subscribers are guaranteed to be reachable by the server by virtue of
     the TCP three-way handshake.  Flooding attacks are possible with any
     protocol, and a benefit of TCP is that there are already established
     industry best practices to guard against SYN flooding and similar attacks
     <xref target="SYN"/> target="SYN" format="default"/> <xref target="RFC4953"/>.</t> target="RFC4953"
     format="default"/>.</t>
      <t>Use of TCP also allows DNS Push Notifications to take advantage of
      current and future developments in TCP, TCP such as
     <xref target="RFC6824">Multipath Multipath TCP (MPTCP)</xref>, (MPTCP)
      <xref target="RFC7413">TCP target="RFC6824" format="default"></xref>, TCP Fast Open (TFO)</xref>, (TFO)
      <xref target="I-D.ietf-tcpm-rack">the target="RFC7413" format="default"></xref>, the TCP RACK fast loss
      detection algorithm</xref>, algorithm <xref target="I-D.ietf-tcpm-rack"
      format="default"></xref>, and so on.</t>
      <t>Transport Layer Security (TLS) <xref target="RFC8446">(TLS)</xref> target="RFC8446"
      format="default"></xref> is well understood, understood and is used by many
      application-layer protocols running over TCP. TLS is designed to prevent
      eavesdropping, tampering, and message forgery. TLS is REQUIRED
      <bcp14>REQUIRED</bcp14> for every connection between a client subscriber
      and server in this protocol specification. Additional security measures
      such as client authentication during TLS negotiation may also be
      employed to increase the trust relationship between client and
      server.</t>

<?rfc needLines="25" ?>
    </section>
    <section title="Protocol Operation"> numbered="true" toc="default">
      <name>Protocol Operation</name>
      <t>The DNS Push Notification protocol is a session-oriented protocol, protocol and
      makes use of
     <xref target="RFC8490">DNS
     DNS Stateful Operations (DSO)</xref>.</t> (DSO) <xref target="RFC8490"
     format="default"></xref>.</t>
      <t>For details of the DSO message format format, refer to the
     <xref target="RFC8490">DNS
     DNS Stateful Oper-ations specification</xref>. Operations specification <xref target="RFC8490"
     format="default"></xref>.
     Those details are not repeated here.</t>
      <t>DNS Push Notification clients and servers MUST <bcp14>MUST</bcp14> support
      DSO.
     A single server can support DNS Queries, DNS Updates, and DNS Push
     Notifications (using DSO) on the same TCP port.</t>
      <t>A DNS Push Notification exchange begins with the client discovering
      the appropriate server, using the procedure described in <xref target="discovery"/>,
      target="discovery" format="default"/>, and then making a TLS/TCP
      connection to it.</t>

     <t>A
      <t>After making the TLS/TCP connection to the server,
     a typical DNS Push Notification client will then immediately issue a DSO
     Keepalive operation to establish the DSO session
     and request a session timeout and/or keepalive interval
     longer than the 15-second default values, but this is not required.
     A DNS Push Notification client MAY <bcp14>MAY</bcp14> issue other requests on
     the
     session first, and only issue a DSO Keepalive
     operation later if it determines that to be necessary.
     Sending either a DSO Keepalive operation or a Push Notification
     subscription request over the TLS/TCP connection to the server signals
     the
     client's support of DSO and serves to establish a DSO session.</t>
      <t>In accordance with the current set of active subscriptions,
     the server sends relevant asynchronous Push Notifications to
     the client. Note that a client MUST <bcp14>MUST</bcp14> be prepared to receive
     (and silently ignore) Push Notifications for subscriptions it
     has previously removed, since there is no way to prevent the
     situation where a Push Notification is in flight from server
     to client while the client's UNSUBSCRIBE message cancelling canceling
     that subscription is simultaneously in flight from client to
     server.</t>

<?rfc needLines="30" ?>
      <section title="Discovery" anchor="discovery"> anchor="discovery" numbered="true" toc="default">
        <name>Discovery</name>
        <t>The first step in establishing a DNS Push Notification
        subscription is to discover an appropriate DNS server that
        supports DNS Push Notifications for the desired zone.</t>
        <t>The client begins by opening a DSO Session session to its normal configured
        DNS recursive resolver and requesting a Push Notification
	subscription.
        This connection is made to TCP port 853, the default port for
        DNS over TLS <xref target="RFC7858">DNS-over-TLS</xref>. target="RFC7858" format="default"></xref>.
        If the request for a Push Notification subscription is successful,
        and the recursive resolver doesn't already have an active subscription
	for that name, type, and class,
        then the recursive resolver will make a corresponding
        Push Notification subscription on the client's behalf.
        Results received are relayed to the client.
        This is closely analogous to how a client sends a normal DNS
        query to its configured DNS recursive resolver resolver, which,
        if it doesn't already have appropriate answer(s) in its cache,
        issues an upstream query to satisfy the request.</t>
        <t>In many contexts, the recursive resolver will be able to handle
        Push Notifications for all names that the client may need to follow.
        Use of VPN tunnels and Private DNS <xref target="RFC8499"/> target="RFC8499"
	format="default"/>
        can create some additional complexity in the client software here;
        the techniques to handle VPN tunnels and Private DNS for DNS Push
        Notifications are the same as those already used to handle this for
        normal DNS queries.</t>

        <t>If the recursive resolver does not support DNS over TLS, or
        supports DNS over TLS but is not listening on TCP port 853, or
        supports DNS over TLS on TCP port 853 but does not support DSO on that
        port, then the DSO Session session establishment will fail <xref target="RFC8490"/>.</t>
        target="RFC8490" format="default"/>.</t>
        <t>If the recursive resolver does support DSO on TCP port 853
        but does not support Push Notification subscriptions,
        then it when the client attempts to create a subscription,
        the server will return the DSO error code DSOTYPENI (11).</t>
        <t>In some cases, the recursive resolver may support DSO and Push
        Notification subscriptions, subscriptions but may not be able
        to subscribe for Push Notifications for a particular name.
        In this case, the recursive resolver should return
        SERVFAIL to the client. This includes being unable
        to establish a connection
        to the zone's DNS Push Notification server or establishing
        a connection but receiving a non success non-success response code.
        In some cases, where the client has a pre-established trust
        relationship with the owner of the zone (that is not handled
        via the usual mechanisms for VPN software) software), the client may
        handle these failures by contacting the zone's DNS Push Notification
	server
        directly.</t>
        <t>In any of the cases described above where the client
        fails to establish a DNS Push Notification subscription via its
        configured recursive resolver, the client should proceed to discover
        the appropriate server for direct communication.  The client MUST
	<bcp14>MUST</bcp14>
        also determine on which TCP port on the server is listening for
        connections, which need not be (and be, and often is not) the typical not,
        TCP port 53 (traditionally used for conventional DNS, DNS) or
        TCP port 853 (traditionally used for DNS over TLS.</t> TLS).</t>
        <t>The discovery algorithm described here is an iterative algorithm,
        which starts with the full name of the record to which the
        client wishes to subscribe. Successive SOA queries are then
        issued, trimming one label each time, until
        the closest enclosing authoritative server is discovered.
        There is also an optimization to enable the client to
        take a "short cut" directly to the SOA record of
        the closest enclosing authoritative server in many cases.</t>

        <t>
          <list style="numbers">
            <t>The
        <ol spacing="normal" type="1">
          <li>The client begins the discovery by sending a DNS query to its
          local resolver, with record type SOA <xref target="RFC1035">SOA</xref> target="RFC1035"
          format="default"></xref> for the record name to which it wishes to
          subscribe.  As an example, suppose the client wishes to subscribe to
          PTR records with the name _ipp._tcp.headoffice.example.com <tt>_ipp._tcp.headoffice.example.com</tt>
	  (to
          discover Internet Printing Protocol (IPP) printers <xref target="RFC8010"/>
          target="RFC8010" format="default"/> <xref target="RFC8011"/> target="RFC8011"
          format="default"/> being advertised in the head office of Example Company.).
          Company).  The client begins by sending an SOA query for _ipp._tcp.headoffice.example.com
          <tt>_ipp._tcp.headoffice.example.com</tt> to the local recursive
	  resolver.
          The goal is to determine the server that is authoritative for the
	  name _ipp._tcp.headoffice.example.com.
          <tt>_ipp._tcp.headoffice.example.com</tt>.  The closest enclosing
	  DNS zone
          containing the name _ipp._tcp.headoffice.example.com <tt>_ipp._tcp.headoffice.example.com</tt> could
	  be example.com,
          <tt>example.com</tt>, or headoffice.example.com, <tt>headoffice.example.com</tt>, or _tcp.headoffice.example.com,
          <tt>_tcp.headoffice.example.com</tt>, or even _ipp._tcp.headoffice.example.com.
          <tt>_ipp._tcp.headoffice.example.com</tt>.  The client does not know
	  in
          advance where the closest enclosing zone cut occurs, which is why it
          uses the iterative procedure described here to discover this information.</t>
          information.</li>
          <li>
            <t>If the requested SOA record exists, it will be returned in the
            Answer section Section with a NOERROR response code, and the client has
            succeeded in discovering the information it needs.
            <vspace />
            </t>
            <t>
            (This language is not placing any new requirements on DNS
	    recursive resolvers.
            This text merely describes the existing operation of the DNS
	    protocol
            <xref target="RFC1034"/> target="RFC1034" format="default"/> <xref target="RFC1035"/>.)</t> target="RFC1035"
	    format="default"/>.)</t>
          </li>

          <li>
            <t>If the requested SOA record does not exist, the client will get
            back a NOERROR/NODATA response or an NXDOMAIN/Name Error response.
            In either case, the local resolver would normally include the SOA
            record for the closest enclosing zone of the requested name in the
            Authority Section.  If the SOA record is received in the Authority
            Section, then the client has succeeded in discovering the
            information it needs.
            <vspace />
            </t>

        <t>
            (This language is not placing any new requirements on DNS
	    recursive resolvers.
            This text merely describes the existing operation of the DNS
	    protocol
            regarding negative responses <xref target="RFC2308"/>.)</t>

            <t>If target="RFC2308"
	    format="default"/>.)</t>
          </li>
          <li>If the client receives a response containing no SOA record, then
          it proceeds with the iterative approach.  The client strips the
          leading label from the current query name, and if the resulting name
          has at least two labels in it, it then the client sends an SOA query for
          that new name, name and processing continues at step 2 above, repeating
          the iterative search until either an SOA is received, received or the query
          name consists of a single label, i.e., a Top Level Top-Level Domain (TLD).  In
          the case of a single-label name (TLD), this is a network
          configuration error, which should not happen, and the client gives
          up.  The client may retry the operation at a later time, time of the
          client's choosing, such as after a change in network attachment.</t>

            <t>Once
	  attachment.</li>
          <li>Once the SOA is known (either by (by virtue of being seen either in the
          Answer Section, Section or in the Authority Section), the client sends a DNS
          query with type SRV <xref target="RFC2782">SRV</xref> target="RFC2782" format="default"></xref>
          for the record name
            <spanx style="verb">_dns&nbhy;push&nbhy;tls._tcp.&lt;zone&gt;</spanx>,
          <tt>_dns&nbhy;push&nbhy;tls._tcp.&lt;zone&gt;</tt>, where
          &lt;zone&gt; is the owner name of the discovered SOA record.</t>

            <t>If record.</li>
          <li>If the zone in question is set up to offer DNS Push Notifications
          Notifications, then this SRV record MUST <bcp14>MUST</bcp14> exist. (If
          this SRV record does not exist exist, then the zone is not correctly
          configured for DNS Push Notifications as specified in this
          document.) The SRV <spanx style="verb">target</spanx> <tt>target</tt> contains the name of the server
          providing DNS Push Notifications for the zone. The port number on
          which to contact the server is in the SRV record <spanx style="verb">port</spanx> <tt>port</tt>
          field. The address(es) of the target host MAY <bcp14>MAY</bcp14> be
          included in the Additional Section, however, the address records SHOULD
          <bcp14>SHOULD</bcp14> be authenticated before use as described below in
          <xref target="tls_name_auth"/> target="tls_name_auth" format="default"/> and in
            <xref target="RFC7673">the the
          specification for using DANE DNS-Based Authentication of Named Entities
	  (DANE) TLSA Records with SRV Records</xref>, Records <xref
          target="RFC7673" format="default"></xref>, if applicable.</t>

            <t applicable.</li>
          <li anchor="SRV">More than one SRV record may be returned. In this
          case, the <spanx style="verb">priority</spanx> <tt>priority</tt> and <spanx style="verb">weight</spanx> <tt>weight</tt> values in the
          returned SRV records are used to determine the order in which to
          contact the servers for subscription requests. As described in <xref target="RFC2782">the the
          SRV specification</xref>, specification <xref target="RFC2782" format="default"></xref>,
          the server with the lowest <spanx style="verb">priority</spanx> <tt>priority</tt> is first contacted. If
          more than one server has the same <spanx style="verb">priority</spanx>, <tt>priority</tt>, the <spanx style="verb">weight</spanx>
          <tt>weight</tt> indicates the weighted probability that the client
          should contact that server. Higher weights have higher probabilities
          of being selected.  If a server is not willing to accept a
          subscription request, or is not reachable within a reasonable time,
          as determined by the client, then a subsequent server is to be contacted.</t>

          </list>
        </t>
          contacted.</li>
        </ol>
        <t>Each time a client makes a new DNS Push Notification subscription,
        it SHOULD <bcp14>SHOULD</bcp14> repeat the discovery process in order to
        determine the preferred DNS server for that subscription at that time.
        If a client already has a DSO session with that DNS server server, the client SHOULD
        <bcp14>SHOULD</bcp14> reuse that existing DSO session for the new subscription,
        subscription; otherwise, a new DSO session is established.  The client MUST
        <bcp14>MUST</bcp14> respect the DNS TTL values on records it receives
        while performing the discovery process and store them in its local
        cache with this lifetime (as it will generally be do anyway for all
        DNS queries it performs).  This means that, as long as the DNS TTL
        values on the authoritative records are set to reasonable values,
        repeated application of the discovery process can be completed
        nearly
	practically
        instantaneously by the client, using only locally-stored locally stored cached
        data.</t>

<?rfc needLines="48" ?>
      </section>

      <section title="DNS anchor="subscribe" numbered="true" toc="default">
        <name>DNS Push Notification SUBSCRIBE" anchor="subscribe"> SUBSCRIBE</name>
        <t>After connecting, and requesting a longer idle timeout and/or
        keepalive interval if necessary, a DNS Push Notification client<vspace /> client then
        indicates its desire to receive DNS Push Notifications for<vspace /> for a given
        domain name by sending a SUBSCRIBE request to the server.<vspace /> server.  A SUBSCRIBE
        request is encoded in a DSO message <xref target="RFC8490"/>.<vspace /> target="RFC8490"
        format="default"/>. This specification defines a primary
        DSO Primary TLV for DNS Push Notification SUBSCRIBE Requests (tentatively DSO
        (DSO Type Code 0x40).</t> 0x0040).</t>
        <t>DSO messages with the SUBSCRIBE TLV as the Primary TLV are
        permitted in TLS early data, provided that the precautions described
	in
        <xref target="early_data"/> target="early_data" format="default"/> are followed.</t>
        <t>The entity that initiates a SUBSCRIBE request is by definition the
        client.  A server MUST NOT <bcp14>MUST NOT</bcp14> send a SUBSCRIBE request
        over an existing session from a client.  If a server does send a
        SUBSCRIBE request over a DSO session initiated by a client, this is a
        fatal error and the client MUST <bcp14>MUST</bcp14> forcibly abort the
        connection immediately.</t>
        <t>Each SUBSCRIBE request generates exactly one SUBSCRIBE response
        from the server.  The entity that initiates a SUBSCRIBE response is by
        definition the server.  A client MUST NOT <bcp14>MUST NOT</bcp14> send a
        SUBSCRIBE response.  If a client does send a SUBSCRIBE response, this
        is a fatal error and the server MUST <bcp14>MUST</bcp14> forcibly abort the
        connection immediately.</t>
        <section title="SUBSCRIBE Request"> numbered="true" toc="default">
          <name>SUBSCRIBE Request</name>
          <t>A SUBSCRIBE request begins with the standard
         <xref target="RFC8490">DSO DSO 12-byte header</xref>, header
          <xref target="RFC8490" format="default"></xref>, followed by the
          SUBSCRIBE primary Primary TLV.  A SUBSCRIBE request is illustrated in <xref target="subscribe_req"/>.</t>
          target="subscribe_req" format="default"/>.</t>
          <t>The MESSAGE ID field MUST <bcp14>MUST</bcp14> be set to a unique value,
          value that the client is not using for any other active operation
          on this DSO session. For the purposes here, a MESSAGE ID is in use
          on this session if either the client has used it in a request for
	  which it
          has not yet received a response, or if the client has used it for a
          subscription which that it has not yet cancelled canceled using UNSUBSCRIBE. In
          the SUBSCRIBE response response, the server MUST <bcp14>MUST</bcp14> echo back the
          MESSAGE ID value unchanged.</t>
          <t>The other header fields MUST <bcp14>MUST</bcp14> be set as described
	  in the
         <xref target="RFC8490">DSO spec-ification</xref>. target="RFC8490" format="default">DSO specification</xref>.
         The DNS OPCODE field contains the OPCODE value for DNS Stateful
	 Operations (6).
         The four count fields must be zero, and the corresponding four
	 sections must be empty (i.e., absent).</t>
          <t>The DSO-TYPE is SUBSCRIBE (tentatively 0x40).</t> (0x0040).</t>
          <t>The DSO-LENGTH is the length of the DSO-DATA that follows, which
	  specifies
         the name, type, and class of the record(s) being sought.</t>
          <figure anchor="subscribe_req">
            <name>SUBSCRIBE Request</name>
            <artwork align="left" anchor="subscribe_req" title="SUBSCRIBE Request"><artwork align="left"><![CDATA[ name="" type="" alt=""><![CDATA[
                                   1  1  1  1  1  1
     0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  \
   |                  MESSAGE ID                   |   \
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
   |QR| OPCODE(6) |         Z          |   RCODE   |    |
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
   |             QDCOUNT (MUST BE ZERO)            |    |
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+     > HEADER
   |             ANCOUNT (MUST BE ZERO)            |    |
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
   |             NSCOUNT (MUST BE ZERO)            |    |
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
   |             ARCOUNT (MUST BE ZERO)            |   /
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  /
   |         DSO-TYPE = SUBSCRIBE (tentatively 0x40) (0x0040)         |
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   |   DSO-LENGTH (number of octets in DSO-DATA)   |
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  \
   \                     NAME                      \   \
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
   |                     TYPE                      |     > DSO-DATA
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
   |                     CLASS                     |   /
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  /]]></artwork></figure>  /]]></artwork>
          </figure>
          <t>The DSO-DATA for a SUBSCRIBE request MUST <bcp14>MUST</bcp14> contain
	  exactly one NAME, TYPE, and CLASS.
         Since SUBSCRIBE requests are sent over TCP, multiple SUBSCRIBE DSO
	 request messages
         can be concatenated in a single TCP stream and packed efficiently
	 into TCP segments.</t>
          <t>If accepted, the subscription will stay in effect until the
          client cancels the subscription using UNSUBSCRIBE or until the DSO
          session between the client and the server is closed.</t>
          <t>SUBSCRIBE requests on a given session MUST <bcp14>MUST</bcp14> be
          unique.  A client MUST NOT <bcp14>MUST NOT</bcp14> send a SUBSCRIBE message
          that duplicates the
         NAME, TYPE name, type and CLASS class of an existing active
          subscription on that DSO session.  For the purpose of this matching,
          the established DNS case-insensitivity case insensitivity for US-ASCII letters <xref
          target="RFC0020" /> format="default"/> applies (e.g., "example.com" and
          "Example.com" are the same).  If a server receives such a duplicate
          SUBSCRIBE message, this is a fatal error and the server MUST
          <bcp14>MUST</bcp14> forcibly abort the connection immediately.</t>
          <t>DNS wildcarding is not supported.
          That is, a wildcard an asterisk character ("*") in a SUBSCRIBE message matches
          only a literal wildcard asterisk character ("*") in the zone, a name and nothing else.</t>

         <t>Aliasing is not supported. That is, else.
          Similarly, a CNAME in a SUBSCRIBE message matches only a literal CNAME
	  record
          with that name in the zone, zone and no other records with the same owner that name.</t>
          <t>A client may SUBSCRIBE to records that are unknown to the server
          at the time of the request (providing that the name falls within one
          of the zone(s) the server is responsible for) for), and this is not an
          error. The server MUST NOT <bcp14>MUST NOT</bcp14> return NXDOMAIN in this
          case. The server MUST <bcp14>MUST</bcp14> accept these requests and send
          Push Notifications if and when matching records are found in the
          future.</t>
          <t>If neither TYPE nor CLASS are ANY (255) (255), then this is a specific
          subscription to changes for the given NAME, TYPE name, type, and CLASS. class. If one
          or both of TYPE or CLASS are ANY (255) (255), then this subscription
          matches any type all types and/or any class, all classes as appropriate.</t>

<?rfc needLines="14" ?>
          <t>NOTE: A little-known quirk of DNS is that in DNS QUERY requests,
          QTYPE and QCLASS 255 mean "ANY" "ANY", not "ALL". They indicate that the
          server should respond with ANY matching records of its choosing, not
          necessarily ALL matching records. This can lead to some surprising
          and unexpected results, where a query returns some valid answers answers,
	  but
          not all of them, and makes QTYPE = 255 (ANY) queries less useful
          than people sometimes imagine.</t>
          <t>When used in conjunction with SUBSCRIBE, TYPE 255 and CLASS 255
          should be interpreted to mean "ALL", not "ANY". After accepting a
          subscription where one or both of TYPE or CLASS are 255, the server MUST
          <bcp14>MUST</bcp14> send Push Notification Updates for ALL record
          changes that match the subscription, not just some of them.</t>
<?rfc needLines="48" ?>
        </section>

        <section title="SUBSCRIBE Response" anchor="subresp"> anchor="subresp" numbered="true" toc="default">
          <name>SUBSCRIBE Response</name>
          <t>A SUBSCRIBE response begins with the standard
         <xref target="RFC8490">DSO
         DSO 12-byte header</xref>. header <xref target="RFC8490" format="default"></xref>.
         The QR bit in the header is set indicating it is a response.
         The header MAY <bcp14>MAY</bcp14> be followed by one or more
         optional TLVs, Additional TLVs such as a Retry Delay Additional TLV.
         A SUBSCRIBE response is illustrated in <xref target="subscribe_resp"/>.</t> target="subscribe_resp"
	 format="default"/>.</t>
          <t>The MESSAGE ID field MUST <bcp14>MUST</bcp14> echo the value given in
          the MESSAGE ID field of the SUBSCRIBE request.  This is how the
          client knows which request is being responded to.</t>
          <t>The other header fields MUST <bcp14>MUST</bcp14> be set as described
          in the <xref target="RFC8490">DSO spec-ification</xref>. target="RFC8490"
          format="default">DSO specification</xref>.  The DNS OPCODE field
	  contains the OPCODE
          value for DNS Stateful Operations (6).  The four count fields must
          be zero, and the corresponding four sections must be empty (i.e.,
          absent).</t>
          <t>A SUBSCRIBE response message MUST NOT <bcp14>MUST NOT</bcp14> include a
	  SUBSCRIBE TLV.
         If a client receives a SUBSCRIBE response message containing a
	 SUBSCRIBE TLV TLV,
         then the response message is processed but the SUBSCRIBE TLV MUST
	 <bcp14>MUST</bcp14> be silently ignored.</t>
          <figure anchor="subscribe_resp">
            <name>SUBSCRIBE Response</name>
            <artwork align="left" anchor="subscribe_resp" title="SUBSCRIBE Response"><artwork align="left"><![CDATA[ name="" type="" alt=""><![CDATA[
                                   1  1  1  1  1  1
     0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  \
   |                  MESSAGE ID                   |   \
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
   |QR| OPCODE(6) |         Z          |   RCODE   |    |
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
   |             QDCOUNT (MUST BE ZERO)            |    |
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+     > HEADER
   |             ANCOUNT (MUST BE ZERO)            |    |
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
   |             NSCOUNT (MUST BE ZERO)            |    |
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
   |             ARCOUNT (MUST BE ZERO)            |   /
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  /
]]>
</artwork></figure>
<?rfc needLines="20" ?>

]]></artwork>
          </figure>
          <t>In the SUBSCRIBE response response, the RCODE indicates whether or not the
          subscription was accepted. Supported RCODEs are as follows:</t>
         <texttable title="SUBSCRIBE
          <table anchor="subscribe_rcodes" align="center">
            <name>SUBSCRIBE Response codes" anchor="subscribe_rcodes">
         <ttcol align="left">Mnemonic</ttcol>
         <ttcol align="center">Value</ttcol>
         <ttcol align="left">Description</ttcol>
         <c>NOERROR</c><c>0</c><c>SUBSCRIBE successful.</c>
         <c>FORMERR</c><c>1</c><c>Server Codes</name>
            <thead>
              <tr>
                <th align="left">Mnemonic</th>
                <th align="center">Value</th>
                <th align="left">Description</th>
              </tr>
            </thead>
            <tbody>
              <tr>
                <td align="left">NOERROR</td>
                <td align="center">0</td>
                <td align="left">SUBSCRIBE successful.</td>
              </tr>
              <tr>
                <td align="left">FORMERR</td>
                <td align="center">1</td>
                <td align="left">Server failed to process request due to a
		malformed request.</c>
         <c>SERVFAIL</c><c>2</c><c>Server request.</td>
              </tr>
              <tr>
                <td align="left">SERVFAIL</td>
                <td align="center">2</td>
                <td align="left">Server failed to process request due to a
		problem with the server.</c>
         <c>NOTIMP</c><c>4</c><c>Server server.</td>
              </tr>
              <tr>
                <td align="left">NOTIMP</td>
                <td align="center">4</td>
                <td align="left">Server does not implement DSO.</c>
         <c>REFUSED</c><c>5</c><c>Server DSO.</td>
              </tr>
              <tr>
                <td align="left">REFUSED</td>
                <td align="center">5</td>
                <td align="left">Server refuses to process request for policy
		or security reasons.</c>
         <c>NOTAUTH</c><c>9</c><c>Server reasons.</td>
              </tr>
              <tr>
                <td align="left">NOTAUTH</td>
                <td align="center">9</td>
                <td align="left">Server is not authoritative for the requested name.</c>
         <c>DSOTYPENI</c><c>11</c><c>SUBSCRIBE
		name.</td>
              </tr>
              <tr>
                <td align="left">DSOTYPENI</td>
                <td align="center">11</td>
                <td align="left">SUBSCRIBE operation not supported.</c>
         </texttable> supported.</td>
              </tr>
            </tbody>
          </table>
          <t>This document specifies only these RCODE values for SUBSCRIBE
          Responses. Servers sending SUBSCRIBE Responses SHOULD <bcp14>SHOULD</bcp14>
          use one of these values. Note that NXDOMAIN is not a valid RCODE in
          response to a SUBSCRIBE Request. However, future circumstances may
          create situations where other RCODE values are appropriate in
          SUBSCRIBE Responses, so clients MUST <bcp14>MUST</bcp14> be prepared to
          accept and handle SUBSCRIBE Responses with any other nonzero RCODE value.</t>
	  error values.</t>
          <t>If the server sends a nonzero RCODE in the SUBSCRIBE response,
	  that means:
           <?rfc subcompact="yes" ?>
           <list style="letters">
             <t>the
          </t>
          <ol spacing="compact" type="a">
            <li>the client is (at least partially) misconfigured, or</t>
             <t>the or</li>
            <li>the server resources are exhausted, or</t>
             <t>there or</li>
            <li>there is some other unknown failure on the server.</t>
           </list>
           <?rfc subcompact="no" ?> server.</li>
          </ol>
          <t>
           In any case, the client shouldn't retry the subscription to this
           server right away. If multiple SRV records were returned as
           described in <xref target="discovery"/>, <xref target="SRV"/>,
           target="SRV" format="default"/>, a subsequent server MAY
           <bcp14>MAY</bcp14> be tried immediately.</t>
          <t>If the client has other successful subscriptions to this server,
          these subscriptions remain even though additional subscriptions may
          be refused. Neither the client nor the server are is required to close
          the connection, although, although either end may choose to do so.</t>
          <t>If the server sends a nonzero RCODE RCODE, then it SHOULD
	  <bcp14>SHOULD</bcp14>
          append a Retry Delay Additional TLV <xref target="RFC8490"/> target="RFC8490"
	  format="default"/>
          to the response specifying a delay before the client attempts this
          operation again. Recommended values for the delay for different
          RCODE values are given below. These recommended values apply both to
          the default values a server should place in the Retry Delay TLV,
	  Additional TLV and
          the default values a client should assume if the server provides no
          Retry Delay Additional TLV.
           <list style="bullets">
             <t>For
          </t>
          <ul spacing="normal" empty="true">
            <li>For RCODE = 1 (FORMERR) (FORMERR), the delay may be any value selected
	    by
            the implementer. A value of five minutes is RECOMMENDED,
            <bcp14>RECOMMENDED</bcp14> to reduce the risk of high load from
            defective clients.</t>

             <t>For clients.</li>
            <li>For RCODE = 2 (SERVFAIL) (SERVFAIL), the delay should be chosen according
            to the level of server overload and the anticipated duration of
            that overload. By default, a value of one minute is RECOMMENDED.
            <bcp14>RECOMMENDED</bcp14>. If a more serious server failure
            occurs, the delay may be longer in accordance with the specific
            problem encountered.</t>

             <t>For encountered.</li>
            <li>For RCODE = 4 (NOTIMP), which occurs on a server that doesn't
	    implement
             <xref target="RFC8490">DNS
             DNS Stateful Operations</xref>, Operations <xref target="RFC8490"
	     format="default"></xref>,
             it is unlikely that the server will begin supporting DSO
             in the next few minutes, so the retry delay SHOULD <bcp14>SHOULD</bcp14>
	     be one hour.
             Note that in such a case, a server that doesn't implement DSO
             is unlikely to place a Retry Delay Additional TLV in its
	     response, so this
             recommended value in particular applies to what a client should
	     assume by default.</t>

             <t>For default.</li>
            <li>For RCODE = 5 (REFUSED), which occurs on a server that
            implements DNS Push Notifications, Notifications but is currently configured to
            disallow DNS Push Notifications, the retry delay may be any value
            selected by the implementer and/or configured by the operator.</t>
             <t>If
            operator.</li>
            <li>If the server being queried is listed in a
             <spanx style="verb">_dns&nbhy;push&nbhy;tls._tcp.&lt;zone&gt;</spanx>
             <tt>_dns&nbhy;push&nbhy;tls._tcp.&lt;zone&gt;</tt>
             SRV record for the zone, then this is a misconfiguration,
             since this server is being advertised as supporting DNS Push
	     Notifications for this zone,
             but the server itself is not currently configured to perform that
	     task.
             Since it is possible that the misconfiguration may be repaired
             at any time, the retry delay should not be set too high.  By
	     default,
             a value of 5 minutes is RECOMMENDED.</t>

             <t>For <bcp14>RECOMMENDED</bcp14>.</li>
            <li>For RCODE = 9 (NOTAUTH), which occurs on a server that
            implements DNS Push Notifications, Notifications but is not configured to be
            authoritative for the requested name, the retry delay may be any
            value selected by the implementer and/or configured by the operator.</t>
             <t>If
            operator.</li>
            <li>If the server being queried is listed in a
             <spanx style="verb">_dns&nbhy;push&nbhy;tls._tcp.&lt;zone&gt;</spanx>
             <tt>_dns&nbhy;push&nbhy;tls._tcp.&lt;zone&gt;</tt>
             SRV record for the zone, then this is a misconfiguration,
             since this server is being advertised as supporting DNS Push
	     Notifications for this zone,
             but the server itself is not currently configured to perform that
	     task.
             Since it is possible that the misconfiguration may be repaired
             at any time, the retry delay should not be set too high.  By
	     default,
             a value of 5 minutes is RECOMMENDED.</t>

             <t>For <bcp14>RECOMMENDED</bcp14>.</li>
            <li>For RCODE = 11 (DSOTYPENI),
             which occurs on a server that implements DSO but doesn't
	     implement DNS Push Notifications,
             it is unlikely that the server will begin supporting DNS Push
	     Notifications
             in the next few minutes, so the retry delay SHOULD <bcp14>SHOULD</bcp14>
	     be one hour.</t>

             <t>For hour.</li>
            <li>For other RCODE values, the retry delay should be
            set by the server as appropriate for that error condition.
            By default, a value of 5 minutes is RECOMMENDED.</t>
           </list>
         </t>
	    <bcp14>RECOMMENDED</bcp14>.</li>
          </ul>
          <t>For RCODE = 9 (NOTAUTH), the time delay applies to requests for
          other names falling within the same zone. Requests for names falling
          within other zones are not subject to the delay. For all other RCODEs
          RCODEs, the time delay applies to all subsequent requests to this
          server.</t>
          <t>After sending an error response response, the server MAY <bcp14>MAY</bcp14>
          allow the session to remain open, or MAY send <bcp14>MAY</bcp14> follow it
	  with
          a DNS Push Notification DSO Retry Delay Operation TLV operation (using the Retry Delay Primary TLV)
          instructing the client to close the session, session as described in the
          <xref target="RFC8490">DSO target="RFC8490" format="default">DSO specification</xref>.
          Clients MUST <bcp14>MUST</bcp14> correctly handle both cases.</t>

<?rfc needLines="48" ?> cases.
          Note that the
          DSO Retry Delay operation (using the Retry Delay Primary TLV)
          is different to the Retry Delay Additional TLV mentioned above.
          </t>
        </section>
      </section>
      <section title="DNS anchor="push" numbered="true" toc="default">
        <name>DNS Push Notification Updates" anchor="push"> Updates</name>
        <t>Once a subscription has been successfully established,
        the server generates PUSH messages to send to the client as
	appropriate.
        In the case that the answer set was already non-empty at the moment
        the subscription was established, an initial PUSH message will be sent
        immediately following the SUBSCRIBE Response. Subsequent changes to
	the
        answer set are then communicated to the client in subsequent PUSH
	messages.</t>
        <t>A client MUST NOT <bcp14>MUST NOT</bcp14> send a PUSH message.
       If a client does send a PUSH message,
       or a PUSH message is sent with the QR bit set indicating that it is a
       response,
       this is a fatal error and the receiver MUST <bcp14>MUST</bcp14> forcibly
       abort the connection immediately.</t>
        <section title="PUSH Message"> numbered="true" toc="default">
          <name>PUSH Message</name>
          <t>A PUSH unidirectional message begins with the standard
         <xref target="RFC8490">DSO
         DSO 12-byte header</xref>, header <xref target="RFC8490" format="default"></xref>,
	 followed by the PUSH primary Primary TLV.
         A PUSH message is illustrated in <xref target="push_msg"/>.</t> target="push_msg"
	 format="default"/>.</t>
          <t>In accordance with the definition of DSO unidirectional messages,
         the MESSAGE ID field MUST <bcp14>MUST</bcp14> be zero.
         There is no client response to a PUSH message.</t>
          <t>The other header fields MUST <bcp14>MUST</bcp14> be set as described
	  in the
         DSO specification <xref target="RFC8490">DSO spec-ification</xref>. target="RFC8490" format="default"></xref>.
         The DNS OPCODE field contains the OPCODE value for DNS Stateful
	 Operations (6).
         The four count fields must be zero, and the corresponding four
	 sections must be empty (i.e., absent).</t>
          <t>The DSO-TYPE is PUSH (tentatively 0x41).</t> (0x0041).</t>
          <t>The DSO-LENGTH is the length of the DSO-DATA that follows, which
	  specifies
         the changes being communicated.</t>
          <t>The DSO-DATA contains one or more change notifications.
         A PUSH Message MUST <bcp14>MUST</bcp14> contain at least one change
	 notification.
         If a PUSH Message is received that contains no change notifications,
         this is a fatal error, error and the client MUST <bcp14>MUST</bcp14> forcibly
	 abort the connection immediately.</t>
          <t>The change notification records are formatted similarly to how
         DNS Resource Records are conventionally expressed in DNS messages,
         as illustrated in <xref target="push_msg"/>, target="push_msg" format="default"/>,
         and are interpreted as described below.</t>

<?rfc needLines="6" ?>
          <t>The TTL field holds an unsigned 32-bit integer <xref target="RFC2181"/>.
	  target="RFC2181" format="default"/>.
         If the TTL is in the range 0 to 2,147,483,647 seconds (0 to 2^31
	 2<sup>31</sup> - 1, or 0x7FFFFFFF),
         then a new DNS Resource Record with the given name, type, class class, and
	 RDATA is added.
         Type and class MUST NOT <bcp14>MUST NOT</bcp14> be 255 (ANY). If either type
	 or class are 255 (ANY) (ANY),
         this is a fatal error, error and the client MUST <bcp14>MUST</bcp14> forcibly
	 abort the connection immediately.
         A TTL of 0 means that this record should be retained for as long as
	 the subscription is active, active
         and should be discarded immediately the moment the subscription is cancelled.</t>
	  canceled.</t>
          <t>If the TTL has the value 0xFFFFFFFF, then the DNS Resource Record
          with the given name, type, class class, and RDATA is removed.  Type and
          class MUST NOT <bcp14>MUST NOT</bcp14> be 255 (ANY). If either type or class
          are 255 (ANY) (ANY), this is a fatal error, error and the client MUST
          <bcp14>MUST</bcp14> forcibly abort the connection immediately.</t>
          <t>If the TTL has the value 0xFFFFFFFE, then this is a 'collective'
          remove notification.  For collective remove notifications notifications, RDLEN MUST
          <bcp14>MUST</bcp14> be zero zero, and consequently consequently, the RDATA MUST
          <bcp14>MUST</bcp14> be empty.  If a change notification is received
          where TTL = 0xFFFFFFFE and RDLEN is not zero, this is a fatal error, error
          and the client MUST <bcp14>MUST</bcp14> forcibly abort the connection
          immediately.</t>

          <t>There are three types of collective remove notification:</t>

         <t>For notification.
          For collective remove notifications,
         if notifications:</t>

<ul>

<li>If CLASS is not 255 (ANY) and TYPE is not 255 (ANY) (ANY), then for the given name
name, this removes all records of the specified type in the specified class.</t>

         <t>For collective remove notifications,
         if class.
</li>

<li>If CLASS is not 255 (ANY) and TYPE is 255 (ANY) (ANY), then for the given name name,
this removes all records of all types in the specified class.</t>

         <t>For collective remove notifications,
         if class.
</li>

<li>If CLASS is 255 (ANY), then for the given name name, this removes all records
of all types in all classes.  In this case case, TYPE MUST <bcp14>MUST</bcp14> be set to
zero on transmission,
transmission and MUST <bcp14>MUST</bcp14> be silently ignored on reception.</t>

<?rfc needLines="19" ?> reception.
</li>

</ul>

          <t>Summary of change notification types:
           <list style="bullets">
          </t>
          <ul spacing="normal">
            <li>
              <t>Remove all RRsets from a name, name in all classes<vspace /> classes:<br/>
             TTL = 0xFFFFFFFE, RDLEN = 0, CLASS = 255 (ANY)</t> (ANY).</t>
            </li>
            <li>
              <t>Remove all RRsets from a name, name in given class:<vspace /> class:<br/>
             TTL = 0xFFFFFFFE, RDLEN = 0, CLASS gives class, TYPE = 255 (ANY)</t>
	      (ANY).</t>
            </li>
            <li>
              <t>Remove specified RRset from a name, name in given class:<vspace /> class:<br/>
             TTL = 0xFFFFFFFE, RDLEN = 0<vspace /> 0,<br/>
             CLASS and TYPE specify the RRset being removed</t> removed.</t>
            </li>
            <li>
              <t>Remove an individual RR from a name:<vspace /> name:<br/>
             TTL = 0xFFFFFFFF<vspace /> 0xFFFFFFFF,<br/>
             CLASS, TYPE, RDLEN RDLEN, and RDATA specify the RR being removed</t> removed.</t>
            </li>
            <li>
              <t>Add individual RR to a name<vspace /> name:<br/>
             TTL &gt;= 0 and TTL &lt;= 0x7FFFFFFF<vspace /> 0x7FFFFFFF,<br/>
             CLASS, TYPE, RDLEN, RDATA RDATA, and TTL specify the RR being added</t>
           </list>
         </t>
	      added.</t>
            </li>
          </ul>
          <t>Note that it is valid for the RDATA of an added or removed DNS
          Resource Record to be empty (zero length).  For example, an <xref target="RFC3123">Address Address
          Prefix List Resource Record</xref> Record <xref target="RFC3123"
          format="default"></xref> may have empty RDATA.  Therefore, a change
          notification with RDLEN = 0 does not automatically indicate a remove
          notification.  If RDLEN = 0 and TTL is the in the range 0 - to
          0x7FFFFFFF, this change notification signals the addition of a
          record with the given name, type, class, and empty RDATA.  If RDLEN
          = 0 and TTL = 0xFFFFFFFF, this change notification signals the
          removal specifically of that single record with the given name,
          type, class, and empty RDATA.</t>
          <t>If the TTL is any value other than 0xFFFFFFFF, 0xFFFFFFFE, or a
	  value in the range 0 - to 0x7FFFFFFF,
         then the receiver SHOULD <bcp14>SHOULD</bcp14> silently ignore this
	 particular change notification record.
         The connection is not terminated and other valid change notification
	 records
         within this PUSH message are processed as usual.</t>

          <t>In the case where a single change affects more than one active
          subscription, only one PUSH message is sent. For example, a PUSH
          message adding a given record may match both a SUBSCRIBE request
          with the same TYPE and a different SUBSCRIBE request with TYPE = 255
          (ANY). It is not the case that two PUSH messages are sent because
          the new record matches two active subscriptions.</t>

          <t>The server <bcp14>SHOULD</bcp14> encode change notifications in
	  the most efficient manner possible.
          For example, when three AAAA records are removed from a given name,
	  and no other AAAA
          records exist for that name, the server <bcp14>SHOULD</bcp14> send a
          "Remove specified RRset from a name in given class" PUSH message,
	  not three separate
          "Remove an individual RR from a name" PUSH messages.
          Similarly, when both an SRV and a TXT record are removed from a
	  given name, and no other
          records of any kind exist for that name in that class, the server
	  <bcp14>SHOULD</bcp14> send a
          "Remove all RRsets from a name in given class" PUSH message, not two
	  separate
          "Remove specified RRset from a name in given class" PUSH
	  messages.</t>

          <t>For efficiency, when generating a PUSH PUSH message, rather than
	  sending
          each change notification as a separate DSO message, a server SHOULD
          <bcp14>SHOULD</bcp14> include as many change notifications as it has
          immediately available to send,
         rather than sending each send to that client, even if those change notification as
          notifications apply to different subscriptions from that
	  client. Conceptually, a separate DSO message. PUSH
          message is a session-level mechanism, not a subscription-level
	  mechanism.
          Once it has exhausted the list of change notifications immediately
	  available to send, send to that client,
          a server SHOULD <bcp14>SHOULD</bcp14> then send the PUSH message immediately,
	  immediately
          rather than waiting speculatively to see if additional change
	  notifications become available.</t>

<?rfc needLines="6" ?>
          <t>For efficiency, when generating a PUSH message, message a server SHOULD
          <bcp14>SHOULD</bcp14> use standard DNS name compression, with
	  offsets
          relative to the beginning of the DNS message <xref target="RFC1035"/>. target="RFC1035"
          format="default"/>.  When multiple change notifications in a single
          PUSH message have the same owner name, this name compression can
          yield significant savings.  Name compression should be performed as
          specified in Section 18.14 of the <xref target="RFC6762">Multicast target="RFC6762" sectionFormat="of"
	    section="18.14">the Multicast DNS specification</xref>, specification</xref>; namely,
	  owner names
          should always be compressed, and names appearing within RDATA should
          be compressed for only the RR types listed below:
         <list style="hanging">
           <t>NS,
          </t>
          <dl newline="false" spacing="normal">
            <dt/>
            <dd>NS, CNAME, PTR, DNAME, SOA, MX, AFSDB, RT, KX, RP, PX, SRV, NSEC</t>
         </list></t>
	    NSEC</dd>
          </dl>
          <t>Servers may generate PUSH messages up to a maximum DNS message
	  length of 16,382 bytes,
         counting from the start of the DSO 12-byte header.
         Including the two-byte length prefix that is used to frame DNS over a
	 byte stream
         like TLS, this makes a total of 16,384 bytes.
         Servers MUST NOT <bcp14>MUST NOT</bcp14> generate PUSH messages larger than
	 this.
         Where the immediately available change notifications
         are sufficient to exceed a DNS message length of 16,382 bytes,
         the change notifications MUST <bcp14>MUST</bcp14> be communicated in
	 separate PUSH messages
         of up to 16,382 bytes each.
         DNS name compression becomes less effective for messages larger than
	 16,384 bytes,
         so little efficiency benefit is gained by sending messages larger
	 than this.</t>
          <t>If a client receives a PUSH message with a DNS message length
	  larger than 16,382 bytes,
         this is a fatal error, error and the client MUST <bcp14>MUST</bcp14> forcibly
	 abort the connection immediately.</t>
          <figure anchor="push_msg">
            <name>PUSH Message</name>
            <artwork align="left" anchor="push_msg" title="PUSH Message"><artwork align="left"><![CDATA[ name="" type="" alt=""><![CDATA[
                                   1  1  1  1  1  1
     0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  \
   |           MESSAGE ID (MUST BE ZERO)           |   \
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
   |QR| OPCODE(6) |         Z          |   RCODE   |    |
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
   |             QDCOUNT (MUST BE ZERO)            |    |
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+     > HEADER
   |             ANCOUNT (MUST BE ZERO)            |    |
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
   |             NSCOUNT (MUST BE ZERO)            |    |
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
   |             ARCOUNT (MUST BE ZERO)            |   /
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  /
   |            DSO-TYPE = PUSH (tentatively 0x41) (0x0041)           |
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   |   DSO-LENGTH (number of octets in DSO-DATA)   |
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  \
   \                     NAME                      \   \
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
   |                     TYPE                      |    |
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
   |                     CLASS                     |    |
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
   |                      TTL                      |    |
   |     (32-bit unsigned big-endian integer)      |     > DSO-DATA
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
   |  RDLEN (16-bit unsigned big-endian integer)   |    |
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
   \           RDATA (sized as necessary)          \    |
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
   :     NAME, TYPE, CLASS, TTL, RDLEN, RDATA      :    |
   :             Repeated As Necessary             :   /
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  /]]></artwork></figure>  /]]></artwork>
          </figure>
          <t>When processing the records received in a PUSH Message, the
	  receiving client MUST <bcp14>MUST</bcp14> validate
         that the records being added or removed correspond with at least one
	 currently active
         subscription on that session. Specifically, the record name MUST
	 <bcp14>MUST</bcp14> match the name given in the
         SUBSCRIBE request, subject to the usual established DNS
	 case-insensitivity for US-ASCII letters.
         For individual additions and removals,
         if the TYPE in the SUBSCRIBE request was not ANY (255) (255),
         then the TYPE of the record must either be CNAME or match the TYPE
	 given in the SUBSCRIBE request, and
         if the CLASS in the SUBSCRIBE request was not ANY (255) (255),
         then the CLASS of the record must match the CLASS given in the
	 SUBSCRIBE request.
         For collective removals, at least one of the records being removed
	 must match an active subscription.
         If a matching active subscription on that session is not found, then
	 that particular
         addition/removal record is silently ignored. Processing of other additions and removal records
         in this message is not affected. The DSO session is not closed. This is to allow for
         the unavoidable race condition where a client sends an outbound UNSUBSCRIBE while
         inbound PUSH messages for that subscription from the server are still in flight.</t>

         <t>In the case where a single change affects more than one active subscription, only one PUSH message is sent. For example, a PUSH message adding a given record may match both a SUBSCRIBE request with the same TYPE and a different SUBSCRIBE request with TYPE = 255 (ANY). It is not the case that two PUSH messages are sent because the new record matches two active subscriptions.</t>

         <t>The server SHOULD encode change notifications in the most efficient manner possible.
         For example, when three AAAA records are removed from a given name, and no other AAAA
         records exist for that name, the server SHOULD send a "remove an RRset from a name"
         PUSH message, not three separate "remove an individual RR from a name" PUSH messages.
         Similarly, when both an SRV and a TXT record are removed from a given name, and no silently ignored. The processing of other
	 additions and removal records of any kind exist
         in this message is not affected. The DSO session is not closed. This
	 is to allow for that name,
         the server SHOULD send a "remove all RRsets
         from unavoidable race condition where a name" PUSH message, not two separate "remove client sends an RRset from a name" outbound
	 UNSUBSCRIBE while
         inbound PUSH messages.</t>

         <t>A messages for that subscription from the server SHOULD combine multiple change notifications are still
	 in a single PUSH message when possible, even if those change notifications apply to different subscriptions. Conceptually, a PUSH message is a session-level mechanism, not a subscription-level mechanism.</t> flight.</t>
          <t>The TTL of an added record is stored by the client.  While the
	  subscription
           is active, active the TTL is not decremented, because a change to the TTL
	   would
           produce a new update.
           For as long as a relevant subscription remains active, the client
           SHOULD
           <bcp14>SHOULD</bcp14> assume that when a record goes away away, the
	   server will notify it
           of that fact.  Consequently, a client does not have to poll to
	   verify
           that the record is still there.  Once a subscription is cancelled canceled
           (individually, or as a result of the DSO session being closed) closed),
	   record
           aging for records covered by the subscription resumes and records
	   are
           removed from the local cache when their  TTL reaches zero.</t>
<?rfc needLines="48" ?>
         </section>
       </section>

      <section title="DNS anchor="unsubscribe" numbered="true" toc="default">
        <name>DNS Push Notification UNSUBSCRIBE" anchor="unsubscribe"> UNSUBSCRIBE</name>
        <t>To cancel an individual subscription without closing the entire DSO
        session, the client sends an UNSUBSCRIBE message over the established
        DSO session to the server.</t>
        <t>The entity that initiates an UNSUBSCRIBE message is by definition
	the client.
       A server MUST NOT <bcp14>MUST NOT</bcp14> send an UNSUBSCRIBE message over an
       existing session from a client.
       If a server does send an UNSUBSCRIBE message over a DSO session
       initiated by a client,
       or an UNSUBSCRIBE message is sent with the QR bit set indicating that
       it is a response,
       this is a fatal error and the receiver MUST <bcp14>MUST</bcp14> forcibly
       abort the connection immediately.</t>
        <section title="UNSUBSCRIBE Message"> numbered="true" toc="default">
          <name>UNSUBSCRIBE Message</name>
          <t>An UNSUBSCRIBE unidirectional message begins with the standard
         <xref target="RFC8490">DSO
         DSO 12-byte header</xref>, header <xref target="RFC8490" format="default"></xref>,
	 followed by the UNSUBSCRIBE primary Primary TLV.
         An UNSUBSCRIBE message is illustrated in <xref target="unsubscribe_msg"/>.</t>
	 target="unsubscribe_msg" format="default"/>.</t>
          <t>In accordance with the definition of DSO unidirectional messages,
         the MESSAGE ID field MUST <bcp14>MUST</bcp14> be zero.
         There is no server response to an UNSUBSCRIBE message.</t>
          <t>The other header fields MUST <bcp14>MUST</bcp14> be set as described
	  in the
         <xref target="RFC8490">DSO spec-ification</xref>. target="RFC8490" format="default">DSO specification</xref>.
         The DNS OPCODE field contains the OPCODE value for DNS Stateful
	 Operations (6).
         The four count fields must be zero, and the corresponding four
	 sections must be empty (i.e., absent).</t>
          <t>The DSO-TYPE is UNSUBSCRIBE (tentatively 0x42).</t> (0x0042).</t>
          <t>The DSO-LENGTH field contains the value 2, the length of the
	  2-octet MESSAGE ID contained in the DSO-DATA.</t>
          <t>The DSO-DATA contains the value previously given in the MESSAGE
	  ID field of an active SUBSCRIBE request.
         This is how the server knows which SUBSCRIBE request is being cancelled.
	 canceled.
         After receipt of the UNSUBSCRIBE message, the SUBSCRIBE request is no
	 longer active.</t>
          <t>It is allowable for the client to issue an UNSUBSCRIBE message
	  for a previous SUBSCRIBE request
         for which the client has not yet received a SUBSCRIBE response.
         This is to allow for the case where a client starts and stops a
	 subscription in less than the
         round-trip time to the server.
         The client is NOT required to wait for the SUBSCRIBE response before
	 issuing the UNSUBSCRIBE message.</t>

<?rfc needLines="6" ?>
          <t>Consequently, it is possible for a server to receive an
	  UNSUBSCRIBE message
         that does not match any currently active subscription.
         This can occur when a client sends a SUBSCRIBE request,
         which subsequently fails and returns an error code,
         but the client sent an UNSUBSCRIBE message before it
         became aware that the SUBSCRIBE request had failed.
         Because of this, servers MUST <bcp14>MUST</bcp14> silently ignore
         UNSUBSCRIBE messages that do not match any currently active
	  subscription.</t>
          <figure anchor="unsubscribe_msg">
            <name>UNSUBSCRIBE Message</name>
            <artwork align="left" anchor="unsubscribe_msg" title="UNSUBSCRIBE Message"><artwork align="left"><![CDATA[ name="" type="" alt=""><![CDATA[
                                   1  1  1  1  1  1
     0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  \
   |           MESSAGE ID (MUST BE ZERO)           |   \
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
   |QR| OPCODE(6) |         Z          |   RCODE   |    |
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
   |             QDCOUNT (MUST BE ZERO)            |    |
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+     > HEADER
   |             ANCOUNT (MUST BE ZERO)            |    |
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
   |             NSCOUNT (MUST BE ZERO)            |    |
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
   |             ARCOUNT (MUST BE ZERO)            |   /
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  /
   |         DSO-TYPE = UNSUBSCRIBE (tentatively 0x42) (0x0042)       |
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   |                DSO-LENGTH (2)                 |
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  \
   |              SUBSCRIBE MESSAGE ID             |   > DSO-DATA
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  /]]></artwork></figure>

<?rfc needLines="48" ?>  /]]></artwork>
          </figure>
        </section>
      </section>
      <section title="DNS anchor="reconfirm" numbered="true" toc="default">
        <name>DNS Push Notification RECONFIRM" anchor="reconfirm"> RECONFIRM</name>
        <t>Sometimes, particularly when used with a Discovery Proxy <xref target="DisProx">Discovery Proxy</xref>,
        target="RFC8766" format="default"></xref>, a DNS Zone may contain
        stale data. When a client encounters data that it believes may be
        stale (e.g., an SRV record referencing a target host+port that is not
        responding to connection requests) requests), the client can send a RECONFIRM
        message to ask the server to re-verify that the data is still
        valid. For a Discovery Proxy, this causes it to issue new Multicast
        DNS queries to ascertain whether the target device is still
        present. How the Discovery Proxy causes these new Multicast DNS
        queries to be issued depends on the details of the underlying
        Multicast DNS implementation being used.  For example, a Discovery
        Proxy built on Apple's dns_sd.h API <xref target="SD-API"/> target="SD-API"
        format="default"/> responds to a DNS Push Notification RECONFIRM
        message by calling the underlying API's DNSServiceReconfirmRecord()
        routine.</t>
        <t>For other types of DNS server, the RECONFIRM operation is currently undefined,
        undefined and SHOULD <bcp14>SHOULD</bcp14> result in a NOERROR response, but otherwise
        it need not cause any other action to occur.</t>
        <t>Frequent use of RECONFIRM operations may be a sign of network
        unreliability, or some kind of misconfiguration, so RECONFIRM
        operations MAY <bcp14>MAY</bcp14> be logged or otherwise communicated to a
        human administrator to assist in detecting and remedying such network
        problems.</t>
        <t>If, after receiving a valid RECONFIRM message, the server
        determines that the disputed records are in fact no longer valid, then
        subsequent DNS PUSH Messages will be generated to inform interested
        clients. Thus, one client discovering that a previously-advertised previously advertised
        device (like a network printer) is no longer present has the side
        effect of informing all other interested clients that the device in
        question is now gone.</t>
        <t>The entity that initiates a RECONFIRM message is by definition the
	client.
       A server MUST NOT <bcp14>MUST NOT</bcp14> send a RECONFIRM message over an
       existing session from a client.
       If a server does send a RECONFIRM message over a DSO session initiated
       by a client,
       or a RECONFIRM message is sent with the QR bit set indicating that it
       is a response,
       this is a fatal error and the receiver MUST <bcp14>MUST</bcp14> forcibly
       abort the connection immediately.</t>

<?rfc needLines="20" ?>
        <section title="RECONFIRM Message"> numbered="true" toc="default">
          <name>RECONFIRM Message</name>
          <t>A RECONFIRM unidirectional message begins with the standard
         <xref target="RFC8490">DSO DSO
          12-byte header</xref>, header <xref target="RFC8490" format="default"></xref>,
          followed by the RECONFIRM primary TLV.<vspace /> Primary TLV.
          A RECONFIRM message is illustrated in <xref target="reconfirm_msg"/>.</t> target="reconfirm_msg"
	  format="default"/>.</t>
          <t>In accordance with the definition of DSO unidirectional messages,
         the MESSAGE ID field MUST <bcp14>MUST</bcp14> be zero.
         There is no server response to a RECONFIRM message.</t>
          <t>The other header fields MUST <bcp14>MUST</bcp14> be set as described
	  in the
         <xref target="RFC8490">DSO spec-ification</xref>. target="RFC8490" format="default">DSO specification</xref>.
         The DNS OPCODE field contains the OPCODE value for DNS Stateful
	 Operations (6).
         The four count fields must be zero, and the corresponding four
	 sections must be empty (i.e., absent).</t>
          <t>The DSO-TYPE is RECONFIRM (tentatively 0x43).</t> (0x0043).</t>
          <t>The DSO-LENGTH is the length of the data that follows, which
	  specifies
         the name, type, class, and content of the record being disputed.</t>

         <t>The
          <t>A DNS Push Notifications RECONFIRM message contains exactly one
	  RECONFIRM Primary TLV.
          The DSO-DATA for in a RECONFIRM message MUST Primary TLV <bcp14>MUST</bcp14> contain
	  exactly one record.
          The DSO-DATA for in a RECONFIRM message Primary TLV has no count field to
	  specify more than one record.
          Since RECONFIRM messages are sent over TCP, multiple RECONFIRM
	  messages
          can be concatenated in a single TCP stream and packed efficiently
	  into TCP segments.</t> segments.
          Note that this means that DNS name compression cannot be used
          between different RECONFIRM messages.
          However, when a client is sending multiple RECONFIRM messages this
	  indicates
          a situation with serious network problems, and this is not expected
	  to occur
          frequently enough that optimizing efficiency in this case is
	  important.
          </t>
          <t>TYPE MUST NOT <bcp14>MUST NOT</bcp14> be the value ANY (255) and CLASS MUST NOT
	  <bcp14>MUST NOT</bcp14> be the value ANY (255).</t>
          <t>DNS wildcarding is not supported.
          That is, a wildcard an asterisk character ("*") in a RECONFIRM message matches
          only a literal wildcard asterisk character ("*") in the zone, a name and nothing else.</t>

         <t>Aliasing is not supported. That is, else.
          Similarly, a CNAME in a RECONFIRM message matches only a literal CNAME
	  record
          with that name in the zone, zone and no other records with the same owner that name.</t>
          <t>Note that there is no RDLEN field,
          since the length of the RDATA can be inferred from DSO-LENGTH,
          so an additional RDLEN field would be redundant.</t>
          <t>Following the same rules as for PUSH messages, DNS name
	  compression SHOULD
          be used within the RDATA of the RECONFIRM message, with offsets
	  relative to the
          beginning of the DNS message <xref target="RFC1035"
	  format="default"/>.</t>
          <figure anchor="reconfirm_msg">
            <name>RECONFIRM Message</name>
            <artwork align="left" anchor="reconfirm_msg" title="RECONFIRM Message"><artwork align="left"><![CDATA[ name="" type="" alt=""><![CDATA[
                                   1  1  1  1  1  1
     0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  \
   |           MESSAGE ID (MUST BE ZERO)           |   \
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
   |QR| OPCODE(6) |         Z          |   RCODE   |    |
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
   |             QDCOUNT (MUST BE ZERO)            |    |
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+     > HEADER
   |             ANCOUNT (MUST BE ZERO)            |    |
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
   |             NSCOUNT (MUST BE ZERO)            |    |
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
   |             ARCOUNT (MUST BE ZERO)            |   /
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  /
   |         DSO-TYPE = RECONFIRM (tentatively 0x43) (0x0043)         |
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   |   DSO-LENGTH (number of octets in DSO-DATA)   |
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  \
   \                     NAME                      \   \
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
   |                     TYPE                      |    |
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+     > DSO-DATA
   |                     CLASS                     |    |
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
   \                     RDATA                     \   /
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  /]]></artwork></figure>

<?rfc needLines="48" ?>  /]]></artwork>
          </figure>
        </section>
      </section>
      <section title="DNS numbered="true" toc="default">
        <name>DNS Stateful Operations TLV Context Summary"> Summary</name>
        <t>This document defines four new DSO TLVs. As recommended in Section 8.2 of the <xref target="RFC8490">DNS
	target="RFC8490" sectionFormat="of"  section="8.2">the DNS Stateful
	Operations specification</xref>, the valid contexts of these
        new TLV types are summarized below.</t>
        <t>The client TLV contexts are:
         <?rfc subcompact="yes" ?>
         <list style="hanging">
           <t hangText="C-P:">Client

        </t>
        <dl newline="false" spacing="compact">
          <dt>C-P:</dt>
          <dd>Client request message, primary TLV</t>
           <t hangText="C-U:">Client unidirectional Primary TLV</dd>
          <dt>C-U:</dt>
          <dd>Client Unidirectional message, primary TLV</t>
           <t hangText="C-A:">Client TLV</dd>
          <dt>C-A:</dt>
          <dd>Client request or unidirectional message, additional TLV</t>
           <t hangText="CRP:">Response Additional TLV</dd>
          <dt>CRP:</dt>
          <dd>Response back to client, primary TLV</t>
           <t hangText="CRA:">Response Primary TLV</dd>
          <dt>CRA:</dt>
          <dd>Response back to client, additional TLV</t>
         </list>
         <?rfc subcompact="no" ?>
       </t>
       <texttable title="DSO Additional TLV</dd>
        </dl>
        <table anchor="tlv_client_contexts" align="center">
          <name>DSO TLV Client Context Summary" anchor="tlv_client_contexts">
       <ttcol Summary</name>
          <thead>
            <tr>
              <th align="right">TLV Type</ttcol>
       <ttcol align="center">C-P</ttcol>
       <ttcol align="center">C-U</ttcol>
       <ttcol align="center">C-A</ttcol>
       <ttcol align="center">CRP</ttcol>
       <ttcol align="center">CRA</ttcol>
       <c>SUBSCRIBE</c><c>X</c><c></c><c></c><c></c><c></c>
       <c>PUSH</c><c></c><c></c><c></c><c></c><c></c>
       <c>UNSUBSCRIBE</c><c></c><c>X</c><c></c><c></c><c></c>
       <c>RECONFIRM</c><c></c><c>X</c><c></c><c></c><c></c>
     </texttable> Type</th>
              <th align="center">C-P</th>
              <th align="center">C-U</th>
              <th align="center">C-A</th>
              <th align="center">CRP</th>
              <th align="center">CRA</th>
            </tr>
          </thead>
          <tbody>
            <tr>
              <td align="right">SUBSCRIBE</td>
              <td align="center">X</td>
              <td align="center"/>
              <td align="center"/>
              <td align="center"/>
              <td align="center"/>
            </tr>
            <tr>
              <td align="right">PUSH</td>
              <td align="center"/>
              <td align="center"/>
              <td align="center"/>
              <td align="center"/>
              <td align="center"/>
            </tr>
            <tr>
              <td align="right">UNSUBSCRIBE</td>
              <td align="center"/>
              <td align="center">X</td>
              <td align="center"/>
              <td align="center"/>
              <td align="center"/>
            </tr>
            <tr>
              <td align="right">RECONFIRM</td>
              <td align="center"/>
              <td align="center">X</td>
              <td align="center"/>
              <td align="center"/>
              <td align="center"/>
            </tr>
          </tbody>
        </table>
        <t>The server TLV contexts are:
       <?rfc subcompact="yes" ?>
       <list style="hanging">
         <t hangText="S-P:">Server

        </t>
        <dl newline="false" spacing="compact">
          <dt>S-P:</dt>
          <dd>Server request message, primary TLV</t>
         <t hangText="S-U:">Server unidirectional Primary TLV</dd>
          <dt>S-U:</dt>
          <dd>Server Unidirectional message, primary TLV</t>
         <t hangText="S-A:">Server TLV</dd>
          <dt>S-A:</dt>
          <dd>Server request or unidirectional message, additional TLV</t>
         <t hangText="SRP:">Response Additional TLV</dd>
          <dt>SRP:</dt>
          <dd>Response back to server, primary TLV</t>
         <t hangText="SRA:">Response Primary TLV</dd>
          <dt>SRA:</dt>
          <dd>Response back to server, additional TLV</t>
       </list>
       <?rfc subcompact="no" ?>
     </t>
     <texttable title="DSO Additional TLV</dd>
        </dl>
        <table anchor="tlv_server_contexts" align="center">
          <name>DSO TLV Server Context Summary" anchor="tlv_server_contexts">
     <ttcol Summary</name>
          <thead>
            <tr>
              <th align="right">TLV Type</ttcol>
     <ttcol align="center">S-P</ttcol>
     <ttcol align="center">S-U</ttcol>
     <ttcol align="center">S-A</ttcol>
     <ttcol align="center">SRP</ttcol>
     <ttcol align="center">SRA</ttcol>
     <c>SUBSCRIBE</c><c></c><c></c><c></c><c></c><c></c>
     <c>PUSH</c><c></c><c>X</c><c></c><c></c><c></c>
     <c>UNSUBSCRIBE</c><c></c><c></c><c></c><c></c><c></c>
     <c>RECONFIRM</c><c></c><c></c><c></c><c></c><c></c>
   </texttable> Type</th>
              <th align="center">S-P</th>
              <th align="center">S-U</th>
              <th align="center">S-A</th>
              <th align="center">SRP</th>
              <th align="center">SRA</th>
            </tr>
          </thead>
          <tbody>
            <tr>
              <td align="right">SUBSCRIBE</td>
              <td align="center"/>
              <td align="center"/>
              <td align="center"/>
              <td align="center"/>
              <td align="center"/>
            </tr>
            <tr>
              <td align="right">PUSH</td>
              <td align="center"/>
              <td align="center">X</td>
              <td align="center"/>
              <td align="center"/>
              <td align="center"/>
            </tr>
            <tr>
              <td align="right">UNSUBSCRIBE</td>
              <td align="center"/>
              <td align="center"/>
              <td align="center"/>
              <td align="center"/>
              <td align="center"/>
            </tr>
            <tr>
              <td align="right">RECONFIRM</td>
              <td align="center"/>
              <td align="center"/>
              <td align="center"/>
              <td align="center"/>
              <td align="center"/>
            </tr>
          </tbody>
        </table>
      </section>
      <section title="Client-Initiated Termination"> numbered="true" toc="default">
        <name>Client-Initiated Termination</name>
        <t>An individual subscription is terminated by sending an UNSUBSCRIBE
        TLV for that specific subscription, or all subscriptions can be cancelled
        canceled at once by the client closing the DSO session. When a client
        terminates an individual subscription (via UNSUBSCRIBE) or all
        subscriptions on that DSO session (by ending the session) session), it is
        signaling to the server that it is no longer interested in receiving
        those particular updates. It is informing the server that the server
        may release any state information it has been keeping with regards to
        these particular subscriptions.</t>
        <t>After terminating its last subscription on a session via
        UNSUBSCRIBE, a client MAY <bcp14>MAY</bcp14> close the session immediately, immediately
        or it may keep it open if it anticipates performing further operations
        on that session in the future. If a client wishes to keep an idle
        session open, it MUST <bcp14>MUST</bcp14> respect the maximum idle time
        required by the server <xref target="RFC8490"/>.</t> target="RFC8490" format="default"/>.</t>
        <t>If a client plans to terminate one or more subscriptions on a
        session and doesn't intend to keep that session open, then as an
        efficiency optimization optimization, it MAY <bcp14>MAY</bcp14> instead choose to
	simply
        close the session, which implicitly terminates all subscriptions on
        that session. This may occur because the client computer is being shut
        down, is going to sleep, the application requiring the subscriptions
        has terminated, or simply because the last active subscription on that
        session has been cancelled.</t> canceled.</t>
        <t>When closing a session, a client should perform an orderly close of
        the TLS session.  Typical APIs will provide a session close method
        that will send a TLS close_notify alert
       (see Section 6.1 of the as described in <xref
	target="RFC8446"
        sectionFormat="of" section="6.1">the TLS 1.3 specification <xref target="RFC8446"/>). specification</xref>.
	This instructs the
        recipient that the sender will not send any more data over the
        session.  After sending the TLS close_notify alert alert, the client MUST
        <bcp14>MUST</bcp14> gracefully close the underlying connection using a
        TCP FIN, FIN so that the TLS close_notify is reliably delivered.  The
        mechanisms for gracefully closing a TCP connection with a TCP FIN vary
        depending on the networking API.  For example, in the BSD Sockets API,
        sending a TCP FIN is achieved by calling "shutdown(s,SHUT_WR)" and
        keeping the socket open until all remaining data has been read from
        it.</t>
        <t>If the session is forcibly closed at the TCP level by sending a
       RST from either end of the connection, data may be lost.</t>
<?rfc needLines="10" ?>
      </section>

      <section title="Client anchor="polling" numbered="true" toc="default">
        <name>Client Fallback to Polling" anchor="polling"> Polling</name>
        <t>There are cases where a client may exhaust all avenues for
        establishing a DNS Push Notification subscription without success.
        This can happen if the client's configured recursive resolver does not
        support DNS over TLS, or supports DNS over TLS but is not listening on
        TCP port 853, or supports DNS over TLS on TCP port 853 but does not
        support DSO on that port, or for some other reason is unable to
        provide a DNS Push Notification subscription.  In this case case, the
	client
        will attempt to communicate directly with an appropriate server, and
        it may be that the zone apex discovery fails, or there is no
        <spanx style="verb">_dns&nbhy;push&nbhy;tls._tcp.&lt;zone&gt;</spanx>
        <tt>_dns&nbhy;push&nbhy;tls._tcp.&lt;zone&gt;</tt> SRV record, or
        the server indicated in the SRV record is misconfigured, overloaded,
	or is
        unresponsive for some other reason.</t>
        <t>Regardless of the reason for the failure, after being unable to
        establish the desired DNS Push Notification subscription, it is likely
        that the client will still wish to know the answer it seeks, even if
        that answer cannot be obtained with the timely change notifications
        provided by DNS Push Notifications.  In such cases cases, it is likely that
        the client will obtain the answer it seeks via a conventional DNS
        query instead, repeated at some interval to detect when the answer
        RRset changes.</t>
        <t>In the case where a client responds to its failure to establish a
        DNS Push Notification subscription by falling back to polling with
        conventional DNS queries instead, the polling rate should be
        controlled to avoid placing excessive burden on the server.  The
        interval between successive DNS queries for the same name, type type, and
        class
        SHOULD <bcp14>SHOULD</bcp14> be at least the minimum of: of 900 seconds (15 minutes),
        minutes) or two seconds more than the TTL of the answer RRset.</t>

        <t>The reason that for TTLs shorter than up to 898 seconds the query should
        not be reissued until two seconds *after* <em>after</em> the answer RRset has expired
        expired, is to ensure that the answer RRset has also expired from the
        cache on the client's configured recursive resolver.  Otherwise
        (particularly if the clocks on the client and the recursive resolver
        do not run at precisely the same rate) rate), there's a risk of a race
        condition where the client queries its configured recursive resolver
        just as the answer RRset has one second remaining in the recursive
        resolver's cache.  The client would then receive a reply telling it
        that the answer RRset has one second remaining, and then remaining; the client
        would then re-query requery the recursive resolver again one second later when later.
        If by this time the answer RRset has actually expires, and only then would expired from the
        recursive resolver's cache, the recursive resolver would then
        issue a new query to fetch new fresh data from the
        authoritative server.  Waiting until the answer RRset has definitely
        expired from the
        the cache on the client's configured recursive resolver
        avoids this race condition and any unnecessary additional queries it
        causes.</t>
        <t>Each time a client is about to reissue its query to discover
        changes to the answer RRset, it should first make a new attempt to
        establish a DNS Push Notification subscription, subscription using previously
        cached DNS answers as appropriate.  After a temporary misconfiguration
        has been remedied, this allows a client that is polling to return to
        using DNS Push Notifications for asynchronous notification of
        changes.</t>
      </section>
    </section>

    <section title="Security Considerations" anchor="Security"> anchor="Security" numbered="true" toc="default">
      <name>Security Considerations</name>
      <t>The Strict Privacy Usage Profile profile for DNS over TLS is REQUIRED
      <bcp14>REQUIRED</bcp14> for DNS Push Notifications <xref target="RFC8310"/>.
      target="RFC8310" format="default"/>. Cleartext connections for DNS Push
      Notifications are not permissible. Since this is a new protocol,
      transition mechanisms from the Opportunistic Privacy profile are
      unnecessary.</t>

      <t>Also, see Section 9 of the DNS over (D)TLS
      <xref target="RFC8310" sectionFormat="of" section="9">the document Usage
      Profiles document <xref target="RFC8310"/> for DNS over (D)TLS</xref>
      for additional
      recommendations for various versions of TLS usage.</t>
      <t>As a consequence of requiring TLS, client certificate authentication
      and verification may also be enforced by the server for stronger
      client-server security or end-to-end security. However, recommendations
      for security in particular deployment scenarios are outside the scope of
      this document.</t>
      <t>DNSSEC is RECOMMENDED <bcp14>RECOMMENDED</bcp14> for the authentication of DNS
      Push Notification servers.  TLS alone does not provide complete
      security.  TLS certificate verification can provide reasonable assurance
      that the client is really talking to the server associated with the
      desired host name, but since the desired host name is learned via a DNS
      SRV query, if the SRV query is subverted then the client may have a
      secure connection to a rogue server.  DNSSEC can provide added
      confidence that the SRV query has not been subverted.</t>

<?rfc needLines="14" ?>
      <section title="Security Services"> numbered="true" toc="default">
        <name>Security Services</name>
        <t>It is the goal of using TLS to provide the following security
	services:
         <list style="hanging">
           <t hangText="Confidentiality:">All
        </t>
        <dl newline="false" spacing="normal">
          <dt>Confidentiality:</dt>
          <dd>All application-layer communication is encrypted with the goal
	  that no party should be able to decrypt it except the intended receiver.</t>
           <t hangText="Data
	  receiver.</dd>
          <dt>Data integrity protection:">Any protection:</dt>
          <dd>Any changes made to the communication in transit are detectable
	  by the receiver.</t>
           <t hangText="Authentication:">An end-point receiver.</dd>
          <dt>Authentication:</dt>
          <dd>An endpoint of the TLS communication is authenticated as the
	  intended entity to communicate with.</t>
           <t hangText="Anti-replay protection:">TLS with.</dd>
          <dt>Anti-replay protection:</dt>
          <dd>TLS provides for the detection of and prevention
           against messages sent previously over a TLS connection (such as DNS
	   Push Notifications).
           If prior messages are re-sent at a later time as a form of a
	   man-in-the-middle attack attack,
           then the receiver will detect this and reject the replayed messages.</t>
         </list>
       </t>
	  messages.</dd>
        </dl>

        <t>Deployment recommendations on the appropriate key lengths and cypher
	cipher suites are beyond the scope of this document. Please refer to
	the current
	TLS Recommendations <xref target="BCP195">TLS Recommendations</xref> target="RFC7525" format="default"></xref>
	for the best current practices.
	Keep in mind that best practices only exist for a snapshot in time time,
	and recommendations will continue to change.
	Updated versions or errata may exist for these recommendations.</t>
      </section>
      <section title="TLS anchor="tls_name_auth" numbered="true" toc="default">
        <name>TLS Name Authentication" anchor="tls_name_auth"> Authentication</name>
        <t>As described in <xref target="discovery"/>, target="discovery" format="default"/>, the
        client discovers the DNS Push Notification server using an SRV lookup
        for the record name <spanx style="verb">_dns&nbhy;push&nbhy;tls._tcp.&lt;zone&gt;</spanx>.
        <tt>_dns&nbhy;push&nbhy;tls._tcp.&lt;zone&gt;</tt>. The server
        connection endpoint SHOULD <bcp14>SHOULD</bcp14> then be authenticated using
        DANE TLSA records for the associated SRV record. This associates the
        target's name and port number with a trusted TLS certificate <xref target="RFC7673"/>.
        target="RFC7673" format="default"/>. This procedure uses the TLS
        Server Name Indication (SNI) extension <xref target="RFC6066"/> target="RFC6066"
        format="default"/> to inform the server of the name the client has
        authenticated through the use of TLSA records. Therefore, if the SRV
        record passes DNSSEC validation and a TLSA record matching the target
        name is useable, usable, an SNI extension must be used for the target name to
        ensure the client is connecting to the server it has authenticated. If
        the target name does not have a usable TLSA record, then the use of
        the SNI extension is optional. See <xref target="RFC8310">Usage Usage Profiles for DNS over TLS and
        DNS over DTLS</xref> DTLS <xref target="RFC8310" format="default"></xref> for more
        information on authenticating domain names.</t>
      </section>
      <section title="TLS anchor="early_data" numbered="true" toc="default">
        <name>TLS Early Data" anchor="early_data"> Data</name>
        <t>DSO messages with the SUBSCRIBE TLV as the Primary TLV are
	permitted in TLS early data.
       Using TLS early data can save one network round trip, trip and can result in
       the client obtaining results faster.</t>
        <t>However, there are some factors to consider before using TLS early
	data.</t>
        <t>TLS Early Data early data is not forward secret.
       In cases where forward secrecy of DNS Push Notification subscriptions
       is required,
       the client should not use TLS Early Data.</t> early data.</t>
        <t>With TLS early data data, there are no guarantees of non-replay between
	connections.
       If packets are duplicated and delayed in the network,
       the later arrivals could be mistaken for new subscription requests.
       Generally
       Generally, this is not a major concern, concern
       since the amount of state generated on the server for
       these spurious subscriptions is small and short-lived, short lived
       since the TCP connection will not complete the three-way handshake.
       Servers MAY <bcp14>MAY</bcp14> choose to implement rate-limiting measures
       that are activated when
       the server detects an excessive number of spurious subscription
	requests.</t>
        <t>For further guidance on use of TLS early data, please see
	discussion of zero round-trip data
       (Section 2.3, Section 8,
       in Sections <xref target="RFC8446" sectionFormat="bare" section="2.3"/>
       and
       <xref target="RFC8446" sectionFormat="bare" section="8"/>, and Appendix E.5)
       in the
       <xref
       target="RFC8446" sectionFormat="bare" section="E.5"/>, of <xref
       target="RFC8446">the TLS 1.3 specification, <xref target="RFC8446"/>.</t> specification</xref>.</t>
      </section>
      <section title="TLS anchor="resumption" numbered="true" toc="default">
        <name>TLS Session Resumption" anchor="resumption"> Resumption</name>
        <t>TLS Session Resumption session resumption <xref target="RFC8446"/> target="RFC8446" format="default"/>
       is permissible on DNS Push Notification servers.
       However, closing the TLS connection terminates the DSO session.
       When the TLS session is resumed, the DNS Push Notification server will
       not
       have any subscription state and will proceed as with any other new DSO
       session.
       Use of TLS Session Resumption session resumption may allow a TLS connection to be set up
       more quickly,
       but the client will still have to recreate any desired
	subscriptions.</t>
<?rfc needLines="30" ?>
      </section>
    </section>
    <section title="IANA Considerations" anchor="IANA"> anchor="IANA" numbered="true" toc="default">
      <name>IANA Considerations</name>
      <t>This document defines a new service name, only applicable for the TCP protocol,
     to be recorded in the IANA Service Type Registry <xref target="RFC6335"/><xref target="SRVTYPE"/>.</t>
     <texttable title="IANA Service Type Assignments" anchor="iana_service_table">
       <ttcol width="25%" align="left">Name</ttcol>
       <ttcol align="center">Port</ttcol>
       <ttcol align="center">Value</ttcol>
       <ttcol align="left">Definition</ttcol>
       <c>DNS Push Notification Service Type</c>
       <c>None</c>
       <c><spanx style="verb">_dns&nbhy;push&nbhy;tls._tcp</spanx></c>
       <c><xref target="discovery"/></c>
     </texttable>

     <t>This document defines four new DNS Stateful Operation TLV types
     to be recorded in the IANA DSO Type Code Registry <xref target="RFC8490"/><xref target="DSOTYPE"/>.</t>
     <texttable title="IANA DSO TLV Type Code Assignments" anchor="iana_tlv_table">
       <ttcol align="left"              >Name</ttcol>
       <ttcol align="center" width="18%">Value</ttcol>
       <ttcol align="center"            >Early Data</ttcol>
       <ttcol align="center" width="28%">Status</ttcol>
       <ttcol align="left"   width="20%">Definition</ttcol>
       <c>SUBSCRIBE</c>
       <c>TBA (0x40)</c>
       <c>OK</c>
       <c>Standards Track</c>
       <c><xref target="subscribe"/></c>
       <c>PUSH</c>
       <c>TBA (0x41)</c>
       <c>NO</c>
       <c>Standards Track</c>
       <c><xref target="push"/></c>
       <c>UNSUBSCRIBE</c>
       <c>TBA (0x42)</c>
       <c>NO</c>
       <c>Standards Track</c>
       <c><xref target="unsubscribe"/></c>
       <c>RECONFIRM</c>
       <c>TBA (0x43)</c>
       <c>NO</c>
       <c>Standards Track</c>
       <c><xref target="reconfirm"/></c>
     </texttable>

     <t>This document defines no new DNS OPCODEs or RCODEs.</t>

<?rfc needLines="12" ?>
   </section>

   <section title="Acknowledgements" anchor="Acknowledgements">
     <t>The authors would like to thank Kiren Sekar and Marc Krochmal for previous work completed in this field.</t>

     <t>This draft has been improved due to comments from
     Ran Atkinson,
     Tim Chown,
     Sara Dickinson,
     Mark Delany,
     Ralph Droms,
     Jan Komissar,
     Eric Rescorla,
     Michael Richardson,
     David Schinazi,
     Manju Shankar Rao,
     Robert Sparks,
     Markus Stenberg,
     Andrew Sullivan,
     Michael Sweet,
     Dave Thaler,
     Brian Trammell,
     Bernie Volz,
     Eric Vyncke,
     Christopher Wood,
     Liang Xia,
     and
     Soraia Zlatkovic.
     Ted Lemon provided clarifying text that was greatly appreciated.</t>
<?rfc needLines="15" ?>
   </section>
 </middle>

 <!--  *****BACK MATTER ***** -->

 <back>
   <!-- References split into informative and normative -->

   <!-- There are 2 ways to insert reference entries from the citation libraries:
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    2. simply use a PI "less than character"?rfc include="reference.RFC.2119.xml"?> here
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      Port Number Registry" <xref target="RFC6335" format="default"/> <xref
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      <table anchor="iana_service_table" align="center">
        <name>IANA Service Type Assignments</name>
        <thead>
          <tr>
            <th align="left">Name</th>
            <th align="center">Port</th>
            <th align="center">Value</th>
            <th align="center">Section</th>
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        </thead>
        <tbody>
          <tr>
            <td align="left">DNS Push Notification Service Type</td>
            <td align="center">None</td>
            <td align="center">
              <tt>_dns&nbhy;push&nbhy;tls._tcp</tt></td>
            <td align="center">
              <xref target="discovery" format="counter"/></td>
          </tr>
        </tbody>
      </table>
      <t>This document defines four new DNS Stateful Operation TLV types,
     which have been recorded in the local
    filing system IANA "DSO Type Codes" registry <xref
     target="RFC8490" format="default"/> <xref target="DSOTYPE"
     format="default"/>.</t>
      <table anchor="iana_tlv_table" align="center">
        <name>IANA DSO TLV Type Code Assignments</name>
        <thead>
          <tr>
            <th align="left">Name</th>
            <th align="center">Value</th>
            <th align="center">Early Data</th>
            <th align="center">Status</th>
            <th align="center">Section</th>
          </tr>
        </thead>
        <tbody>
          <tr>
            <td align="left">SUBSCRIBE</td>
            <td align="center">0x0040</td>
            <td align="center">OK</td>
            <td align="center">Standards Track</td>
            <td align="center">
              <xref target="subscribe" format="counter"/></td>
          </tr>
          <tr>
            <td align="left">PUSH</td>
            <td align="center">0x0041</td>
            <td align="center">NO</td>
            <td align="center">Standards Track</td>
            <td align="center">
              <xref target="push" format="counter"/></td>
          </tr>
          <tr>
            <td align="left">UNSUBSCRIBE</td>
            <td align="center">0x0042</td>
            <td align="center">NO</td>
            <td align="center">Standards Track</td>
            <td align="center">
              <xref target="unsubscribe" format="counter"/></td>
          </tr>
          <tr>
            <td align="left">RECONFIRM</td>
            <td align="center">0x0043</td>
            <td align="center">NO</td>
            <td align="center">Standards Track</td>
            <td align="center">
              <xref target="reconfirm" format="counter"/></td>
          </tr>
        </tbody>
      </table>
      <t>This document defines no new DNS OPCODEs or remote ones accessed by http (http://domain/dir/... ).-->

   <references title="Normative References">
     &RFC0020;
     &RFC0768;
     &RFC0793;
     &RFC1034;
     &RFC1035;
     &RFC1123;
     &RFC2119;
     &RFC2136;
     &RFC2181;
     &RFC2782;
     &RFC6066;
     <?rfc include="reference.RFC.6335" ?>
     &RFC6895;
     &RFC7673;
     &RFC7766;
     &RFC7858;
     &RFC8174;
     &RFC8310;
     &RFC8446;
     &RFC8490; RCODEs.</t>
    </section>

  </middle>

  <back>

<displayreference target="RFC7525" to="BCP195"/>

<displayreference target="I-D.ietf-tcpm-rack" to="TCPRACK"/>

    <references>
      <name>References</name>
      <references>
        <name>Normative References</name>
        <xi:include
	    href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.0020.xml"/>
        <xi:include
	    href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.0768.xml"/>
        <xi:include
	    href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.0793.xml"/>
        <xi:include
	    href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.1034.xml"/>
        <xi:include
	    href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.1035.xml"/>
        <xi:include
	    href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.1123.xml"/>
        <xi:include
	    href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"/>
        <xi:include
	    href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2136.xml"/>
        <xi:include
	    href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2181.xml"/>
        <xi:include
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        <xi:include
	    href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6066.xml"/>
        <xi:include
	    href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6335.xml"/>
        <xi:include
	    href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6895.xml"/>
        <xi:include
	    href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7673.xml"/>
        <xi:include
	    href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7766.xml"/>
        <xi:include
	    href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7858.xml"/>
        <xi:include
	    href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"/>
        <xi:include
	    href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8310.xml"/>
        <xi:include
	    href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8446.xml"/>
        <xi:include
	    href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8490.xml"/>

        <reference anchor="SRVTYPE"
     target="http://www.iana.org/assignments/service-names-port-numbers/">
		   target="https://www.iana.org/assignments/service-names-port-numbers/">
          <front>
            <title>Service Name and Transport Protocol Port Number
	    Registry</title>
         <author/>
         <date/>
            <author><organization>IANA</organization></author>
          </front>
        </reference>

        <reference anchor="DSOTYPE"
		   target="https://www.iana.org/assignments/dns-parameters/">
          <front>
         <title>DSO Type Code Registry</title>
         <author/>
         <date/>
            <title>Domain Name System (DNS) Parameters</title>
            <author><organization>IANA</organization></author>
          </front>
        </reference>

      </references>

      <references>
        <name>Informative References</name>
        <xi:include
	    href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7525.xml"/>
        <xi:include
	    href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2308.xml"/>
        <xi:include
	    href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.3123.xml"/>
        <xi:include
	    href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4287.xml"/>
        <xi:include
	    href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4953.xml"/>
        <xi:include
	    href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6281.xml"/>
        <xi:include
	    href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6762.xml"/>
        <xi:include
	    href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6763.xml"/>
        <xi:include
	    href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6824.xml"/>
        <xi:include
	    href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6886.xml"/>
        <xi:include
	    href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6887.xml"/>
        <xi:include
	    href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7413.xml"/>

        <xi:include
	    href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8499.xml"/>

        <xi:include
	    href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8010.xml"/>
        <xi:include
	    href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8011.xml"/>

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        <xi:include
	    href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml3/reference.I-D.ietf-tcpm-rack.xml"/>

<!-- draft-sekar-dns-llq ; companion document 8764 -->
<?rfc needLines="9" ?>

   <references title="Informative References">
     <reference anchor="BCP195" target="http://www.rfc-editor.org/info/bcp195"><front>
           <title>Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)</title>
           <author initials="Y." surname="Sheffer" fullname="Yaron Sheffer"/>
           <author initials="R." surname="Holz" fullname="Ralph Holz"/>
           <author initials="P." surname="Saint-Andre" fullname="Peter Saint-Andre"/>
           <date year="2015" month="May"/>
         </front><seriesInfo name="BCP" value="195"/><seriesInfo name="RFC" value="7525"/></reference>

     &RFC2308;
     &RFC3123;
     &RFC4287;
     &RFC4953;
     &RFC6281;
     &RFC6762;
     &RFC6763;
     &RFC6824;
     &RFC6886;
     &RFC6887;
     &RFC7413;
     &RFC7719;
     &RFC8010;
     &RFC8011;
     &RFC8499;

     &I-D.ietf-tcpm-rack;

<reference anchor='LLQ'> anchor='RFC8764' target="https://www.rfc-editor.org/info/rfc8764">
  <front>
<title>DNS
    <title>Apple's DNS Long-Lived Queries</title> Queries Protocol</title>

    <author initials='S' surname='Cheshire' fullname='Stuart Cheshire'>
      <organization />
    </author>

    <author initials='M' surname='Krochmal' fullname='Marc Krochmal'>
      <organization />
    </author>

    <date month='March' day='4' year='2019' year='2020' />

<abstract><t>DNS Long-Lived Queries (LLQ) is a protocol for extending the DNS protocol to support change notification, thus allowing clients to learn about changes to DNS data without polling the server.  From 2007 onwards, LLQ was implemented in Apple products including Mac OS X, Bonjour for Windows, and AirPort wireless base stations.  In 2019, the LLQ protocol was superseded by the IETF Standards Track RFC "DNS Push Notifications", which builds on experience gained with the LLQ protocol to create a superior replacement.</t></abstract>
  </front>

  <seriesInfo name='Internet-Draft' value='draft-sekar-dns-llq-03' />
<format type='TXT'
        target='http://www.ietf.org/internet-drafts/draft-sekar-dns-llq-03.txt' name='RFC' value='8764' />
  <seriesInfo name="DOI" value="10.17487/RFC8764"/>
</reference>

<!-- I-D.draft-ietf-dnssd-hybrid-10; companion document RFC 8766 -->

<reference anchor='DisProx'> anchor='RFC8766' target="https://www.rfc-editor.org/info/rfc8766">
<front>
<title>Discovery Proxy for Multicast DNS-Based Service Discovery</title>

<author initials='S' surname='Cheshire' fullname='Stuart Cheshire'>
    <organization />
</author>

<date month='March' day='24' year='2019' year='2020' />

<abstract><t>This document specifies a network proxy that uses Multicast DNS to automatically populate the wide-area unicast Domain Name System namespace with records describing devices and services found on the local link.</t></abstract>
</front>

<seriesInfo name='Internet-Draft' value='draft-ietf-dnssd-hybrid-10' />
<format type='TXT'
        target='http://www.ietf.org/internet-drafts/draft-ietf-dnssd-hybrid-10.txt' /> name="RFC" value="8766"/>
<seriesInfo name="DOI" value="10.17487/RFC8766"/>
</reference>

        <reference anchor='SYN'> anchor="SYN"
		   target="https://www.cisco.com/web/about/ac123/ac147/archived_issues/ipj_9-4/ipj_9-4.pdf">
          <front>
            <title>Defenses Against TCP SYN Flooding Attacks</title>

            <author initials='W.' surname='Eddy' fullname='Wesley Eddy'> initials="W." surname="Eddy" fullname="Wesley Eddy">
              <organization>Verizon Federal Network Systems</organization>
              <address>
                <email>weddy@grc.nasa.gov</email>
              </address>
            </author>
            <date year='2006' month='December' /> year="2006" month="December"/>
            <keyword>TCP</keyword>
          </front>
       <seriesInfo name="The
	    <refcontent>The Internet Protocol Journal," value='Cisco Systems' />
       <seriesInfo name='Volume' value='9' />
       <seriesInfo name='Number' value='4' />
       <format type='PDF' octets='882020' target="http://www.cisco.com/web/about/ac123/ac147/archived_issues/ipj_9-4/ipj_9-4.pdf" />
       <format type='HTML' octets='65566' target="http://www.cisco.com/web/about/ac123/ac147/archived_issues/ipj_9-4/syn_flooding_attacks.html" /> Journal</refcontent>
	      <refcontent>Cisco Systems</refcontent>
	        <refcontent>Volume 9</refcontent>
		  <refcontent>Number 4</refcontent>
        </reference>

        <reference anchor='obs' target="https://en.wikipedia.org/wiki/Observer_pattern"> anchor="OBS"
		      target="https://en.wikipedia.org/w/index.php?title=Observer_pattern&amp;oldid=939702131">
          <front>
            <title>Observer Pattern</title>
          <author/>
          <date/> pattern</title>
            <author>
<organization>Wikipedia
</organization>
	    </author>
            <date month="February" year="2020"/>
          </front>
        </reference>

        <reference anchor='SD-API' anchor="SD-API"
		   target="https://opensource.apple.com/source/mDNSResponder/mDNSResponder-878.70.2/mDNSShared/dns_sd.h.auto.html">
          <front>
         <title>dns_sd.h API</title>
          <author/>
          <date/>
            <title>dns_sd.h</title>
            <author>
<organization>Apple Inc.
</organization>
	    </author>

          </front>
        </reference>

        <reference anchor="XEP0060"> anchor="XEP0060"
		   target="https://xmpp.org/extensions/xep-0060.html">
          <front>
            <title>Publish-Subscribe</title>
            <author initials="P." surname="Millard" fullname="Peter Millard">
              <organization/>
              <address>
                <email/>
              </address>
            </author>
            <author initials="P." surname="Saint-Andre" fullname="Peter
								  Saint-Andre">
              <organization/>
              <address>
                <email>peter@andyet.net</email>
              </address>
            </author>
            <author initials="R." surname="Meijer" fullname="Ralph Meijer">
              <organization/>
              <address>
                <email>ralphm@ik.nu</email>
              </address>
            </author>
            <date day="01" month="July" year="2010"/>  month="October" year="2019"/>
          </front>
       <seriesInfo name="XSF XEP" value="0060"/>
       <format type="HTML" target="http://xmpp.org/extensions/xep-0060.html"/>
<refcontent>XSF XEP 0060
</refcontent>
	</reference>
      </references>
    </references>

    <section anchor="Acknowledgments" numbered="false" toc="default">
      <name>Acknowledgments</name>
      <t>The authors would like to thank <contact fullname="Kiren Sekar"/> and
      <contact fullname="Marc Krochmal"/> for previous work completed in this
      field.</t>
      <t>This document has been improved due to comments from
     <contact fullname="Ran Atkinson"/>,
     <contact fullname="Tim Chown"/>,
     <contact fullname="Sara Dickinson"/>,
     <contact fullname="Mark Delany"/>,
     <contact fullname="Ralph Droms"/>,
     <contact fullname="Jan Komissar"/>,
     <contact fullname="Eric Rescorla"/>,
     <contact fullname="Michael Richardson"/>,
     <contact fullname="David Schinazi"/>,
     <contact fullname="Manju Shankar Rao"/>,
     <contact fullname="Robert Sparks"/>,
     <contact fullname="Markus Stenberg"/>,
     <contact fullname="Andrew Sullivan"/>,
     <contact fullname="Michael Sweet"/>,
     <contact fullname="Dave Thaler"/>,
     <contact fullname="Brian Trammell"/>,
     <contact fullname="Bernie Volz"/>,
     <contact fullname="√Čric Vyncke"/>,
     <contact fullname="Christopher Wood"/>,
     <contact fullname="Liang Xia"/>,
     and
     <contact fullname="Soraia Zlatkovic"/>.
     <contact fullname="Ted Lemon"/> provided clarifying text that was greatly
      appreciated.</t>
    </section>
  </back>
</rfc>