Extended
Optional Parameters Length for BGP OPEN MessagePalo Alto Networksenchen@paloaltonetworks.comJuniper Networksjgs@juniper.net
General
IDRIDRBGP
The Optional Parameters in the BGP OPEN message as defined in the base
BGP specification are limited to 255 octets due to a one-octet length
field.
BGP capabilities are carried in this field and may foreseeably
exceed 255 octets in the future, leading to concerns about this
limitation.
This document updates RFC 4271 by extending, in a backward-compatible manner, the length of the Optional Parameters in a BGP OPEN message. The Parameter Length field of individual Optional Parameters is also extended.
Introduction
The Optional Parameters Length field in the BGP OPEN message is defined
in the base BGP specification as one
octet, thus limiting the Optional Parameters field in the OPEN message
to 255 octets. Since BGP capabilities are
carried in the Optional Parameters field, and new BGP capabilities
continue to be introduced, the limitation is a concern for BGP
development.
This document updates by extending the length of the Optional Parameters in BGP
OPEN in a
backward-compatible manner. This is done by using Optional Parameter type code 255 as a
distinguished value, which indicates an extended Optional Parameters
Length field follows and that the parsing of the BGP OPEN should be
modified according to these procedures. In this case, the Parameter Length
field of the individual Optional Parameters in the BGP OPEN message is
also extended.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED",
"MAY", and "OPTIONAL" in this document are to be interpreted as
described in BCP 14
when, and only when, they appear in all capitals, as shown here.
Update to RFC 4271
This document reserves Optional Parameter type code 255 as the "Extended
Length".
In the event that the length of the Optional Parameters in the BGP OPEN
message does not exceed 255, the encodings of the
base BGP specificationSHOULD be used without alteration.
Configuration MAY override this to force the extended format to be used
in all cases; this might be used, for example, to test that a peer
supports this specification. (In any case, an implementation MUST accept
an OPEN message that uses the encoding of this specification even if the
length of the Optional Parameters is 255 or less.)
However, if the length of the Optional Parameters in the BGP OPEN message
does exceed 255, the OPEN message MUST be encoded according to the
procedure below.
The Non-Extended Optional Parameters Length field (Non-Ext OP Len.)
SHOULD be set to 255 on transmission and, in any event, MUST NOT be set to
0; it MUST be ignored on receipt once the use of the extended format is
determined positively by inspection of the Non-Extended Optional
Parameters Type (Non-Ext OP Type) field.
The subsequent one-octet field (which would be the first Optional
Parameter Type field in the non-extended format and is called "Non-Ext
OP Type" in the figure above) MUST be set to 255 on transmission. On
receipt, a value of 255 for this field is the indication that the
extended format is in use.
In this extended encoding, the subsequent two-octet field, termed
the "Extended Optional Parameters Length field", is an unsigned
integer indicating the total length of the Optional Parameters field
in octets. If the value of this field is zero, no Optional
Parameters are present.
Likewise, in that situation, the Optional Parameters encoding is modified
to be the following:
The rules for encoding Optional Parameters are unchanged with respect
to those given in , except that the Parameter Length field is extended to be a two-octet unsigned integer.
In parsing an OPEN message, if the one-octet Optional Parameters
Length field (labeled "Non-Ext OP Len." in ) is non-zero, a BGP speaker
MUST use the value of the octet following the one-octet Optional
Parameters Length field (labeled "Non-Ext OP Type" in ) to determine both the encoding of the Optional
Parameters length and the size of the Parameter Length field of
individual Optional Parameters. If the value of the "Non-Ext OP Type"
field is 255, then the encoding described above is used for the Optional
Parameters length. Otherwise, the encoding defined in is used.
Backward Compatibility
If a BGP speaker supporting this specification (a "new speaker") is
peering with one that does not (an "old speaker"), no interoperability
issues arise unless the new speaker needs to encode Optional Parameters
whose length exceeds 255. In that case, it will transmit an OPEN
message that the old speaker will interpret as containing an Optional
Parameter with type code 255. Since the old speaker will
not recognize that type code by definition, the old speaker is expected to close
the connection with a NOTIFICATION with an error code of "OPEN Message
Error" and an error subcode of "Unsupported Optional Parameters", according
to .
Although the Optional Parameter type code 255 is used in this
specification as the indication that the extended encoding is in use, it
is not a bona fide Optional Parameter type code in the usual sense and MUST
NOT be used other than as described above.
If encountered other than as the Non-Ext OP Type, it MUST be
treated as an unrecognized Optional Parameter and handled according to
.
It is not considered an error to receive an OPEN message whose Extended
Optional Parameters Length value is less than or equal to 255. It is not
considered a fatal error to receive an OPEN message whose (non-extended)
Optional Parameters Length value is not 255 and whose first Optional
Parameter type code is 255 -- in this case, the encoding of this
specification MUST be used for decoding the message.
IANA Considerations
IANA has assigned value 255 as the Extended Length type code in the "BGP OPEN Optional
Parameter Types" registry.
Security Considerations
This extension to BGP does not change the underlying security or
confidentiality issues
inherent in the existing BGP .
ReferencesNormative ReferencesInformative ReferencesAcknowledgements
The authors would like to thank and for
discussing various options to enlarge the Optional Parameters field. We
would also like to thank , , ,
, , , , and for their
valuable comments.