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IETF RFC 5492
Capabilities Advertisement with BGP-4
Last modified on Wednesday, February 25th, 2009
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Network Working Group J. Scudder
Request for Comments: 5492 Juniper Networks
Obsoletes: 3392 R. Chandra
Category: Standards Track Sonoa Systems
February 2009
Capabilities Advertisement with BGP-4
 Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Please review these documents carefully, as they describe your rights
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Without obtaining an adequate license from the person(s) controlling
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than English.
Abstract
This document defines an Optional Parameter, called Capabilities,
that is expected to facilitate the introduction of new capabilities
in the Border Gateway Protocol (BGP) by providing graceful capability
advertisement without requiring that BGP peering be terminated.
This document obsoletes RFC 3392.
Scudder & Chandra Standards Track PAGE 1 
RFC 5492 Capabilities Advertisement February 2009
1. Introduction
The base BGP-4 specification [RFC 4271] requires that when a BGP
speaker receives an OPEN message with one or more unrecognized
Optional Parameters, the speaker must terminate the BGP peering.
This complicates the introduction of new capabilities in BGP.
This specification defines an Optional Parameter and processing rules
that allow BGP speakers to communicate capabilities in an OPEN
message. A pair of BGP speakers that supports this specification can
establish the peering even when presented with unrecognized
capabilities, so long as all capabilities required to support the
peering are supported.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC 2119].
3. Overview of Operations
When a BGP speaker [RFC 4271] that supports capabilities advertisement
sends an OPEN message to its BGP peer, the message MAY include an
Optional Parameter, called Capabilities. The parameter lists the
capabilities supported by the speaker.
A BGP speaker determines the capabilities supported by its peer by
examining the list of capabilities present in the Capabilities
Optional Parameter carried by the OPEN message that the speaker
receives from the peer.
A BGP speaker that supports a particular capability may use this
capability with its peer after the speaker determines (as described
above) that the peer supports this capability. Simply put, a given
capability can be used on a peering if that capability has been
advertised by both peers. If either peer has not advertised it, the
capability cannot be used.
A BGP speaker determines that its peer doesn't support capabilities
advertisement if, in response to an OPEN message that carries the
Capabilities Optional Parameter, the speaker receives a NOTIFICATION
message with the Error Subcode set to Unsupported Optional Parameter.
(This is a consequence of the base BGP-4 specification [RFC 4271] and
not a new requirement.) In this case, the speaker SHOULD attempt to
re-establish a BGP connection with the peer without sending to the
peer the Capabilities Optional Parameter.
Scudder & Chandra Standards Track PAGE 2
RFC 5492 Capabilities Advertisement February 2009
If a BGP speaker that supports a certain capability determines that
its peer doesn't support this capability, the speaker MAY send a
NOTIFICATION message to the peer and terminate peering (see Section
"Extensions to Error Handling" for more details). For example, a BGP
speaker may need to terminate peering if it established peering to
exchange IPv6 routes and determines that its peer does not support
Multiprotocol Extensions for BGP-4 [RFC 4760]. The Error Subcode in
the NOTIFICATION message is then set to Unsupported Capability. The
message MUST contain the capability or capabilities that cause the
speaker to send the message. The decision to send the message and
terminate the peering is local to the speaker. If terminated, such
peering SHOULD NOT be re-established automatically.
If a BGP speaker receives from its peer a capability that it does not
itself support or recognize, it MUST ignore that capability. In
particular, the Unsupported Capability NOTIFICATION message MUST NOT
be generated and the BGP session MUST NOT be terminated in response
to reception of a capability that is not supported by the local
speaker.
4. Capabilities Optional Parameter (Parameter Type 2):
This is an Optional Parameter that is used by a BGP speaker to convey
to its BGP peer the list of capabilities supported by the speaker.
The encoding of BGP Optional Parameters is specified in Section 4.2
of [RFC 4271]. The parameter type of the Capabilities Optional
Parameter is 2.
The parameter contains one or more triples <Capability Code,
Capability Length, Capability Value>, where each triple is encoded as
shown below:
+------------------------------+
| Capability Code (1 octet) |
+------------------------------+
| Capability Length (1 octet) |
+------------------------------+
| Capability Value (variable) |
~ ~
+------------------------------+
The use and meaning of these fields are as follows:
Capability Code:
Capability Code is a one-octet unsigned binary integer that
unambiguously identifies individual capabilities.
Scudder & Chandra Standards Track PAGE 3
RFC 5492 Capabilities Advertisement February 2009
Capability Length:
Capability Length is a one-octet unsigned binary integer that
contains the length of the Capability Value field in octets.
Capability Value:
Capability Value is a variable-length field that is interpreted
according to the value of the Capability Code field.
BGP speakers SHOULD NOT include more than one instance of a
capability with the same Capability Code, Capability Length, and
Capability Value. Note, however, that processing of multiple
instances of such capability does not require special handling, as
additional instances do not change the meaning of the announced
capability; thus, a BGP speaker MUST be prepared to accept such
multiple instances.
BGP speakers MAY include more than one instance of a capability (as
identified by the Capability Code) with non-zero Capability Length
field, but with different Capability Value and either the same or
different Capability Length. Processing of these capability
instances is specific to the Capability Code and MUST be described in
the document introducing the new capability.
The Capabilities Optional Parameter (OPEN Optional Parameter Type 2)
SHOULD only be included in the OPEN message once. If the BGP speaker
wishes to include multiple capabilities in the OPEN message, it
SHOULD do so as discussed above -- by listing all those capabilities
as TLVs within a single Capabilities Optional Parameter. However,
for backward compatibility, a BGP speaker MUST be prepared to receive
an OPEN message that contains multiple Capabilities Optional
Parameters, each of which contains one or more capabilities TLVs.
The set of capabilities should be processed in the same way in either
case, whether it is enumerated within a single Capabilities Optional
Parameter of the OPEN message or split across multiple Capabilities
Optional Parameters.
5. Extensions to Error Handling
This document defines a new Error Subcode, Unsupported Capability.
The value of this Subcode is 7. The Data field in the NOTIFICATION
message MUST list the set of capabilities that causes the speaker to
send the message. Each such capability is encoded in the same way as
it would be encoded in the OPEN message.
Scudder & Chandra Standards Track PAGE 4
RFC 5492 Capabilities Advertisement February 2009
As explained in the "Overview of Operations" section, the Unsupported
Capability NOTIFICATION is a way for a BGP speaker to complain that
its peer does not support a required capability without which the
peering cannot proceed. It MUST NOT be used when a BGP speaker
receives a capability that it does not understand; such capabilities
MUST be ignored.
6. IANA Considerations
This document defines a Capability Optional Parameter along with a
Capability Code field. IANA maintains the registry for Capability
Code values. Capability Code value 0 is reserved. Capability Code
values 1 through 63 are to be assigned by IANA using the "IETF
Review" policy defined in [RFC 5226]. Capability Code values 64
through 127 are to be assigned by IANA using the "First Come First
Served" policy defined in [RFC 5226]. Capability Code values 128
through 255 are for "Private Use" as defined in [RFC 5226].
IANA created and maintains a registry for OPEN message Optional
Parameters called "BGP OPEN Optional Parameter Types". Optional
Parameters are identified by the Parameter Type, which is a one-octet
unsigned integer. Values (0 reserved, 1-255) are to be allocated
according to the "IETF Review" policy as defined in [RFC 5226].
The registry has been populated with the two Parameter Type codes
that are currently defined:
o Parameter Type 1: Authentication (deprecated) [RFC 4271] [RFC 5492]
o Parameter Type 2: Capabilities [RFC 5492]
7. Security Considerations
This extension to BGP does not change the underlying security issues
inherent in the existing BGP [RFC 4272].
8. Acknowledgments
The authors would like to thank members of the IDR Working Group and
the IESG and its Directorates for their review and comments.
Scudder & Chandra Standards Track PAGE 5
RFC 5492 Capabilities Advertisement February 2009
9. References
9.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
[RFC 5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
9.2. Informative References
[RFC 4272] Murphy, S., "BGP Security Vulnerabilities Analysis",
RFC 4272, January 2006.
[RFC 4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760,
January 2007.
Scudder & Chandra Standards Track PAGE 6
RFC 5492 Capabilities Advertisement February 2009
Appendix A. Comparison between RFC 2842 and RFC 3392
In addition to several minor editorial changes, RFC 3392 also
clarified how to handle multiple instances of the same capability.
Appendix B. Comparison between RFC 3392 and This Document
This document makes minor editorial changes and updated references,
clarifies the use of the Unsupported Optional Parameter NOTIFICATION
message, clarifies behavior when the Capabilities Parameter is
included in the OPEN message multiple times, and clarifies
requirements by changing a number of SHOULDs to MUSTs.
Authors' Addresses
John G. Scudder
Juniper Networks
EMail: jgs@juniper.net
Ravi Chandra
Sonoa Systems
EMail: rchandra@sonoasystems.com
Scudder & Chandra Standards Track PAGE 7
Capabilities Advertisement with BGP-4
RFC TOTAL SIZE: 13581 bytes
PUBLICATION DATE: Wednesday, February 25th, 2009
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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