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Capabilities Advertisement with BGP-4
RFC 5492

Document Type RFC - Draft Standard (February 2009)
Updated by RFC 8810
Obsoletes RFC 3392
Authors Ravi Chandra , John Scudder
Last updated 2020-07-29
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
IESG Responsible AD David Ward
Send notices to (None)
RFC 5492
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.

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   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.


   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.

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1.  Introduction

   The base BGP-4 specification [RFC4271] 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",
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Overview of Operations

   When a BGP speaker [RFC4271] 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 [RFC4271] 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.

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   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 [RFC4760].  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

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 [RFC4271].  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.

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      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.

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   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 [RFC5226].  Capability Code values 64
   through 127 are to be assigned by IANA using the "First Come First
   Served" policy defined in [RFC5226].  Capability Code values 128
   through 255 are for "Private Use" as defined in [RFC5226].

   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 [RFC5226].

   The registry has been populated with the two Parameter Type codes
   that are currently defined:

   o  Parameter Type 1: Authentication (deprecated) [RFC4271] [RFC5492]

   o  Parameter Type 2: Capabilities [RFC5492]

7.  Security Considerations

   This extension to BGP does not change the underlying security issues
   inherent in the existing BGP [RFC4272].

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.

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9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
              Protocol 4 (BGP-4)", RFC 4271, January 2006.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

9.2.  Informative References

   [RFC4272]  Murphy, S., "BGP Security Vulnerabilities Analysis",
              RFC 4272, January 2006.

   [RFC4760]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
              "Multiprotocol Extensions for BGP-4", RFC 4760,
              January 2007.

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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


   Ravi Chandra
   Sonoa Systems


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