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Versions: 00 01 02 03 04 05 rfc5492                                     
Internet Engineering Task Force                               J. Scudder
Internet-Draft                                          Juniper Networks
Intended status: Standards Track                              R. Chandra
Expires: May 22, 2009                                      Sonoa Systems
                                                       November 18, 2008

                 Capabilities Advertisement with BGP-4

Status of this Memo

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   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.

1.  Introduction

   The base BGP-4 specification [RFC4271] requires that when a BGP
   speaker receives an OPEN message with one or more unrecognized

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   Optional Parameters, the speaker must terminate BGP peering.  This
   complicates introduction of new capabilities in BGP.

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.

   If a BGP speaker that supports a certain capability requires that
   this capability be used on a peering but 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).  The Error Subcode in the message
   is set to Unsupported Capability.  The message SHOULD contain the
   capability (capabilities) that causes the speaker to send the
   message.  The decision to send the message and terminate peering is
   local to the speaker.  If terminated, such peering SHOULD NOT be re-

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   established automatically.  An example of when this procedure might
   be followed is if a BGP speaker is attempting to establish an IPv6
   peering but determines that its peer does not support Multiprotocol
   Extensions for BGP-4 [RFC4760].

   If a BGP speaker receives from its peer a capability which it does
   not itself support or recognize, it SHOULD ignore that capability.
   In particular, the Unsupported Capability NOTIFICATION message MUST
   NOT be generated in response to reception of a capability which 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 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 field that unambiguously
         identifies individual capabilities.

      Capability Length:

         Capability Length is a one octet field 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

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   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

   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 which contains multiple Capabilities Optional
   Parameters, each of which contains one or more capabilities TLVs.
   The set of capabilities should be processed the same in either case,
   whether it is enumerated within a single Capabilities Optional
   Parameter of the OPEN message, or split across multiple.

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 SHOULD list the set of capabilities that cause the speaker to
   send the message.  Each such capability is encoded the same way as it
   would be encoded in the OPEN message.

   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 which it does not understand; such capabilities
   SHOULD 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

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   Consensus" 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].

7.  Security Considerations

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

8.  Acknowledgements

   The authors would like to thank members of the IDR Working Group for
   their review and comments.

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.

Appendix A.  Comparison with RFC 2842

   In addition to several minor editorial changes, this document also
   clarifies how to handle multiple instances of the same capability.

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Appendix B.  Comparison with RFC 3392

   In addition to minor editorial changes and updated references, this
   document also clarifies the use of the Unsupported Optional Parameter
   NOTIFICATION message and clarifies behavior when the Capabilities
   parameter is included in the OPEN message multiple times.

Authors' Addresses

   John G. Scudder
   Juniper Networks

   Email: jgs@juniper.net

   Ravi Chandra
   Sonoa Systems

   Email: rchandra@sonoasystems.com

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