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
  Provisions Relating to IETF Documents in effect on the date of
  publication of this document (http://trustee.ietf.org/license-info).
  Please review these documents carefully, as they describe your rights
  and restrictions with respect to this document.

  This document may contain material from IETF Documents or IETF
  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.

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.



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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",
  "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
  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
  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 [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

  EMail: [email protected]


  Ravi Chandra
  Sonoa Systems

  EMail: [email protected]


























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