Network Working Group                                        R. Chandra
Request for Comments: 2842                        Redback Networks Inc.
Category: Standards Track                                    J. Scudder
                                                          cisco Systems
                                                               May 2000


                 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) The Internet Society (2000).  All Rights Reserved.

Abstract

   Currently BGP-4 [BGP-4] requires that when a BGP speaker receives an
   OPEN message with one or more unrecognized Optional Parameters, the
   speaker must terminate BGP peering. This complicates introduction of
   new capabilities in BGP.

   This document defines new Optional Parameter, called Capabilities,
   that is expected to facilitate introduction of new capabilities in
   BGP by providing graceful capability advertisement without requiring
   that BGP peering be terminated.

1. Overview of Operations

   When a BGP speaker 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.

   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.
   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 determines that
   its peer doesn't support this capability, the speaker may send a
   NOTIFICATION message to the peer, and terminate peering. 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.  Such peering should not be re-
   established automatically.

2. 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.

   A particular capability, as identified by its Capability Code, may
   occur more than once within the Optional Parameter.

3. Extensions to Error Handling

   This document defines new Error Subcode - Unsupported Capability.
   The value of this Subcode is 7. The Data field in the NOTIFICATION
   message lists the set of capabilities that cause the speaker to send
   the message.  Each such capability is encoded the same way as it was
   encoded in the received OPEN message.

4. IANA Considerations

   Section 4 defines a Capability Optional Parameter along with an
   Capability Code field. IANA is expected to create and maintain 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 Consensus" policy defined in RFC2434. Capability
   Code values 64 through 127 are to be assigned by IANA, using the
   "First  Come First Served" policy defined in RFC2434. Capability Code
   values 128 through 255 are for "Private Use" as defined in RFC2434.

5. Security Considerations

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

6. Acknowledgements

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

7. References

   [BGP-4]      Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
                (BGP-4)", RFC 1771, March 1995.

   [Heffernan]  Heffernan, A., "Protection of BGP Sessions via the TCP
                MD5 Signature Option", RFC 2385, August 1998.

8. Authors' Addresses

   Ravi Chandra
   Redback Networks Inc.
   350, Holger Way
   San Jose, CA 95134

   EMail: rchandra@redback.com


   John G. Scudder
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA 95134

   EMail: jgs@cisco.com

9. Full Copyright Statement

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Acknowledgement

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