





Network Working Group                                         R. Chandra
Request for Comments: 3392                              Redback Networks
Obsoletes: 2842                                               J. Scudder
Category: Standards Track                                  Cisco Systems
                                                           November 2002


                 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 (2002).  All Rights Reserved.

Abstract

   This document defines a new 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 2842.

1. Introduction

   Currently 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.

2. Specification of Requirements

   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].









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3. Overview of Operations

   When a BGP speaker [BGP-4] 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 (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-established automatically.


















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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
   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 announced
   capability.

   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.




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5. 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 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.

6. IANA Considerations

   This document defines a Capability Optional Parameter along with an
   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
   Consensus" policy defined in RFC 2434.  Capability Code values 64
   through 127 are to be assigned by IANA, using the "First Come First
   Served" policy defined in RFC 2434.  Capability Code values 128
   through 255 are for "Private Use" as defined in RFC 2434.

7. Security Considerations

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

8. Acknowledgements

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

9. Comparison with RFC 2842

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

10. 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.

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







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11. 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



































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12.  Full Copyright Statement

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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