Network Working Group                                      Ravi Chandra
Internet Draft                                            Cisco Systems
Expiration Date: October 1998                           John G. Scudder
                                        Internet Engineering Group, LLC

                  Capabilities Negotiation with BGP-4


1. Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as ``work in progress.''

   To view the entire list of current Internet-Drafts, please check the
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2. 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 negotiation without requiring
   that BGP peering be terminated.

   The proposed parameter is backward compatible - a router that
   supports the parameter can maintain BGP peering with a router that
   doesn't support the parameter.

Chandra, Scudder                                                [Page 1]

Internet Draft     draft-ietf-idr-bgp4-cap-neg-01.txt         April 1998

3. Overview of Operations

   When a BGP speaker that supports capabilities negotiation 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 may use a particular capability when peering with
   another speaker only if both speakers support that capability.  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 determines that its peer doesn't support capabilities
   negotiation, 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.

   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 is local to
   the 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)  |

Chandra, Scudder                                                [Page 2]

Internet Draft     draft-ietf-idr-bgp4-cap-neg-01.txt         April 1998

   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.

   This document reserves Capability Codes 128-255 for vendor-specific

   This document reserves value 0.

   Capability Codes (other than those reserved for vendor specific use)
   are assigned only by the IETF consensus process and IESG approval.

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 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.

Chandra, Scudder                                                [Page 3]

Internet Draft     draft-ietf-idr-bgp4-cap-neg-01.txt         April 1998

6. Security Considerations

   This extension to BGP does not change the underlying security issues.

7. Acknowledgements

   To be supplied.

8. References

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

9. Author Information

   Ravi Chandra
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA 95134

   John G. Scudder
   Internet Engineering Group, LLC
   122 S. Main, Suite 280
   Ann Arbor, MI 48104

Chandra, Scudder                                                [Page 4]