Network Working Group                                      Ravi Chandra
Internet Draft                                            Cisco Systems
Expiration Date: February 1999                          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.

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   and may be updated, replaced, or obsoleted by other documents at any
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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.

       Chandra, Scudder                                                [Page 1]

       Internet Draft     draft-ietf-idr-bgp4-cap-neg-02.txt        August 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 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
          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.
          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.

       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:

       Chandra, Scudder                                                [Page 2]

       Internet Draft     draft-ietf-idr-bgp4-cap-neg-02.txt        August 1998

             | 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.

          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.

       Chandra, Scudder                                                [Page 3]

       Internet Draft     draft-ietf-idr-bgp4-cap-neg-02.txt        August 1998

       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.

       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]