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

3. Overview of Operations

   When a BGP speaker that supports capabilities negotiation sends an
   OPEN message to its BGP peer, the message includes may include an Optional
   Parameter, called Capabilities. The parameter lists the capabilities
   supported by the speaker. The speaker can mark a listed capability as
   "Required", which means that if the peer doesn't recognize/support
   the capability, the BGP peering shall be terminated.

   When the peer receives the OPEN message, if the message contains the
   Capabilities Optional Parameter, the peer checks whether it supports
   all of the listed capabilities marked as R, and if not, sends a
   NOTIFICATION message, and terminates peering. The Error Subcode in
   the message is set to Unsupported Capability. The message should
   contain all the capabilities marked as R that are not supported by
   the peer.  If the peer doesn't support a capability that is not
   marked as R, the peer should not use this as a reason to terminate

   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 peer sends to speaker
   receives from the speaker. 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)  |
   The use and meaning of these fields are as follows:

      Capability Code:

         Capability Code is a one octet field that unambiguously
         identifies individual capabilities.

         The high-order bit of this field is used to mark the capability
         as "Required" (if the bit is set to 1).

      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 are marked as Required,
   but are either unsupported or unrecognized by cause the BGP speaker that
   sends to send
   the message.  Each such capability is encoded the same way as it was
   encoded in the received OPEN message.

6. Security Considerations

   Security issues are

   This extension to BGP does not discussed in this document. 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