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Network Working Group                                    Ruchi Kapoor
Internet Draft                                         Gargi Nalawade
Expires: December 2003                                Chandra Appanna

                                                        Cisco Systems

                     BGPv4 SAFI-Specific Attribute


1. Status of this Memo

      This document is an Internet-Draft and is in full conformance with
      all provisions of Section 10 of RFC2026.

      Internet-Drafts are working documents of the Internet Engineering
      Task Force (IETF), its areas, and its working groups.  Note that
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      Internet-Drafts are draft documents valid for a maximum of six
      months and may be updated, replaced, or obsoleted by other
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      as reference material or to cite them other than as "work in

      The list of current Internet-Drafts can be accessed at

      The list of Internet-Draft Shadow Directories can be accessed at

   2. Abstract

      This document defines a new BGP attribute called the "SAFI
      [IANA-SAFI] Specific Attribute" (SSA). This attribute consists
      of a set of TLVs (it MUST contain atleast one TLV; it MAY contain
      more than one TLV). Each Subsequent Address Family Identifier
      (SAFI) that uses this attribute MUST define the Typecodes that
      are valid for the respective SAFI.

   3. Introduction

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      This document defines a new BGP attribute called the "SAFI
      Specific Attribute" (SSA). The value part of the SSA consists
      of a set of TLVs (it MUST contain atleast one TLV; it MAY contain
      more than one TLVs). The intent behind SSA is to introduce an
      attribute, which is flexible enough so that it has utility for
      multiple applications.

      The SSA provides a general way of encoding application (AFI/SAFI)
      specific data. AFI/SAFIs may encode data in the form of TLVs. The
      AFI/SAFI specific data does not necessarily have global meaning
      across all AFI/SAFIs, i.e. some typecodes of the TLVs could be
      relevant to only one AFI/SAFI. Each AFI/SAFI MUST specify the
      Typecodes that are valid for the respective AFI/SAFI.

      The SSA TLV is a variable length entity.

   4. Format of the SAFI-Specific Attribute

      The format of the BGP SSA attribute will be as follows:

      |  Attr. Flags  | Attr.Type Code|
                  0                   1
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
      |1|1|0|?|UNUSED | Type = TBD    | Length (2 Octets)             |
      |                                                               |
      |                      Attribute Value                          |
      |                                                               |

   The BGP SSA is a variable length, optional transitive attribute. The
   Value field of the attribute may contain one or more SSA TLVs. The
   Value field of the SSA Attribute is encoded as follows and may
   contain one or more tuples of the following:

      - Type Field  : 2 octets
      - Length Field: 2 octets
      - Value Field : Variable

       0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

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   |       Type                    |       Length                  |
   |                         Value                                 |

   Type Field: The format of the Type Field is as shown below:

             0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
            |T|                             |

            T - Transitive bit

            Value 1: The SSA type is transitive across ASes

            Value 0: The SSA type is non-transitive across ASes

            Remaining 15 bits: Indicate the type of the SSA TLV.

   Length Field: The Length is 2 octets long and indicates the length
   of the Value field.

   The value contained in this Length field MUST not exceed the total
   length of the BGP SSA Attribute minus the total length of any prior

   Value Field: The Value Field contains a TLV where the Types of the
   TLV are defined by the respective SAFIs.  The TLV in the Value field
   of this Attribute will be of the form :

      - Type Field  : 2 octets
      - Length Field: 2 octets
      - Value Field : Variable

   The Value field will have a fixed part as specified by the
   respective SAFI-Specific Types and may have a variable part in the
   form of Sub-TLVs encoded as follows :

      - Sub-Type Field  : 1 octets
      - Length Field: 1 octets
      - Value Field : Variable

5.  Operation

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   A BGP speaker that supports a SAFI, MUST accept any SSA TLVs that
   are defined as valid for the SAFI. If the SSA TLV is transitive,
   the BGP speaker MUST forward it even if it does not understand it.
   Operation rules are governed by those for the SAFI. This attribute
   only requires that it be forwarded even when it is not understood.

   When sending an UPDATE message for this SAFI to a peer, a BGP speaker
   MUST only advertise the SSA TLVs that are defined and are valid for
   that SAFI.

   A Capability has not been defined for this attribute intentionally.
   For newer SAFIs, it is concieved that presence of the SAFI would
   imply understanding the SSA attribute and the SSA TLVs. If existing
   SAFIs make use of this Attribute, a capability may be defined
   depending on the operational rules defined when the valid Typecodes
   for this attribute are defined for that SAFI.

6. Security Considerations

   This extension to BGP does not change the underlying security

7. Acknowledgements

   We would like to thank Dan Tappan, Arjun Sreekantiah, Jim Guichard,
   Shyam Suri and John Scudder for their comments and suggestions.

8. Normative References

   [BGP-4]  Rekhter, Y. and T. Li (editors), "A Border Gateway Protocol
   4 (BGP-4)", Internet Draft draft-ietf-idr-bgp4-17.txt, January 2002.

   [BGP-CAP] Chandra, R., Scudder, J., "Capabilities Advertisement with
   BGP-4", draft-ietf-idr-rfc2842bis-02.txt, April 2002.

   [IANA-AFI] http://www.iana.org/assignments/address-family-numbers.

   [IANA-SAFI] http://www.iana.org/assignments/safi-namespace.

9. Authors' Addresses

      Ruchi Kapoor
      Cisco Systems, Inc
      170 West Tasman Drive

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      San Jose, CA 95134

      Gargi Nalawade
      Cisco Systems, Inc
      170 West Tasman Drive
      San Jose, CA 95134

      Chandra Appanna
      Cisco Systems, Inc
      170 West Tasman Drive
      San Jose, CA 95134
      Email: achandra@cisco.com

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12. Expiration Date

   This memo is filed as <draft-kapoor-nalawade-idr-bgp-ssa-00.txt>, and
   expires Dec, 2003.

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