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Network Working Group                                          D. Walton
Internet Draft                                                 A. Retana
Expiration Date: January 2009                                    E. Chen
                                                           Cisco Systems
                                                              J. Scudder
                                                        Juniper Networks


                 Advertisement of Multiple Paths in BGP

                   draft-walton-bgp-add-paths-06.txt


Status of this Memo

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Abstract

   In this document we propose a BGP extension that allows the
   advertisement of multiple paths for the same address prefix without
   the new paths implicitly replacing any previous ones.  The essence of
   the extension is that each path is identified by a path identifier in
   addition to the address prefix.








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1. Introduction

   The BGP specification [RFC4271] defines an "Update-Send Process" to
   advertise the routes chosen by the Decision Process to other BGP
   speakers.  No provisions are made to allow the advertisement of
   multiple paths for the same address prefix, or Network Layer
   Reachability Information (NLRI).  In fact, a route with the same NLRI
   as a previously advertised route implicitly replaces the previous
   advertisement.

   In this document we propose a BGP extension that allows the
   advertisement of multiple paths for the same address prefix without
   the new paths implicitly replacing any previous ones.  The essence of
   the extension is that each path is identified by a path identifier in
   addition to the address prefix.


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 [RFC2119].


3. How to Identify a Path

   As defined in [RFC4271], a path refers to the information reported in
   the path attribute field of an UPDATE message.  As the procedures
   specified in [RFC4271] allow only the advertisement of one path for a
   particular address prefix, a path for an address prefix from a BGP
   peer can be keyed on the address prefix.

   In order for a BGP speaker to advertise multiple paths for the same
   address prefix, a new identifier (termed "Path Identifier" hereafter)
   needs to be introduced so that a particular path for an address
   prefix can be identified by the combination of the address prefix and
   the Path Identifier.

   The assignment of the Path Identifier for a path by a BGP speaker is
   purely a local matter.  However, the Path Identifier MUST be assigned
   in such a way that the BGP speaker is able to use the (prefix, path
   identifier) to uniquely identify a path advertised to a neighbor.  A
   BGP speaker that re-advertises a route MUST generate its own Path
   Identifier to be associated with the re-advertised route.  A BGP
   speaker that receives a route SHOULD NOT assume that the identifier
   carries any particular semantics; it SHOULD be treated as an opaque
   value.




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4. Extended NLRI Encodings

   In order to carry the Path Identifier in an UPDATE message, the
   existing NLRI encodings are extended by prepending the Path
   Identifier field, which is of four-octets.

   For example, the NLRI encodings specified in [RFC4271, RFC4760] are
   extended as the following:


               +--------------------------------+
               | Path Identifier (4 octets)     |
               +--------------------------------+
               | Length (1 octet)               |
               +--------------------------------+
               | Prefix (variable)              |
               +--------------------------------+


   and the NLRI encoding specified in [RFC3107] is extended as the
   following:


               +--------------------------------+
               | Path Identifier (4 octets)     |
               +--------------------------------+
               | Length (1 octet)               |
               +--------------------------------+
               | Label (3 octets)               |
               +--------------------------------+
               | ...                            |
               +--------------------------------+
               | Prefix (variable)              |
               +--------------------------------+


   The usage of the extended NLRI encodings is specified in the
   Operation section.













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5. ADD-PATH Capability

   The ADD-PATH Capability is a new BGP capability [RFC3392].  The
   Capability Code for this capability is specified in the IANA
   Considerations section of this document. The Capability Length field
   of this capability is variable. The Capability Value field consists
   of one or more of the following tuples:


             +------------------------------------------------+
             | Address Family Identifier (2 octets)           |
             +------------------------------------------------+
             | Subsequent Address Family Identifier (1 octet) |
             +------------------------------------------------+
             | Send/Receive (1 octet)                         |
             +------------------------------------------------+


   The meaning and use of the fields are as follows:

       Address Family Identifier (AFI):

          This field is the same as the one used in [RFC4760].

       Subsequent Address Family Identifier (SAFI):

          This field is the same as the one used in [RFC4760].

       Send/Receive:

          This field indicates whether the sender is (a) willing to
          receive multiple paths from its peer (value 1), (b) would
          like to send multiple paths to its peer (value 2), or (c)
          both (value 3) for the <AFI, SAFI>.


6. Operation

   The Path Identifier specified in the previous section can be used to
   advertise multiple paths for the same address prefix without
   subsequent advertisements replacing the previous ones.  Apart from
   the fact that this is now possible, the route advertisement rules of
   [RFC4271] are not changed.  In particular, a new advertisement for a
   given address prefix and a given path identifier replaces a previous
   advertisement for the given address prefix and the given path
   identifier.

   A BGP speaker that is willing to receive multiple paths from its



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   peer, or would like to send multiple paths to its peer, SHOULD
   advertise the ADD-PATH Capability to the peer using BGP Capabilities
   advertisement [RFC3392].

   A BGP speaker MUST follow the existing procedures in generating an
   UPDATE message for a particular <AFI, SAFI> to a peer unless the BGP
   speaker advertises the ADD-PATH Capability to the peer indicating its
   desire to send multiple paths for the <AFI, SAFI>, and also receives
   the ADD-PATH Capability from the peer indicating its willingness to
   receive multiple paths for the <AFI, SAFI>, in which case the speaker
   MUST generate a route update for the <AFI, SAFI> based on the
   combination of the address prefix and the Path Identifier, and use
   the extended NLRI encodings specified in this document.  The peer
   SHALL act accordingly in processing an UPDATE message related to a
   particular <AFI, SAFI>.


7. Applications

   The BGP extension specified in this document can be used by a BGP
   speaker to advertise multiple paths in certain applications.  The
   availability of the additional paths can help reduce or eliminate
   persistent route oscillations [RFC3345].  It can also help with
   optimal routing and routing convergence in a network.  The
   applications are detailed in separate documents.


8. Deployment Considerations

   The extension proposed in this document provides a mechanism for a
   BGP speaker to advertise multiple paths over a BGP session.  Care
   needs to be taken in its deployment to ensure consistent routing and
   forwarding in a network, the details of which will be described in
   separate application documents.


9. IANA Considerations

   IANA needs to assign a capability number for the ADD-PATH Capability
   described in this document.











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10. Security Considerations

   This document introduces no new security concerns to BGP or other
   specifications referenced in this document.


11. Acknowledgments

   We would like to thank David Cook and Naiming Shen for their
   contributions to the design and development of the extension, and for
   co-authoring drafts that lead to the current document.

   Many people have made valuable comments and suggestions, including
   Dave Meyer, Srihari Sangli, Eric Rosen, Dan Tappan, Robert Raszuk,
   Mark Turner, Danny McPherson, Eugene Kim, Pradosh Mohapatra, Rex
   Fernando, and Keyur Patel.


12. Normative References

   [RFC4271] Rekhter, Y., T. Li, and S. Hares, "A Border Gateway
   Protocol 4 (BGP-4)," RFC 4271, January 2006.

   [RFC3392] Chandra, R. and J. Scudder, "Capabilities Advertisement
   with BGP-4," RFC 3392, November 2002.

   [RFC4760] Bates, T., Chandra, R., Rekhter, Y., and D. Katz,
   "Multiprotocol Extensions for BGP-4", RFC 4760, January 2007.

   [RFC3107] Rekhter, R. and E. Rosen, "Carrying Label Information in
   BGP-4," RFC 3107, May 2001.

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


13. Informative References

   [RFC3345] McPherson, D., V. Gill, D. Walton, and A. Retana, "Border
   Gateway Protocol (BGP) Persistent Route Oscillation Condition", RFC
   3345, August 2002.










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14. Authors' Addresses

   Daniel Walton
   Cisco Systems, Inc.
   7025 Kit Creek Rd.
   Research Triangle Park, NC 27709

   Email: dwalton@cisco.com


   Alvaro Retana
   Cisco Systems, Inc.
   7025 Kit Creek Rd.
   Research Triangle Park, NC 27709

   Email: aretana@cisco.com


   Enke Chen
   Cisco Systems, Inc.
   170 W. Tasman Dr.
   San Jose, CA 95134

   Email: enkechen@cisco.com


   John Scudder
   Juniper Networks

   Email: jgs@juniper.net


15. Intellectual Property Considerations

   This section is taken from Section 5 of RFC 3668.

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
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   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of



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   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at ietf-
   ipr@ietf.org.


16. Copyright Notice

   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
   THE INTERNET ENGINEERING TASK FORCE DISCLAIM 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.

























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