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<meta name="DC.Description.Abstract" content="In this document we propose a BGP extension that allows the" />
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Network Working Group                                          D. Walton
Internet Draft                                                   E. Chen
Intended Status: Standards Track                           Cisco Systems
Expiration Date: December 18, 2012                             A. Retana
                                                     Hewlett-Packard Co.
                                                              J. Scudder
                                                        Juniper Networks
                                                           June 17, 2012


                 <span class="h1">Advertisement of Multiple Paths in BGP</span>

                    <a href="./draft-ietf-idr-add-paths-07.txt">draft-ietf-idr-add-paths-07.txt</a>


Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of <a href="./bcp78">BCP 78</a> and <a href="./bcp79">BCP 79</a>.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   This Internet-Draft will expire on December 18, 2012.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors. All rights reserved.

   This document is subject to <a href="./bcp78">BCP 78</a> and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) in effect on the date of
   publication of this document. Please review these documents
   carefully, as they describe your rights and restrictions with respect



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   to this document. Code Components extracted from this document must
   include Simplified BSD License text as described in <a href="#section-4">Section 4</a>.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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.


<span class="h2"><a class="selflink" name="section-1" href="#section-1">1</a>. Introduction</span>

   The BGP specification [<a href="./rfc4271" title="&quot;A Border Gateway Protocol 4 (BGP-4),&quot;">RFC4271</a>] 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.


<span class="h3"><a class="selflink" name="section-1.1" href="#section-1.1">1.1</a>. Specification of Requirements</span>

   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 [<a href="./rfc2119" title="&quot;Key words for use in RFCs to Indicate Requirement Levels,&quot;">RFC2119</a>].















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<span class="h2"><a class="selflink" name="section-2" href="#section-2">2</a>. How to Identify a Path</span>

   As defined in [<a href="./rfc4271" title="&quot;A Border Gateway Protocol 4 (BGP-4),&quot;">RFC4271</a>], a path refers to the information reported in
   the path attribute field of an UPDATE message.  As the procedures
   specified in [<a href="./rfc4271" title="&quot;A Border Gateway Protocol 4 (BGP-4),&quot;">RFC4271</a>] 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.


<span class="h2"><a class="selflink" name="section-3" href="#section-3">3</a>. Extended NLRI Encodings</span>

   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, <a href="./rfc4760">RFC4760</a>] are
   extended as the following:


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


   and the NLRI encoding specified in [<a href="./rfc3107" title="&quot;Carrying Label Information in BGP-4,&quot;">RFC3107</a>] is extended as the
   following:





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               +--------------------------------+
               | 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.


<span class="h2"><a class="selflink" name="section-4" href="#section-4">4</a>. ADD-PATH Capability</span>

   The ADD-PATH Capability is a new BGP capability [<a href="./rfc5492" title="&quot;Capabilities Advertisement with BGP-4&quot;">RFC5492</a>].  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 [<a href="./rfc4760" title="&quot;Multiprotocol Extensions for BGP-4&quot;">RFC4760</a>].

       Subsequent Address Family Identifier (SAFI):

          This field is the same as the one used in [<a href="./rfc4760" title="&quot;Multiprotocol Extensions for BGP-4&quot;">RFC4760</a>].

       Send/Receive:

          This field indicates whether the sender is (a) willing to



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          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 &lt;AFI, SAFI&gt;.


<span class="h2"><a class="selflink" name="section-5" href="#section-5">5</a>. Operation</span>

   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
   [<a href="./rfc4271" title="&quot;A Border Gateway Protocol 4 (BGP-4),&quot;">RFC4271</a>] 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
   peer, or would like to send multiple paths to its peer, SHOULD
   advertise the ADD-PATH Capability to the peer using BGP Capabilities
   advertisement [<a href="./rfc5492" title="&quot;Capabilities Advertisement with BGP-4&quot;">RFC5492</a>].

   A BGP speaker MUST follow the existing procedures in generating an
   UPDATE message for a particular &lt;AFI, SAFI&gt; to a peer unless the BGP
   speaker advertises the ADD-PATH Capability to the peer indicating its
   desire to send multiple paths for the &lt;AFI, SAFI&gt;, and also receives
   the ADD-PATH Capability from the peer indicating its willingness to
   receive multiple paths for the &lt;AFI, SAFI&gt;, in which case the speaker
   MUST generate a route update for the &lt;AFI, SAFI&gt; 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 &lt;AFI, SAFI&gt;.

   A BGP speaker SHOULD include the bestpath when more than one path are
   advertised to a neighbor unless the bestpath is a path received from
   that neighbor.

   When deployed as a provider edge router or a peering router that
   interacts with external neighbors, a BGP speaker usually advertises
   at most one path to the internal neighbors in a network.  In the case
   the speaker is configured to advertise multiple paths to the internal
   neighbors, it should include the Edge_Discriminator attribute defined
   in [<a href="#ref-FAST-CONV" title="&quot;Fast Connectivity Restoration Using BGP Add-path&quot;">FAST-CONV</a>] in order to make the route selection consistent inside
   the network.

   As the Path Identifiers are locally assigned, and may or may not be
   persistent across a control plane restart of a BGP speaker, an
   implementation SHOULD take special care so that the underlying



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   forwarding plane of a "Receiving Speaker" as described in [<a href="./rfc4724" title="&quot;Graceful Restart Mechanism for BGP&quot;">RFC4724</a>]
   is not affected during the graceful restart of a BGP session.


<span class="h2"><a class="selflink" name="section-6" href="#section-6">6</a>. Applications</span>

   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 [<a href="./rfc3345" title="&quot;Border Gateway Protocol (BGP) Persistent Route Oscillation Condition&quot;">RFC3345</a>].  It can also help with
   optimal routing and routing convergence in a network.  The
   applications are detailed in separate documents.


<span class="h2"><a class="selflink" name="section-7" href="#section-7">7</a>. Deployment Considerations</span>

   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.


<span class="h2"><a class="selflink" name="section-8" href="#section-8">8</a>. IANA Considerations</span>

   IANA has assigned capability number 69 for the ADD-PATH Capability
   described in this document.  This registration is in the BGP
   Capability Codes registry.


<span class="h2"><a class="selflink" name="section-9" href="#section-9">9</a>. Security Considerations</span>

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


<span class="h2"><a class="selflink" name="section-10" href="#section-10">10</a>. Acknowledgments</span>

   We would like to thank David Cook and Naiming Shen for their
   contributions to the design and development of the extension.

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






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<span class="h2"><a class="selflink" name="section-11" href="#section-11">11</a>. References</span>


<span class="h3"><a class="selflink" name="section-11.1" href="#section-11.1">11.1</a>. Normative References</span>

   [<a name="ref-RFC4271" id="ref-RFC4271">RFC4271</a>] Rekhter, Y., T. Li, and S. Hares, "A Border Gateway
   Protocol 4 (BGP-4)," <a href="./rfc4271">RFC 4271</a>, January 2006.

   [<a name="ref-RFC5492" id="ref-RFC5492">RFC5492</a>] Scudder, J. and R. Chandra, "Capabilities Advertisement
   with BGP-4", <a href="./rfc5492">RFC 5492</a>, February 2009.

   [<a name="ref-RFC4760" id="ref-RFC4760">RFC4760</a>] Bates, T., Chandra, R., Rekhter, Y., and D. Katz,
   "Multiprotocol Extensions for BGP-4", <a href="./rfc4760">RFC 4760</a>, January 2007.

   [<a name="ref-RFC3107" id="ref-RFC3107">RFC3107</a>] Rekhter, R. and E. Rosen, "Carrying Label Information in
   BGP-4," <a href="./rfc3107">RFC 3107</a>, May 2001.

   [<a name="ref-RFC2119" id="ref-RFC2119">RFC2119</a>] Bradner, S., "Key words for use in RFCs to Indicate
   Requirement Levels," <a href="./rfc2119">RFC 2119</a>, <a href="./bcp14">BCP 14</a>, March 1997.

   [<a name="ref-RFC4724" id="ref-RFC4724">RFC4724</a>] Sangli, S., E. Chen, R. Fernando, J. Scudder, and Y.
   Rekhter, "Graceful Restart Mechanism for BGP", <a href="./rfc4724">RFC 4724</a>, January
   2007.

   [<a name="ref-FAST-CONV" id="ref-FAST-CONV">FAST-CONV</a>] Mohapatra, P., R. Fernando, C. Filsfils, R. Raszuk, "Fast
   Connectivity Restoration Using BGP Add-path", Work in Progress, March
   2011.


<span class="h3"><a class="selflink" name="section-11.2" href="#section-11.2">11.2</a>. Informative References</span>

   [<a name="ref-RFC3345" id="ref-RFC3345">RFC3345</a>] McPherson, D., V. Gill, D. Walton, and A. Retana, "Border
   Gateway Protocol (BGP) Persistent Route Oscillation Condition", <a href="./rfc3345">RFC</a>
   <a href="./rfc3345">3345</a>, August 2002.


<span class="h2"><a class="selflink" name="section-12" href="#section-12">12</a>. Authors' Addresses</span>

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

   Email: dwalton@cisco.com


   Alvaro Retana
   Hewlett-Packard Co.



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<span class="grey">INTERNET DRAFT       <a href="./draft-ietf-idr-add-paths-07.txt">draft-ietf-idr-add-paths-07.txt</a>           June 2012</span>


   2610 Wycliff Road
   Raleigh, NC  27607

   Email: alvaro.retana@hp.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

































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