



Internet Engineering Task Force (IETF)                         D. Walton
Request for Comments: 7911                              Cumulus Networks
Category: Standards Track                                      A. Retana
ISSN: 2070-1721                                                  E. Chen
                                                     Cisco Systems, Inc.
                                                              J. Scudder
                                                        Juniper Networks
                                                               July 2016


                 Advertisement of Multiple Paths in BGP

Abstract

   This document defines 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.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7911.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Specification of Requirements
   2.  How to Identify a Path
   3.  Extended NLRI Encodings
   4.  ADD-PATH Capability
   5.  Operation
   6.  Deployment Considerations
   7.  IANA Considerations
   8.  Security Considerations
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Acknowledgments
   Authors' Addresses

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.

   This document defines 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.

   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 by providing
   potential alternate or backup paths, respectively.

1.1.  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].

2.  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.

3.  Extended NLRI Encodings

   In order to carry the Path Identifier in an UPDATE message, the NLRI
   encoding MUST be extended by prepending the Path Identifier field,
   which is of four octets.

   For example, the NLRI encoding specified in [RFC4271] is extended as
   the following:

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

   The usage of the extended NLRI encodings is specified in Section 5.

4.  ADD-PATH Capability

   The ADD-PATH Capability is a BGP capability [RFC5492], with
   Capability Code 69.  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) able to receive
         multiple paths from its peer (value 1), (b) able to send
         multiple paths to its peer (value 2), or (c) both (value 3) for
         the <AFI, SAFI>.

      -  If any other value is received, then the capability SHOULD be
         treated as not understood and ignored [RFC5492].

   A BGP speaker that wishes to indicate support for multiple AFI/SAFIs
   MUST do so by including the information in a single instance of the
   ADD-PATH Capability.

5.  Operation

   The Path Identifier specified in Section 3 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 same address prefix and Path Identifier.  If a BGP speaker
   receives a message to withdraw a prefix with a Path Identifier not
   seen before, it SHOULD silently ignore it.

   For a BGP speaker to be able to send multiple paths to its peer, that
   BGP speaker MUST advertise the ADD-PATH Capability with the Send/
   Receive field set to either 2 or 3, and MUST receive from its peer
   the ADD-PATH Capability with the Send/Receive field set to either 1
   or 3, for the corresponding <AFI, SAFI>.

   A BGP speaker MUST follow the procedures defined in [RFC4271] when
   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 ability to send multiple paths for the <AFI, SAFI>,
   and also receives the ADD-PATH Capability from the peer indicating
   its ability 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>.

   A BGP speaker SHOULD include the best route [RFC4271] when more than
   one path is advertised to a neighbor, unless it is a path received
   from that neighbor.

   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
   forwarding plane of a "Receiving Speaker" as described in [RFC4724]
   is not affected during the graceful restart of a BGP session.

6.  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 [ADDPATH].

   The only explicit indication that the encoding described in Section 3
   is in use in a particular BGP session is the exchange of Capabilities
   described in Section 4.  If the exchange is successful [RFC5492],
   then the BGP speakers will be able to process all BGP UPDATES
   properly, as described in Section 5.  However, if, for example, a
   packet analyzer is used on the wire to examine an active BGP session,
   it may not be able to properly decode the BGP UPDATES because it
   lacks prior knowledge of the exchanged Capabilities.

   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
   where the speaker is configured to advertise multiple paths to the
   internal neighbors, and additional information is needed for the
   application, the speaker could use attributes such as the
   Edge_Discriminator attribute [FAST].  The use of that type of
   additional information is outside the scope of this document.

7.  IANA Considerations

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

8.  Security Considerations

   This document defines a BGP extension that allows the advertisement
   of multiple paths for the same address prefix without the new paths
   implicitly replacing any previous ones.  As a result, multiple paths
   for a large number of prefixes may be received by a BGP speaker,
   potentially depleting memory resources or even causing network-wide
   instability, which can be considered a denial-of-service attack.
   Note that this is not a new vulnerability, but one that is present in
   the base BGP specification [RFC4272].

   The use of the ADD-PATH Capability is intended to address specific
   needs related to, for example, eliminating route oscillations that
   were induced by the MULTI_EXIT_DISC (MED) attribute [STOP-OSC].
   While describing the applications for the ADD-PATH Capability is
   outside the scope of this document, users are encouraged to examine
   their behavior and potential impact by studying the best practices
   described in [ADDPATH].

   Security concerns in the base operation of BGP [RFC4271] also apply.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <https://www.rfc-editor.org/info/rfc4271>.

   [RFC4760]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
              "Multiprotocol Extensions for BGP-4", RFC 4760,
              DOI 10.17487/RFC4760, January 2007,
              <https://www.rfc-editor.org/info/rfc4760>.

   [RFC5492]  Scudder, J. and R. Chandra, "Capabilities Advertisement
              with BGP-4", RFC 5492, DOI 10.17487/RFC5492, February
              2009, <https://www.rfc-editor.org/info/rfc5492>.

9.2.  Informative References

   [ADDPATH]  Uttaro, J., Francois, P., Patel, K., Haas, J., Simpson,
              A., and R. Fragassi, "Best Practices for Advertisement of
              Multiple Paths in IBGP", Work in Progress, draft-ietf-idr-
              add-paths-guidelines-08, 25 April 2016.

   [FAST]     Mohapatra, P., Fernando, R., Filsfils, C., and R. Raszuk,
              "Fast Connectivity Restoration Using BGP Add-path", Work
              in Progress, draft-pmohapat-idr-fast-conn-restore-03, 22
              January 2013.

   [RFC3345]  McPherson, D., Gill, V., Walton, D., and A. Retana,
              "Border Gateway Protocol (BGP) Persistent Route
              Oscillation Condition", RFC 3345, DOI 10.17487/RFC3345,
              August 2002, <https://www.rfc-editor.org/info/rfc3345>.

   [RFC4272]  Murphy, S., "BGP Security Vulnerabilities Analysis",
              RFC 4272, DOI 10.17487/RFC4272, January 2006,
              <https://www.rfc-editor.org/info/rfc4272>.

   [RFC4724]  Sangli, S., Chen, E., Fernando, R., Scudder, J., and Y.
              Rekhter, "Graceful Restart Mechanism for BGP", RFC 4724,
              DOI 10.17487/RFC4724, January 2007,
              <https://www.rfc-editor.org/info/rfc4724>.

   [STOP-OSC] Walton, D., Retana, A., Chen, E., and J. Scudder, "BGP
              Persistent Route Oscillation Solutions", Work in
              Progress, draft-ietf-idr-route-oscillation-stop-03, 30
              April 2016.

Acknowledgments

   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, Mark Turner, Jeff Haas, Jay Borkenhagen, Mach Chen, Denis
   Ovsienko, Carlos Pignataro, Meral Shirazipour, and Kathleen Moriarty.

Authors' Addresses

   Daniel Walton
   Cumulus Networks
   185 E. Dana Street
   Mountain View, CA 94041
   United States of America
   Email: dwalton@cumulusnetworks.com


   Alvaro Retana
   Cisco Systems, Inc.
   Kit Creek Rd.
   Research Triangle Park, NC 27709
   United States of America
   Email: aretana@cisco.com


   Enke Chen
   Cisco Systems, Inc.
   170 W. Tasman Dr.
   San Jose, CA 95134
   United States of America
   Email: enkechen@cisco.com


   John Scudder
   Juniper Networks
   1194 N. Mathilda Ave
   Sunnyvale, CA 94089
   United States of America
   Email: jgs@juniper.net
