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.ds LH RFC 7911
.ds CH ADD-PATH
.ds RH July 2016
.ds LF Walton, et al.
.ds CF Standards Track
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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
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Advertisement of Multiple Paths in BGP

Abstract
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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.
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Status of This Memo
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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\02 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.
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Copyright Notice
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Copyright (c) 2016 IETF Trust and the persons identified as the
document authors.  All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
\%(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document.  Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document.  Code Components extracted from this document must
include Simplified BSD License text as described in Section\04.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
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   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Specification of Requirements . . . . . . . . . . . . . .   2
   2.  How to Identify a Path  . . . . . . . . . . . . . . . . . . .   3
   3.  Extended NLRI Encodings . . . . . . . . . . . . . . . . . . .   3
   4.  ADD-PATH Capability . . . . . . . . . . . . . . . . . . . . .   4
   5.  Operation . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   6.  Deployment Considerations . . . . . . . . . . . . . . . . . .   5
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8
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1.  Introduction
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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.
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1.1.  Specification of Requirements
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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].
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2.  How to Identify a Path
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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.
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3.  Extended NLRI Encodings
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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:
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                  +--------------------------------+
                  | Path Identifier (4 octets)     |
                  +--------------------------------+
                  | Length (1 octet)               |
                  +--------------------------------+
                  | Prefix (variable)              |
                  +--------------------------------+
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The usage of the extended NLRI encodings is specified in Section\05.
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4.  \%ADD-PATH Capability
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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:
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                +------------------------------------------------+
                | Address Family Identifier (2 octets)           |
                +------------------------------------------------+
                | Subsequent Address Family Identifier (1 octet) |
                +------------------------------------------------+
                | Send/Receive (1 octet)                         |
                +------------------------------------------------+
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The meaning and use of the fields are as follows:
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Address Family Identifier (AFI):
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This field is the same as the one used in [RFC4760].
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Subsequent Address Family Identifier (SAFI):
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This field is the same as the one used in [RFC4760].
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Send/Receive:
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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].
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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.
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5.  Operation
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The Path Identifier specified in Section\03 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
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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.
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6.  Deployment Considerations
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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\03
is in use in a particular BGP session is the exchange of Capabilities
described in Section\04.  If the exchange is successful [RFC5492],
then the BGP speakers will be able to process all BGP UPDATES
properly, as described in Section\05.  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.
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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.
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7.  IANA Considerations
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IANA has assigned the value 69 for the \%ADD-PATH Capability described
in this document.  This registration is in the "Capability Codes"
registry.
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8.  Security Considerations
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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.
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9.  References
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9.1.  Normative References
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[RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP\014, RFC\02119,
DOI\010.17487/RFC2119, March 1997,
\%<https://www.rfc-editor.org/info/rfc2119>.
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[RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 \%(BGP-4)", RFC\04271,
DOI\010.17487/RFC4271, January 2006,
\%<https://www.rfc-editor.org/info/rfc4271>.
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[RFC4760]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for \%BGP-4", RFC\04760,
DOI\010.17487/RFC4760, January 2007,
\%<https://www.rfc-editor.org/info/rfc4760>.
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[RFC5492]  Scudder, J. and R. Chandra, "Capabilities Advertisement
with \%BGP-4", RFC\05492, DOI\010.17487/RFC5492, February
2009, \%<https://www.rfc-editor.org/info/rfc5492>.
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9.2.  Informative References
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[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,\0draft-ietf-idr-
\%add-paths-guidelines-08, April 2016.
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[FAST]     Mohapatra, P., Fernando, R., Filsfils, C., and R. Raszuk,
"Fast Connectivity Restoration Using BGP \%Add-path", Work
in \%Progress,\0draft-pmohapat-idr-fast-conn-restore-03,
January 2013.
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[RFC3345]  McPherson, D., Gill, V., Walton, D., and A. Retana,
"Border Gateway Protocol (BGP) Persistent Route
Oscillation Condition", RFC\03345, DOI\010.17487/RFC3345,
August 2002, \%<https://www.rfc-editor.org/info/rfc3345>.
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[RFC4272]  Murphy, S., "BGP Security Vulnerabilities Analysis",
RFC\04272, DOI\010.17487/RFC4272, January 2006,
\%<https://www.rfc-editor.org/info/rfc4272>.
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[RFC4724]  Sangli, S., Chen, E., Fernando, R., Scudder, J., and Y.
Rekhter, "Graceful Restart Mechanism for BGP", RFC\04724,
DOI\010.17487/RFC4724, January 2007,
\%<https://www.rfc-editor.org/info/rfc4724>.
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\%[STOP-OSC]
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Walton, D., Retana, A., Chen, E., and J. Scudder, "BGP
Persistent Route Oscillation Solutions", Work in
\%Progress,\0draft-ietf-idr-route-oscillation-stop-03, April
2016.
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Acknowledgments
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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.
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Authors' Addresses
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   Daniel Walton
   Cumulus Networks
   185 E. Dana Street
   Mountain View, CA  94041
   United States of America

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

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

   Email: enkechen@cisco.com
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   John Scudder
   Juniper Networks
   1194 N. Mathilda Ave
   Sunnyvale, CA  94089
   United States of America

   Email: jgs@juniper.net
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