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IETF RFC 7911
Last modified on Tuesday, July 12th, 2016
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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
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/RFC 7911.
Copyright Notice
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 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Walton, et al. Standards Track PAGE 1 
RFC 7911 ADD-PATH July 2016
Table of Contents
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
1. Introduction
The BGP specification [RFC 4271] 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 [RFC 3345]. 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 [RFC 2119].
Walton, et al. Standards Track PAGE 2
RFC 7911 ADD-PATH July 2016
2. How to Identify a Path
As defined in [RFC 4271], a path refers to the information reported in
the Path Attribute field of an UPDATE message. As the procedures
specified in [RFC 4271] 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 [RFC 4271] 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.
Walton, et al. Standards Track PAGE 3
RFC 7911 ADD-PATH July 2016
4. ADD-PATH Capability
The ADD-PATH Capability is a BGP capability [RFC 5492], 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 [RFC 4760].
Subsequent Address Family Identifier (SAFI):
This field is the same as the one used in [RFC 4760].
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 [RFC 5492].
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 [RFC 4271] are
not changed. In particular, a new advertisement for a given address
prefix and a given Path Identifier replaces a previous advertisement
Walton, et al. Standards Track PAGE 4
RFC 7911 ADD-PATH July 2016
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 [RFC 4271] 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 [RFC 4271] 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 [RFC 4724]
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 [RFC 5492],
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.
Walton, et al. Standards Track PAGE 5
RFC 7911 ADD-PATH July 2016
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 [RFC 4272].
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 [RFC 4271] also apply.
9. References
9.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<http://www.rfc-editor.org/info/RFC 2119>.
Walton, et al. Standards Track PAGE 6
RFC 7911 ADD-PATH July 2016
[RFC 4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC 4271, January 2006,
<http://www.rfc-editor.org/info/RFC 4271>.
[RFC 4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760,
DOI 10.17487/RFC 4760, January 2007,
<http://www.rfc-editor.org/info/RFC 4760>.
[RFC 5492] Scudder, J. and R. Chandra, "Capabilities Advertisement
with BGP-4", RFC 5492, DOI 10.17487/RFC 5492, February
2009, <http://www.rfc-editor.org/info/RFC 5492>.
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, 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,
January 2013.
[RFC 3345] McPherson, D., Gill, V., Walton, D., and A. Retana,
"Border Gateway Protocol (BGP) Persistent Route
Oscillation Condition", RFC 3345, DOI 10.17487/RFC 3345,
August 2002, <http://www.rfc-editor.org/info/RFC 3345>.
[RFC 4272] Murphy, S., "BGP Security Vulnerabilities Analysis",
RFC 4272, DOI 10.17487/RFC 4272, January 2006,
<http://www.rfc-editor.org/info/RFC 4272>.
[RFC 4724] Sangli, S., Chen, E., Fernando, R., Scudder, J., and Y.
Rekhter, "Graceful Restart Mechanism for BGP", RFC 4724,
DOI 10.17487/RFC 4724, January 2007,
<http://www.rfc-editor.org/info/RFC 4724>.
[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, April 2016.
Walton, et al. Standards Track PAGE 7
RFC 7911 ADD-PATH July 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
Walton, et al. Standards Track PAGE 8
RFC TOTAL SIZE: 17537 bytes
PUBLICATION DATE: Tuesday, July 12th, 2016
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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