Network Working Group W. Kumari
Internet-Draft Google, Inc.
Intended status: Standards Track K. Sriram
Expires: March 6, 2020 USA NIST
September 3, 2019
Deprecation of AS_SET and AS_CONFED_SET in BGP
draft-ietf-idr-deprecate-as-set-confed-set-00
Abstract
BCP 172 (i.e., RFC 6472) recommends not using AS_SET and
AS_CONFED_SET in Border Gateway Protocol (BGP). This document
updates RFC 4271 and proscribes the use of the AS_SET and
AS_CONFED_SET types of path segments in the AS_PATH. This is done to
simplify the design and implementation of BGP and to make the
semantics of the originator of a route clearer. This will also
simplify the design, implementation, and deployment of various BGP
security mechanisms.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on March 6, 2020.
Copyright Notice
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document authors. All rights reserved.
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to this document. Code Components extracted from this document must
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Recommendation to Network Operators . . . . . . . . . . . . . 3
4. Security Considerations . . . . . . . . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 4
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 4
7.1. Normative References . . . . . . . . . . . . . . . . . . 4
7.2. Informative References . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
BCP 172 [RFC6472] makes a recommendation for not using AS_SET and
AS_CONFED_SET in Border Gateway Protocol (BGP). This document
advances the recommendation to a standards requirement in BGP. It
updates [RFC4271] and proscribes the use of the AS_SET and
AS_CONFED_SET types of path segments in the AS_PATH.
The AS_SET path segment in the AS_PATH attribute (Sections 4.3 and
5.1.2 of [RFC4271]) is created by a router that is performing route
aggregation and contains an unordered set of Autonomous Systems
(ASes) that the update has traversed. The AS_CONFED_SET path segment
(see [RFC5065]) in the AS_PATH attribute is created by a router that
is performing route aggregation and contains an unordered set of
Member AS Numbers in the local confederation that the update has
traversed. It is very similar to AS_SETs but is used within a
confederation.
By performing aggregation, a router is combining multiple existing
routes into a single new route. The aggregation together with the
use of AS_SET blurs the semantics of origin AS for the prefix being
announced. Therefore, the aggregation with AS_SET (or AS_CONFED_SET)
can cause operational issues, such as not being able to authenticate
a route origin for the aggregate prefix in new BGP security
technologies such as those that take advantage of X.509 extensions
for IP addresses and AS identifiers [RFC3779] [RFC6811] [RFC8205].
This in turn could result in reachability problems for the aggregated
prefix and its components (i.e., more-specific prefixes). The
aggregation as described above could also create traffic engineering
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issues, because the precise path information for the component
prefixes are not preserved.
From analysis of past Internet routing data, it is apparent that
aggregation that involves AS_SETs is very seldom used in practice on
the public Internet [Analysis] and when it is used, it is often used
incorrectly -- reserved AS numbers ([RFC1930]) and/or only a single
AS in the AS_SET are by far the most common cases. Because the
aggregation involving AS_SETs is very rarely used, the reduction in
table size provided by said aggregation is extremely small, and any
advantage thereof is outweighed by additional complexity in BGP. As
noted above, said aggregation also poses impediments to
implementation of new BGP security technologies.
In the past, AS_SET had been used in a few rare cases to allow route
aggregation where two or more providers could form the same aggregate
prefix, using the exact match of the other's aggregate prefix in some
advertisement and configuring the aggregation differently elsewhere.
The key to configuring this correctly was to form the aggregate at
the border in the outbound BGP policy and omit prefixes from the AS
that the aggregate was being advertised to. The AS_SET therefore
allowed this practice without the loss of BGP's AS_PATH loop
protection. This use of AS_SET served a purpose that fell in line
with the original intended use. Without the use of AS_SET,
aggregates must always contain only less-specific prefixes (not less
than or equal to) and must never aggregate an exact match.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Recommendation to Network Operators
Operators MUST NOT generate any new announcements containing AS_SETs
or AS_CONFED_SETs. If they have already announced routes with
AS_SETs or AS_CONFED_SETs in them, then they MUST withdraw those
routes and re-announce routes for the component prefixes (i.e., the
more-specific prefixes subsumed by the previously aggregated prefix)
without AS_SETs or CONFED_SETs in the updates. Route aggregation
that was previously performed by proxy aggregation (i.e., without the
use of AS_SETs) is still possible under some conditions. When doing
this, operators MUST form the aggregate at the border in the outbound
BGP policy and omit any prefixes from the AS that the aggregate was
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being advertised to. As with any change, the operator should
understand the full implications of the change.
It is worth noting that new BGP security technologies (such as those
that take advantage of X.509 extensions for IP addresses and AS
identifiers [RFC3779] [RFC6811] [RFC8205]) might not support routes
with AS_SETs/AS_CONFED_SETs in them, and may treat routes containing
them as infeasible. Future BGP implementations may also do the same.
It is expected that, even before the deployment of these new or
future technologies, operators may filter routes with AS_SETs/
AS_CONFED_SETs in them. Other than making that observation, this
document is not intended to make any recommendation for how an
implementation should behave when receiving a route with AS_SET or
AS_CONFED_SET in it. This document's focus is entirely on the sender
side, as discussed in the preceding paragraph.
4. Security Considerations
This document obsoletes the use of aggregation techniques that create
AS_SETs or AS_CONFED_SETs. Obsoleting these path segment types from
BGP and removal of the related code from implementations would
potentially decrease the attack surface for BGP.
5. IANA Considerations
This document requires no IANA actions.
6. Acknowledgements
The authors would like to thank Tony Li, Randy Bush, John Scudder,
Curtis Villamizar, Danny McPherson, Chris Morrow, Tom Petch, Ilya
Varlashkin, Douglas Montgomery, Enke Chen, Florian Weimer, Jakob
Heitz, John Leslie, Keyur Patel, Paul Jakma, Rob Austein, Russ
Housley, Sandra Murphy, Steve Bellovin, Steve Kent, Steve Padgett,
Alfred Hoenes, and Alvaro Retana.
7. References
7.1. 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>.
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[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>.
[RFC5065] Traina, P., McPherson, D., and J. Scudder, "Autonomous
System Confederations for BGP", RFC 5065,
DOI 10.17487/RFC5065, August 2007,
<https://www.rfc-editor.org/info/rfc5065>.
7.2. Informative References
[Analysis]
Sriram, K. and D. Montgomery, "Measurement Data on AS_SET
and AGGREGATOR: Implications for {Prefix, Origin}
Validation Algorithms", SIDR WG presentation, IETF 78,
July 2010, <http://www.nist.gov/itl/antd/upload/
AS_SET_Aggregator_Stats.pdf>.
[RFC1930] Hawkinson, J. and T. Bates, "Guidelines for creation,
selection, and registration of an Autonomous System (AS)",
BCP 6, RFC 1930, DOI 10.17487/RFC1930, March 1996,
<https://www.rfc-editor.org/info/rfc1930>.
[RFC3779] Lynn, C., Kent, S., and K. Seo, "X.509 Extensions for IP
Addresses and AS Identifiers", RFC 3779,
DOI 10.17487/RFC3779, June 2004,
<https://www.rfc-editor.org/info/rfc3779>.
[RFC6472] Kumari, W. and K. Sriram, "Recommendation for Not Using
AS_SET and AS_CONFED_SET in BGP", BCP 172, RFC 6472,
DOI 10.17487/RFC6472, December 2011,
<https://www.rfc-editor.org/info/rfc6472>.
[RFC6811] Mohapatra, P., Scudder, J., Ward, D., Bush, R., and R.
Austein, "BGP Prefix Origin Validation", RFC 6811,
DOI 10.17487/RFC6811, January 2013,
<https://www.rfc-editor.org/info/rfc6811>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8205] Lepinski, M., Ed. and K. Sriram, Ed., "BGPsec Protocol
Specification", RFC 8205, DOI 10.17487/RFC8205, September
2017, <https://www.rfc-editor.org/info/rfc8205>.
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Authors' Addresses
Warren Kumari
Google, Inc.
1600 Amphitheatre Parkway
Mountain View, CA 94043
US
Phone: +1 571 748 4373
Email: warren@kumari.net
Kotikalapudi Sriram
USA NIST
100 Bureau Drive
Gaithersburg, MD 20899
US
Phone: +1 301 975 3973
Email: sriram.ietf@gmail.com
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