Deprecation of AS_SET and AS_CONFED_SET in BGP
draft-ietf-idr-deprecate-as-set-confed-set-01

Network Working Group                                          W. Kumari
Internet-Draft                                              Google, Inc.
Obsoletes: 6472 (if approved)                                  K. Sriram
Updates: 4271 5065 (if approved)                             L. Hannachi
Intended status: Standards Track                                USA NIST
Expires: May 1, 2020                                    October 29, 2019

             Deprecation of AS_SET and AS_CONFED_SET in BGP
             draft-ietf-idr-deprecate-as-set-confed-set-01

Abstract

   BCP 172 (i.e., RFC 6472) recommends not using AS_SET and
   AS_CONFED_SET in the Border Gateway Protocol.  This document advances
   this recommendation to a standards requirement in BGP; it 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.  This document (if approved) updates
   RFC 4271 and RFC 5065 by eliminating AS_SET and AS_CONFED_SET types,
   and obsoletes RFC 6472.

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 May 1, 2020.

Copyright Notice

   Copyright (c) 2019 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

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  Recommendation to Network Operators . . . . . . . . . . . . .   3
   4.  Updates to Existing RFCs  . . . . . . . . . . . . . . . . . .   4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   BCP 172 [RFC6472] makes a recommendation for not using AS_SET (see
   [RFC4271]) and AS_CONFED_SET (see [RFC5065]) in the Border Gateway
   Protocol (BGP).  This document advances the BCP recommendation to a
   standards requirement in BGP; it 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] [RFC6480] [RFC6811]

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   [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 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 -- only a single AS in the AS_SET are by far the most
   common cases.  Also, very often the same AS appears in the
   AS_SEQUENCE and the AS_SET in the BGP update.  The occurrence of
   reserved AS numbers ([IANA-SP-ASN]) is also somewhat frequent.
   Because the aggregation involving AS_SETs is very rarely used, the
   reduction in table size provided by this is extremely small, and any
   advantage thereof is outweighed by additional complexity in BGP.  As
   noted above, AS_SETs also pose 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

   Network 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 routes subsumed by the previously aggregated route)
   without AS_SETs or AS_CONFED_SETs in the updates.  A sending router

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   MUST not generate a BGP UPDATE with AS_SET or AS_CONFED_SET.  A
   receiving router MUST treat the announced routes in a BGP UPDATE with
   AS_SET or AS_CONFED_SET as withdrawn routes.

   If a network operator wishes to consider a BGP UPDATE with AS_SET or
   AS_CONFED_SET for path selection, they MAY have a feature (knob) in
   the router to opt to do so.  The operator should understand the full
   implications of choosing this option.

   Route aggregation that was previously performed by proxy aggregation
   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 is 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] [RFC6480] [RFC6811] [RFC8205]) might not
   support routes with AS_SETs/AS_CONFED_SETs in them, and may treat
   routes containing them as infeasible even before the updated BGP in
   this document is implemented.

4.  Updates to Existing RFCs

   This document eliminates AS_PATH segment type 1, namely, AS_SET that
   is specified in Section 4.3 of [RFC4271].  That is, in a future
   specification of BGP -- one that would obsolete RFC 4271 -- the use
   of AS_SET will not be specified.

   This document also eliminates AS_PATH segment type 4, namely,
   AS_CONFED_SET that is specified in Section 3 of [RFC5065].  That is,
   in a future specification of Autonomous System Confederations for BGP
   -- one that would obsolete RFC 5065 -- the use of AS_CONFED_SET will
   not be specified.

5.  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.  Deployments of new
   BGP security technologies [RFC6480] [RFC6811] [RFC8205] benefit
   greatly if AS_SET and AS_CONFED_SET are not used in BGP.

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6.  IANA Considerations

   This document requires no IANA actions.

7.  Acknowledgements

   The authors would like to thank Jeffery Haas, John Heasley, Job
   Snijders, Jared Mauch, Jakob Heitz, Keyur Patel, Douglas Montgomery,
   Randy Bush, Susan Hares, John Scudder, Curtis Villamizar, Danny
   McPherson, Chris Morrow, Tom Petch, Ilya Varlashkin, Enke Chen, Tony
   Li, Florian Weimer, John Leslie, Paul Jakma, Rob Austein, Russ
   Housley, Sandra Murphy, Steve Bellovin, Steve Kent, Steve Padgett,
   Alfred Hoenes, and Alvaro Retana for comments and suggestions.

8.  References

8.1.  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>.

   [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>.

8.2.  Informative References

   [Analysis]
              Hannachi, L. and K. Sriram, "Detailed analysis of AS_SETs
              in BGP updates", NIST Robust Inter-domain Routing Project
              Website , October 2019,
              <https://www.nist.gov/sites/default/files/
              documents/2019/10/23/detailed-as_set-analysis.txt>.

   [IANA-SP-ASN]
              "Special-Purpose Autonomous System (AS) Numbers",
              <https://www.iana.org/assignments/iana-as-numbers-special-
              registry/iana-as-numbers-special-registry.xhtml>.

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   [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>.

   [RFC6480]  Lepinski, M. and S. Kent, "An Infrastructure to Support
              Secure Internet Routing", RFC 6480, DOI 10.17487/RFC6480,
              February 2012, <https://www.rfc-editor.org/info/rfc6480>.

   [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>.

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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   Lilia Hannachi
   USA NIST
   100 Bureau Drive
   Gaithersburg, MD  20899
   US

   Phone: +1 301 975 3259
   Email: lilia.hannachi@nist.gov

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