Recommendation for Not Using AS_SET and AS_CONFED_SET in BGP
draft-ietf-idr-deprecate-as-sets-06

Network Working Group                                          W. Kumari
Internet-Draft                                              Google, Inc.
Intended status: BCP                                           K. Sriram
Expires: April 9, 2012                                         U.S. NIST
                                                        October 07, 2011

      Recommendation for Not Using AS_SET and AS_CONFED_SET in BGP
                  draft-ietf-idr-deprecate-as-sets-06

Abstract

   This document recommends against the use of the AS_SET and
   AS_CONFED_SET types of the AS_PATH in BGPv4.  This is done to
   simplify the design and implementation of the BGP protocol and to
   make the semantics of the originator of a route more clear.  This
   will also simplify the design, implementation and deployment of
   ongoing work in the Secure Inter-Domain Routing Working Group.

Status of this Memo

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   document authors.  All rights reserved.

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   the Trust Legal Provisions and are provided without warranty as
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Requirements notation . . . . . . . . . . . . . . . . . . . . . 4
   3.  Recommendation to Network Operators . . . . . . . . . . . . . . 4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 4
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 5
   7.  Informative References  . . . . . . . . . . . . . . . . . . . . 5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 5

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1.  Introduction

   The AS_SET path segment type of the AS_PATH attribute ([RFC4271],
   Section 4.3) is created by a router that is performing route
   aggregation and contains an unordered set of ASs that the update has
   traversed.  The AS_CONFED_SET path type ([RFC5065]) of 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, in essence, combining
   multiple existing routes into a single new route.  This type of
   aggregation blurs the semantics of what it means to originate a
   route.  Said aggregation can therefore 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 the "X.509 Extensions for IP Addresses and AS
   Identifiers" [RFC3779]).  This in turn would result in reachability
   problems for aggregated prefix and its components (i.e., more
   specifics).  Said aggregation also creates traffic engineering issues
   because the precise path information for the component prefixes is
   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 network [analysis] and, when it is used, it is usually
   used incorrectly -- reserved AS numbers ([RFC1930]) and / or only a
   single AS in the AS_SET are by far the most common case.  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 the BGP
   protocol.  As noted above, said aggregation also poses impediments to
   implementation of said 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 prefix,
   using the exact match of the other's 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 which fell in line with the original
   intended use.  Without 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.

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2.  Requirements notation

   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].

3.  Recommendation to Network Operators

   It is RECOMMENDED that operators 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 SHOULD
   withdraw those routes and re-announce routes for the component
   prefixes (i.e., the more specifics of the previously aggregated
   prefix) without AS_SETs in the updates.  This involves undoing the
   aggregation that was previously performed (with AS_SETs), and
   announcing more specifics (without AS_SETs).  Route aggregation that
   was previously performed by proxy aggregation (i.e., without the use
   of AS_SETs) is still possible under some conditions.  As with any
   change, the operator should understand the full implications of the
   change.

   It is worth noting that new technologies (such as those that take
   advantage of the "X.509 Extensions for IP Addresses and AS
   Identifiers" [RFC3779]) might not support routes with AS_SETs /
   AS_CONFED_SETs in them, and may treat as infeasible routes containing
   them.  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
   operator 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.  IANA Considerations

   This document requires no IANA actions.

5.  Security Considerations

   This document discourages the use of aggregation techniques that
   create AS_SETs.  Future work may update the protocol to remove
   support for the AS_SET path segment type of the AS_PATH attribute.
   This will remove complexity and code that is not exercised very
   often, which decreases the attack surface.  This will also simplify

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   the design and implementation of the RPKI and systems that will rely
   on it.

6.  Acknowledgements

   The authors would like to thank Tony Li, Randy Bush, John Scudder,
   Curtis Villamizar, Danny McPherson, Chris Morrow, Tom Petch, Ilya
   Varlashkin as well as 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 Hones, Alvaro Retana, everyone in IDR and everyone else who
   provided input

   Apologies to those who we may have missed, it was not intentional.

7.  Informative References

   [RFC1930]  Hawkinson, J. and T. Bates, "Guidelines for creation,
              selection, and registration of an Autonomous System (AS)",
              BCP 6, RFC 1930, March 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3779]  Lynn, C., Kent, S., and K. Seo, "X.509 Extensions for IP
              Addresses and AS Identifiers", RFC 3779, June 2004.

   [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
              Protocol 4 (BGP-4)", RFC 4271, January 2006.

   [RFC5065]  Traina, P., McPherson, D., and J. Scudder, "Autonomous
              System Confederations for BGP", RFC 5065, August 2007.

   [analysis]
              Sriram, K., "Measurement Data on AS_SET and AGGREGATOR,
              SIDR WG presentation, IETF-78", July 2010, < http://
              www.antd.nist.gov/~ksriram/AS_SET_Aggregator_Stats.pdf>.

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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
   U.S. NIST
   100 Bureau Drive
   Gaithersburg, MD  20899
   US

   Phone: +1 301 975 3973
   Email: ksriram@nist.gov

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