Internet Engineering Task Force (IETF)                         W. Kumari
Request for Comments: 6472                                  Google, Inc.
BCP: 172                                                       K. Sriram
Category: Best Current Practice                                U.S. NIST
ISSN: 2070-1721                                            December 2011


      Recommendation for Not Using AS_SET and AS_CONFED_SET in BGP

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 BGP 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

   This memo documents an Internet Best Current Practice.

   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
   BCPs is available in Section 2 of RFC 5741.

   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/rfc6472.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................2
   2. Requirements Notation ...........................................3
   3. Recommendation to Network Operators .............................3
   4. Security Considerations .........................................4
   5. Acknowledgements ................................................4
   6. References ......................................................4
      6.1. Normative References .......................................4
      6.2. Informative References .....................................4

1.  Introduction

   The AS_SET path segment type of 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 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 the 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 BGP.  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 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 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 additional 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.  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 future work will remove complexity and code that are not
   exercised very often, thereby decreasing the attack surface.  This
   future work will also simplify the design and implementation of the
   Resource Public Key Infrastructure (RPKI) and systems that will
   rely on it.

5.  Acknowledgements

   The authors would like to thank Tony Li, Randy Bush, John Scudder,
   Curtis Villamizar, Danny McPherson, Chris Morrow, Tom Petch, and 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 Hoenes, Alvaro Retana, everyone in the IDR working group, and
   everyone else who provided input.

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

6.  References

6.1.  Normative References

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

6.2.  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, <www.antd.nist.gov/~ksriram/
               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, March 1996.

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

   [RFC4271]   Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "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.

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