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Internet Engineering Task Force (IETF)                         W. KumariRequest for Comments: 6472                                  Google, Inc.BCP: 172                                                       K. SriramCategory: Best Current Practice                                U.S. NISTISSN: 2070-1721                                            December 2011Recommendation for Not Using AS_SET and AS_CONFED_SET in BGPAbstract   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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6472.Copyright Notice   Copyright (c) 2011 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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.Kumari & Sriram           Best Current Practice                 [Page 1]

RFC 6472          AS_SET, AS_CONFED_SET Use Deprecation    December 2011Table of Contents1. Introduction ....................................................22. Requirements Notation ...........................................33. Recommendation to Network Operators .............................34. Security Considerations .........................................45. Acknowledgements ................................................46. References ......................................................46.1. Normative References .......................................46.2. Informative References .....................................41.  Introduction   The AS_SET path segment type of the AS_PATH attribute (Sections4.3   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.Kumari & Sriram           Best Current Practice                 [Page 2]

RFC 6472          AS_SET, AS_CONFED_SET Use Deprecation    December 2011   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.Kumari & Sriram           Best Current Practice                 [Page 3]

RFC 6472          AS_SET, AS_CONFED_SET Use Deprecation    December 20114.  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.  References6.1.  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.Kumari & Sriram           Best Current Practice                 [Page 4]

RFC 6472          AS_SET, AS_CONFED_SET Use Deprecation    December 2011   [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.govKumari & Sriram           Best Current Practice                 [Page 5]