Recommendation for Not Using AS_SET and AS_CONFED_SET in BGP
RFC 6472

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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.

   This document is subject to BCP 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 2011

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.

Kumari & Sriram           Best Current Practice                 [Page 2]
RFC 6472          AS_SET, AS_CONFED_SET Use Deprecation    December 2011