Network Working Group                                          W. Kumari
Internet-Draft                                              Google, Inc.
Intended status: Informational BCP                                           K. Sriram
Expires: November 2, 2011 January 28, 2012                                      U.S. NIST
                                                             May 1,
                                                           July 27, 2011

          Deprecation of the use of BGP AS_SET, AS_CONFED_SET.


   This document deprecates 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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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on November 2, 2011. January 28, 2012.

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Table of Contents

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

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
   which can cause operational issues that include reachability problems
   and traffic engineering issues.

   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.  The
   reduction in table size provided by the aggregation is outweighed by
   additional complexity in the BGP protocol and confusion regarding
   what exactly is meant by originating a route.

   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 others 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 AS_SET aggregates must always contain only less specific
   prefixes (not less than or equal to), and must never aggregate an
   exact match.  Since this practice is thought to no longer be widely
   used, it is thought to be safe to deprecate the use of AS_SET.

2.  Requirements notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  Recommendation to Network Operators

   Operators are strongly advised to 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 and re-announce those prefixes without AS_SETs in the
   updates.  This may require undoing the aggregation that was
   previously performed, and announcing more specifics.  Route
   aggregation that was previously performed by proxy aggregation is
   still possible under some conditions without the use of AS_SETs.  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]) may 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
   technologies, operators may begin filtering routes that contain

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 will 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
   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.

              Sriram, K., "Measurement Data on AS_SET and AGGREGATOR,
              SIDR WG presentation, IETF-78", July 2010, < http://

Authors' Addresses

   Warren Kumari
   Google, Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA  94043

   Phone: +1 571 748 4373
   Kotikalapudi Sriram
   U.S. NIST
   100 Bureau Drive
   Gaithersburg, MD  20899

   Phone: +1 301 975 3973