rfc3065.txt

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Network Working Group                                          P. Traina
Request for Comments: 3065                        Juniper Networks, Inc.
Obsoletes: 1965                                             D. McPherson
Category: Standards Track                           Amber Networks, Inc.
                                                              J. Scudder
                                                     Cisco Systems, Inc.
                                                           February 2001


                Autonomous System Confederations for BGP

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

Abstract

   The Border Gateway Protocol (BGP) is an inter-autonomous system
   routing protocol designed for Transmission Control Protocol/Internet
   Protocol (TCP/IP) networks.  BGP requires that all BGP speakers
   within a single autonomous system (AS) must be fully meshed.  This
   represents a serious scaling problem that has been well documented in
   a number of proposals.

   This document describes an extension to BGP which may be used to
   create a confederation of autonomous systems that is represented as a
   single autonomous system to BGP peers external to the confederation,
   thereby removing the "full mesh" requirement.  The intention of this
   extension is to aid in policy administration and reduce the
   management complexity of maintaining a large autonomous system.

1. Specification of Requirements

   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 [RFC 2119].







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2. Introduction

   As currently defined, BGP requires that all BGP speakers within a
   single AS must be fully meshed.  The result is that for n BGP
   speakers within an AS n*(n-1)/2 unique IBGP sessions are required.
   This "full mesh" requirement clearly does not scale when there are a
   large number of IBGP speakers within the autonomous system, as is
   common in many networks today.

   This scaling problem has been well documented and a number of
   proposals have been made to alleviate this [3,5].  This document
   represents another alternative in alleviating the need for a "full
   mesh" and is known as "Autonomous System Confederations for BGP", or
   simply, "BGP Confederations".  It can also be said the BGP
   Confederations MAY provide improvements in routing policy control.

   This document is a revision of RFC 1965 [4] and it includes editorial
   changes, clarifications and corrections based on the deployment
   experience with BGP Confederations.  These revisions are summarized
   in Appendix A.

3. Terms and Definitions

   AS Confederation

      A collection of autonomous systems advertised as a single AS
      number to BGP speakers that are not members of the confederation.

   AS Confederation Identifier

      An externally visible autonomous system number that identifies the
      confederation as a whole.

   Member-AS

      An autonomous system that is contained in a given AS
      confederation.

   Member-AS Number

      An autonomous system number visible only internal to a BGP
      confederation.

4. Discussion

   It may be useful to subdivide autonomous systems with a very large
   number of BGP speakers into smaller domains for purposes of
   controlling routing policy via information contained in the BGP



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   AS_PATH attribute.  For example, one may choose to consider all BGP
   speakers in a geographic region as a single entity.  In addition to
   potential improvements in routing policy control, if techniques such
   as those presented here or in [5] are not employed, [1] requires BGP
   speakers in the same autonomous system to establish a full mesh of
   TCP connections among all speakers for the purpose of exchanging
   exterior routing information.  In autonomous systems the number of
   intra-domain connections that need to be maintained by each border
   router can become significant.

   Subdividing a large autonomous system allows a significant reduction
   in the total number of intra-domain BGP connections, as the
   connectivity requirements simplify to the model used for inter-domain
   connections.

   Unfortunately subdividing an autonomous system may increase the
   complexity of routing policy based on AS_PATH information for all
   members of the Internet.  Additionally, this division increases the
   maintenance overhead of coordinating external peering when the
   internal topology of this collection of autonomous systems is
   modified.

   Finally, dividing a large AS may unnecessarily increase the length of
   the sequence portions of the AS_PATH attribute.  Several common BGP
   implementations can use the number of "AS hops" required to reach a
   given destination as part of the path selection criteria.  While this
   is not an optimal method of determining route preference, given the
   lack of other in-band information, it provides a reasonable default
   behavior which is widely used across the Internet.  Therefore,
   division of an autonomous system into separate systems may adversely
   affect optimal routing of packets through the Internet.

   However, there is usually no need to expose the internal topology of
   this divided autonomous system, which means it is possible to regard
   a collection of autonomous systems under a common administration as a
   single entity or autonomous system when viewed from outside the
   confines of the confederation of autonomous systems itself.

5. AS_CONFED Segment Type Extension

   Currently, BGP specifies that the AS_PATH attribute is a well-known
   mandatory attribute that is composed of a sequence of AS path
   segments.  Each AS path segment is represented by a triple <path
   segment type, path segment length, path segment value>.

   In [1], the path segment type is a 1-octet long field with the two
   following values defined:




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   Value     Segment Type

      1       AS_SET: unordered set of ASs a route in the
              UPDATE message has traversed

      2       AS_SEQUENCE: ordered set of ASs a route in
              the UPDATE message has traversed

   This document reserves two additional segment types:

      3       AS_CONFED_SEQUENCE: ordered set of Member AS Numbers
              in the local confederation that the UPDATE message has
              traversed

      4       AS_CONFED_SET: unordered set of Member AS Numbers in
              the local confederation that the UPDATE message has
              traversed

6. Operation

   A member of a BGP confederation will use its AS Confederation ID in
   all transactions with peers that are not members of its
   confederation.  This confederation identifier is considered to be the
   "externally visible" AS number and this number is used in OPEN
   messages and advertised in the AS_PATH attribute.

   A member of a BGP confederation will use its Member AS Number in all
   transactions with peers that are members of the same confederation as
   the given router.

   A BGP speaker receiving an AS_PATH attribute containing an autonomous
   system matching its own confederation shall treat the path in the
   same fashion as if it had received a path containing its own AS
   number.

   A BGP speaker receiving an AS_PATH attribute containing an
   AS_CONFED_SEQUENCE or AS_CONFED_SET which contains its own Member AS
   Number shall treat the path in the same fashion as if it had received
   a path containing its own AS number.

6.1. AS_PATH Modification Rules

   Section 5.1.2 of [1] is replaced with the following text:

   When a BGP speaker propagates a route which it has learned from
   another BGP speaker's UPDATE message, it shall modify the route's
   AS_PATH attribute based on the location of the BGP speaker to which
   the route will be sent:



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   a) When a given BGP speaker advertises the route to another BGP
      speaker located in its own autonomous system, the advertising
      speaker shall not modify the AS_PATH attribute associated with the
      route.

   b) When a given BGP speaker advertises the route to a BGP speaker
      located in a neighboring autonomous system that is a member of the
      local autonomous system confederation, then the advertising
      speaker shall update the AS_PATH attribute as follows:

      1) if the first path segment of the AS_PATH is of type
         AS_CONFED_SEQUENCE, the local system shall prepend its own AS
         number as the last element of the sequence (put it in the
         leftmost position).

      2) if the first path segment of the AS_PATH is not of type
         AS_CONFED_SEQUENCE the local system shall prepend a new path
         segment of type AS_CONFED_SEQUENCE to the AS_PATH, including
         its own confederation identifier in that segment.

   c) When a given BGP speaker advertises the route to a BGP speaker
      located in a neighboring autonomous system that is not a member of
      the current autonomous system confederation, the advertising
      speaker shall update the AS_PATH attribute as follows:

      1) if the first path segment of the AS_PATH is of type
         AS_CONFED_SEQUENCE, that segment and any immediately following
         segments of the type AS_CONFED_SET or AS_CONFED_SEQUENCE are
         removed from the AS_PATH attribute, leaving the sanitized
         AS_PATH attribute to be operated on by steps 2, or 3.

      2) if the first path segment of the remaining AS_PATH is of type
         AS_SEQUENCE, the local system shall prepend its own
         confederation ID as the last element of the sequence (put it in
         the leftmost position).

      3) if there are no path segments following the removal of the
         first AS_CONFED_SET/AS_CONFED_SEQUENCE segments, or if the
         first path segment of the remaining AS_PATH is of type AS_SET
         the local system shall prepend a new path segment of type
         AS_SEQUENCE to the AS_PATH, including its own confederation ID
         in that segment.









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   When a BGP speaker originates a route:

   a) the originating speaker shall include an empty AS_PATH attribute
      in all UPDATE messages sent to BGP speakers located in its own
      Member AS Number.  (An empty AS_PATH attribute is one whose length
      field contains the value zero).

   b) the originating speaker shall include its own Member AS Number in
      an AS_CONFED_SEQUENCE segment of the AS_PATH attribute of all
      UPDATE messages sent to BGP speakers located in neighboring
      Member-AS that are members of the local confederation (i.e., the
      originating speaker's Member AS Number will be the only entry in
      the AS_PATH attribute).

   c) the originating speaker shall include its own autonomous system in
      an AS_SEQUENCE segment of the AS_PATH attribute of all UPDATE
      messages sent to BGP speakers located in neighboring autonomous
      systems that are not members of the local confederation.  (In this
      case, the autonomous system number of the originating speaker's
      member confederation will be the only entry in the AS_PATH
      attribute).

7. Common Administration Issues