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rfc:rfc3065

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].

Traina, et al. Standards Track [Page 1] RFC 3065 Autonomous System Confederations for BGP February 2001

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

Traina, et al. Standards Track [Page 2] RFC 3065 Autonomous System Confederations for BGP February 2001

 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:

Traina, et al. Standards Track [Page 3] RFC 3065 Autonomous System Confederations for BGP February 2001

 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:

Traina, et al. Standards Track [Page 4] RFC 3065 Autonomous System Confederations for BGP February 2001

 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.

Traina, et al. Standards Track [Page 5] RFC 3065 Autonomous System Confederations for BGP February 2001

 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

 It is reasonable for member ASs of a confederation to share a common
 administration and IGP information for the entire confederation.
 It shall be legal for a BGP speaker to advertise an unchanged
 NEXT_HOP and MULTI_EXIT_DISCRIMINATOR (MED) attribute to peers in a
 neighboring AS within the same confederation.  In addition, the
 restriction against sending the LOCAL_PREFERENCE attribute to peers
 in a neighboring AS within the same confederation is removed.  Path
 selection criteria for information received from members inside a
 confederation MUST follow the same rules used for information
 received from members inside the same autonomous system, as specified
 in [1].

8. Compatability Considerations

 All BGP speakers participating in a confederation must recognize the
 AS_CONFED_SET and AS_CONFED_SEQUENCE segment type extensions to the
 AS_PATH attribute.
 Any BGP speaker not supporting these extensions will generate a
 notification message specifying an "UPDATE Message Error" and a sub-
 code of "Malformed AS_PATH".

Traina, et al. Standards Track [Page 6] RFC 3065 Autonomous System Confederations for BGP February 2001

 This compatibility issue implies that all BGP speakers participating
 in a confederation MUST support BGP confederations.  However, BGP
 speakers outside the confederation need not support these extensions.

9. Deployment Considerations

 BGP confederations have been widely deployed throughout the Internet
 for a number of years and are supported by multiple vendors.
 Improper configuration of BGP confederations can cause routing
 information within an AS to be duplicated unnecessarily.  This
 duplication of information will waste system resources, cause
 unnecessary route flaps, and delay convergence.
 Care should be taken to manually filter duplicate advertisements
 caused by reachability information being relayed through multiple
 member autonomous systems based upon the topology and redundancy
 requirements of the confederation.
 Additionally, confederations (as well as route reflectors), by
 excluding different reachability information from consideration at
 different locations in a confederation, have been shown to cause
 permanent oscillation between candidate routes when using the tie
 breaking rules required by BGP [1].  Care must be taken when
 selecting MED values and tie breaking policy to avoid these
 situations.
 One potential way to avoid this is by configuring inter-Member-AS IGP
 metrics higher than intra-Member-AS IGP metrics and/or using other
 tie breaking policies to avoid BGP route selection based on
 incomparable MEDs.

10. Security Considerations

 This extension to BGP does not change the underlying security issues
 inherent in the existing BGP, such as those defined in [6].

11. Acknowledgments

 The general concept of BGP confederations was taken from IDRP's
 Routing Domain Confederations [2].  Some of the introductory text in
 this document was taken from [5].
 The authors would like to acknowledge Bruce Cole of Juniper Networks
 for his implementation feedback and extensive analysis of the
 limitations of the protocol extensions described in this document and
 [5].  We would also like to acknowledge Srihari Ramachandra of Cisco
 Systems, Inc., for his feedback.

Traina, et al. Standards Track [Page 7] RFC 3065 Autonomous System Confederations for BGP February 2001

 Finally, we'd like to acknowledge Ravi Chandra and Yakov Rekhter for
 providing constructive and valuable feedback on earlier versions of
 this document.

12. References

 [1] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-4)", RFC
     1771, March 1995.
 [2] Kunzinger, C., Editor, "Inter-Domain Routing Protocol", ISO/IEC
     10747, October 1993.
 [3] Haskin, D., "A BGP/IDRP Route Server alternative to a full mesh
     routing", RFC 1863, October 1995.
 [4] Traina, P. "Autonomous System Confederations for BGP", RFC 1965,
     June 1996.
 [5] Bates, T., Chandra, R. and E. Chen, "BGP Route Reflection An
     Alternative to Full Mesh IBGP", RFC 2796, April 2000.
 [6] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
     Signature Option", RFC 2385, August 1998.

Traina, et al. Standards Track [Page 8] RFC 3065 Autonomous System Confederations for BGP February 2001

13. Authors' Addresses

 Paul Traina
 Juniper Networks, Inc.
 1194 N. Mathilda Ave.
 Sunnyvale, CA 94089 USA
 Phone: +1 408 745-2000
 EMail: pst+confed@juniper.net
 Danny McPherson
 Amber Networks, Inc.
 48664 Milmont Drive
 Fremont, CA 94538
 Phone: +1 510.687.5226
 EMail:  danny@ambernetworks.com
 John G. Scudder
 Cisco Systems, Inc.
 170 West Tasman Drive
 San Jose, CA 95134
 Phone: +1 734.669.8800
 EMail: jgs@cisco.com

Traina, et al. Standards Track [Page 9] RFC 3065 Autonomous System Confederations for BGP February 2001

Appendix A: Comparison with RFC 1965

 The most notable change from [1] is that of reversing the values
 AS_CONFED_SEQUENCE(4) and AS_CONFED_SET(3) to those defined in
 section "AS_CONFED Segment Type Extension".  The reasoning for this
 is that in the initial implementation, which was already widely
 deployed, they were implemented backwards from [4], and as such,
 subsequent implementations implemented them backwards as well.  In
 order to foster interoperability and compliance with deployed
 implementations, they've therefore been changed here as well.
 The "Compatibility Discussion" was removed and incorporated into
 other discussions in the document.  Also, the mention of hierarchical
 confederations is removed.  The use of the term "Routing Domain
 Identifier" was replaced with Member AS Number.
 Finally, the "Deployment Considerations" section was expanded a few
 subtle grammar changes were made and a bit more introductory text was
 added.

Traina, et al. Standards Track [Page 10] RFC 3065 Autonomous System Confederations for BGP February 2001

Full Copyright Statement

 Copyright (C) The Internet Society (2001).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Traina, et al. Standards Track [Page 11]

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