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Network Working Group                                          P. TrainaRequest for Comments: 3065                        Juniper Networks, Inc.Obsoletes:1965                                             D. McPhersonCategory: Standards Track                           Amber Networks, Inc.                                                              J. Scudder                                                     Cisco Systems, Inc.                                                           February 2001Autonomous System Confederations for BGPStatus 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 20012. 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 ofRFC 1965 [4] and it includes editorial   changes, clarifications and corrections based on the deployment   experience with BGP Confederations.  These revisions are summarized   inAppendix 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 BGPTraina, 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              traversed6. 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)",RFC1771, 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 200113. 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.comTraina, et al.              Standards Track                     [Page 9]

RFC 3065        Autonomous System Confederations for BGP   February 2001Appendix A: Comparison withRFC 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 2001Full 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]