Network Working Group                                        Paul Traina
INTERNET DRAFT                                    Juniper Networks, Inc.
                                                         Danny McPherson
                                                                     TCB
                                                         John G. Scudder
SEPTEMBER 2002                                       Cisco Systems, Inc.



                Autonomous System Confederations for BGP
                  <draft-mcpherson-rfc3065bis-00.txt>

1. Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet- Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
        http://www.ietf.org/shadow.html.

2. 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



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   management complexity of maintaining a large autonomous system.


3. 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 [8].

4. 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 [3,6].  This document
   presents another alternative alleviating the need for a "full mesh"
   and is known as "Autonomous System Confederations for BGP", or
   simply, "BGP Confederations".  It has also been observed that BGP
   Confederations may provide improvements in routing policy control.

   This document is a revision of RFC 3065 [5], which is itself a
   revision to RFC 1965 [4].  It includes editorial changes,
   clarifications and corrections based on deployment experience with
   BGP Confederations.  These revisions are summarized in Appendices A
   and B.

5. 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



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   An autonomous system number visible only within a BGP confederation.

6. 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
   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 [6] 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.

   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.













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7. 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:

   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 specifies 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

8. Operation

   A member of a BGP confederation will use its AS Confederation
   Identifier in all transactions with peers that are not members of its
   confederation.  This confederation identifier is 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 BGP speaker.

   A BGP speaker receiving an AS_PATH attribute containing an autonomous
   system matching its own AS Confederation Identifier 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



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   a path containing its own AS number.

8.1. AS_PATH Modification Rules

   When implementing BGP Confederations 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:

   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
      configured 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, the local system shall prepend its own
         Member-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 Member-AS Number 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 configured autonomous system confederation, the advertising
      speaker shall update the AS_PATH attribute as follows:

      1) if any path segments of the AS_PATH are of the type
         AS_CONFED_SEQUENCE or AS_CONFED_SET, those segments shall
         be 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
         AS Confederation Identifier 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



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         first AS_CONFED_SET/AS_CONFED_SEQUENCE segments, or if the
         path segment of the remaining AS_PATH is not of type
         AS_SEQUENCE the local system shall prepend a new path segment
         of type AS_SEQUENCE to the AS_PATH, including its own AS
         Confederation Identifier in that segment.

   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 residing within the
      same autonomous system.  (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-ASs 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 AS Confederation
      Identifier 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 originating speaker's AS
      Confederation Identifier will be the only entry in the AS_PATH
      attribute).

9. Error Handling

   It is an error for a BGP speaker to receive an update message with an
   AS_PATH attribute which contains AS_CONFED_SEQUENCE or AS_CONFED_SET
   segments from a neighbor which is not located in the same
   confederation.  If a BGP speaker receives such an update message, it
   SHALL treat the message as having a malformed AS_PATH according to
   the procedures of [1] Section 6.3 ("UPDATE message error handling").















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10. 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].

11. Compatability Considerations

   All BGP speakers participating as member of 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".

   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.

12. 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-ASs based upon the topology and redundancy requirements of the
   confederation.

   Additionally, confederations (as well as route reflectors), by



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   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.

13. Security Considerations

   This extension to BGP does not change the underlying security issues
   inherent in the existing BGP, such as those defined in [7].

14. 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 [6].

   The authors would like to acknowledge Bruce Cole 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, Alex Zinin, Naresh
   Kumar Paliwal, Jeffrey Haas and Bruno Rijsman for their feedback and
   suggestions.

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


15. 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.



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   [5] Traina, P., McPherson, D. and Scudder, J., "Autonomous System
       Confederations for BGP", RFC 3065, February 2001.

   [6] Bates, T., Chandra, R. and E. Chen, "BGP Route Reflection An
       Alternative to Full Mesh IBGP", RFC 2796, April 2000.

   [7] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
       Signature Option", RFC 2385, August 1998.

   [8] Bradner, S., "Key words for use in RFCs to Indicate Requirement
       Levels", RFC 2119, March 1997.


16. 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
   TCB

   EMail:  danny@tcb.net


   John G. Scudder
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA 95134

   Phone: +1 734.302.4128
   EMail: jgs@cisco.com














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17. Appendix A: Comparison with RFC 1965

   The most notable change from [4] 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.  The use of the term "Routing
   Domain Identifier" was replaced with Member-AS Number.

   The mention of hierarchical confederations was removed due to the
   fact that it is not actually possible to deploy confederations
   hierarchically.  That is, a Member-AS of confederation A can only be
   a simple autonomous system, it cannot itself be a confederation B
   (whose internal topology is hidden from confederation A).

   Finally, the "Deployment Considerations" section was expanded a few
   subtle grammar changes were made and a bit more introductory text was
   added.

18. Appendix B: Comparison with RFC 3065

   Added discussion regarding inability to nest confederations.

   Added text regarding not propagating confederation attributes beyond
   confederation boundaries.

   Finally, made use of "Member-AS" and "AS Confederation Identifier"
   terminology more consistent.

















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