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rfc:rfc5065

Network Working Group P. Traina Request for Comments: 5065 Blissfully Retired Obsoletes: 3065 D. McPherson Category: Standards Track Arbor Networks

                                                            J. Scudder
                                                      Juniper Networks
                                                           August 2007
              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 IETF Trust (2007).

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 that 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.
 This document obsoletes RFC 3065.

Traina, et al. Standards Track [Page 1] RFC 5065 August 2007

Table of Contents

 1. Introduction ....................................................3
    1.1. Specification of Requirements ..............................3
    1.2. Terminology ................................................3
 2. Discussion ......................................................4
 3. AS_CONFED Segment Type Extension ................................5
 4. Operation .......................................................5
    4.1. AS_PATH Modification Rules .................................6
 5. Error Handling ..................................................8
    5.1. Error Handling .............................................8
    5.2. MED and LOCAL_PREF Handling ................................8
    5.3. AS_PATH and Path Selection .................................9
 6. Compatibility Considerations ...................................10
 7. Deployment Considerations ......................................10
 8. Security Considerations ........................................10
 9. Acknowledgments ................................................11
 10. References ....................................................11
    10.1. Normative References .....................................11
    10.2. Informative References ...................................11
 Appendix A. Aggregate Routing Information .........................13
 Appendix B. Changes from RFC 3065 .................................13

Traina, et al. Standards Track [Page 2] RFC 5065 August 2007

1. Introduction

 As originally 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 Internal BGP (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, such as [RFC2796] and
 [RFC1863] (made historic by [RFC4223]).  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, and obsoletes, [RFC3065], which is
 itself a revision of [RFC1965].  It includes editorial changes,
 terminology clarifications, and more explicit protocol specifications
 based on extensive implementation and deployment experience with BGP
 Confederations.

1.1. 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 [RFC2119].

1.2. Terminology

 AS Confederation
    A collection of autonomous systems represented and advertised as a
    single AS number to BGP speakers that are not members of the local
    BGP confederation.
 AS Confederation Identifier
    An externally visible autonomous system number that identifies a
    BGP confederation as a whole.
 Member Autonomous System (Member-AS)
    An autonomous system that is contained in a given AS
    confederation.  Note that "Member Autonomous System" and "Member-
    AS" are used entirely interchangeably throughout this document.

Traina, et al. Standards Track [Page 3] RFC 5065 August 2007

 Member-AS Number
    An autonomous system number identifier visible only within a BGP
    confederation, and used to represent a Member-AS within that
    confederation.

2. 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 [RFC4456] are not
 employed, [BGP-4] 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.

Traina, et al. Standards Track [Page 4] RFC 5065 August 2007

3. 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 [BGP-4], the path segment type is a 1-octet field with the two
 following values defined:
 Value     Segment Type
   1       AS_SET: unordered set of autonomous systems that a route in
           the UPDATE message has traversed
   2       AS_SEQUENCE: ordered set of autonomous systems that a route
           in the UPDATE message has traversed
 This document specifies two additional segment types:
   3       AS_CONFED_SEQUENCE: ordered set of Member Autonomous
           Systems in the local confederation that the UPDATE message
           has traversed
   4       AS_CONFED_SET: unordered set of Member Autonomous Systems
           in the local confederation that the UPDATE message has
           traversed

4. Operation

 A member of a BGP confederation MUST use its AS Confederation
 Identifier in all transactions with peers that are not members of its
 confederation.  This AS 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 MUST use its Member-AS Number in all
 transactions with peers that are members of the same confederation as
 the local 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.

Traina, et al. Standards Track [Page 5] RFC 5065 August 2007

 A BGP speaker receiving an AS_PATH attribute containing an
 AS_CONFED_SEQUENCE or AS_CONFED_SET that 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.

4.1. AS_PATH Modification Rules

 When implementing BGP confederations, Section 5.1.2 of [BGP-4] is
 replaced with the following text:
 AS_PATH is a well-known mandatory attribute.  This attribute
 identifies the autonomous systems through which routing information
 carried in this UPDATE message has passed.  The components of this
 list can be AS_SETs, AS_SEQUENCEs, AS_CONFED_SETs or
 AS_CONFED_SEQUENCES.
 When a BGP speaker propagates a route it learned from another BGP
 speaker's UPDATE message, it modifies 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 Member-AS, 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 confederation, the advertising speaker updates 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 prepends its own Member-AS
       number as the last element of the sequence (put it in the
       leftmost position with respect to the position of octets in the
       protocol message).  If the act of prepending will cause an
       overflow in the AS_PATH segment (i.e., more than 255 ASs), it
       SHOULD prepend a new segment of type AS_CONFED_SEQUENCE and
       prepend its own AS number to this new segment.
    2) if the first path segment of the AS_PATH is not of type
       AS_CONFED_SEQUENCE, the local system prepends a new path
       segment of type AS_CONFED_SEQUENCE to the AS_PATH, including
       its own Member-AS Number in that segment.
    3) if the AS_PATH is empty, the local system creates a path
       segment of type AS_CONFED_SEQUENCE, places its own Member-AS
       Number into that segment, and places that segment into the
       AS_PATH.

Traina, et al. Standards Track [Page 6] RFC 5065 August 2007

 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 local 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 MUST be
       removed from the AS_PATH attribute, leaving the sanitized
       AS_PATH attribute to be operated on by steps 2, 3 or 4.
    2) if the first path segment of the remaining AS_PATH is of type
       AS_SEQUENCE, the local system prepends its own AS Confederation
       Identifier as the last element of the sequence (put it in the
       leftmost position with respect to the position of octets in the
       protocol message).  If the act of prepending will cause an
       overflow in the AS_PATH segment (i.e., more than 255 ASs), it
       SHOULD prepend a new segment of type AS_SEQUENCE and prepend
       its own AS number to this new segment.
    3) if the first path segment of the remaining AS_PATH is of type
       AS_SET, the local system prepends a new path segment of type
       AS_SEQUENCE to the AS_PATH, including its own AS Confederation
       Identifier in that segment.
    4) if the remaining AS_PATH is empty, the local system creates a
       path segment of type AS_SEQUENCE, places its own AS
       Confederation Identifier into that segment, and places that
       segment into the AS_PATH.
 When a BGP speaker originates a route then:
 a) the originating speaker includes its own AS Confederation
    Identifier in a path segment, of type AS_SEQUENCE, in 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 AS Confederation Identifier of
    the originating speaker's autonomous system will be the only entry
    the path segment, and this path segment will be the only segment
    in the AS_PATH attribute.
 b) the originating speaker includes its own Member-AS Number in a
    path segment, of type AS_CONFED_SEQUENCE, in the AS_PATH attribute
    of all UPDATE messages sent to BGP speakers located in neighboring
    Member Autonomous Systems that are members of the local
    confederation.  In this case, the Member-AS Number of the
    originating speaker's autonomous system will be the only entry the
    path segment, and this path segment will be the only segment in
    the AS_PATH attribute.

Traina, et al. Standards Track [Page 7] RFC 5065 August 2007

 c) the originating speaker includes an empty AS_PATH attribute in all
    UPDATE messages sent to BGP speakers residing within the same
    Member-AS.  (An empty AS_PATH attribute is one whose length field
    contains the value zero).
 Whenever the modification of the AS_PATH attribute calls for
 including or prepending the AS Confederation Identifier or Member-AS
 Number of the local system, the local system MAY include/prepend more
 than one instance of that value in the AS_PATH attribute.  This is
 controlled via local configuration.

5. Error Handling

 A BGP speaker MUST NOT transmit updates containing AS_CONFED_SET or
 AS_CONFED_SEQUENCE attributes to peers that are not members of the
 local confederation.
 It is an error for a BGP speaker to receive an UPDATE message with an
 AS_PATH attribute that contains AS_CONFED_SEQUENCE or AS_CONFED_SET
 segments from a neighbor that 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 [BGP-4], Section 6.3 ("UPDATE Message Error
 Handling").
 It is a error for a BGP speaker to receive an update message from a
 confederation peer that is not in the same Member-AS that does not
 have AS_CONFED_SEQUENCE as the first segment.  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 [BGP-4], Section
 6.3 ("UPDATE Message Error Handling").

5.1. Common Administrative Issues

 It is reasonable for Member Autonomous Systems of a confederation to
 share a common administration and Interior Gateway Protocol (IGP)
 information for the entire confederation.  It is also reasonable for
 each Member-AS to run an independent IGP.  In the latter case, the
 NEXT_HOP may need to be set using policy (i.e., by default it is
 unchanged).

5.2. MED and LOCAL_PREF Handling

 It SHALL be legal for a BGP speaker to advertise an unchanged
 NEXT_HOP and MULTI_EXIT_DISC (MED) attribute to peers in a
 neighboring Member-AS of the local confederation.

Traina, et al. Standards Track [Page 8] RFC 5065 August 2007

 MEDs of two routes SHOULD only be compared if the first autonomous
 systems in the first AS_SEQUENCE in both routes are the same -- i.e.,
 skip all the autonomous systems in the AS_CONFED_SET and
 AS_CONFED_SEQUENCE.  An implementation MAY provide the ability to
 configure path selection such that MEDs of two routes are comparable
 if the first autonomous systems in the AS_PATHs are the same,
 regardless of AS_SEQUENCE or AS_CONFED_SEQUENCE in the AS_PATH.
 An implementation MAY compare MEDs received from a Member-AS via
 multiple paths.  An implementation MAY compare MEDs from different
 Member Autonomous Systems of the same confederation.
 In addition, the restriction against sending the LOCAL_PREF attribute
 to peers in a neighboring autonomous system within the same
 confederation is removed.

5.3. AS_PATH and Path Selection

 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 [BGP-4].
 In addition, the following rules SHALL be applied:
 1) If the AS_PATH is internal to the local confederation (i.e., there
    are only AS_CONFED_* segments), consider the neighbor AS to be the
    local AS.
 2) Otherwise, if the first segment in the path that is not an
    AS_CONFED_SEQUENCE or AS_CONFED_SET is an AS_SEQUENCE, consider
    the neighbor AS to be the leftmost AS_SEQUENCE AS.
 3) When comparing routes using AS_PATH length, CONFED_SEQUENCE and
    CONFED_SETs SHOULD NOT be counted.
 4) When comparing routes using the internal (IBGP learned) versus
    external (EBGP learned) rules, treat a route that is learned from
    a peer that is in the same confederation (not necessarily the same
    Member-AS) as "internal".

Traina, et al. Standards Track [Page 9] RFC 5065 August 2007

6. Compatibility Considerations

 All BGP speakers participating as members 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.

7. 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 [RFC3345] to
 cause permanent oscillation between candidate routes when using the
 tie-breaking rules required by BGP [BGP-4].  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.

8. Security Considerations

 This extension to BGP does not change the underlying security issues
 inherent in the existing BGP protocol, such as those described in
 [RFC2385] and [BGP-VULN].

Traina, et al. Standards Track [Page 10] RFC 5065 August 2007

9. Acknowledgments

 The general concept of BGP confederations was taken from IDRP's
 Routing Domain Confederations [ISO10747].  Some of the introductory
 text in this document was taken from [RFC2796].
 The authors would like to acknowledge Jeffrey Haas for his extensive
 feedback on this document.  We'd also like to thank Bruce Cole,
 Srihari Ramachandra, Alex Zinin, Naresh Kumar Paliwal, Jeffrey Haas,
 Cengiz Alaettinoglu, Mike Hollyman, 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.

10. References

10.1. Normative References

 [BGP-4]    Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
            Border Gateway Protocol 4 (BGP-4)", RFC 4271, January
            2006.
 [RFC1965]  Traina, P., "Autonomous System Confederations for BGP",
            RFC 1965, June 1996.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC3065]  Traina, P., McPherson, D., and J. Scudder, "Autonomous
            System Confederations for BGP", RFC 3065, February 2001.

10.2. Informative References

 [ISO10747] Kunzinger, C., Editor, "Inter-Domain Routing Protocol",
            ISO/IEC 10747, October 1993.
 [RFC1863]  Haskin, D., "A BGP/IDRP Route Server alternative to a full
            mesh routing", RFC 1863, October 1995.
 [RFC2385]  Heffernan, A., "Protection of BGP Sessions via the TCP MD5
            Signature Option", RFC 2385, August 1998.
 [RFC3345]  McPherson, D., Gill, V., Walton, D., and A. Retana,
            "Border Gateway Protocol (BGP) Persistent Route
            Oscillation Condition", RFC 3345, August 2002.

Traina, et al. Standards Track [Page 11] RFC 5065 August 2007

 [RFC4223]  Savola, P., "Reclassification of RFC 1863 to Historic",
            RFC 4223, October 2005.
 [RFC4272]  Murphy, S., "BGP Security Vulnerabilities Analysis", RFC
            4272, January 2006.
 [RFC4456]  Bates, T., Chen, E., and R. Chandra, "BGP Route
            Reflection: An Alternative to Full Mesh Internal BGP
            (IBGP)", RFC 4456, April 2006.

Traina, et al. Standards Track [Page 12] RFC 5065 August 2007

Appendix A. Aggregate Routing Information

 As a practical matter, aggregation as discussed in [BGP-4], Section
 9.2.2.2, is not generally employed within confederations.  However,
 in the event that such aggregation is performed within a
 confederation, the rules of [BGP-4] should be followed, making the
 necessary substitutions between AS_SET and AS_CONFED_SET and
 similarly, AS_SEQUENCE and AS_CONFED_SEQUENCE.  Confederation-type
 segments (AS_CONFED_SET and AS_CONFED_SEQUENCE) MUST be kept separate
 from non-confederation segments (AS_SET and AS_SEQUENCE).  An
 implementation could also choose to provide a form of aggregation
 wherein non-confederation segments are aggregated as discussed in
 [BGP-4], Section 9.2.2.2, and confederation-type segments are not
 aggregated.
 Support for aggregation of confederation-type segments is not
 mandatory.

Appendix B. Changes from RFC 3065

 The primary trigger for an update to RFC 3065 was regarding issues
 associated with AS path segment handling, in particular what to do
 when interacting with BGP peers external to a confederation and to
 ensure AS_CONFED_[SET|SEQUENCE] segment types are not propagated to
 peers outside of a confederation.
 As such, the "Error Handling" section above was added and applies not
 only to BGP confederation speakers, but to all BGP speakers.
 Other changes are mostly trivial and surrounding some clarification
 and consistency in terminology and denoting that
 AS_CONFED_[SET|SEQUENCE] Segment Type handling should be just as it
 is in the base BGP specification [BGP-4].

Authors' Addresses

 Paul Traina
 Blissfully Retired
 Email: bgp-confederations@st04.pst.org
 Danny McPherson
 Arbor Networks
 EMail: danny@arbor.net
 John G. Scudder
 Juniper Networks
 EMail: jgs@juniper.net

Traina, et al. Standards Track [Page 13] RFC 5065 August 2007

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Traina, et al. Standards Track [Page 14]

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