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Versions: (RFC 3065) 00 01 02 04 05 06 RFC 5065

INTERNET-DRAFT                                   Paul Traina
                                          Blissfully Retired
                                             Danny McPherson
                                              Arbor Networks
                                                John Scudder
                                            Juniper Networks
Expires: August 2007                           February 2007

                Autonomous System Confederations for BGP
                   <draft-ietf-idr-rfc3065bis-06.txt>



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Copyright Notice

   Copyright (C) The IETF Trust (2007).





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                                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.



































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Table of Contents


   1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . .   4
    1.1. Specification of Requirements . . . . . . . . . . . . . . .   4
    1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . .   4
   2. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3. AS_CONFED Segment Type Extension . . . . . . . . . . . . . . .   6
   4. Operation. . . . . . . . . . . . . . . . . . . . . . . . . . .   6
    4.1. AS_PATH Modification Rules. . . . . . . . . . . . . . . . .   7
   5. Error Handling . . . . . . . . . . . . . . . . . . . . . . . .   8
    5.1. Common Administrative Issues. . . . . . . . . . . . . . . .   9
    5.2. MED and LOCAL_PREF Handling . . . . . . . . . . . . . . . .   9
    5.3. AS_PATH and Path Selection. . . . . . . . . . . . . . . . .  10
   6. Compatability Considerations . . . . . . . . . . . . . . . . .  10
   7. Deployment Considerations. . . . . . . . . . . . . . . . . . .  11
   8. Security Considerations. . . . . . . . . . . . . . . . . . . .  11
   9. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . .  12
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   11. References. . . . . . . . . . . . . . . . . . . . . . . . . .  14
    11.1. Normative References . . . . . . . . . . . . . . . . . . .  14
    11.2. Informative References . . . . . . . . . . . . . . . . . .  14
   12. Appendix A: Aggregate Routing Information . . . . . . . . . .  15
   13. Appendix B: Changes From RFC 3065 . . . . . . . . . . . . . .  15
   14. Authors' Addresses. . . . . . . . . . . . . . . . . . . . . .  15


























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1.  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 [RFC 1863, RFC 2796].
   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], which is itself a revision
   to [RFC 1965].  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 [RFC 2119].



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.



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   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.

   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 [RFC 2796] 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



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   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.



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 long field with the
   two following values defined:

   Value     Segment Type

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

     2       AS_SEQUENCE: ordered set of autonomous systems 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



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   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.

   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.



4.1.  AS_PATH Modification Rules


   When implementing BGP Confederations Section 5.1.2 of [BGP-4] 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 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 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



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      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 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
         first AS_CONFED_SET/AS_CONFED_SEQUENCE segments, or if the
         first 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 Member-AS.  (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 Autonomous Systems 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).



5.  Error Handling


   A BGP speaker MUST NOT transmit updates containing AS_CONFED_SET or



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   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 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 [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 which 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 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.

   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



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   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 which 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 which is in the same confederation (not necessarily the
      same Member-AS) as "internal".



6.  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



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   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 [RFC 3365] 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
   [RFC 2385] and [BGP-VULN].









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9.  Acknowledgments


   The general concept of BGP confederations was taken from IDRP's
   Routing Domain Confederations [ISO 10747].  Some of the introductory
   text in this document was taken from [RFC 2796].

   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.



































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10.  IANA Considerations


This spefication introduces no new IANA considerations and therefore
requires no actions on the part of IANA.














































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11.  References




11.1.  Normative References


[BGP-4] Rekhter, Y., Li, T., and Hares, S., "A Border Gateway
    Protocol 4", RFC 4271.

[RFC 1965] Traina, P. "Autonomous System Confederations for BGP",
    RFC 1965, June 1996.

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



11.2.  Informative References


[ISO 10747] Kunzinger, C., Editor, "Inter-Domain Routing Protocol",
    ISO/IEC 10747, October 1993.

[RFC 1863] Haskin, D., "A BGP/IDRP Route Server alternative to a
    full mesh routing", RFC 1863, October 1995.

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

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

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

[RFC 3365] McPherson, D., Gill, V., Walton, D., Retana, A., "Border
    Gateway Protocol (BGP) Persistent Route Oscillation Condition",
    RFC 3345, August 2002.

[BGP-VULN] Murphy, S., "BGP Security Vulnerabilities Analysis",
    Internet-Draft, "Work in Progress".








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12.  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.



13.  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 explicitly call attention 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].




14.  Authors' Addresses








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   Paul Traina
   Blissfully Retired
   Email: bgp-confederations <possibly at> st04.pst.org

   Danny McPherson
   Arbor Networks
   EMail:  danny@arbor.net

   John G. Scudder
   Juniper Networks
   EMail: jgs@juniper.net



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