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Network Working Group                                          P. TrainaRequest for Comments: 1965                                 cisco SystemsCategory: Experimental                                         June 1996Autonomous System Confederations for BGPStatus of this Memo   This memo defines an Experimental Protocol for the Internet   community.  This memo does not specify an Internet standard of any   kind.  Discussion and suggestions for improvement are requested.   Distribution of this memo is unlimited.Abstract   Border Gateway Protocol [1] is an inter-autonomous system routing   protocol designed for TCP/IP networks.   This document describes an extension to BGP which may be used to   create a confederation of autonomous systems which is represented as   one single autonomous system to BGP peers external to the   confederation.   The intention of this extension is to aid in policy administration   and reduce the management complexity of maintaining a large   autonomous system.   The extension this document describes is widely deployed in the   Internet today.Introduction   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 chose to consider all BGP   speakers in a geographic region as a single entity.   In addition to improvements in routing policy control, current   techniques for deploying BGP among speakers in the same autonomous   system 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 theTraina                        Experimental                      [Page 1]

RFC 1965               AS Confederations for BGP               June 1996   connectivity requirements simplify to the model used for inter-domain   connections.   Unfortunately subdividing an autonomous system may increase the   complexity of policy routing 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 "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.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.Overview   IDRP[2] has the concept of a routing domain confederation.  An IDRP   routing domain confederation appears to IDRP speakers external to the   confederation as a single administrative entity.  This extension is   based upon that work.Traina                        Experimental                      [Page 2]

RFC 1965               AS Confederations for BGP               June 1996   In IDRP, routing domain confederations may be nested within each   other or disjoint portions of still larger confederations.  The   algorithm BGP defines for additions to the AS_PATH attribute imposes   an additional restriction that AS confederations must be strictly   hierarchical in nature.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 type/length/value   triple.   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 reserves two additional segment types:             3         AS_CONFED_SET: unordered set of ASs in the local                              confederation that the UPDATE message                              has traversed             4         AS_CONFED_SEQUENCE: ordered set of ASs in the                              local confederation that the UPDATE                              message has traversedOperation   A member of a BGP confederation will use its confederation identifier   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 routing domain   identifier (the internally visible AS number) in all transactions   with peers that are members of the same confederation as the given   router.Traina                        Experimental                      [Page 3]

RFC 1965               AS Confederations for BGP               June 1996   A BGP speaker receiving an AS_PATH attribute containing a   confederation ID matching its own confederation shall treat the path   in the same fashion as if it had received a path containing its own   AS number.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:      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 routing domain 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, that segment and any immediately         following segments of the type AS_CONFED_SET are removed from         the AS_PATH attribute, leaving the sanitized AS_PATH         attribute to be operated on by steps 2, or 3.Traina                        Experimental                      [Page 4]

RFC 1965               AS Confederations for BGP               June 1996         2) if the first path segment of the remaining AS_PATH is of         type AS_SEQUENCE, the local system shall prepend its own         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 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         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 located in      its own autonomous system. (An empty AS_PATH attribute is one      whose length field contains the value zero).      b) the originating speaker shall include its own AS number in an      AS_CONFED_SEQUENCE segment of the AS_PATH attribute of all      UPDATE messages sent to BGP speakers located in neighboring      autonomous systems that are members of the local confederation.      (In this case, the AS number of the originating speaker's member      autonomous system number will be the only entry in the AS_PATH      attribute).      c) the originating speaker shall include its own confederation      identifier in a 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 confederation identifier of      the originating speaker's member confederation will be the only      entry in the AS_PATH attribute).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 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 may follow the same rules used for information received   from members inside the same autonomous system.Traina                        Experimental                      [Page 5]

RFC 1965               AS Confederations for BGP               June 1996Compatibility   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".   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.Compatibility Discussion   We considered the use of a distinct, optional, transitive attribute   to carry AS confederation information as opposed to specifying new   types in the existing AS path attribute.  This would relax the   requirement that all BGP speakers participating in a confederation to   allow the use of legacy units provided they have no external (i.e.   neither inter-AS nor intra-confederation) connectivity.   At the time of this writing, an implementation of this extension as   documented is widely deployed throughout the Internet,  therefore the   value of any change that is incompatible with this document must be   weighed against the benefit gained from a relaxation of this   restriction.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.Security Considerations   Security issues are not discussed in this memo.Acknowledgments   Ravi Chandra and Yakov Rekhter reviewed this document and provided   constructive and valuable comments.Traina                        Experimental                      [Page 6]

RFC 1965               AS Confederations for BGP               June 1996Author's Address   Paul Traina   cisco Systems, Inc.   170 W. Tasman Dr.   San Jose, CA 95134   EMail: pst@cisco.comTraina                        Experimental                      [Page 7]