Autonomous System Confederations for BGP

Status 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 the 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.

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 traversed

Operation

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.

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.

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.

Compatibility

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.

Author's Address

Paul Traina
cisco Systems, Inc.
170 W. Tasman Dr.
San Jose, CA 95134

EMail:

          pst@cisco.com