Network Working Group P. Traina
Request for Comments: 3065 Juniper Networks, Inc.
Obsoletes: 1965 D. McPherson
Category: Standards Track Amber Networks, Inc.
J. Scudder
Cisco Systems, Inc.
February 2001
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 Internet Society (2001). All Rights Reserved.
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.
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 [RFC2119].
2. 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 [3,5]. This document
represents another alternative in alleviating the need for a "full
mesh" and is known as "Autonomous System Confederations for BGP", or
simply, "BGP Confederations". It can also be said the BGP
Confederations MAY provide improvements in routing policy control.
This document is a revision of RFC1965 [4] and it includes editorial
changes, clarifications and corrections based on the deployment
eXPerience with BGP Confederations. These revisions are summarized
in Appendix A.
3. 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
An autonomous system number visible only internal to a BGP
confederation.
4. 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 [5] 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.
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 "AS 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.
5. 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 reserves 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
6. Operation
A member of a BGP confederation will use its AS Confederation ID 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 Member 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 an autonomous
system matching its own confederation 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.
6.1. 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 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, that segment and any immediately following
segments of the type AS_CONFED_SET or AS_CONFED_SEQUENCE 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 ID 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 ID
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
Member AS Number. (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-AS 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 autonomous system 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 autonomous system number of the originating speaker's
member confederation will be the only entry in the AS_PATH
attribute).
7. 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].
8. Compatability Considerations
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.
9. 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 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.
10. Security Considerations
This extension to BGP does not change the underlying security issues
inherent in the existing BGP, such as those defined in [6].
11. 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 [5].
The authors would like to acknowledge Bruce Cole of Juniper Networks
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 of Cisco
Systems, Inc., for his feedback.
Finally, we'd like to acknowledge Ravi Chandra and Yakov Rekhter for
providing constructive and valuable feedback on earlier versions of
this document.
12. 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", RFC1863, October 1995.
[4] Traina, P. "Autonomous System Confederations for BGP", RFC1965,
June 1996.
[5] Bates, T., Chandra, R. and E. Chen, "BGP Route Reflection An
Alternative to Full Mesh IBGP", RFC2796, April 2000.
[6] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC2385, August 1998.
13. 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
Amber Networks, Inc.
48664 Milmont Drive
Fremont, CA 94538
Phone: +1 510.687.5226
EMail: danny@ambernetworks.com
John G. Scudder
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
Phone: +1 734.669.8800
EMail: jgs@cisco.com
Appendix A: Comparison with RFC1965
The most notable change from [1] 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. Also, the mention of hierarchical
confederations is removed. The use of the term "Routing Domain
Identifier" was replaced with Member AS Number.
Finally, the "Deployment Considerations" section was expanded a few
suBTle grammar changes were made and a bit more introductory text was
added.
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