INTERNET-DRAFT Paul Traina
Danny McPherson
Arbor Networks, Inc.
John Scudder
Cisco Systems, Inc.
Expires: April 2006 October 2005
Autonomous System Confederations for BGP
<draft-ietf-idr-rfc3065bis-05.txt>
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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. 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. . . . . . . . . . . . . . . . . . . . . . . . 11
10. References. . . . . . . . . . . . . . . . . . . . . . . . . . 13
10.1. Normative References . . . . . . . . . . . . . . . . . . . 13
10.2. Informative References . . . . . . . . . . . . . . . . . . 13
11. Authors' Addresses. . . . . . . . . . . . . . . . . . . . . . 14
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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. 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.
Member-AS Number
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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
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.
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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
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.
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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
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.
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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
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
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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
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.
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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
in a confederation MUST support BGP confederations. However, BGP
speakers outside the confederation need not support these extensions.
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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]
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].
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 Bruce Cole for his
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implementation feedback and extensive analysis of the limitations of
the protocol extensions described in this document and [RFC 3065].
We would also like to acknowledge Srihari Ramachandra, Alex Zinin,
Naresh Kumar Paliwal, Jeffrey Haas, Cengiz Alaettinoglu 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. References
10.1. Normative References
[BGP-4] Rekhter, Y., Li, T., and Hares, S., "A Border Gateway
Protocol 4", Internet-Draft, "Work in Progress".
[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.
10.2. Informative References
[ISO 10747] Kunzinger, C., Editor, "Inter-Domain Routing Protocol",
ISO/IEC 10747, October 1993.
[RFC 1771] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
(BGP-4)", RFC 1771, March 1995.
[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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11. Authors' Addresses
Paul Traina
EMail: pst+confed@spamcatcher.bogus.com
Danny McPherson
Arbor Networks, Inc.
Phone: +1 303.470.9257
EMail: danny@arbor.net
John G. Scudder
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
Phone: +1 734.302.4128
EMail: jgs@cisco.com
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