Internet DRAFT - draft-chen-as4bytes
draft-chen-as4bytes
Network Working Group Enke Chen
Internet Draft Redback Networks, Inc.
Expiration Date: May 2001 Yakov Rekhter
cisco Systems
BGP support for four-octet AS number space
draft-chen-as4bytes-00.txt
1. Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026 except that the right to
produce derivative works is not granted.
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The list of current Internet-Drafts can be accessed at
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The list of Internet-Draft Shadow Directories can be accessed at
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2. Abstract
Currently the Autonomous System number is encoded in BGP as a two-
octets field. This document describes extentions to BGP to carry the
Autonomous System number as a four-octets field.
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Internet Draft draft-chen-as4bytes-00.txt November 2000
3. Protocol Extensions
BGP carries the Autonomous System number in the My Autonomous System
field of the OPEN message, in the AS_PATH attribute of the UPDATE
message, and in the AGGREGATOR attribute of the UPDATE message.
A BGP speaker that is capable of supporting 4-octets Autonomous
System numbers uses BGP Capability Advertisements [BGP-CAP] to
advertise it to its neighbors (either internal or external). The BGP
Capability code for the 4-octets Autonomous System number capability
is [TBD]. The Capability that is used by a BGP speaker to convey to
its BGP peer the 4-octets Autonomous System number capability carries
the 4-octets Autonomous System number of the speaker in the
Capability Value field of the Optional Parameter. The Capability
Length field of the Capability is set to 4.
To carry AS path information expressed in terms of 4-octets
Autonomous Systems numbers, we use the existing AS_PATH attribute,
except that each AS in this attribute is expressed not as a 2-octets,
but as a 4-octets entity. The same applies to the AGGREGATOR
attribute - we use the same attribute, except that the AS carried in
this attribute is encoded as a 4-octets entity.
Currently assigned 2-octets Autonomous System numbers are converted
into 4-octets Autonomous System numbers by setting the high-order 2
octets of the 4-octets field to zero.
4. Operations
A BGP speaker that supports 4-octets Autonomous System numbers may
advertise this to its peers using the BGP Capability Advertisements.
A BGP speaker that advertises such capability to a particular peer,
and receives from that peer the advertisement of such capability MUST
encode Autonomous System numbers as 4-octets entities in both the
AS_PATH and the AGGREGATOR attributes in the updates it sends to the
peer, and MUST assume that these attributes in the updates received
from the peer encode Autonomous System numbers as 4-octets entities.
In all other cases the speaker MUST encode Autonomous System numbers
as 2-octets entities in both the AS_PATH and the AGGREGATOR attribute
in the updates it sends to the peer, and MUST assume that these
attributes in the updates received from the peer encoded Autonomous
System numbers as 2-octets entities.
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Internet Draft draft-chen-as4bytes-00.txt November 2000
5. Transition
In an ideal case we assume that all the BGP speakers in the Internet
will become capable of supporting 4-octets Autonomous System numbers
before we'll exhaust the current (2-octets) Autonomous System number
space. Assignment of 4-octets Autonomous System numbers would start
only after we'll exhaust the current Autonomous System number space,
and thus only after all the BGP speakers in the Internet will become
capable of supporting 4-octets Autonomous System numbers.
During the transition a BGP speaker that supports 4-octets Autonomous
System numbers may have some of its peers capable of supporting 4-
octets Autonomous System number, and others capable of supporting
only 2-octets Autonomous System number. In this case the speaker may
need to convert between the 2 and the 4 octets Autonomous System
numbers, as a route received from one peer may carry Autonomous
System numbers encoded as 4 octets, while re-advertising this route
to other peer may require the speaker to encode Autonomous System
numbers as 2 octets. Note that as long as no Autonomous System is
assigned a 4-octets Autonomous System number this conversion is
straightforward.
In a less than ideal case we need to assume that not all the BGP
speakers in the Internet will become capable of supporting 4-octets
Autonomous System numbers prior to the exhaustion of the current (2-
octets) Autonomous System number space. In this case conversion from
4-octets to 2-octets may not be possible for any of the 4-octets
assigned Autonomous System number. One way to deal with this
situation is to truncate the AS_PATH information as to exclude all of
the 4-octets assigned Autonomous System numbers, although doing this
may result in a formation of forwarding loops.
6. Security Considerations
Security issues are not discussed in this document.
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Internet Draft draft-chen-as4bytes-00.txt November 2000
7. Acknowledgements
TBD.
8. References
9. Author Information
Enke Chen
Redback Networks, Inc.
350 Holger Way
San Jose, CA 95134
e-mail: enke@redback.com
Yakov Rekhter
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
e-mail: yakov@cisco.com
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