Network Working Group Q. Vohra

Network Working Group Q. Vohra
Request for Comments: 4893 Juniper Networks
Category: Standards Track E. Chen
Cisco Systems
May 2007

BGP Support for Four-octet AS Number Space

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 IETF Trust (2007).

Abstract

Currently the Autonomous System (AS) number is encoded as a two-octet
entity in BGP. This document describes extensions to BGP to carry the
Autonomous System number as a four-octet entity.

1. Introduction

Currently the Autonomous System number is encoded as a two-octet
entity in BGP [BGP]. To prepare for the anticipated exhaustion of
the two-octet AS numbers, this document describes extensions to BGP
to carry the Autonomous System number as a four-octet entity.

More specifically, this document defines a new BGP capability, Four-
octet AS Number Capability, that can be used by a BGP speaker to
indicate its support for the four-octet AS numbers. Two new
attributes, AS4_PATH and AS4_AGGREGATOR, are introduced that can be
used to propagate four-octet based AS path information across BGP
speakers that do not support the four-octet AS numbers. This
document also specifies mechanisms for constructing the AS path
information from the AS_PATH attribute and the AS4_PATH attribute.

The extensions proposed in this document allow a gradual transition
from 2-octet AS numbers to 4-octet AS numbers.

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RFC 4893 BGP Support for Four-octet AS Number Space May 2007

2. 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].

3. Protocol Extensions

For the purpose of this document we define a BGP speaker that does
not support the new 4-octet AS number extensions as an OLD BGP
speaker, and a BGP speaker which supports the new 4-octet AS number
extensions as a NEW BGP speaker.

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. BGP also carries the Autonomous System number in the BGP
Communities attribute.

A NEW BGP speaker uses BGP Capability Advertisements [RFC2842] to
advertise to its neighbors (either internal or external) that it
supports 4-octet AS number extensions, as specified in this document.

The Capability that is used by a BGP speaker to convey to its BGP
peer the 4-octet Autonomous System number capability, also carries
the 4-octet Autonomous System number of the speaker in the Capability
Value field of the Capability Optional Parameter. The Capability
Length field of the Capability is set to 4.

NEW BGP speakers carry AS path information expressed in terms of 4-
octet Autonomous Systems numbers by using the existing AS_PATH
attribute, except that each AS number in this attribute is encoded
not as a 2-octet, but as a 4-octet entity. The same applies to the
AGGREGATOR attribute - NEW BGP speakers use the same attribute,
except that the AS carried in this attribute is encoded as a 4-octet
entity.

To preserve AS path information with 4-octet AS numbers across OLD
BGP speakers, this document defines a new AS path attribute, called
AS4_PATH. This is an optional transitive attribute that contains the
AS path encoded with 4-octet AS numbers. The AS4_PATH attribute has
the same semantics as the AS_PATH attribute, except that it is
optional transitive, and it carries 4-octet AS numbers.

To prevent the possible propagation of confederation path segments
outside of a confederation, the path segment types AS_CONFED_SEQUENCE
and AS_CONFED_SET [RFC3065] are declared invalid for the AS4_PATH
attribute.

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RFC 4893 BGP Support for Four-octet AS Number Space May 2007

Similarly, this document defines a new aggregator attribute called
AS4_AGGREGATOR, which is optional transitive. The AS4_AGGREGATOR
attribute has the same semantics as the AGGREGATOR attribute, except
that it carries a 4-octet AS number.

Currently assigned 2-octet Autonomous System numbers are converted
into 4-octet Autonomous System numbers by setting the two high-order
octets of the 4-octet field to zero. Such a 4-octet AS number is
said to be mappable to a 2-octet AS number.

To represent 4-octet AS numbers (which are not mapped from 2-octets)
as 2-octet AS numbers in the AS path information encoded with 2-octet
AS numbers, this document reserves a 2-octet AS number. We denote
this special AS number as AS_TRANS for ease of description in the
rest of this specification. This AS number is also placed in the "My
Autonomous System" field of the OPEN message originated by a NEW BGP
speaker, if the speaker does not have a (globally unique) 2-octet AS
number.

4. Operations

4.1. Interaction Between NEW BGP Speakers

A BGP speaker that supports 4-octet Autonomous System numbers SHOULD
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-octet 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-octet entities.

The new attributes, AS4_PATH and AS4_AGGREGATOR SHOULD NOT be carried
in the UPDATE messages between NEW BGP peers. A NEW BGP speaker that
receives the AS4_PATH and AS4_AGGREGATOR path attributes in an UPDATE
message from a NEW BGP speaker SHOULD discard these path attributes
and continue processing the UPDATE message.

4.2. Interaction Between NEW and OLD BGP Speakers

4.2.1. BGP Peering

Note that peering between a NEW BGP speaker and an OLD one is
possible only if the NEW BGP speaker has a 2-octet AS number.
However, this document does not assume that an Autonomous System with
NEW speakers has to have a globally unique 2-octet AS number --
AS_TRANS could be used instead (even if a multiple Autonomous System
would use it).

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RFC 4893 BGP Support for Four-octet AS Number Space May 2007

4.2.2. Generating Updates

When communicating with an OLD BGP speaker, a NEW speaker MUST send
the AS path information in the AS_PATH attribute encoded with 2-octet
AS numbers. The NEW speaker MUST also send the AS path information
in the AS4_PATH attribute (encoded with 4-octet AS numbers), except
for the case where the entire AS path information is composed of 2-
octet AS numbers only. In this case, the NEW speaker SHOULD NOT send
the AS4_PATH attribute.

In the AS_PATH attribute encoded with 2-octet AS numbers, non-
mappable 4-octet AS numbers are represented by the well-known 2-octet
AS number, AS_TRANS. This will preserve the path length property of
the AS path information and also help in updating the AS path
information received on a NEW BGP speaker from an OLD speaker, as
explained in the next section.

The NEW speaker constructs the AS4_PATH attribute from the
information carried in the AS_PATH attribute. In the case where the
AS_PATH attribute contains either AS_CONFED_SEQUENCE or AS_CONFED_SET
path segments, the NEW speaker, when constructing the AS4_PATH
attribute from the AS_PATH attribute, MUST exclude such path
segments. The AS4_PATH attribute will be carried across a series of
OLD BGP speakers without modification and will help preserve the
truly 4-octet AS numbers in the AS path information.

Similarly, if the NEW speaker has to send the AGGREGATOR attribute,
and if the aggregating Autonomous System's AS number is truly 4-
octets, then the speaker constructs the AS4_AGGREGATOR attributes by
taking the attribute length and attribute value from the AGGREGATOR
attribute and placing them into the attribute length and attribute
value of the AS4_AGGREGATOR attribute, and sets the AS number field
in the existing AGGREGATOR attribute to the reserved AS number,
AS_TRANS. Note that if the AS number is 2-octets only, then the
AS4_AGGREGATOR attribute SHOULD NOT be sent.

4.2.3. Processing Received Updates

When a NEW BGP speaker receives an update from an OLD one, it should
be prepared to receive the AS4_PATH attribute along with the existing
AS_PATH attribute. If the AS4_PATH attribute is also received, both
the attributes will be used to construct the exact AS path
information, and therefore the information carried by both the
attributes will be considered for AS path loop detection.

Note that a route may have traversed a series of autonomous systems
with 2-octet AS numbers and OLD BGP speakers only. In that case, if
the route carries the AS4_PATH attribute, this attribute must have

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RFC 4893 BGP Support for Four-octet AS Number Space May 2007

remained unmodified since the route left the last NEW BGP speaker.
The trailing AS path information (representing autonomous systems
with 2-octet AS numbers and OLD BGP speakers only) is contained only
in the current AS_PATH attribute (encoded in the leading part of the
AS_PATH attribute).

Under certain conditions, it may not be possible to reconstruct the
entire AS path information from the AS_PATH and the AS4_PATH
attributes of a route. This occurs when two or more routes that
carry the AS4_PATH attribute are aggregated by an OLD BGP speaker,
and the AS4_PATH attribute of at least one of these routes carries at
least one 4-octet AS number (as oppose to a 2-octet AS number that is
encoded in 4 octets). Depending on the implementation, either the
AS4_PATH attribute would be lost during route aggregation, or both
the AS_PATH attribute and the AS4_PATH attribute would contain valid,
partial information that cannot be combined seamlessly, resulting in
incomplete AS path information in these cases.

A NEW BGP speaker should also be prepared to receive the
AS4_AGGREGATOR attribute along with the AGGREGATOR attribute from an
OLD BGP speaker. When both the attributes are received, if the AS
number in the AGGREGATOR attribute is not AS_TRANS, then:

- the AS4_AGGREGATOR attribute and the AS4_PATH attribute SHALL
be ignored,

- the AGGREGATOR attribute SHALL be taken as the information
about the aggregating node, and

- the AS_PATH attribute SHALL be taken as the AS path
information.

Otherwise,

- the AGGREGATOR attribute SHALL be ignored,

- the AS4_AGGREGATOR attribute SHALL be taken as the information
about the aggregating node, and

- the AS path information would need to be constructed, as in all
other cases.

In order to construct the AS path information, it would be necessary
to first calculate the number of AS numbers in the AS_PATH and
AS4_PATH attributes using the method specified in Section 9.1.2.2
[BGP] and [RFC3065] for route selection.

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RFC 4893 BGP Support for Four-octet AS Number Space May 2007

If the number of AS numbers in the AS_PATH attribute is less than the
number of AS numbers in the AS4_PATH attribute, then the AS4_PATH
attribute SHALL be ignored, and the AS_PATH attribute SHALL be taken
as the AS path information.

If the number of AS numbers in the AS_PATH attribute is larger than
or equal to the number of AS numbers in the AS4_PATH attribute, then
the AS path information SHALL be constructed by taking as many AS
numbers and path segments as necessary from the leading part of the
AS_PATH attribute, and then prepending them to the AS4_PATH attribute
so that the AS path information has an identical number of AS numbers
as the AS_PATH attribute. Note that a valid AS_CONFED_SEQUENCE or
AS_CONFED_SET path segment SHALL be prepended if it is either the
leading path segment or adjacent to a path segment that is prepended.

5. Handling BGP Communities

As specified in [RFC1997], when the high-order two-octets of the
community attribute is neither 0x0000 nor 0xffff, these two octets
encode the Autonomous System number. Quite clearly this would not
work for BGP speakers that use 4-octets Autonomous System numbers.
Such BGP speakers should use the Four-octet AS Specific Extended
Communities [AS-EXT-COM] instead.

6. Transition

The scheme described in this document allows a gradual transition
from 2-octet AS numbers to 4-octet AS numbers. One can upgrade one
Autonomous System or one BGP speaker at a time.

To simplify transition, this document assumes that an Autonomous
System could start using a 4-octet AS number only after all the BGP
speakers within that Autonomous System have been upgraded to support
4-octet AS numbers.

An OLD BGP speaker MUST NOT use AS_TRANS as its Autonomous System
number.

A non-mappable 4-octet AS number cannot be used as a "Member AS
Number" of a BGP Confederation until all the BGP speakers within the
Confederation have transitioned to support 4-octet AS numbers.

In an environment where an Autonomous System that has OLD BGP
speakers peers with two or more Autonomous Systems that have NEW BGP
speakers and use AS_TRANS (rather than having a globally unique AS
number), use of Multi-Exit Discriminators by the Autonomous System

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RFC 4893 BGP Support for Four-octet AS Number Space May 2007

with the OLD speakers may result in a situation where Multi-Exit
Discriminator will influence route selection among the routes that
were received from different neighboring Autonomous Systems.

Under certain conditions, it may not be possible to reconstruct the
entire AS path information from the AS_PATH and the AS4_PATH
attributes of a route. This occurs when two or more routes that
carry the AS4_PATH attribute are aggregated by an OLD BGP speaker,
and the AS4_PATH attribute of at least one of these routes carries at
least one 4-octet AS number (as oppose to a 2-octet AS number that is
encoded in 4 octets). When such aggregation results in creating a
route that is less specific than any of the component routes (route
whose Network Layer Reachability Information (NLRI) covers NLRI of
all the component routes), loss of the AS path information does not
create a risk of a routing loop. In all other cases, loss of the AS
path information does create a risk of a routing loop.

7. IANA Considerations

This document expands the pool for AS numbers from 0 - 65535 to 0 -
4294967295. The AS numbers are managed by the IANA "Autonomous
System Numbers" registry. Other than expanding the AS number pool,
this document does not propose any modifications to the existing
policies and procedures pertaining to the AS number allocation.

This document uses a BGP Capability code to indicate that a BGP
speaker supports the 4-octet AS numbers. The Capability Code 65 has
been assigned by IANA per RFC 2842.

In addition, this document introduces two new BGP optional transitive
attributes, and their type codes have been assigned by the IANA. The
first one is the AS4_PATH attribute, value 17, which preserves the AS
path information with 4-octet AS numbers across old BGP speakers.
The second one is the AS4_AGGREGATOR attribute, value 18, which is
similar in use to the current AGGREGATOR attribute, but it carries a
4-octet AS number.

Finally, this document introduces a reserved 2-octet AS number --
AS_TRANS. The AS number 23456 has been assigned by the IANA for
AS_TRANS.

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RFC 4893 BGP Support for Four-octet AS Number Space May 2007

8. Security Considerations

This extension to BGP does not change the underlying security issues
inherent in the existing BGP, except for the following:

The inconsistency between the AS_PATH attribute and the AS4_PATH
attribute can create loss of the AS path information, and potential
routing loops in certain cases as discussed in the document. This
could be exploited by an attacker.

9. Acknowledgments

The authors would like to thank Yakov Rekhter, Chaitanya Kodeboyina,
and Jeffrey Haas for the numerous discussions that went into the
making of this document.

10. Normative References

[BGP] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271, January
2006.

[RFC1997] Chandra, R., Traina, P., and T. Li, "BGP Communities
Attribute", RFC 1997, August 1996.

[RFC3392] Chandra, R. and J. Scudder, "Capabilities Advertisement
with BGP-4", RFC 3392, November 2002.

[RFC3065] Traina, P., McPherson, D., and J. Scudder, "Autonomous
System Confederations for BGP", RFC 3065, February 2001.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

11. Informative References

[AS-EXT-COM] Rekhter, Y., Ramachandra, S., and D. Tappan, "Four-octet
AS Specific BGP Extended Community", Work in Progress,
April 2007.

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RFC 4893 BGP Support for Four-octet AS Number Space May 2007

Authors' Addresses

Quaizar Vohra
Juniper Networks
1194 N.Mathilda Ave
Sunnyvale, CA 94089

EMail: [email protected]

Enke Chen
Cisco Systems, Inc.
170 W. Tasman Dr.
San Jose, CA 95134

EMail: [email protected]

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RFC 4893 BGP Support for Four-octet AS Number Space May 2007

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