4-octet AS community: 4-Octet AS Specific BGP Extended Community
A BGP extended community format that carries a four-octet Autonomous System number.
Rekhter, Y., Sangli, S., and D. Tappan, "4-Octet AS Specific BGP Extended Community", RFC 5668, DOI 10.17487/RFC5668, October 2009, <https://www.rfc-editor.org/info/rfc5668>.
@misc{rfc5668,
series = {Request for Comments},
number = 5668,
howpublished = {RFC 5668},
publisher = {RFC Editor},
doi = {10.17487/RFC5668},
url = {https://www.rfc-editor.org/info/rfc5668},
author = {Y. Rekhter and S. Sangli and D. Tappan},
title = {{4-Octet AS Specific BGP Extended Community}},
pagetotal = 5,
year = 2009,
month = oct,
} RFC text
Contents
Network Working Group Y. Rekhter
Request for Comments: 5668 Juniper Networks
Category: Standards Track S. Sangli
Cisco Systems
D. Tappan
Consultant
October 2009
4-Octet AS Specific BGP Extended Community
Abstract
This document defines a new type of a BGP extended community, which
carries a 4-octet Autonomous System (AS) number.
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) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
1. Introduction
This document defines a new type of BGP extended community [RFC4360]:
a 4-octet AS specific extended community. This type of extended
community is similar to the 2-octet AS specific extended community,
except that it can carry a 4-octet Autonomous System number.
1.1. 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 RFC 2119 [RFC2119].
2. 4-Octet AS Specific Extended Community
This is an extended type with a Type field comprising 2 octets and a
Value field comprising 6 octets.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x02 or 0x42 | Sub-Type | Global Administrator :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Global Administrator (cont.) | Local Administrator |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The value of the high-order octet of this extended type is either
0x02 (for transitive communities) or 0x42 (for non-transitive
communities). The low-order octet of this extended type is used to
indicate sub-types.
The Value field consists of 2 sub-fields:
Global Administrator sub-field: 4 octets
This sub-field contains a 4-octet Autonomous System number
assigned by IANA.
Local Administrator sub-field: 2 octets
The organization identified by the Autonomous System number in
the Global Administrator sub-field can encode any information
in this sub-field. The format and meaning of the value encoded
in this sub-field should be defined by the sub-type of the
community.
3. Considerations for 2-Octet Autonomous Systems
As per [RFC4893], a 2-octet Autonomous System number can be converted
into a 4-octet Autonomous System number by setting the 2 high-order
octets of the 4-octet field to zero.
As a consequence, at least in principle, an Autonomous System that
uses a 2-octet Autonomous System number could use either 2-octet or
4-octet AS specific extended communities. This is undesirable, as
both communities would be treated as different, even if they had the
same Sub-Type and Local Administrator values.
Therefore, for backward compatibility with existing deployments and
to avoid inconsistencies between 2-octet and 4-octet specific
extended communities, Autonomous Systems that use 2-octet Autonomous
System numbers SHOULD use 2-octet AS specific extended communities
rather than 4-octet AS specific extended communities.
4. IANA Considerations
This document defines a class of extended communities, called 4-octet
AS specific extended communities, for which the IANA has created and
will maintain a registry entitled Four-octet AS Specific Extended
Community. All the communities in this class are of extended Types.
Future assignments are to be made using the "First Come First Served"
policy defined in [RFC5226]. The Type values for the transitive
communities of the 4-octet AS specific extended community class are
0x0200-0x02ff; for the non-transitive communities of that class, they
are 0x4200-0x42ff. Assignments consist of a name and the value.
This document makes the following assignments for the 4-octet AS
specific extended community:
Name Type Value
---- ----------
four-octet AS specific Route Target 0x0202
four-octet AS specific Route Origin 0x0203
5. Security Considerations
This document does not add new security issues. All the security
considerations for BGP extended communities apply here. At the time
that this document was written, there were significant efforts
underway to improve the security properties of BGP. For examples of
documents that have been produced up to this time of publication, see
[RFC4593] and [SIDR].
There is a potential serious issue if a malformed, optional
transitive attribute is received. This issue and the steps to avoid
it are discussed in [OPT_TRANS].
6. Acknowledgements
Thanks to Bruno Decraene for his contributions to this document.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
Communities Attribute", RFC 4360, February 2006.
[RFC4893] Vohra, Q. and E. Chen, "BGP Support for Four-octet AS
Number Space", RFC 4893, May 2007.
7.2. Informative References
[OPT_TRANS] Scudder, J., and E. Chen, "Error Handling for Optional
Transitive BGP Attributes", Work in Progress, April 2009.
[RFC4593] Barbir, A., Murphy, S., and Y. Yang, "Generic Threats to
Routing Protocols", RFC 4593, October 2006.
[SIDR] Lepinski, M. and S. Kent, "An Infrastructure to Support
Secure Internet Routing", Work in Progress, July 2009.
Authors' Addresses
Yakov Rekhter
Juniper Networks, Inc.
EMail: yakov@juniper.net
Srihari R. Sangli
Cisco Systems, Inc.
EMail: rsrihari@cisco.com
Dan Tappan
Boxborough MA
EMail: Dan.Tappan@Gmail.com
ISO Plain Language
On this page
§1Why a Four-Octet Community Is Needed
A BGP extended community carries information associated with routes. This RFC defines a four-octet AS-specific extended community, similar to the two-octet form but able to carry a four-octet Autonomous System (AS) number.
The new community keeps the same general purpose as the older AS-specific form. It has room for a four-octet AS number.
§2Fields in the Extended Community
The extended type has a two-octet Type field and a six-octet Value field. The high-order type octet is 0x02 for transitive communities or 0x42 for non-transitive communities; the low-order octet identifies the subtype.
The Value field contains a four-octet Global Administrator field with an IANA-assigned AS number and a two-octet Local Administrator field. The organization identified by the AS number can encode information in the local field, whose format and meaning should be defined by the subtype.
The type identifies whether the community is transitive and which subtype it uses. The value carries the AS number and a smaller field whose meaning comes from that subtype.
§3Compatibility With Two-Octet AS Numbers
A two-octet AS number can be represented in the four-octet field by setting its two high-order octets to zero. But a two-octet and four-octet community with the same subtype and Local Administrator value would still be treated as different communities.
To preserve backward compatibility and avoid inconsistencies, ASes using two-octet AS numbers SHOULD use the two-octet AS-specific communities instead.
A two-octet AS could fit in the new format, but the two community forms would not be treated as identical. The RFC recommends that two-octet ASes continue using the two-octet form.
§4Registered Types and Security
The RFC establishes an IANA registry for four-octet AS-specific extended communities. It assigns 0x0202 to the four-octet AS-specific Route Target and 0x0203 to the four-octet AS-specific Route Origin. The registry also defines ranges for transitive and non-transitive type values.
The RFC adds no new security issues beyond those for BGP extended communities. It notes a potential serious issue involving malformed optional transitive attributes and refers to separate guidance on that issue.
IANA records the subtype values defined here. The memo says the usual BGP extended-community security considerations still apply.