4-octet AS community: 4-Octet AS Specific BGP Extended Community
A BGP extended community that can carry a 4-octet AS number, with two route-target and route-origin types.
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.
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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
AI ELI5
On this page
§1Introduction
This document defines a new type of BGP extended community (RFC 4360): the 4-octet AS specific extended community. It is similar to the 2-octet AS specific extended community, except that it can carry a 4-octet Autonomous System number.
It is the same kind of tag as before, only with room for a longer network number.
§2Format of the Community
This is an extended type with a Type field of 2 octets and a Value field of 6 octets. The high-order octet of the type is 0x02 for transitive communities or 0x42 for non-transitive ones. The low-order octet is the sub-type.
The Value field has two sub-fields: a 4-octet Global Administrator holding an AS number assigned by IANA, and a 2-octet Local Administrator in which the organization identified by that AS can encode any information. Its format and meaning should be defined by the sub-type.
The tag has a part for the network's number and a part the network can fill in however it wants.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
TYPE | Type (high) | 1 byte | 0x02 or 0x42 | 0x02 transitive or 0x42 non-transitive. |
SUBTYPE | Sub-Type | 1 byte | Indicates the sub-type. | |
GLOBAL | Global Administrator | 4 bytes | 4-octet AS number assigned by IANA. | |
LOCAL | Local Administrator | 2 bytes | Any information the AS owner chooses. |
§3Two-Octet Autonomous Systems
Under RFC 4893, a 2-octet AS number becomes a 4-octet one by setting the 2 high-order octets to zero. So an AS with a 2-octet number could in principle use either kind of community, but the two would be treated as different even with the same sub-type and Local Administrator values.
For backward compatibility and to avoid inconsistencies, ASes that use 2-octet AS numbers SHOULD use 2-octet AS specific extended communities rather than 4-octet ones.
If your network number is still short, keep using the short tag so nobody gets confused.
§4IANA and Security
IANA created and maintains a registry called Four-octet AS Specific Extended Community. Future assignments use First Come First Served. Transitive types are 0x0200 to 0x02ff and non-transitive types are 0x4200 to 0x42ff.
This document assigns 0x0202 to the four-octet AS specific Route Target and 0x0203 to the Route Origin. It adds no new security issues; the considerations for BGP extended communities apply, and it points to separate work on handling malformed optional transitive attributes.
An official list says which tag numbers mean what, and this RFC claims two of them.