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.

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.

   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).

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

   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.

   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

   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.

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.

Authors' Addresses

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

   EMail: quaizar.vohra@gmail.com


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

   EMail: enkechen@cisco.com

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