Network Working Group                                  Quaizar Vohra
Internet Draft                                      Juniper Networks
Expiration Date: June 2005 January 2006                              Enke Chen
                                                       Cisco Systems

               BGP support for four-octet AS number space



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2. Abstract

   Currently the Autonomous System number is encoded in BGP [BGP] as a
   two-octets field. This document describes extensions to BGP to carry
   the Autonomous System number as a four-octets field.

3. Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

4. Protocol Extensions

   For the purpose of this document lets define a BGP speaker which
   doesnt support the new 4-octets AS number extensions as an OLD BGP
   speaker, and a BGP speaker which supports the new 4-octets 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

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

   The Capability that is used by a BGP speaker to convey to its BGP
   peer the 4-octets Autonomous System number capability, also carries
   the 4-octets 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-octets Autonomous Systems numbers by using the existing AS_PATH
   attribute, except that each AS number in this attribute is encoded
   not as a 2-octets, but as a 4-octets 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-octets

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

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

   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. Such a 4-octets AS number is
   said to be mappable to a 2-octets AS number.

   To represent 4-octets AS numbers (which are not mapped from 2-octets)
   as 2-octets AS numbers in the AS path information encoded with
   2-octets AS numbers, this document reserves a 2-octets AS number.
   Lets 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 doesn’t doesnt have a
   (globally unique) 2-octets AS number.

5. Operations

5.1. Interaction between NEW BGP speakers

   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.

   The new attributes, NEW_AS_PATH and NEW_AGGREGATOR should not be
   carried in the UPDATE messages between NEW BGP peers. A NEW BGP
   speaker that receives the NEW_AS_PATH and NEW_AGGREGATOR path
   attributes in an UPDATE message from a NEW BGP speaker should discard
   these path attributes and continue processing the UPDATE message.

5.2. Interaction between NEW and OLD BGP speaker

5.2.1. 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-octets AS number.
   However, this document doesn’t doesnt assume that an Autonomous System with
   NEW speakers has to have a globally unique 2-octets AS number -
   AS_TRANS could be used instead (even if multiple Autonomous System
   would use it).

5.2.2. 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-octets AS numbers. The NEW speaker also MUST send the AS path
   information in the NEW_AS_PATH attribute (encoded with 4-octets AS
   numbers), except for the case where the entire AS path information is
   composed of 2-octets AS numbers only. In this case the NEW speaker
   should not send the NEW_AS_PATH attribute.

   In the AS_PATH attribute encoded with 2-octets AS numbers, non-
   mappable 4-octets AS numbers are represented by the well known
   2-octets AS number, AS_TRANS. This will preserve the path length
   property of the AS path information; and will 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 NEW_AS_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 NEW_AS_PATH
   attribute from the AS_PATH attribute, must exclude such path
   segments. The NEW_AS_PATH attribute will be carried across a series
   of OLD BGP speakers without modification and will help preserve the
   truely 4-octets 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 Systems AS number is truely
   4-octets, the speaker constructs the NEW_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 NEW_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
   NEW_AGGREGATE attribute should not be sent.

5.2.3. Processing Received Updates

   When a NEW BGP speaker receives an update from an OLD one, it should
   be prepared to receive the NEW_AS_PATH attribute along with the
   existing AS_PATH attribute. If NEW_AS_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-octets AS numbers and OLD BGP speakers only. In that case, if
   the route carries a NEW_AS_PATH attribute, this attribute may not
   have been updated since the route left the last NEW BGP speaker. The
   trailing AS path information (representing autonomous systems with
   2-octets 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). This AS path information should be prepended to
   the NEW_AS_PATH attribute to construct the exact AS path information.

   Similarly, a NEW BGP speaker should be prepared to receive the
   NEW_AGGREGATOR attribute from an OLD BGP speaker. In that case, the
   AGGREGATOR attribute is ignored and the NEW_AGGREGATOR contains the
   exact information about the aggregating node.

5.3. Interaction between OLD BGP speakers

   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.

6. 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 routers that use 4-octets Autonomous System numbers. Such
   routers should use the Extended Communities [EXT-COMM] attribute

7. Transition

   The scheme described in this document allows a gradual transition
   from 2-octets AS numbers to 4-octets AS numbers. One can upgrade one
   Autonomous system or one router at a time.

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

   An OLD BGP speaker should not use AS_TRANS as its Autonomous System

   A non-mappable 4-octets AS number can not be used as a "Member AS
   Number" of a BGP Confederation until all the BGP speakers within the
   Confederation transitioned to support 4-octets 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 NEW_AS_PATH
   attributes of a route. This occurs when two or more routes that carry
   the NEW_AS_PATH attribute are aggregated by an OLD BGP speaker, and
   the NEW_AS_PATH attribute of at least one of these routes carries at
   least one 4-octets AS number (as oppose to a 2-octets 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 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.

8. IANA Consideration

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

   In addition, this document introduces two new BGP optional transitive
   attributes. The first is the NEW_AS_PATH attribute, which preserves
   the AS path information with 4-octet AS numbers across old BGP
   speakers. The second is the NEW_AGGREGATOR attribute, which is
   similar in use to the current AGGREGATOR attribute but it carries
   4-octet AS numbers.  The Type Codes for these attributes has been
   assigned by IANA.

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

9. Security Considerations

   Security issues are not discussed in this document.

10. Acknowledgments

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

11. References

   [BGP] Rekhter, Y., Li, T., and Hares, S., "A Border Gateway Protocol
   4 (BGP-4)", draft-ietf-idr-bgp4-26.txt, October 2004.

   [EXT-COM] Ramachandra, S., Tappan, D., and Rekter Y. "BGP Extended
   Communities Attribute", draft-ramachandra-bgp-ext-communities-07.txt,
   March 2004. draft-ramachandra-bgp-ext-communities-09.txt,
   July 2005.

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

   [RFC2842] Chandra, R., and Scudder, J., "Capabilities Advertisement
   with BGP-4", RFC 2842, May 2000.

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

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

12. Author Information

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

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

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