Network Working Group Enke Chen Internet Draft Redback Networks, Inc. Expiration Date: May 2001 Yakov Rekhter cisco Systems BGP support for four-octet AS number space draft-chen-as4bytes-00.txt 1. Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026 except that the right to produce derivative works is not granted. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as ``work in progress.'' The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. 2. Abstract Currently the Autonomous System number is encoded in BGP as a two- octets field. This document describes extentions to BGP to carry the Autonomous System number as a four-octets field. draft-chen-as4bytes-00.txt [Page 1] Internet Draft draft-chen-as4bytes-00.txt November 2000 3. Protocol Extensions 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. A BGP speaker that is capable of supporting 4-octets Autonomous System numbers uses BGP Capability Advertisements [BGP-CAP] to advertise it to its neighbors (either internal or external). The BGP Capability code for the 4-octets Autonomous System number capability is [TBD]. The Capability that is used by a BGP speaker to convey to its BGP peer the 4-octets Autonomous System number capability carries the 4-octets Autonomous System number of the speaker in the Capability Value field of the Optional Parameter. The Capability Length field of the Capability is set to 4. To carry AS path information expressed in terms of 4-octets Autonomous Systems numbers, we use the existing AS_PATH attribute, except that each AS in this attribute is expressed not as a 2-octets, but as a 4-octets entity. The same applies to the AGGREGATOR attribute - we use the same attribute, except that the AS carried in this attribute is encoded as a 4-octets entity. 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. 4. Operations 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. 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. draft-chen-as4bytes-00.txt [Page 2] Internet Draft draft-chen-as4bytes-00.txt November 2000 5. Transition In an ideal case we assume that all the BGP speakers in the Internet will become capable of supporting 4-octets Autonomous System numbers before we'll exhaust the current (2-octets) Autonomous System number space. Assignment of 4-octets Autonomous System numbers would start only after we'll exhaust the current Autonomous System number space, and thus only after all the BGP speakers in the Internet will become capable of supporting 4-octets Autonomous System numbers. During the transition a BGP speaker that supports 4-octets Autonomous System numbers may have some of its peers capable of supporting 4- octets Autonomous System number, and others capable of supporting only 2-octets Autonomous System number. In this case the speaker may need to convert between the 2 and the 4 octets Autonomous System numbers, as a route received from one peer may carry Autonomous System numbers encoded as 4 octets, while re-advertising this route to other peer may require the speaker to encode Autonomous System numbers as 2 octets. Note that as long as no Autonomous System is assigned a 4-octets Autonomous System number this conversion is straightforward. In a less than ideal case we need to assume that not all the BGP speakers in the Internet will become capable of supporting 4-octets Autonomous System numbers prior to the exhaustion of the current (2- octets) Autonomous System number space. In this case conversion from 4-octets to 2-octets may not be possible for any of the 4-octets assigned Autonomous System number. One way to deal with this situation is to truncate the AS_PATH information as to exclude all of the 4-octets assigned Autonomous System numbers, although doing this may result in a formation of forwarding loops. 6. Security Considerations Security issues are not discussed in this document. draft-chen-as4bytes-00.txt [Page 3] Internet Draft draft-chen-as4bytes-00.txt November 2000 7. Acknowledgements TBD. 8. References 9. Author Information Enke Chen Redback Networks, Inc. 350 Holger Way San Jose, CA 95134 e-mail: enke@redback.com Yakov Rekhter Cisco Systems, Inc. 170 West Tasman Drive San Jose, CA 95134 e-mail: yakov@cisco.com draft-chen-as4bytes-00.txt [Page 4]