BGP is an inter-autonomous system routing protocol designed for TCP/IP internets. Version 1 of the BGP protocol was published in RFC 1105. Since then BGP Versions 2, 3, and 4 have been developed. Version 2 was documented in RFC 1163. Version 3 is documented in RFC 1267. The changes between versions 1, 2 and 3 are explained in Appendix 2 of . All of the functionality that was present in the previous versions is present in version 4.
BGP version 2 removed from the protocol the concept of "up", "down", and "horizontal" relations between autonomous systems that were present in version 1. BGP version 2 introduced the concept of path attributes. In addition, BGP version 2 clarified parts of the protocol that were "under-specified".
BGP version 3 lifted some of the restrictions on the use of the NEXT_HOP path attribute, and added the BGP Identifier field to the BGP OPEN message. It also clarifies the procedure for distributing BGP routes between the BGP speakers within an autonomous system.
BGP version 4 redefines the (previously class-based) network layer reachability portion of the updates to specify prefixes of arbitrary length in order to represent multiple classful networks in a single entry as discussed in . BGP version 4 has also modified the AS- PATH attribute so that sets of autonomous systems, as well as individual ASs may be described. In addition, BGP version for has redescribed the INTER-AS METRIC attribute as the MULTI-EXIT DISCRIMINATOR and added new LOCAL-PREFERENCE and AGGREGATOR attributes.
Possible applications of BGP in the Internet are documented in .
The BGP protocol was developed by the IDR Working Group of the Internet Engineering Task Force. This Working Group has a mailing list, email@example.com, where discussions of protocol features and operation are held. The IDR Working Group meets regularly during the quarterly Internet Engineering Task Force conferences. Reports of these meetings are published in the IETF's Proceedings.