The routing protocol used to connect autonomous systems on the Internet. BGP was introduced as a replacement for the earlier Exterior Gateway Protocol (EGP); unlike its predecessor it allows fully decentralized routing, with no part of an inter-network having a privileged position or being especially important to the operation of the whole. Version 4, which has been used since 1994, supports Classless Inter-Domain Routing. Its use on the Internet is obligatory, but it is often used within autonomous systems to link the subnetworks they administer.

http://tools.ietf.org/html/rfc4271 The Border Gateway Protocol (version 4) specification