RFC 1654 (rfc1654) - Page 3 of 56

A Border Gateway Protocol 4 (BGP-4)

RFC 1654                         BGP-4                         July 1994


   BGP runs over a reliable transport protocol.  This eliminates the
   need to implement explicit update fragmentation, retransmission,
   acknowledgement, and sequencing.  Any authentication scheme used by
   the transport protocol may be used in addition to BGP's own
   authentication mechanisms.  The error notification mechanism used in
   BGP assumes that the transport protocol supports a "graceful" close,
   i.e., that all outstanding data will be delivered before the
   connection is closed.

   BGP uses TCP [4] as its transport protocol.  TCP meets BGP's
   transport requirements and is present in virtually all commercial
   routers and hosts.  In the following descriptions the phrase
   "transport protocol connection" can be understood to refer to a TCP
   connection.  BGP uses TCP port 179 for establishing its connections.

   This memo uses the term `Autonomous System' (AS) throughout.  The
   classic definition of an Autonomous System is a set of routers under
   a single technical administration, using an interior gateway protocol
   and common metrics to route packets within the AS, and using an
   exterior gateway protocol to route packets to other ASs.  Since this
   classic definition was developed, it has become common for a single
   AS to use several interior gateway protocols and sometimes several
   sets of metrics within an AS.  The use of the term Autonomous System
   here stresses the fact that, even when multiple IGPs and metrics are
   used, the administration of an AS appears to other ASs to have a
   single coherent interior routing plan and presents a consistent
   picture of what networks are reachable through it.

   The planned use of BGP in the Internet environment, including such
   issues as topology, the interaction between BGP and IGPs, and the
   enforcement of routing policy rules is presented in a companion
   document [5].  This document is the first of a series of documents
   planned to explore various aspects of BGP application.  Please send
   comments to the BGP mailing list (iwg@ans.net).

3.  Summary of Operation

   Two systems form a transport protocol connection between one another.
   They exchange messages to open and confirm the connection parameters.
   The initial data flow is the entire BGP routing table.  Incremental
   updates are sent as the routing tables change.  BGP does not require
   periodic refresh of the entire BGP routing table.  Therefore, a BGP
   speaker must retain the current version of the entire BGP routing
   tables of all of its peers for the duration of the connection.
   KeepAlive messages are sent periodically to ensure the liveness of
   the connection.  Notification messages are sent in response to errors
   or special conditions.  If a connection encounters an error
   condition, a notification message is sent and the connection is



Rekhter & Li