Section 1. Introduction

The Border Gateway Protocol (BGP) is an inter-Autonomous System
   routing protocol.
   The primary function of a BGP speaking system is to exchange network
   reachability information with other BGP systems.  This network
   reachability information includes information on the list of
   Autonomous Systems (ASes) that reachability information traverses.
   This information is sufficient for constructing a graph of AS
   connectivity for this reachability, from which routing loops may be
   pruned and, at the AS level, some policy decisions may be enforced.
   BGP-4 provides a set of mechanisms for supporting Classless Inter-
   Domain Routing (CIDR) [RFC1518, RFC1519].  These mechanisms include
   support for advertising a set of destinations as an IP prefix and
   eliminating the concept of network "class" within BGP.  BGP-4 also
   introduces mechanisms that allow aggregation of routes, including
   aggregation of AS paths.
   Routing information exchanged via BGP supports only the destination-
   based forwarding paradigm, which assumes that a router forwards a
   packet based solely on the destination address carried in the IP
   header of the packet.  This, in turn, reflects the set of policy
   decisions that can (and cannot) be enforced using BGP.  BGP can
   support only those policies conforming to the destination-based
   forwarding paradigm.
1.1.  Definition of Commonly Used Terms
   This section provides definitions for terms that have a specific
   meaning to the BGP protocol and that are used throughout the text.
   Adj-RIB-In
      The Adj-RIBs-In contains unprocessed routing information that has
      been advertised to the local BGP speaker by its peers.
   Adj-RIB-Out
      The Adj-RIBs-Out contains the routes for advertisement to specific
      peers by means of the local speaker's UPDATE messages.
   Autonomous System (AS)
      The classic definition of an Autonomous System is a set of routers
      under a single technical administration, using an interior gateway
      protocol (IGP) and common metrics to determine how to route
      packets within the AS, and using an inter-AS routing protocol to
      determine how to route packets to other ASes.  Since this classic
      definition was developed, it has become common for a single AS to
      use several IGPs and, sometimes, several sets of metrics within an
      AS.  The use of the term Autonomous System stresses the fact that,
      even when multiple IGPs and metrics are used, the administration
      of an AS appears to other ASes to have a single coherent interior
      routing plan, and presents a consistent picture of the
      destinations that are reachable through it.
   BGP Identifier
      A 4-octet unsigned integer that indicates the BGP Identifier of
      the sender of BGP messages.  A given BGP speaker sets the value of
      its BGP Identifier to an IP address assigned to that BGP speaker.
      The value of the BGP Identifier is determined upon startup and is
      the same for every local interface and BGP peer.
   BGP speaker
      A router that implements BGP.
   EBGP
      External BGP (BGP connection between external peers).
   External peer
      Peer that is in a different Autonomous System than the local
      system.
   Feasible route
      An advertised route that is available for use by the recipient.
   IBGP
      Internal BGP (BGP connection between internal peers).
   Internal peer
      Peer that is in the same Autonomous System as the local system.
   IGP
      Interior Gateway Protocol - a routing protocol used to exchange
      routing information among routers within a single Autonomous
      System.
   Loc-RIB
      The Loc-RIB contains the routes that have been selected by the
      local BGP speaker's Decision Process.
   NLRI
      Network Layer Reachability Information.
   Route
      A unit of information that pairs a set of destinations with the
      attributes of a path to those destinations.  The set of
      destinations are systems whose IP addresses are contained in one
      IP address prefix carried in the Network Layer Reachability
      Information (NLRI) field of an UPDATE message.  The path is the
      information reported in the path attributes field of the same
      UPDATE message.
   RIB
      Routing Information Base.
   Unfeasible route
      A previously advertised feasible route that is no longer available
      for use.
1.2.  Specification of Requirements
   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].