Network Working Group                                         Y. Rekhter
Request for Comments: 1771        T.J. Watson Research Center, IBM Corp.
Obsoletes: 1654                                                    T. Li
Category: Standards Track                                  cisco Systems
                                                                 Editors
                                                              March 1995


                  A Border Gateway Protocol 4 (BGP-4)

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   This document, together with its companion document, "Application of
   the Border Gateway Protocol in the Internet", define an inter-
   autonomous system routing protocol for the Internet.

1. Acknowledgements

   This document was originally published as RFC 1267 in October 1991,
   jointly authored by Kirk Lougheed (cisco Systems) and Yakov Rekhter
   (IBM).

   We would like to express our thanks to Guy Almes (ANS), Len Bosack
   (cisco Systems), and Jeffrey C. Honig (Cornell University) for their
   contributions to the earlier version of this document.

   We like to explicitly thank Bob Braden (ISI) for the review of the
   earlier version of this document as well as his constructive and
   valuable comments.

   We would also like to thank Bob Hinden, Director for Routing of the
   Internet Engineering Steering Group, and the team of reviewers he
   assembled to review the previous version (BGP-2) of this document.
   This team, consisting of Deborah Estrin, Milo Medin, John Moy, Radia
   Perlman, Martha Steenstrup, Mike St. Johns, and Paul Tsuchiya, acted
   with a strong combination of toughness, professionalism, and
   courtesy.

   This updated version of the document is the product of the IETF IDR
   Working Group with Yakov Rekhter and Tony Li as editors. Certain
   sections of the document borrowed heavily from IDRP [7], which is the
   OSI counterpart of BGP. For this credit should be given to the ANSI
   X3S3.3 group chaired by Lyman Chapin (BBN) and to Charles Kunzinger
   (IBM Corp.) who was the IDRP editor within that group.  We would also
   like to thank Mike Craren (Proteon, Inc.), Dimitry Haskin (Bay
   Networks, Inc.), John Krawczyk (Bay Networks, Inc.), and Paul Traina
   (cisco Systems) for their insightful comments.

   We would like to specially acknowledge numerous contributions by
   Dennis Ferguson (MCI).

   The work of Yakov Rekhter was supported in part by the National
   Science Foundation under Grant Number NCR-9219216.

2.  Introduction

   The Border Gateway Protocol (BGP) is an inter-Autonomous System
   routing protocol.  It is built on experience gained with EGP as
   defined in RFC 904 [1] and EGP usage in the NSFNET Backbone as
   described in RFC 1092 [2] and RFC 1093 [3].

   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 (ASs) that reachability information traverses.
   This information is sufficient to construct a graph of AS
   connectivity from which routing loops may be pruned and some policy
   decisions at the AS level may be enforced.

   BGP-4 provides a new set of mechanisms for supporting classless
   interdomain routing.  These mechanisms include support for
   advertising an IP prefix and eliminates the concept of network
   "class" within BGP.  BGP-4 also introduces mechanisms which allow
   aggregation of routes, including aggregation of AS paths.  These
   changes provide support for the proposed supernetting scheme [8, 9].

   To characterize the set of policy decisions that can be enforced
   using BGP, one must focus on the rule that a BGP speaker advertise to
   its peers (other BGP speakers which it communicates with) in
   neighboring ASs only those routes that it itself uses.  This rule
   reflects the "hop-by-hop" routing paradigm generally used throughout
   the current Internet.  Note that some policies cannot be supported by
   the "hop-by-hop" routing paradigm and thus require techniques such as
   source routing to enforce.  For example, BGP does not enable one AS
   to send traffic to a neighboring AS intending that the traffic take a
   different route from that taken by traffic originating in the

   neighboring AS.  On the other hand, BGP can support any policy
   conforming to the "hop-by-hop" routing paradigm.  Since the current
   Internet uses only the "hop-by-hop" routing paradigm and since BGP
   can support any policy that conforms to that paradigm, BGP is highly
   applicable as an inter-AS routing protocol for the current Internet.

   A more complete discussion of what policies can and cannot be
   enforced with BGP is outside the scope of this document (but refer to
   the companion document discussing BGP usage [5]).

   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 document 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 destinations 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 (bgp@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
   closed.

   The hosts executing the Border Gateway Protocol need not be routers.
   A non-routing host could exchange routing information with routers
   via EGP or even an interior routing protocol.  That non-routing host
   could then use BGP to exchange routing information with a border
   router in another Autonomous System.  The implications and
   applications of this architecture are for further study.

   If a particular AS has multiple BGP speakers and is providing transit
   service for other ASs, then care must be taken to ensure a consistent
   view of routing within the AS.  A consistent view of the interior
   routes of the AS is provided by the interior routing protocol.  A
   consistent view of the routes exterior to the AS can be provided by
   having all BGP speakers within the AS maintain direct BGP connections
   with each other.  Using a common set of policies, the BGP speakers
   arrive at an agreement as to which border routers will serve as
   exit/entry points for particular destinations outside the AS.  This
   information is communicated to the AS's internal routers, possibly
   via the interior routing protocol.  Care must be taken to ensure that
   the interior routers have all been updated with transit information
   before the BGP speakers announce to other ASs that transit service is
   being provided.

   Connections between BGP speakers of different ASs are referred to as
   "external" links.  BGP connections between BGP speakers within the
   same AS are referred to as "internal" links.  Similarly, a peer in a
   different AS is referred to as an external peer, while a peer in the
   same AS may be described as an internal peer.

3.1 Routes: Advertisement and Storage

   For purposes of this protocol a route is defined as a unit of
   information that pairs a destination with the attributes of a path to
   that destination:

      - Routes are advertised between a pair of BGP speakers in UPDATE
      messages:  the destination is the systems whose IP addresses are
      reported in the Network Layer Reachability Information (NLRI)
      field, and the the path is the information reported in the path
      attributes fields of the same UPDATE message.

      - Routes are stored in the Routing Information Bases (RIBs):
      namely, the Adj-RIBs-In, the Loc-RIB, and the Adj-RIBs-Out. Routes
      that will be advertised to other BGP speakers must be present in
      the Adj-RIB-Out; routes that will be used by the local BGP speaker
      must be present in the Loc-RIB, and the next hop for each of these
      routes must be present in the local BGP speaker's forwarding
      information base; and routes that are received from other BGP
      speakers are present in the Adj-RIBs-In.

   If a BGP speaker chooses to advertise the route, it may add to or
   modify the path attributes of the route before advertising it to a
   peer.

   BGP provides mechanisms by which a BGP speaker can inform its peer
   that a previously advertised route is no longer available for use.
   There are three methods by which a given BGP speaker can indicate
   that a route has been withdrawn from service:

      a) the IP prefix that expresses destinations for a previously
      advertised route can be advertised in the WITHDRAWN ROUTES field
      in the UPDATE message, thus marking the associated route as being
      no longer available for use

      b) a replacement route with the same Network Layer Reachability
      Information can be advertised, or

      c) the BGP speaker - BGP speaker connection can be closed, which
      implicitly removes from service all routes which the pair of
      speakers had advertised to each other.

3.2 Routing Information Bases

   The Routing Information Base (RIB) within a BGP speaker consists of
   three distinct parts:

      a) Adj-RIBs-In: The Adj-RIBs-In store routing information that has
      been learned from inbound UPDATE messages. Their contents
      represent routes that are available as an input to the Decision
      Process.

      b) Loc-RIB: The Loc-RIB contains the local routing information
      that the BGP speaker has selected by applying its local policies
      to the routing information contained in its Adj-RIBs-In.

      c) Adj-RIBs-Out: The Adj-RIBs-Out store the information that the
      local BGP speaker has selected for advertisement to its peers. The
      routing information stored in the Adj-RIBs-Out will be carried in
      the local BGP speaker's UPDATE messages and advertised to its
      peers.

   In summary, the Adj-RIBs-In contain unprocessed routing information
   that has been advertised to the local BGP speaker by its peers; the
   Loc-RIB contains the routes that have been selected by the local BGP
   speaker's Decision Process; and the Adj-RIBs-Out organize the routes
   for advertisement to specific peers by means of the local speaker's
   UPDATE messages.

   Although the conceptual model distinguishes between Adj-RIBs-In,
   Loc-RIB, and Adj-RIBs-Out, this neither implies nor requires that an
   implementation must maintain three separate copies of the routing
   information. The choice of implementation (for example, 3 copies of
   the information vs 1 copy with pointers) is not constrained by the
   protocol.