draft-ietf-idr-bgp4-18.txt

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Network Working Group                                      Y. Rekhter
INTERNET DRAFT                                       Juniper Networks
                                                                T. Li
                                               Procket Networks, Inc.
                                                             S. Hares
                                            NextHop Technologies, Inc.
                                                              Editors



                  A Border Gateway Protocol 4 (BGP-4)
                      <draft-ietf-idr-bgp4-18.txt>


Status of this Memo


   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as ``work in progress.''

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.


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 RFC2119 [RFC2119].








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                           Table of Contents


   Abstract  . . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   1. Definition of commonly used terms  . . . . . . . . . . . . . .   4
   2. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .   6
   3. Summary of Operation . . . . . . . . . . . . . . . . . . . . .   7
   3.1 Routes: Advertisement and Storage . . . . . . . . . . . . . .   9
   3.2 Routing Information Bases . . . . . . . . . . . . . . . . . .  10
   4. Message Formats  . . . . . . . . . . . . . . . . . . . . . . .  11
   4.1 Message Header Format . . . . . . . . . . . . . . . . . . . .  11
   4.2 OPEN Message Format . . . . . . . . . . . . . . . . . . . . .  12
   4.3 UPDATE Message Format . . . . . . . . . . . . . . . . . . . .  14
   4.4 KEEPALIVE Message Format  . . . . . . . . . . . . . . . . . .  21
   4.5 NOTIFICATION Message Format . . . . . . . . . . . . . . . . .  21
   5. Path Attributes  . . . . . . . . . . . . . . . . . . . . . . .  23
   5.1 Path Attribute Usage  . . . . . . . . . . . . . . . . . . . .  25
   5.1.1 ORIGIN  . . . . . . . . . . . . . . . . . . . . . . . . . .  25
   5.1.2 AS_PATH . . . . . . . . . . . . . . . . . . . . . . . . . .  25
   5.1.3 NEXT_HOP  . . . . . . . . . . . . . . . . . . . . . . . . .  26
   5.1.4 MULTI_EXIT_DISC . . . . . . . . . . . . . . . . . . . . . .  28
   5.1.5 LOCAL_PREF  . . . . . . . . . . . . . . . . . . . . . . . .  28
   5.1.6 ATOMIC_AGGREGATE  . . . . . . . . . . . . . . . . . . . . .  29
   5.1.7 AGGREGATOR  . . . . . . . . . . . . . . . . . . . . . . . .  30
   6. BGP Error Handling . . . . . . . . . . . . . . . . . . . . . .  30
   6.1 Message Header error handling . . . . . . . . . . . . . . . .  30
   6.2 OPEN message error handling . . . . . . . . . . . . . . . . .  31
   6.3 UPDATE message error handling . . . . . . . . . . . . . . . .  32
   6.4 NOTIFICATION message error handling . . . . . . . . . . . . .  34
   6.5 Hold Timer Expired error handling . . . . . . . . . . . . . .  34
   6.6 Finite State Machine error handling . . . . . . . . . . . . .  34
   6.7 Cease . . . . . . . . . . . . . . . . . . . . . . . . . . . .  34
   6.8 BGP connection collision detection  . . . . . . . . . . . . .  35
   7. BGP Version Negotiation  . . . . . . . . . . . . . . . . . . .  36
   8. BGP Finite State machine . . . . . . . . . . . . . . . . . . .  36
   8.1 Events for the BGP FSM  . . . . . . . . . . . . . . . . . . .  37
   8.1.1   Administrative Events . . . . . . . . . . . . . . . . . .  37
   8.1.2 Timer Events  . . . . . . . . . . . . . . . . . . . . . . .  38
   8.1.3 TCP connection based Events . . . . . . . . . . . . . . . .  39
   8.1.4 BGP Messages based Events . . . . . . . . . . . . . . . . .  41
   8.2 Description of FSM  . . . . . . . . . . . . . . . . . . . . .  43
   8.2.1 FSM Definition  . . . . . . . . . . . . . . . . . . . . . .  43
   8.2.1.1 Terms "active" and "passive"  . . . . . . . . . . . . . .  43
   8.2.1.2 FSM and collision detection . . . . . . . . . . . . . . .  44
   8.2.2 Finite State Machine  . . . . . . . . . . . . . . . . . . .  44
   9. UPDATE Message Handling  . . . . . . . . . . . . . . . . . . .  57
   9.1 Decision Process  . . . . . . . . . . . . . . . . . . . . . .  58
   9.1.1 Phase 1: Calculation of Degree of Preference  . . . . . . .  59



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   9.1.2 Phase 2: Route Selection  . . . . . . . . . . . . . . . . .  60
   9.1.2.1 Route Resolvability Condition . . . . . . . . . . . . . .  61
   9.1.2.2 Breaking Ties (Phase 2) . . . . . . . . . . . . . . . . .  62
   9.1.3 Phase 3: Route Dissemination  . . . . . . . . . . . . . . .  64
   9.1.4 Overlapping Routes  . . . . . . . . . . . . . . . . . . . .  65
   9.2 Update-Send Process . . . . . . . . . . . . . . . . . . . . .  66
   9.2.1 Controlling Routing Traffic Overhead  . . . . . . . . . . .  67
   9.2.1.1 Frequency of Route Advertisement  . . . . . . . . . . . .  67
   9.2.1.2 Frequency of Route Origination  . . . . . . . . . . . . .  68
   9.2.2 Efficient Organization of Routing Information . . . . . . .  68
   9.2.2.1 Information Reduction . . . . . . . . . . . . . . . . . .  68
   9.2.2.2 Aggregating Routing Information . . . . . . . . . . . . .  69
   9.3 Route Selection Criteria  . . . . . . . . . . . . . . . . . .  72
   9.4 Originating BGP routes  . . . . . . . . . . . . . . . . . . .  72
   10. BGP Timers  . . . . . . . . . . . . . . . . . . . . . . . . .  72
   Appendix A. Comparison with RFC1771 . . . . . . . . . . . . . . .  73
   Appendix B. Comparison with RFC1267 . . . . . . . . . . . . . . .  74
   Appendix C. Comparison with RFC 1163  . . . . . . . . . . . . . .  75
   Appendix D. Comparison with RFC 1105  . . . . . . . . . . . . . .  75
   Appendix E. TCP options that may be used with BGP . . . . . . . .  76
   Appendix F. Implementation Recommendations  . . . . . . . . . . .  76
   Appendix F.1 Multiple Networks Per Message  . . . . . . . . . . .  76
   Appendix F.2 Reducing route flapping  . . . . . . . . . . . . . .  77
   Appendix F.3 Path attribute ordering  . . . . . . . . . . . . . .  77
   Appendix F.4 AS_SET sorting . . . . . . . . . . . . . . . . . . .  77
   Appendix F.5 Control over version negotiation . . . . . . . . . .  78
   Appendix F.6 Complex AS_PATH aggregation  . . . . . . . . . . . .  78
   Security Considerations . . . . . . . . . . . . . . . . . . . . .  79
   References  . . . . . . . . . . . . . . . . . . . . . . . . . . .  79
   Authors Information . . . . . . . . . . . . . . . . . . . . . . .  80





















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Abstract

   The Border Gateway Protocol (BGP) is an inter-Autonomous System rout-
   ing protocol.

   The primary function of a BGP speaking system is to exchange network
   reachability information with other BGP systems. This network reacha-
   bility information includes information on the list of Autonomous
   Systems (ASs) that reachability information traverses.  This informa-
   tion 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 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 which 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 deci-
   sions that can (and can not) be enforced using BGP. BGP can support
   only the policies conforming to the destination-based forwarding
   paradigm.


1. Definition of commonly used terms

   This section provides definition for terms that have a specific mean-
   ing to the BGP protocol and that are used throughout the text.

   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 pack-
      ets within the AS, and using an inter-AS routing protocol to
      determine how to route packets to other ASs. 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 here stresses the fact
      that, even when multiple IGPs and metrics are used, the adminis-
      tration 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.




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RFC DRAFT                                                   October 2002


   BGP speaker
      A router that implements BGP.

   BGP Identifier
      A 4-octet unsigned integer indicating 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 on startup and is the
      same for every local interface and every BGP peer.

   Internal peer
      Peer that is in the same Autonomous System as the local system.

   IBGP
      Internal BGP (BGP connection between internal peers).

   External peer
      Peer that is in a different Autonomous System than the local sys-
      tem.

   EBGP
      External BGP (BGP connection between external peers).

   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 destina-
      tions are systems whose IP addresses are contained in one IP
      address prefix carried in the Network Layer Reachability Informa-
      tion (NLRI) field of an UPDATE message. The path is the informa-
      tion reported in the path attributes field of the same UPDATE mes-
      sage.

   RIB
      Routing Information Base.

   Adj-RIB-In
      The Adj-RIBs-In contain unprocessed routing information that has
      been advertised to the local BGP speaker by its peers.

   Loc-RIB
      The Loc-RIB contains the routes that have been selected by the
      local BGP speaker's Decision Process.

   Adj-RIB-Out
      The Adj-RIBs-Out contains the routes for advertisement to specific



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RFC DRAFT                                                   October 2002


      peers by means of the local speaker's UPDATE messages.

   IGP
      Interior Gateway Protocol - a routing protocol used to exchange
      routing information among routers within a single Autonomous Sys-
      tem.

   Feasible route
      A route that is available for use.

   Unfeasible route
      A previously advertised feasible route that is no longer available
      for use.


2. Acknowledgments

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

   We would like to express our thanks to Guy Almes, Len Bosack, and
   Jeffrey C. Honig for their contributions to the earlier version
   (BGP-1) of this document.

   We would like to specially acknowledge numerous contributions by Den-
   nis Ferguson to the earlier version of this document.

   We like to explicitly thank Bob Braden for the review of the earlier
   version (BGP-2) of this document as well as his constructive and