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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-bgp4-22.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.


Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.



Abstract

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




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   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 for this reachability 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
   decisions that can (and can not) be enforced using BGP. BGP can
   support only the policies conforming to the destination-based
   forwarding paradigm.

   This specification covers only the exchange of IP version 4 network
   reachability information.


























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                           Table of Contents


   1. Definition of commonly used terms  . . . . . . . . . . . . . .   5
   2. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .   7
   Specification of Requirements . . . . . . . . . . . . . . . . . .   8
   3. Summary of Operation . . . . . . . . . . . . . . . . . . . . .   8
   3.1 Routes: Advertisement and Storage . . . . . . . . . . . . . .   9
   3.2 Routing Information Bases . . . . . . . . . . . . . . . . . .  10
   4. Message Formats  . . . . . . . . . . . . . . . . . . . . . . .  12
   4.1 Message Header Format . . . . . . . . . . . . . . . . . . . .  12
   4.2 OPEN Message Format . . . . . . . . . . . . . . . . . . . . .  13
   4.3 UPDATE Message Format . . . . . . . . . . . . . . . . . . . .  15
   4.4 KEEPALIVE Message Format  . . . . . . . . . . . . . . . . . .  22
   4.5 NOTIFICATION Message Format . . . . . . . . . . . . . . . . .  22
   5. Path Attributes  . . . . . . . . . . . . . . . . . . . . . . .  24
   5.1 Path Attribute Usage  . . . . . . . . . . . . . . . . . . . .  26
   5.1.1 ORIGIN  . . . . . . . . . . . . . . . . . . . . . . . . . .  26
   5.1.2 AS_PATH . . . . . . . . . . . . . . . . . . . . . . . . . .  26
   5.1.3 NEXT_HOP  . . . . . . . . . . . . . . . . . . . . . . . . .  27
   5.1.4 MULTI_EXIT_DISC . . . . . . . . . . . . . . . . . . . . . .  29
   5.1.5 LOCAL_PREF  . . . . . . . . . . . . . . . . . . . . . . . .  30
   5.1.6 ATOMIC_AGGREGATE  . . . . . . . . . . . . . . . . . . . . .  30
   5.1.7 AGGREGATOR  . . . . . . . . . . . . . . . . . . . . . . . .  31
   6. BGP Error Handling . . . . . . . . . . . . . . . . . . . . . .  31
   6.1 Message Header error handling . . . . . . . . . . . . . . . .  31
   6.2 OPEN message error handling . . . . . . . . . . . . . . . . .  32
   6.3 UPDATE message error handling . . . . . . . . . . . . . . . .  33
   6.4 NOTIFICATION message error handling . . . . . . . . . . . . .  35
   6.5 Hold Timer Expired error handling . . . . . . . . . . . . . .  35
   6.6 Finite State Machine error handling . . . . . . . . . . . . .  35
   6.7 Cease . . . . . . . . . . . . . . . . . . . . . . . . . . . .  35
   6.8 BGP connection collision detection  . . . . . . . . . . . . .  36
   7. BGP Version Negotiation  . . . . . . . . . . . . . . . . . . .  37
   8. BGP Finite State machine . . . . . . . . . . . . . . . . . . .  38
   8.1 Events for the BGP FSM  . . . . . . . . . . . . . . . . . . .  39
   8.1.1 Optional Events linked to Optional Session attributes . . .  39
   8.1.2   Administrative Events . . . . . . . . . . . . . . . . . .  44
   8.1.3 Timer Events  . . . . . . . . . . . . . . . . . . . . . . .  47
   8.1.4 TCP connection based Events . . . . . . . . . . . . . . . .  49
   8.1.5 BGP Messages based Events . . . . . . . . . . . . . . . . .  51
   8.2 Description of FSM  . . . . . . . . . . . . . . . . . . . . .  53
   8.2.1 FSM Definition  . . . . . . . . . . . . . . . . . . . . . .  53
   8.2.1.1 Terms "active" and "passive"  . . . . . . . . . . . . . .  54
   8.2.1.2 FSM and collision detection . . . . . . . . . . . . . . .  54
   8.2.1.3  FSM and Optional Attributes  . . . . . . . . . . . . . .  55
   8.2.1.4 FSM Event numbers . . . . . . . . . . . . . . . . . . . .  55




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   8.2.1.5 FSM actions that are implementation dependent . . . . . .  56
   8.2.2 Finite State Machine  . . . . . . . . . . . . . . . . . . .  56
   9. UPDATE Message Handling  . . . . . . . . . . . . . . . . . . .  72
   9.1 Decision Process  . . . . . . . . . . . . . . . . . . . . . .  73
   9.1.1 Phase 1: Calculation of Degree of Preference  . . . . . . .  74
   9.1.2 Phase 2: Route Selection  . . . . . . . . . . . . . . . . .  74
   9.1.2.1 Route Resolvability Condition . . . . . . . . . . . . . .  76
   9.1.2.2 Breaking Ties (Phase 2) . . . . . . . . . . . . . . . . .  77
   9.1.3 Phase 3: Route Dissemination  . . . . . . . . . . . . . . .  79
   9.1.4 Overlapping Routes  . . . . . . . . . . . . . . . . . . . .  80
   9.2 Update-Send Process . . . . . . . . . . . . . . . . . . . . .  81
   9.2.1 Controlling Routing Traffic Overhead  . . . . . . . . . . .  82
   9.2.1.1 Frequency of Route Advertisement  . . . . . . . . . . . .  82
   9.2.1.2 Frequency of Route Origination  . . . . . . . . . . . . .  83
   9.2.2 Efficient Organization of Routing Information . . . . . . .  83
   9.2.2.1 Information Reduction . . . . . . . . . . . . . . . . . .  83
   9.2.2.2 Aggregating Routing Information . . . . . . . . . . . . .  84
   9.3 Route Selection Criteria  . . . . . . . . . . . . . . . . . .  86
   9.4 Originating BGP routes  . . . . . . . . . . . . . . . . . . .  87
   10. BGP Timers  . . . . . . . . . . . . . . . . . . . . . . . . .  87
   Appendix A. Comparison with RFC1771 . . . . . . . . . . . . . . .  88
   Appendix B. Comparison with RFC1267 . . . . . . . . . . . . . . .  89
   Appendix C. Comparison with RFC 1163  . . . . . . . . . . . . . .  90
   Appendix D. Comparison with RFC 1105  . . . . . . . . . . . . . .  90
   Appendix E. TCP options that may be used with BGP . . . . . . . .  91
   Appendix F. Implementation Recommendations  . . . . . . . . . . .  91
   Appendix F.1 Multiple Networks Per Message  . . . . . . . . . . .  91
   Appendix F.2 Reducing route flapping  . . . . . . . . . . . . . .  92
   Appendix F.3 Path attribute ordering  . . . . . . . . . . . . . .  92
   Appendix F.4 AS_SET sorting . . . . . . . . . . . . . . . . . . .  92
   Appendix F.5 Control over version negotiation . . . . . . . . . .  93
   Appendix F.6 Complex AS_PATH aggregation  . . . . . . . . . . . .  93
   Security Considerations . . . . . . . . . . . . . . . . . . . . .  94
   IANA Considerations . . . . . . . . . . . . . . . . . . . . . . .  94
   IPR Notice  . . . . . . . . . . . . . . . . . . . . . . . . . . .  95
   Full Copyright Notice . . . . . . . . . . . . . . . . . . . . . .  95
   Normative References  . . . . . . . . . . . . . . . . . . . . . .  96
   Non-normative References  . . . . . . . . . . . . . . . . . . . .  96
   Authors Information . . . . . . . . . . . . . . . . . . . . . . .  98












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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 for this
   reachability 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.

   Adj-RIB-In
      The Adj-RIBs-In contain 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 pack-
      ets within the AS, and using an inter-AS routing protocol to
      determine how to route packets to other ASs. Since this classic



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      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.

   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.

   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 sys-
      tem.

   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 Sys-
      tem.

   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



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      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.

   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
   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 earlier 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 cour-
   tesy.

   Certain sections of the document borrowed heavily from IDRP
   [IS10747], which is the OSI counterpart of BGP. For this credit
   should be given to the ANSI X3S3.3 group chaired by Lyman Chapin and
   to Charles Kunzinger who was the IDRP editor within that group.

   We would also like to thank Benjamin Abarbanel, Enke Chen, Edward
   Crabbe, Mike Craren, Vincent Gillet, Eric Gray, Jeffrey Haas, Dimitry
   Haskin, John Krawczyk, David LeRoy, Dan Massey, Jonathan Natale, Dan
   Pei, Mathew Richardson, John Scudder, John Stewart III, Dave Thaler,



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   Paul Traina, Russ White, Curtis Villamizar, and Alex Zinin for their
   comments.

   We would like to specially acknowledge Andrew Lange for his help in
   preparing the final version of this document.

   Finally, we would like to thank all the members of the IDR Working
   Group for their ideas and support they have given to this document.


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].



3. Summary of Operation

   The Border Gateway Protocol (BGP) is an inter-Autonomous System rout-
   ing protocol. It is built on experience gained with EGP as defined in
   [RFC904] and EGP usage in the NSFNET Backbone as described in
   [RFC1092] and [RFC1093].

   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.

   In the context of this document we assume that a BGP speaker adver-
   tises to its peers only those routes that it itself uses (in this
   context a BGP speaker is said to "use" a BGP route if it is the most
   preferred BGP route and is used in forwarding). All other cases are
   outside the scope of this document.

   In the context of this document the term "IP address" refers to an IP
   Version 4 address [RFC791].

   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. Note that some



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   policies can not be supported by the destination-based forwarding
   paradigm, and thus require techniques such as source routing (aka
   explicit routing) to be enforced. Such policies can not be enforced
   using BGP either. For example, BGP does not enable one AS to send
   traffic to a neighboring AS for forwarding to some destination
   (reachable through but) beyond that neighboring AS intending that the
   traffic take a different route to that taken by the traffic originat-
   ing in the neighboring AS (for that same destination).  On the other
   hand, BGP can support any policy conforming to the destination-based
   forwarding paradigm.

   BGP-4 provides a new 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, includ-
   ing aggregation of AS paths.

   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
   (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 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 met-
   rics are used, the administration of an AS appears to other ASs to
   have a single coherent interior routing plan and presents a consis-
   tent picture of what destinations are reachable through it.

   BGP uses TCP [RFC793] as its transport protocol. This eliminates the
   need to implement explicit update fragmentation, retransmission,
   acknowledgment, and sequencing. BGP listens on TCP port 179.  The
   error notification mechanism used in BGP assumes that TCP supports a
   "graceful" close, i.e., that all outstanding data will be delivered
   before the connection is closed.

   Two systems form a TCP connection between one another. They exchange
   messages to open and confirm the connection parameters.

   The initial data flow is the portion of the BGP routing table that is
   allowed by the export policy, called the Adj-Ribs-Out (see 3.2).
   Incremental updates are sent as the routing tables change. BGP does
   not require periodic refresh of the routing table. To allow local
   policy changes to have the correct effect without resetting  any BGP
   connections, a BGP speaker SHOULD either (a) retain the current ver-
   sion of the routes advertised to it by all of its peers for the



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   duration of the connection, or (b) make use of the Route Refresh
   extension [RFC2918].

   KEEPALIVE messages may be 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.

   A peer in a different AS is referred to as an external peer, while a
   peer in the same AS is referred to as an internal peer. Internal BGP
   and external BGP are commonly abbreviated IBGP and EBGP.

   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 IGP used within the AS. For the
   purpose of this document, it is assumed that a consistent view of the
   routes exterior to the AS is provided by having all BGP speakers
   within the AS maintain IBGP with each other.

   This document specifies the base behavior of the BGP protocol. This
   behavior can and is modified by extension specifications.  When the
   protocol is extended the new behavior is fully documented in the
   extension specifications.


3.1 Routes: Advertisement and Storage

   For the purpose of this protocol, a route is defined as 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 mes-
   sage, and the path is the information reported in the path attributes
   field of the same UPDATE message.

   Routes are advertised between BGP speakers in UPDATE messages.  Mul-
   tiple routes that have the same path attributes can be advertised in
   a single UPDATE message by including multiple prefixes in the NLRI
   field of the UPDATE message.

   Routes are stored in the Routing Information Bases (RIBs): namely,
   the Adj-RIBs-In, the Loc-RIB, and the Adj-RIBs-Out, as described in
   Section 3.2.

   If a BGP speaker chooses to advertise a previously received route, it
   MAY add to or modify the path attributes of the route before adver-
   tising it to a peer.



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   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 the destination 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 NLRI 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.

   Changing attribute(s) of a route is accomplished by advertising a
   replacement route. The replacement route carries new (changed)
   attributes and has the same address prefix as the original route.


3.2 Routing Information Base

   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 received from other BGP
      speakers. 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. These are
      the routes that will be used by the local BGP speaker. The next
      hop for each of these routes MUST be resolvable via the local BGP
      speaker's Routing Table.

      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



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   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.

   Routing information that the BGP speaker uses to forward packets (or
   to construct the forwarding table that is used for packet forwarding)
   is maintained in the Routing Table. The Routing Table accumulates
   routes to directly connected networks, static routes, routes learned
   from the IGP protocols, and routes learned from BGP.  Whether or not
   a specific BGP route should be installed in the Routing Table, and
   whether a BGP route should override a route to the same destination
   installed by another source is a local policy decision, not specified
   in this document. Besides actual packet forwarding, the Routing Table
   is used for resolution of the next-hop addresses specified in BGP
   updates (see Section 5.1.3).


4. Message Formats

   This section describes message formats used by BGP.

   BGP messages are sent over a TCP connection. A message is processed
   only after it is entirely received. The maximum message size is 4096
   octets. All implementations are required to support this maximum mes-
   sage size. The smallest message that may be sent consists of a BGP
   header without a data portion, or 19 octets.

   All multi-octet fields are in network byte order.


4.1 Message Header Format

   Each message has a fixed-size header. There may or may not be a data
   portion following the header, depending on the message type. The lay-
   out of these fields is shown below:










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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                                                               +
      |                                                               |
      +                                                               +
      |                           Marker                              |
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Length               |      Type     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


      Marker:

         This 16-octet field is included for compatibility; it MUST be
         set to all ones.

      Length:

         This 2-octet unsigned integer indicates the total length of the
         message, including the header, in octets. Thus, e.g., it allows
         one to locate in the TCP stream the (Marker field of the) next
         message. The value of the Length field MUST always be at least
         19 and no greater than 4096, and MAY be further constrained,
         depending on the message type. No "padding" of extra data after
         the message is allowed, so the Length field MUST have the
         smallest value required given the rest of the message.

      Type:

         This 1-octet unsigned integer indicates the type code of the
         message. This document defines the following type codes:

                                    1 - OPEN
                                    2 - UPDATE
                                    3 - NOTIFICATION
                                    4 - KEEPALIVE

         [RFC2918] defines one more type code.


4.2 OPEN Message Format

   After a TCP connection is established, the first message sent by each
   side is an OPEN message. If the OPEN message is acceptable, a



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   KEEPALIVE message confirming the OPEN is sent back.

   In addition to the fixed-size BGP header, the OPEN message contains
   the following fields:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+
       |    Version    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     My Autonomous System      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Hold Time           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         BGP Identifier                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Opt Parm Len  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |             Optional Parameters (variable)                    |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


      Version:

         This 1-octet unsigned integer indicates the protocol version
         number of the message. The current BGP version number is 4.

      My Autonomous System:

         This 2-octet unsigned integer indicates the Autonomous System
         number of the sender.

      Hold Time:

         This 2-octet unsigned integer indicates the number of seconds
         that the sender proposes for the value of the Hold Timer. Upon
         receipt of an OPEN message, a BGP speaker MUST calculate the
         value of the Hold Timer by using the smaller of its configured
         Hold Time and the Hold Time received in the OPEN message. The
         Hold Time MUST be either zero or at least three seconds.  An
         implementation MAY reject connections on the basis of the Hold
         Time.  The calculated value indicates the maximum number of
         seconds that may elapse between the receipt of successive
         KEEPALIVE, and/or UPDATE messages by the sender.

      BGP Identifier:



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         This 4-octet unsigned integer indicates the BGP Identifier of
         the sender. A given BGP speaker sets the value of its BGP Iden-
         tifier 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.

      Optional Parameters Length:

         This 1-octet unsigned integer indicates the total length of the
         Optional Parameters field in octets. If the value of this field
         is zero, no Optional Parameters are present.

      Optional Parameters:

         This field contains a list of optional parameters, where each
         parameter is encoded as a <Parameter Type, Parameter Length,
         Parameter Value> triplet.

               0                   1
               0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...
               |  Parm. Type   | Parm. Length  |  Parameter Value (variable)
               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...

         Parameter Type is a one octet field that unambiguously identi-
         fies individual parameters. Parameter Length is a one octet
         field that contains the length of the Parameter Value field in
         octets.  Parameter Value is a variable length field that is
         interpreted according to the value of the Parameter Type field.

         [RFC3392] defines the Capabilities Optional Parameter.

   The minimum length of the OPEN message is 29 octets (including mes-
   sage header).


4.3 UPDATE Message Format


   UPDATE messages are used to transfer routing information between BGP
   peers. The information in the UPDATE message can be used to construct
   a graph describing the relationships of the various Autonomous Sys-
   tems. By applying rules to be discussed, routing information loops
   and some other anomalies may be detected and removed from inter-AS
   routing.

   An UPDATE message is used to advertise feasible routes sharing common
   path attributes to a peer, or to withdraw multiple unfeasible routes



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   from service (see 3.1). An UPDATE message MAY simultaneously adver-
   tise a feasible route and withdraw multiple unfeasible routes from
   service. The UPDATE message always includes the fixed-size BGP
   header, and also includes the other fields as shown below (note, some
   of the shown fields may not be present in every UPDATE message):

      +-----------------------------------------------------+
      |   Withdrawn Routes Length (2 octets)                |
      +-----------------------------------------------------+
      |   Withdrawn Routes (variable)                       |
      +-----------------------------------------------------+
      |   Total Path Attribute Length (2 octets)            |
      +-----------------------------------------------------+
      |   Path Attributes (variable)                        |
      +-----------------------------------------------------+
      |   Network Layer Reachability Information (variable) |
      +-----------------------------------------------------+


      Withdrawn Routes Length:

         This 2-octets unsigned integer indicates the total length of
         the Withdrawn Routes field in octets.  Its value allows the
         length of the Network Layer Reachability Information field to
         be determined as specified below.

         A value of 0 indicates that no routes are being withdrawn from
         service, and that the WITHDRAWN ROUTES field is not present in
         this UPDATE message.

      Withdrawn Routes:


         This is a variable length field that contains a list of IP
         address prefixes for the routes that are being withdrawn from
         service. Each IP address prefix is encoded as a 2-tuple of the
         form <length, prefix>, whose fields are described below:

                  +---------------------------+
                  |   Length (1 octet)        |
                  +---------------------------+
                  |   Prefix (variable)       |
                  +---------------------------+

         The use and the meaning of these fields are as follows:

         a) Length:




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            The Length field indicates the length in bits of the IP
            address prefix. A length of zero indicates a prefix that
            matches all IP addresses (with prefix, itself, of zero
            octets).

         b) Prefix:

            The Prefix field contains an IP address prefix followed by
            the minimum number of trailing bits needed to make the end
            of the field fall on an octet boundary. Note that the value
            of trailing bits is irrelevant.

      Total Path Attribute Length:

         This 2-octet unsigned integer indicates the total length of the
         Path Attributes field in octets. Its value allows the length of
         the Network Layer Reachability field to be determined as speci-
         fied below.

         A value of 0 indicates that no Network Layer Reachability
         Information field is present in this UPDATE message.

      Path Attributes:

         A variable length sequence of path attributes is present in
         every UPDATE message, except for an UPDATE message that carries
         only the withdrawn routes. Each path attribute is a triple
         <attribute type, attribute length, attribute value> of variable
         length.

         Attribute Type is a two-octet field that consists of the
         Attribute Flags octet followed by the Attribute Type Code
         octet.

               0                   1
               0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               |  Attr. Flags  |Attr. Type Code|
               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         The high-order bit (bit 0) of the Attribute Flags octet is the
         Optional bit. It defines whether the attribute is optional (if
         set to 1) or well-known (if set to 0).

         The second high-order bit (bit 1) of the Attribute Flags octet
         is the Transitive bit. It defines whether an optional attribute
         is transitive (if set to 1) or non-transitive (if set to 0).
         For well-known attributes, the Transitive bit MUST be set to 1.



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         (See Section 5 for a discussion of transitive attributes.)

         The third high-order bit (bit 2) of the Attribute Flags octet
         is the Partial bit. It defines whether the information con-
         tained in the optional transitive attribute is partial (if set
         to 1) or complete (if set to 0). For well-known attributes and
         for optional non-transitive attributes the Partial bit MUST be
         set to 0.

         The fourth high-order bit (bit 3) of the Attribute Flags octet
         is the Extended Length bit. It defines whether the Attribute
         Length is one octet (if set to 0) or two octets (if set to 1).

         The lower-order four bits of the Attribute Flags octet are
         unused. They MUST be zero when sent and MUST be ignored when
         received.

         The Attribute Type Code octet contains the Attribute Type Code.
         Currently defined Attribute Type Codes are discussed in Section
         5.

         If the Extended Length bit of the Attribute Flags octet is set
         to 0, the third octet of the Path Attribute contains the length
         of the attribute data in octets.

         If the Extended Length bit of the Attribute Flags octet is set
         to 1, then the third and the fourth octets of the path
         attribute contain the length of the attribute data in octets.

         The remaining octets of the Path Attribute represent the
         attribute value and are interpreted according to the Attribute
         Flags and the Attribute Type Code. The supported Attribute Type
         Codes, their attribute values and uses are the following:

         a)   ORIGIN (Type Code 1):

            ORIGIN is a well-known mandatory attribute that defines the
            origin of the path information.  The data octet can assume
            the following values:

                  Value      Meaning

                  0         IGP - Network Layer Reachability Information
                               is interior to the originating AS

                  1         EGP - Network Layer Reachability Information
                               learned via the EGP protocol [RFC904]




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                  2         INCOMPLETE - Network Layer Reachability
                               Information learned by some other means

            Usage of this attribute is defined in 5.1.1.

         b) AS_PATH (Type Code 2):

            AS_PATH is a well-known mandatory attribute that is composed
            of a sequence of AS path segments. Each AS path segment is
            represented by a triple <path segment type, path segment
            length, path segment value>.

            The path segment type is a 1-octet long field with the fol-
            lowing values defined:

                  Value      Segment Type

                  1         AS_SET: unordered set of ASs a route in the
                               UPDATE message has traversed

                  2         AS_SEQUENCE: ordered set of ASs a route in
                               the UPDATE message has traversed

            The path segment length is a 1-octet long field containing
            the number of ASs (not the number of octets) in the path
            segment value field.

            The path segment value field contains one or more AS num-
            bers, each encoded as a 2-octets long field.

            Usage of this attribute is defined in 5.1.2.

         c)   NEXT_HOP (Type Code 3):

            This is a well-known mandatory attribute that defines the
            (unicast) IP address of the router that SHOULD be used as
            the next hop to the destinations listed in the Network Layer
            Reachability Information field of the UPDATE message.

            Usage of this attribute is defined in 5.1.3.


         d) MULTI_EXIT_DISC (Type Code 4):

            This is an optional non-transitive attribute that is a four
            octet unsigned integer. The value of this attribute MAY be
            used by a BGP speaker's Decision Process to discriminate
            among multiple entry points to a neighboring autonomous



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            system.

            Usage of this attribute is defined in 5.1.4.

         e) LOCAL_PREF (Type Code 5):

            LOCAL_PREF is a well-known attribute that is a four octet
            unsigned integer. A BGP speaker uses it to inform its other
            internal peers of the advertising speaker's degree of pref-
            erence for an advertised route.

            Usage of this attribute is defined in 5.1.5.

         f) ATOMIC_AGGREGATE (Type Code 6)

            ATOMIC_AGGREGATE is a well-known discretionary attribute of
            length 0.

            Usage of this attribute is defined in 5.1.6.

         g) AGGREGATOR (Type Code 7)

            AGGREGATOR is an optional transitive attribute of length 6.
            The attribute contains the last AS number that formed the
            aggregate route (encoded as 2 octets), followed by the IP
            address of the BGP speaker that formed the aggregate route
            (encoded as 4 octets).  This SHOULD be the same address as
            the one used for the BGP Identifier of the speaker.

            Usage of this attribute is defined in 5.1.7.

      Network Layer Reachability Information:

         This variable length field contains a list of IP address pre-
         fixes. The length in octets of the Network Layer Reachability
         Information is not encoded explicitly, but can be calculated
         as:

            UPDATE message Length - 23 - Total Path Attributes Length -
            Withdrawn Routes Length

         where UPDATE message Length is the value encoded in the fixed-
         size BGP header, Total Path Attribute Length and Withdrawn
         Routes Length are the values encoded in the variable part of
         the UPDATE message, and 23 is a combined length of the fixed-
         size BGP header, the Total Path Attribute Length field and the
         Withdrawn Routes Length field.




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         Reachability information is encoded as one or more 2-tuples of
         the form <length, prefix>, whose fields are described below:

                  +---------------------------+
                  |   Length (1 octet)        |
                  +---------------------------+
                  |   Prefix (variable)       |
                  +---------------------------+

         The use and the meaning of these fields are as follows:

         a) Length:

            The Length field indicates the length in bits of the IP
            address prefix. A length of zero indicates a prefix that
            matches all IP addresses (with prefix, itself, of zero
            octets).

         b) Prefix:

            The Prefix field contains an IP address prefix followed by
            enough trailing bits to make the end of the field fall on an
            octet boundary. Note that the value of the trailing bits is
            irrelevant.

   The minimum length of the UPDATE message is 23 octets -- 19 octets
   for the fixed header + 2 octets for the Withdrawn Routes Length + 2
   octets for the Total Path Attribute Length (the value of Withdrawn
   Routes Length is 0 and the value of Total Path Attribute Length is
   0).

   An UPDATE message can advertise at most one set of path attributes,
   but multiple destinations, provided that the destinations share these
   attributes. All path attributes contained in a given UPDATE message
   apply to all destinations carried in the NLRI field of the UPDATE
   message.

   An UPDATE message can list multiple routes to be withdrawn from ser-
   vice.  Each such route is identified by its destination (expressed as
   an IP prefix), which unambiguously identifies the route in the con-
   text of the BGP speaker - BGP speaker connection to which it has been
   previously advertised.

   An UPDATE message might advertise only routes to be withdrawn from
   service, in which case it will not include path attributes or Network
   Layer Reachability Information. Conversely, it may advertise only a
   feasible route, in which case the WITHDRAWN ROUTES field need not be
   present.



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   An UPDATE message SHOULD NOT include the same address prefix in the
   WITHDRAWN ROUTES and Network Layer Reachability Information fields,
   however a BGP speaker MUST be able to process UPDATE messages in this
   form. A BGP speaker SHOULD treat an UPDATE message of this form as if
   the WITHDRAWN ROUTES doesn't contain the address prefix.


4.4 KEEPALIVE Message Format


   BGP does not use any TCP-based keep-alive mechanism to determine if
   peers are reachable. Instead, KEEPALIVE messages are exchanged
   between peers often enough as not to cause the Hold Timer to expire.
   A reasonable maximum time between KEEPALIVE messages would be one
   third of the Hold Time interval. KEEPALIVE messages MUST NOT be sent
   more frequently than one per second. An implementation MAY adjust the
   rate at which it sends KEEPALIVE messages as a function of the Hold
   Time interval.

   If the negotiated Hold Time interval is zero, then periodic KEEPALIVE
   messages MUST NOT be sent.

   A KEEPALIVE message consists of only message header and has a length
   of 19 octets.


4.5 NOTIFICATION Message Format


   A NOTIFICATION message is sent when an error condition is detected.
   The BGP connection is closed immediately after sending it.

   In addition to the fixed-size BGP header, the NOTIFICATION message
   contains the following fields:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Error code    | Error subcode |   Data (variable)             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


      Error Code:

         This 1-octet unsigned integer indicates the type of NOTIFICA-
         TION. The following Error Codes have been defined:

            Error Code       Symbolic Name               Reference



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              1         Message Header Error             Section 6.1

              2         OPEN Message Error               Section 6.2

              3         UPDATE Message Error             Section 6.3

              4         Hold Timer Expired               Section 6.5

              5         Finite State Machine Error       Section 6.6

              6         Cease                            Section 6.7


      Error subcode:

         This 1-octet unsigned integer provides more specific informa-
         tion about the nature of the reported error.  Each Error Code
         may have one or more Error Subcodes associated with it. If no
         appropriate Error Subcode is defined, then a zero (Unspecific)
         value is used for the Error Subcode field.

         Message Header Error subcodes:

                               1 - Connection Not Synchronized.
                               2 - Bad Message Length.
                               3 - Bad Message Type.

         OPEN Message Error subcodes:

                               1 - Unsupported Version Number.
                               2 - Bad Peer AS.
                               3 - Bad BGP Identifier.
                               4 - Unsupported Optional Parameter.
                               5 - [Deprecated - see Appendix A].
                               6 - Unacceptable Hold Time.

         UPDATE Message Error subcodes:

                               1 - Malformed Attribute List.
                               2 - Unrecognized Well-known Attribute.
                               3 - Missing Well-known Attribute.
                               4 - Attribute Flags Error.
                               5 - Attribute Length Error.
                               6 - Invalid ORIGIN Attribute.
                               7 - [Deprecated - see Appendix A].
                               8 - Invalid NEXT_HOP Attribute.
                               9 - Optional Attribute Error.
                              10 - Invalid Network Field.



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                              11 - Malformed AS_PATH.


      Data:

         This variable-length field is used to diagnose the reason for
         the NOTIFICATION. The contents of the Data field depend upon
         the Error Code and Error Subcode. See Section 6 below for more
         details.

         Note that the length of the Data field can be determined from
         the message Length field by the formula:

                  Message Length = 21 + Data Length

   The minimum length of the NOTIFICATION message is 21 octets (includ-
   ing message header).


5. Path Attributes


   This section discusses the path attributes of the UPDATE message.

   Path attributes fall into four separate categories:

               1. Well-known mandatory.
               2. Well-known discretionary.
               3. Optional transitive.
               4. Optional non-transitive.

   BGP implementations MUST recognize all well-known attributes.  Some
   of these attributes are mandatory and MUST be included in every
   UPDATE message that contains NLRI. Others are discretionary and MAY
   or MAY NOT be sent in a particular UPDATE message.

   Once a BGP peer has updated any well-known attributes, it MUST pass
   these attributes in any updates it transmits to its peers.

   In addition to well-known attributes, each path MAY contain one or
   more optional attributes. It is not required or expected that all BGP
   implementations support all optional attributes. The handling of an
   unrecognized optional attribute is determined by the setting of the
   Transitive bit in the attribute flags octet. Paths with unrecognized
   transitive optional attributes SHOULD be accepted. If a path with
   unrecognized transitive optional attribute is accepted and passed
   along to other BGP peers, then the unrecognized transitive optional
   attribute of that path MUST be passed along with the path to other



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   BGP peers with the Partial bit in the Attribute Flags octet set to 1.
   If a path with recognized transitive optional attribute is accepted
   and passed along to other BGP peers and the Partial bit in the
   Attribute Flags octet is set to 1 by some previous AS, it MUST NOT be
   set back to 0 by the current AS. Unrecognized non-transitive optional
   attributes MUST be quietly ignored and not passed along to other BGP
   peers.

   New transitive optional attributes MAY be attached to the path by the
   originator or by any other BGP speaker in the path. If they are not
   attached by the originator, the Partial bit in the Attribute Flags
   octet is set to 1. The rules for attaching new non-transitive
   optional attributes will depend on the nature of the specific
   attribute. The documentation of each new non-transitive optional
   attribute will be expected to include such rules. (The description of
   the MULTI_EXIT_DISC attribute gives an example.) All optional
   attributes (both transitive and non-transitive) MAY be updated (if
   appropriate) by BGP speakers in the path.

   The sender of an UPDATE message SHOULD order path attributes within
   the UPDATE message in ascending order of attribute type. The receiver
   of an UPDATE message MUST be prepared to handle path attributes
   within the UPDATE message that are out of order.

   The same attribute (attribute with the same type) can not appear more
   than once within the Path Attributes field of a particular UPDATE
   message.

   The mandatory category refers to an attribute which MUST be present
   in both IBGP and EBGP exchanges if NLRI are contained in the UPDATE
   message.  Attributes classified as optional for the purpose of the
   protocol extension mechanism may be purely discretionary, or discre-
   tionary, required, or disallowed in certain contexts.

        attribute           EBGP                    IBGP
         ORIGIN             mandatory               mandatory
         AS_PATH            mandatory               mandatory
         NEXT_HOP           mandatory               mandatory
         MULTI_EXIT_DISC    discretionary           discretionary
         LOCAL_PREF         see Section 5.1.5       required
         ATOMIC_AGGREGATE   see Section 5.1.6 and 9.1.4
         AGGREGATOR         discretionary           discretionary









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5.1 Path Attribute Usage


   The usage of each BGP path attribute is described in the following
   clauses.



5.1.1 ORIGIN


   ORIGIN is a well-known mandatory attribute. The ORIGIN attribute is
   generated by the speaker that originates the associated routing
   information. Its value SHOULD NOT be changed by any other speaker.


5.1.2 AS_PATH


   AS_PATH is a well-known mandatory attribute. This attribute identi-
   fies the autonomous systems through which routing information carried
   in this UPDATE message has passed. The components of this list can be
   AS_SETs or AS_SEQUENCEs.

   When a BGP speaker propagates a route which it has learned from
   another BGP speaker's UPDATE message, it modifies the route's AS_PATH
   attribute based on the location of the BGP speaker to which the route
   will be sent:

      a) When a given BGP speaker advertises the route to an internal
      peer, the advertising speaker SHALL NOT modify the AS_PATH
      attribute associated with the route.

      b) When a given BGP speaker advertises the route to an external
      peer, then the advertising speaker updates the AS_PATH attribute
      as follows:

         1) if the first path segment of the AS_PATH is of type
         AS_SEQUENCE, the local system prepends its own AS number as the
         last element of the sequence (put it in the leftmost position
         with respect to the position of octets in the protocol mes-
         sage). If the act of prepending will cause an overflow in the
         AS_PATH segment, i.e. more than 255 ASs, it SHOULD prepend a
         new segment of type AS_SEQUENCE and prepend its own AS number
         to this new segment.

         2) if the first path segment of the AS_PATH is of type AS_SET,
         the local system prepends a new path segment of type



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         AS_SEQUENCE to the AS_PATH, including its own AS number in that
         segment.

   When a BGP speaker originates a route then:

      a) the originating speaker includes its own AS number in a path
      segment of type AS_SEQUENCE in the AS_PATH attribute of all UPDATE
      messages sent to an external peer. (In this case, the AS number of
      the originating speaker's autonomous system will be the only entry
      the path segment, and this path segment will be the only segment
      in the AS_PATH attribute).

      b) the originating speaker includes an empty AS_PATH attribute in
      all UPDATE messages sent to internal peers.  (An empty AS_PATH
      attribute is one whose length field contains the value zero).

   Whenever the modification of the AS_PATH attribute calls for includ-
   ing or prepending the AS number of the local system, the local system
   MAY include/prepend more than one instance of its own AS number in
   the AS_PATH attribute. This is controlled via local configuration.


5.1.3 NEXT_HOP


   The NEXT_HOP is a well-known mandatory attribute that defines the IP
   address of the router that SHOULD be used as the next hop to the des-
   tinations listed in the UPDATE message. The NEXT_HOP attribute is
   calculated as follows.

      1) When sending a message to an internal peer, if the route is not
      locally originated the BGP speaker SHOULD NOT modify the NEXT_HOP
      attribute, unless it has been explicitly configured to announce
      its own IP address as the NEXT_HOP. When announcing a locally
      originated route to an internal peer, the BGP speaker SHOULD use
      as the NEXT_HOP the interface address of the router through which
      the announced network is reachable for the speaker; if the route
      is directly connected to the speaker, or the interface address of
      the router through which the announced network is reachable for
      the speaker is the internal peer's address, then the BGP speaker
      SHOULD use for the NEXT_HOP attribute its own IP address (the
      address of the interface that is used to reach the peer).

      2) When sending a message to an external peer X, and the peer is
      one IP hop away from the speaker:

         - If the route being announced was learned from an internal
         peer or is locally originated, the BGP speaker can use for the



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         NEXT_HOP attribute an interface address of the internal peer
         router (or the internal router) through which the announced
         network is reachable for the speaker, provided that peer X
         shares a common subnet with this address. This is a form of
         "third party" NEXT_HOP attribute.

         - Otherwise, if the route being announced was learned from an
         external peer, the speaker can use in the NEXT_HOP attribute an
         IP address of any adjacent router (known from the received
         NEXT_HOP attribute) that the speaker itself uses for local
         route calculation, provided that peer X shares a common subnet
         with this address. This is a second form of "third party"
         NEXT_HOP attribute.

         - Otherwise, if the external peer to which the route is being
         advertised shares a common subnet with one of the interfaces of
         the announcing BGP speaker, the speaker MAY use the IP address
         associated with such an interface in the NEXT_HOP attribute.
         This is known as a "first party" NEXT_HOP attribute.

         - By default (if none of the above conditions apply), the BGP
         speaker SHOULD use in the NEXT_HOP attribute the IP address of
         the interface that the speaker uses to establish the BGP con-
         nection to peer X.

      3) When sending a message to an external peer X, and the peer is
      multiple IP hops away from the speaker (aka "multihop EBGP"):

         - The speaker MAY be configured to propagate the NEXT_HOP
         attribute.  In this case when advertising a route that the
         speaker learned from one of its peers, the NEXT_HOP attribute
         of the advertised route is exactly the same as the NEXT_HOP
         attribute of the learned route (the speaker just doesn't modify
         the NEXT_HOP attribute).

         - By default, the BGP speaker SHOULD use in the NEXT_HOP
         attribute the IP address of the interface that the speaker uses
         to establish the BGP connection to peer X.

   Normally the NEXT_HOP attribute is chosen such that the shortest
   available path will be taken. A BGP speaker MUST be able to support
   disabling advertisement of third party NEXT_HOP attributes to handle
   imperfectly bridged media.

   A route originated by a BGP speaker SHALL NOT be advertised to a peer
   using an address of that peer as NEXT_HOP. A BGP speaker SHALL NOT
   install a route with itself as the next hop.




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   The NEXT_HOP attribute is used by the BGP speaker to determine the
   actual outbound interface and immediate next-hop address that SHOULD
   be used to forward transit packets to the associated destinations.

   The immediate next-hop address is determined by performing a recur-
   sive route lookup operation for the IP address in the NEXT_HOP
   attribute using the contents of the Routing Table, selecting one
   entry if multiple entries of equal cost exist.  The Routing Table
   entry which resolves the IP address in the NEXT_HOP attribute will
   always specify the outbound interface. If the entry specifies an
   attached subnet, but does not specify a next-hop address, then the
   address in the NEXT_HOP attribute SHOULD be used as the immediate
   next-hop address.  If the entry also specifies the next-hop address,
   this address SHOULD be used as the immediate next-hop address for
   packet forwarding.


5.1.4 MULTI_EXIT_DISC


   The MULTI_EXIT_DISC is an optional non-transitive attribute which is
   intended to be used on external (inter-AS) links to discriminate
   among multiple exit or entry points to the same neighboring AS.  The
   value of the MULTI_EXIT_DISC attribute is a four octet unsigned num-
   ber which is called a metric. All other factors being equal, the exit
   point with lower metric SHOULD be preferred. If received over EBGP,
   the MULTI_EXIT_DISC attribute MAY be propagated over IBGP to other
   BGP speakers within the same AS (see also 9.1.2.2).  The
   MULTI_EXIT_DISC attribute received from a neighboring AS MUST NOT be
   propagated to other neighboring ASs.

   A BGP speaker MUST implement a mechanism based on local configuration
   which allows the MULTI_EXIT_DISC attribute to be removed from a
   route. If a BGP speaker is configured to remove the MULTI_EXIT_DISC
   attribute from a route, then this removal MUST be done prior to
   determining the degree of preference of the route and performing
   route selection (Decision Process phases 1 and 2).

   An implementation MAY also (based on local configuration) alter the
   value of the MULTI_EXIT_DISC attribute received over EBGP.  If a BGP
   speaker is configured to alter the value of the MULTI_EXIT_DISC
   attribute received over EBGP, then altering the value MUST be done
   prior to determining the degree of preference of the route and per-
   forming route selection (Decision Process phases 1 and 2). See Sec-
   tion 9.1.2.2 for necessary restrictions on this.






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5.1.5 LOCAL_PREF


   LOCAL_PREF is a well-known attribute that SHALL be included in all
   UPDATE messages that a given BGP speaker sends to the other internal
   peers. A BGP speaker SHALL calculate the degree of preference for
   each external route based on the locally configured policy, and
   include the degree of preference when advertising a route to its
   internal peers. The higher degree of preference MUST be preferred.  A
   BGP speaker uses the degree of preference learned via LOCAL_PREF in
   its Decision Process (see Section 9.1.1).

   A BGP speaker MUST NOT include this attribute in UPDATE messages that
   it sends to external peers, except for the case of BGP Confederations
   [RFC3065]. If it is contained in an UPDATE message that is received
   from an external peer, then this attribute MUST be ignored by the
   receiving speaker, except for the case of BGP Confederations
   [RF3065].


5.1.6 ATOMIC_AGGREGATE


   ATOMIC_AGGREGATE is a well-known discretionary attribute.

   When a BGP speaker aggregates several routes for the purpose of
   advertisement to a particular peer, the AS_PATH of the aggregated
   route normally includes an AS_SET formed from the set of ASs from
   which the aggregate was formed.  In many cases the network adminis-
   trator can determine that the aggregate can safely be advertised
   without the AS_SET and not form route loops.

   If an aggregate excludes at least some of the AS numbers present in
   the AS_PATH of the routes that are aggregated as a result of dropping
   the AS_SET, the aggregated route, when advertised to the peer, SHOULD
   include the ATOMIC_AGGREGATE attribute.

   A BGP speaker that receives a route with the ATOMIC_AGGREGATE
   attribute SHOULD NOT remove the attribute from the route when propa-
   gating it to other speakers.

   A BGP speaker that receives a route with the ATOMIC_AGGREGATE
   attribute MUST NOT make any NLRI of that route more specific (as
   defined in 9.1.4) when advertising this route to other BGP speakers.

   A BGP speaker that receives a route with the ATOMIC_AGGREGATE
   attribute needs to be aware of the fact that the actual path to des-
   tinations, as specified in the NLRI of the route, while having the



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   loop-free property, may not be the path specified in the AS_PATH
   attribute of the route.


5.1.7 AGGREGATOR


   AGGREGATOR is an optional transitive attribute which MAY be included
   in updates which are formed by aggregation (see Section 9.2.2.2). A
   BGP speaker which performs route aggregation MAY add the AGGREGATOR
   attribute which SHALL contain its own AS number and IP address. The
   IP address SHOULD be the same as the BGP Identifier of the speaker.


6. BGP Error Handling.


   This section describes actions to be taken when errors are detected
   while processing BGP messages.

   When any of the conditions described here are detected, a NOTIFICA-
   TION message with the indicated Error Code, Error Subcode, and Data
   fields is sent, and the BGP connection is closed, unless it is
   explicitly stated that no NOTIFICATION message is to be sent and the
   BGP connection is not to be closed. If no Error Subcode is specified,
   then a zero MUST be used.

   The phrase "the BGP connection is closed" means that the TCP connec-
   tion has been closed, the associated Adj-RIB-In has been cleared, and
   that all resources for that BGP connection have been deallocated.
   Entries in the Loc-RIB associated with the remote peer are marked as
   invalid.  The local system recalculates its best routes for the des-
   tinations of the routes marked as invalid, and before the invalid
   routes are deleted from the system advertises to its peers either
   withdraws for the routes marked as invalid, or the new best routes
   before the invalid routes are deleted from the system.

   Unless specified explicitly, the Data field of the NOTIFICATION mes-
   sage that is sent to indicate an error is empty.


6.1 Message Header error handling.


   All errors detected while processing the Message Header MUST be indi-
   cated by sending the NOTIFICATION message with Error Code Message
   Header Error. The Error Subcode elaborates on the specific nature of
   the error.



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   The expected value of the Marker field of the message header is all
   ones. If the Marker field of the message header is not as expected,
   then a synchronization error has occurred and the Error Subcode MUST
   be set to Connection Not Synchronized.

   If at least one of the following is true:

      - if the Length field of the message header is less than 19 or
      greater than 4096, or

      - if the Length field of an OPEN message is less than the minimum
      length of the OPEN message, or

      - if the Length field of an UPDATE message is less than the mini-
      mum length of the UPDATE message, or

      - if the Length field of a KEEPALIVE message is not equal to 19,
      or

      - if the Length field of a NOTIFICATION message is less than the
      minimum length of the NOTIFICATION message,

   then the Error Subcode MUST be set to Bad Message Length. The Data
   field MUST contain the erroneous Length field.

   If the Type field of the message header is not recognized, then the
   Error Subcode MUST be set to Bad Message Type. The Data field MUST
   contain the erroneous Type field.


6.2 OPEN message error handling.


   All errors detected while processing the OPEN message MUST be indi-
   cated by sending the NOTIFICATION message with Error Code OPEN Mes-
   sage Error. The Error Subcode elaborates on the specific nature of
   the error.

   If the version number contained in the Version field of the received
   OPEN message is not supported, then the Error Subcode MUST be set to
   Unsupported Version Number. The Data field is a 2-octets unsigned
   integer, which indicates the largest locally supported version number
   less than the version the remote BGP peer bid (as indicated in the
   received OPEN message), or if the smallest locally supported version
   number is greater than the version the remote BGP peer bid, then the
   smallest locally supported version number.

   If the Autonomous System field of the OPEN message is unacceptable,



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   then the Error Subcode MUST be set to Bad Peer AS. The determination
   of acceptable Autonomous System numbers is outside the scope of this
   protocol.

   If the Hold Time field of the OPEN message is unacceptable, then the
   Error Subcode MUST be set to Unacceptable Hold Time. An implementa-
   tion MUST reject Hold Time values of one or two seconds.  An imple-
   mentation MAY reject any proposed Hold Time. An implementation which
   accepts a Hold Time MUST use the negotiated value for the Hold Time.

   If the BGP Identifier field of the OPEN message is syntactically
   incorrect, then the Error Subcode MUST be set to Bad BGP Identifier.
   Syntactic correctness means that the BGP Identifier field represents
   a valid unicast IP host address.

   If one of the Optional Parameters in the OPEN message is not recog-
   nized, then the Error Subcode MUST be set to Unsupported Optional
   Parameters.

   If one of the Optional Parameters in the OPEN message is recognized,
   but is malformed, then the Error Subcode MUST be set to 0 (Unspe-
   cific).


6.3 UPDATE message error handling.


   All errors detected while processing the UPDATE message MUST be indi-
   cated by sending the NOTIFICATION message with Error Code UPDATE Mes-
   sage Error. The error subcode elaborates on the specific nature of
   the error.

   Error checking of an UPDATE message begins by examining the path
   attributes. If the Withdrawn Routes Length or Total Attribute Length
   is too large (i.e., if Withdrawn Routes Length + Total Attribute
   Length + 23 exceeds the message Length), then the Error Subcode MUST
   be set to Malformed Attribute List.

   If any recognized attribute has Attribute Flags that conflict with
   the Attribute Type Code, then the Error Subcode MUST be set to
   Attribute Flags Error. The Data field MUST contain the erroneous
   attribute (type, length and value).

   If any recognized attribute has Attribute Length that conflicts with
   the expected length (based on the attribute type code), then the
   Error Subcode MUST be set to Attribute Length Error. The Data field
   MUST contain the erroneous attribute (type, length and value).




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   If any of the mandatory well-known attributes are not present, then
   the Error Subcode MUST be set to Missing Well-known Attribute. The
   Data field MUST contain the Attribute Type Code of the missing well-
   known attribute.

   If any of the mandatory well-known attributes are not recognized,
   then the Error Subcode MUST be set to Unrecognized Well-known
   Attribute.  The Data field MUST contain the unrecognized attribute
   (type, length and value).

   If the ORIGIN attribute has an undefined value, then the Error Sub-
   code MUST be set to Invalid Origin Attribute. The Data field MUST
   contain the unrecognized attribute (type, length and value).

   If the NEXT_HOP attribute field is syntactically incorrect, then the
   Error Subcode MUST be set to Invalid NEXT_HOP Attribute.  The Data
   field MUST contain the incorrect attribute (type, length and value).
   Syntactic correctness means that the NEXT_HOP attribute represents a
   valid IP host address.

   The IP address in the NEXT_HOP MUST meet the following criteria to be
   considered semantically correct:

      a) It MUST NOT be the IP address of the receiving speaker

      b) In the case of an EBGP where the sender and receiver are one IP
      hop away from each other, either the IP address in the NEXT_HOP
      MUST be the sender's IP address (that is used to establish the BGP
      connection), or the interface associated with the NEXT_HOP IP
      address MUST share a common subnet with the receiving BGP speaker.

   If the NEXT_HOP attribute is semantically incorrect, the error SHOULD
   be logged, and the route SHOULD be ignored. In this case, a NOTIFICA-
   TION message SHOULD NOT be sent, and connection SHOULD NOT be closed.

   The AS_PATH attribute is checked for syntactic correctness. If the
   path is syntactically incorrect, then the Error Subcode MUST be set
   to Malformed AS_PATH.

   If the UPDATE message is received from an external peer, the local
   system MAY check whether the leftmost (with respect to the position
   of octets in the protocol message) AS in the AS_PATH attribute is
   equal to the autonomous system number of the peer that sent the mes-
   sage. If the check determines that this is not the case, the Error
   Subcode MUST be set to Malformed AS_PATH.

   If an optional attribute is recognized, then the value of this
   attribute MUST be checked. If an error is detected, the attribute



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   MUST be discarded, and the Error Subcode MUST be set to Optional
   Attribute Error.  The Data field MUST contain the attribute (type,
   length and value).

   If any attribute appears more than once in the UPDATE message, then
   the Error Subcode MUST be set to Malformed Attribute List.

   The NLRI field in the UPDATE message is checked for syntactic valid-
   ity. If the field is syntactically incorrect, then the Error Subcode
   MUST be set to Invalid Network Field.

   If a prefix in the NLRI field is semantically incorrect (e.g., an
   unexpected multicast IP address), an error SHOULD be logged locally,
   and the prefix SHOULD be ignored.

   An UPDATE message that contains correct path attributes, but no NLRI,
   SHALL be treated as a valid UPDATE message.


6.4 NOTIFICATION message error handling.


   If a peer sends a NOTIFICATION message, and the receiver of the mes-
   sage detects an error in that message, the receiver can not use a
   NOTIFICATION message to report this error back to the peer. Any such
   error, such as an unrecognized Error Code or Error Subcode, SHOULD be
   noticed, logged locally, and brought to the attention of the adminis-
   tration of the peer. The means to do this, however, lies outside the
   scope of this document.


6.5 Hold Timer Expired error handling.


   If a system does not receive successive KEEPALIVE and/or UPDATE
   and/or NOTIFICATION messages within the period specified in the Hold
   Time field of the OPEN message, then the NOTIFICATION message with
   Hold Timer Expired Error Code is sent and the BGP connection is
   closed.


6.6 Finite State Machine error handling.


   Any error detected by the BGP Finite State Machine (e.g., receipt of
   an unexpected event) is indicated by sending the NOTIFICATION message
   with Error Code Finite State Machine Error.




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6.7 Cease.


   In absence of any fatal errors (that are indicated in this section),
   a BGP peer MAY choose at any given time to close its BGP connection
   by sending the NOTIFICATION message with Error Code Cease. However,
   the Cease NOTIFICATION message MUST NOT be used when a fatal error
   indicated by this section does exist.

   A BGP speaker MAY support the ability to impose an (locally config-
   ured) upper bound on the number of address prefixes the speaker is
   willing to accept from a neighbor. When the upper bound is reached,
   the speaker (under control of local configuration) either (a) dis-
   cards new address prefixes from the neighbor (while maintaining BGP
   connection with the neighbor), or (b) terminates the BGP connection
   with the neighbor. If the BGP speaker decides to terminate its BGP
   connection with a neighbor because the number of address prefixes
   received from the neighbor exceeds the locally configured upper
   bound, then the speaker MUST send to the neighbor a NOTIFICATION mes-
   sage with the Error Code Cease. The speaker MAY also log this
   locally.


6.8 BGP connection collision detection.


   If a pair of BGP speakers try simultaneously to establish a BGP con-
   nection to each other, then two parallel connections between this
   pair of speakers might well be formed. If the source IP address used
   by one of these connections is the same as the destination IP address
   used by the other, and the destination IP address used by the first
   connection is the same as the source IP address used by the other, we
   refer to this situation as connection collision.  Clearly in the
   presence of connection collision, one of these connections MUST be
   closed.

   Based on the value of the BGP Identifier a convention is established
   for detecting which BGP connection is to be preserved when a colli-
   sion does occur. The convention is to compare the BGP Identifiers of
   the peers involved in the collision and to retain only the connection
   initiated by the BGP speaker with the higher-valued BGP Identifier.

   Upon receipt of an OPEN message, the local system MUST examine all of
   its connections that are in the OpenConfirm state. A BGP speaker MAY
   also examine connections in an OpenSent state if it knows the BGP
   Identifier of the peer by means outside of the protocol. If among
   these connections there is a connection to a remote BGP speaker whose
   BGP Identifier equals the one in the OPEN message, and this



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   connection collides with the connection over which the OPEN message
   is received then the local system performs the following collision
   resolution procedure:

      1. The BGP Identifier of the local system is compared to the BGP
      Identifier of the remote system (as specified in the OPEN mes-
      sage).  Comparing BGP Identifiers is done by converting them to
      host byte order and treating them as (4-octet long) unsigned inte-
      gers.

      2. If the value of the local BGP Identifier is less than the
      remote one, the local system closes the BGP connection that
      already exists (the one that is already in the OpenConfirm state),
      and accepts the BGP connection initiated by the remote system.

      3. Otherwise, the local system closes newly created BGP connection
      (the one associated with the newly received OPEN message), and
      continues to use the existing one (the one that is already in the
      OpenConfirm state).

   Unless allowed via configuration, a connection collision with an
   existing BGP connection that is in Established state causes closing
   of the newly created connection.

   Note that a connection collision can not be detected with connections
   that are in Idle, or Connect, or Active states.

   Closing the BGP connection (that results from the collision resolu-
   tion procedure) is accomplished by sending the NOTIFICATION message
   with the Error Code Cease.


7. BGP Version Negotiation


   BGP speakers MAY negotiate the version of the protocol by making mul-
   tiple attempts to open a BGP connection, starting with the highest
   version number each supports. If an open attempt fails with an Error
   Code OPEN Message Error, and an Error Subcode Unsupported Version
   Number, then the BGP speaker has available the version number it
   tried, the version number its peer tried, the version number passed
   by its peer in the NOTIFICATION message, and the version numbers that
   it supports. If the two peers do support one or more common versions,
   then this will allow them to rapidly determine the highest common
   version. In order to support BGP version negotiation, future versions
   of BGP MUST retain the format of the OPEN and NOTIFICATION messages.





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8. BGP Finite State machine (FSM)


      The data structures and FSM described in this document are
      conceptual and do not have to be implemented precisely as described
      here, as long as the implementations support the described
      functionality and their externally visible behavior is the same.

      This section specifies the BGP operation in terms of a Finite State
      Machine (FSM).  The section falls into 2 parts:

            1) Description of Events for the State machine (Section 8.1)
            2) Description of the FSM (Section 8.2)

      Session attributes required (mandatory) for each connection are:

            1) State
            2) ConnectRetryCounter
            3) ConnectRetryTimer
            4) ConnectRetryTime
            5) HoldTimer
            6) HoldTime
            7) KeepaliveTimer
            8) KeepaliveTime

      The state session attribute indicates what state the BGP FSM
      is in.  The ConnectRetryCounter indicates the number of times
      a BGP peer has tried to establish a peer session.

      The mandatory attributes related to timers are described in
      section 10.  Each timer has a "timer" and a "time" (the initial
      value).

      The optional Session attributes are listed below. These optional
      attributes may be supported either per connection or per local sys-
      tem:

           1) AcceptConnectionsUnconfiguredPeers
           2) AllowAutomaticStart
           3) AllowAutomaticStop
           4) CollisionDetectEstablishedState
           5) DampPeerOscillations
           6) DelayOpen
           7) DelayOpenTime
           8) DelayOpenTimer
           9) IdleHoldTime
          10) IdleHoldTimer
          11) PassiveTcpEstablishment



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          12) SendNOTIFICATIONwithoutOPEN
          13) TrackTcpState

     The optional session attributes support different features of the BGP
     functionality that have implications for the BGP FSM state
     transitions.   Two groups of the attributes which relate to timers are:
           group 1: DelayOpen, DelayOpenTime, DelayOpenTimer
           group 2: DampPeerOscillations, IdleHoldTime, IdleHoldTimer

     The first parameter (DelayOpen, DampPeerOscillations) is an
     optional attribute that indicates that the Timer function is
     active.  The "Time" value specifies the initial value for "Timer"
     (DelayOpenTime, IdleHoldTime). The "Timer" specifies the actual timer.

     Please refer to section 8.1.1 for an explanation
     of the interaction between these optional attributes and the events
     signaled to the state machine.  Section 8.2.1.4 also provides
     a short overview of the different types of optional attributes
     (flags or timers).





8.1 Events for the BGP FSM



8.1.1 Optional Events linked to Optional Session attributes



      The Inputs to the BGP FSM are events.  Events can either be
      mandatory or optional.  Some optional events are linked to
      optional session attributes.  Optional session attributes enable
      several groups of FSM functionality.

      The description below describes the linkage between FSM
      functionality, events and the optional session attributes.


      Group 1: Automatic Administrative Events (Start/Stop)

           Optional Session Attributes: AllowAutomaticStart, AllowAutomaticStop,
                                        DampPeerOscillations, IdleHoldTime,
                                        IdleHoldTimer

           Option 1:    AllowAutomaticStart



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           Description: A BGP peer connection can be started and stopped
                        by administrative control.  This administrative
                        control can either be manual, based on
                        operator intervention, or under the control
                        of logic specific to a BGP implementation.
                        The term "automatic" refers to a start being
                        issued to the BGP peer connection FSM when
                        such logic determines that the BGP peer
                        connection should be restarted.


                        The AllowAutomaticStart attribute specifies
                        that this BGP connection supports automatic
                        starting of the BGP connection.

                        If the BGP implementation supports
                        AllowAutomaticStart, the peer may be
                        repeatedly restarted. Three other options
                        control the rate at which the automatic
                        restart occurs: DampPeerOscillations,
                        IdleHoldTime, and the IdleHoldTimer.

                        The DampPeerOscillations option specifies
                        that the implementation engages additional
                        logic to damp the oscillations of BGP peers
                        in the face of sequences of automatic start
                        and automatic stop.  IdleHoldTime specifies
                        how long the BGP peer is held in the Idle
                        state prior to allowing the next automatic
                        restart.  The IdleHoldTimer is the timer
                        that runs to hold the peer in Idle state.

                        An example of DampPeerOscillations logic
                        is an increase of the  IdleHoldTime value
                        if a BGP peer oscillates connectivity
                        (connected/disconnected) repeatedly
                        within a time period.   To engage this
                        logic, a peer could connected and disconnect
                        10 times within 5 minutes.  The IdleHoldTime
                        value would be reset from 0 to 120 seconds.

           Values:      TRUE or FALSE


           Option 2:    AllowAutomaticStop

           Description: This BGP peer session optional attribute
                        indicates that the BGP connection allows



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                        "automatic" stopping of the BGP connection.
                        An "automatic" stop is defined as a stop under
                        the control of implementation specific logic.
                        The implementation specific logic is outside
                        the scope of this specification.

           Values:      TRUE or FALSE


           Option 3:    DampPeerOscillations

           Description: The DampPeerOscillations optional session
                        attribute indicates that this BGP connection
                        is using logic that damps BGP peer oscillations
                        in the Idle State.

           Value:       TRUE or FALSE

           Option 4:    IdleHoldTime

           Description: The IdleHoldTime is the value
                        that is set in the IdleHoldtimer.

           Values:      Time in seconds


           Option 5:    IdleHoldTimer

           Description: The IdleHoldTimer aids in controlling BGP peer
                        oscillation. The IdleHoldTimer is used to keep
                        the BGP peer in Idle for a particular duration.
                        The IdleHoldTimer_Expires event is described
                        in section 8.1.3.

           Values:      Time in seconds



      Group 2: Unconfigured Peers

           Optional Session Attributes: AcceptConnectionsUnconfiguredPeers

           Option 1:    AcceptConnectionsUnconfiguredPeers

           Description: The BGP FSM optionally allows the acceptance of BGP
                        peer connections from neighbors that are not
                        pre-configured. The
                        "AcceptConnectionsUnconfiguredPeers" optional



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                        session attribute allows the FSM to support
                        the state transitions that allow the
                        implementation to accept or reject these
                        unconfigured peers.

                        The AcceptConnectionsUnconfiguredPeers has
                        security implications. Please refer to the
                        BGP Vulnerabilities document[BGP_VULN] for
                        details.

           Value:       True or False

      Group 3: TCP processing

           Optional Session Attributes: PassiveTcpEstablishment, TrackTcpState

           Option 1:    PassiveTcpEstablishment

           Description: This option indicates that the BGP FSM will passively
                        wait for the remote BGP peer to establish the BGP
                        TCP connection.

           value:       TRUE or FALSE

           Option 2:    TrackTcpState

           Description: The BGP FSM normally tracks the end result of a TCP
                        connection attempt rather than individual TCP messages.
                        Optionally, the BGP FSM can support additional
                        interaction with the TCP connection negotiation. The
                        interaction with the TCP events may increase the
                        amount of logging the BGP peer connection
                        requires and the number of BGP FSM changes.

           Value:       TRUE or FALSE


      Group 4:  BGP Message Processing


           Optional Session Attributes: DelayOpen, DelayOpenTime,
                                        DelayOpenTimer,
                                        SendNOTIFICATIONwithoutOPEN,
                                        CollisionDetectEstablishedState

           Option 1:     DelayOpen

           Description: The DelayOpen optional session attribute allows



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                        implementations to be configured to delay
                        sending an OPEN message for a specific time
                        period (DelayOpenTime).  The delay allows
                        the remote BGP Peer time to send the first
                        OPEN message.

           Value:       TRUE or FALSE


           Option 2:    DelayOpenTime

           Description: The DelayOpenTime is the initial value that is
                        set in the DelayOpenTimer.

           Value:       Time in seconds

           Option 3:    DelayOpenTimer

           Description: The DelayOpenTimer optional session attribute
                        specifies a time that the local system will wait
                        prior to sending an OPEN message on the connection.

           Value:       Time in seconds


           Option 4:    SendNOTIFICATIONwithoutOPEN

           Description: The SendNOTIFICATIONwithoutOPEN allows a peer to
                        send a NOTIFICATION without first sending an
                        OPEN message. Without this optional session
                        attribute, the BGP connection assumes that an
                        OPEN message must be sent by a peer prior
                        to the peer sending a NOTIFICATION message.

           Value:       True or False

           Option 5:    CollisionDetectEstablishedState

           Description: Normally, a Detect Collision (6.8) will
                        be ignored in the Established state.  This
                        optional session attribute indicates that
                        this BGP connection processes
                        collisions in the Established state.

           Value:       True or False


      Note: The optional session attributes clarify the BGP FSM description



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            for existing features of BGP implementations.  The optional
            session attributes may be pre-defined for an implementation
            and not readable via management interfaces for existing
            correct implementations.  As newer BGP MIBs (version 2
            and beyond) are supported, these fields will be accessible
            via a management interface.




8.1.2 Administrative Events


   An administrative event is an event in which the operator interface
   and BGP Policy engine signal the BGP finite state machine to start or
   stop the BGP state machine.  The basic start and stop indication are
   augmented by optional connection attributes to signal a certain type
   of start or stop mechanism to the BGP FSM.  An example of this combi-
   nation is event 5, AutomaticStart_with_PassiveTcpEstablishment.  With
   this event, the BGP implementation signals to the BGP FSM that the
   implementation is using an Automatic Start with option to use a Pas-
   sive TCP Establishment.  The Passive TCP establishment signals that
   this BGP FSM will wait for the remote side to start the TCP estab-
   lishment.

   Please note that only Event 1 (ManualStart) and Event 2 (ManualStop)
   are mandatory administrative events. All other administrative events
   are optional (Events 3-8). Each event below has a name, definition,
   status (mandatory or optional), and what optional session attributes
   SHOULD be set at each stage. When generating Event 1 through Event 8
   for the BGP FSM, the conditions specified in the "Optional Attribute
   Status" section are verified.  If any of these conditions are not
   satisfied, then the local system should log a FSM error.

   The settings of optional session attributes may be implicit in some
   implementations and therefore may not be set explicitly by an exter-
   nal operator action. Section 8.2.1.5 describes these implicit set-
   tings of the optional session attributes. The administrative states
   described below may also be implicit in some implementations and not
   directly configurable by an external operator.

          Event1: ManualStart

                 Definition: Local system administrator manually starts peer
                             connection.

                 Status:     Mandatory




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                 Optional
                 Attribute
                 Status:     The PassiveTcpEstablishment attribute SHOULD be
                             set to FALSE.

          Event2: ManualStop

                 Definition: Local system administrator manually
                             stops the peer connection.

                 Status:     Mandatory

                 Optional
                 Attribute
                 Status:     No interaction with any optional attributes.

          Event3: AutomaticStart

                 Definition: Local system automatically starts the
                             BGP connection.


                 Status:     Optional, depending on local system

                 Optional
                 Attribute
                 Status:     1) The AllowAutomaticStart attribute SHOULD be set
                                to TRUE if this event occurs.
                             2) If the PassiveTcpEstablishment optional session
                                attribute is supported, it SHOULD be set to FALSE.
                             3) If the DampPeerOscillations is supported, it
                                SHOULD be set to FALSE when this event occurs.


          Event4: ManualStart_with_PassiveTcpEstablishment

                 Definition: Local system administrator manually starts peer
                             connection, but has PassiveTcpEstablishment
                             enabled.  The PassiveTcpEstablishment optional
                             attribute indicates that the peer will listen prior
                             to establishing the connection.

                 Status:     Optional, depending on local system

                 Optional
                 Attribute
                 Status:     1) The PassiveTcpEstablishment attribute SHOULD
                                be set to TRUE if this event occurs.



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                             2) The DampPeerOscillations attribute SHOULD be
                                set to FALSE when this event occurs.


          Event5: AutomaticStart_with_PassiveTcpEstablishment

                 Definition: Local system automatically starts the
                             BGP connection with the PassiveTcpEstablishment
                             enabled.  The PassiveTcpEstablishment
                             optional attribute indicates
                             that the peer will listen prior to
                             establishing a connection.

                 Status:     Optional, depending on local system

                 Optional
                 Attribute
                 Status:     1) The AllowAutomaticStart attribute SHOULD
                                be set to TRUE.
                             2) The PassiveTcpEstablishment attribute SHOULD
                                be set to TRUE
                             3) If the DampPeerOscillations attribute is
                                supported, the DampPeerOscillations SHOULD
                                be set to FALSE.



          Event6: AutomaticStart_with_DampPeerOscillations

                 Definition: Local system automatically starts the
                             BGP peer connection with peer oscillation
                             damping enabled. The exact method of damping
                             persistent peer oscillations is left up to the
                             implementation and is outside the scope of
                             this document.

                 Status:     Optional, depending on local system.

                 Optional
                 Attribute
                 Status:     1) The AllowAutomaticStart attribute SHOULD
                                be set to TRUE.
                             2) The DampPeerOscillations attribute SHOULD
                                be set to TRUE.
                             3) The PassiveTcpEstablishment attribute
                                SHOULD be set to FALSE.





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         Event 7: AutomaticStart_with_DampPeerOscillations_and_
                           PassiveTcpEstablishment

                 Definition: Local system automatically starts the
                             BGP peer connection with peer oscillation
                             damping enabled and PassiveTcpEstablishment
                             enabled.  The exact method of damping
                             persistent peer oscillations is left up to the
                             implementation and is outside the scope of
                             this document.

                 Status:     Optional, depending on local system

                 Optional
                 Attributes
                 Status:     1) The AllowAutomaticStart attribute
                                SHOULD be set to TRUE.
                             2) The DampPeerOscillations attribute SHOULD
                                be set to TRUE.
                             3) The PassiveTcpEstablishment attribute
                                SHOULD be set to TRUE.

          Event8: AutomaticStop

                 Definition: Local system automatically stops the
                             BGP connection.

                             An example of an automatic stop event is
                             exceeding the number of prefixes for a given
                             peer and the local system  automatically
                             disconnecting the peer.


                 Status:     Optional, depending on local system

                 Optional
                 Attribute
                 Status:     1) The AllowAutomaticStop attribute
                               SHOULD be TRUE



8.1.3 Timer Events




          Event9: ConnectRetryTimer_Expires



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                 Definition: An event generated when the ConnectRetryTimer
                             expires.

                 Status:     Mandatory

          Event10: HoldTimer_Expires

                 Definition: An event generated when the HoldTimer expires.

                 Status:     Mandatory

          Event11: KeepaliveTimer_Expires

                 Definition: An event generated when the KeepaliveTimer expires.
                 Status:     Mandatory

          Event12: DelayOpenTimer_Expires

                 Definition: An event generated when the DelayOpenTimer expires.

                 Status:     Optional

                 Optional
                 Attribute
                 Status:     If this event occurs,
                             1) DelayOpen attribute SHOULD be set to TRUE,
                             2) DelayOpenTime attribute SHOULD be supported,
                             3) DelayOpenTimer SHOULD be supported,


         Event13: IdleHoldTimer_Expires

                Definition:  An event generated when the IdleHoldTimer
                             expires indicating that the BGP connection has
                             completed waiting for the back-off period
                             to prevent BGP peer oscillation.

                             The IdleHoldTimer is only used when the
                             persistent peer oscillation damping
                             function is enabled by setting the
                             DampPeerOscillations optional attribute
                             to TRUE.

                             Implementations not implementing the
                             persistent peer oscillation damping
                             function may not have the IdleHoldTimer.





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                 Status:     Optional

                 Optional
                 Attribute
                 Status:     If this event occurs:
                             1) DampPeerOscillations attribute SHOULD be set
                                to TRUE.
                             2) IdleHoldTimer SHOULD have just expired.


8.1.4 TCP Connection based Events



          Event14: TcpConnection_Valid

                 Definition: Event indicating the local system reception of
                             a TCP connection request with a valid
                             source IP address and TCP port and a valid
                             destination IP address and TCP Port. The
                             definition of invalid source and invalid
                             destination IP address is left to the
                             implementation.

                             BGP's destination port SHOULD be port 179
                             as defined by IANA.

                             TCP connection request is denoted by the
                             local system receiving a TCP SYN.

                 Status:     Optional

                 Optional
                 Attribute
                 Status:     1) The TrackTcpState attribute SHOULD be set to
                                TRUE if this event occurs.

          Event15: Tcp_CR_Invalid

                 Definition: Event indicating the local system reception
                             of a TCP connection request with either
                             an invalid source address or port
                             number or an invalid destination
                             address or port number.

                             BGP destination port number SHOULD be 179
                             as defined by IANA.




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                             A TCP connection request occurs when
                             the local system receives a TCP
                             SYN.

                 Status:     Optional

                 Optional
                 Attribute
                 Status:     1) The TrackTcpState attribute should be set to
                                TRUE if this event occurs.


          Event16: Tcp_CR_Acked

                 Definition: Event indicating the local system's request
                             to establish a TCP connection to the remote
                             peer.

                             The local system's TCP connection sent a TCP
                             SYN, and received a TCP SYN/ACK message,
                             and sent a TCP ACK.

                 Status:     Mandatory

          Event17: TcpConnectionConfirmed

                 Definition: Event indicating that the local system has
                             received a confirmation that the TCP
                             connection has been established by the
                             remote site.

                             The remote peer's TCP engine sent a TCP SYN.
                             The local peer's TCP engine sent a SYN, ACK
                             message and now has received a final ACK.

                 Status:     Mandatory

          Event18: TcpConnectionFails

                 Definition: Event indicating that the local system has
                             received a TCP connection failure notice.

                             The remote BGP peer's TCP machine could have
                             sent a FIN.  The local peer would respond
                             with a FIN-ACK. Another alternative is that
                             the local peer indicated a timeout in the
                             TCP connection and downed the connection.




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                 Status:     Mandatory


8.1.5 BGP Message-based Events



          Event19: BGPOpen

                 Definition: An event is generated when a valid OPEN
                             message has been received.

                 Status:     Mandatory

                 Optional
                 Attribute
                 Status:     1) The DelayOpen optional attribute SHOULD
                                be set to FALSE.
                             2) The DelayOpenTimer SHOULD not be running.


          Event20: BGPOpen with DelayOpenTimer running

                 Definition: An event is generated when a valid OPEN
                             message has been received for a peer
                             that has a successfully established
                             transport connection and is currently
                             delaying the sending of a BGP open
                             message.


                 Status:     Optional

                 Optional
                 Attribute
                 Status:     1) The DelayOpen attribute SHOULD be
                               set to TRUE.
                             2) The DelayOpenTimer SHOULD be running.


          Event21: BGPHeaderErr

                 Definition: An event is generated when a received
                             BGP message header is not valid.

                 Status:     Mandatory

          Event22: BGPOpenMsgErr



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                 Definition: An event is generated when an OPEN message
                             has been received with errors.

                 Status:     Mandatory


          Event23: OpenCollisionDump

                 Definition: An event generated administratively
                             when a connection collision has been
                             detected while processing an incoming
                             OPEN message and this connection has been
                             selected to be disconnected. See section
                             6.8 for more information on collision
                             detection.

                             Event23 is an administrative action
                             generated by implementation logic
                             that determines that this connection
                             needs to be dropped per the rules in
                             section 6.8.  This event may occur if the FSM
                             is implemented as two linked state machines.


                 Status:     Optional

                 Optional
                 Attribute
                 Status:     If the state machine is to process this
                             event in Established state,
                              1) CollisionDetectEstablishedState
                                  optional attribute SHOULD be set to TRUE


                              Please note: The OpenCollisionDump event can occur
                              in Idle, Connect, Active, OpenSent, OpenConfirm
                              without any optional attributes being set.

          Event24: NotifMsgVerErr

                 Definition: An event is generated when a
                             NOTIFICATION message with "version
                             error" is received.

                 Status:     Mandatory

          Event25: NotifMsg




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                 Definition: An event is generated when a
                             NOTIFICATION message is received and
                             the error code is anything but
                             "version error".

                 Status:     Mandatory

          Event26: KeepAliveMsg

                 Definition: An event is generated when a KEEPALIVE
                             message is received.

                 Status:     Mandatory

          Event27: UpdateMsg

                 Definition: An event is generated when a valid
                             UPDATE message is received.

                 Status:     Mandatory

          Event28: UpdateMsgErr

                 Definition: An event is generated when an invalid
                             UPDATE message is received.

                 Status:     Mandatory


8.2 Description of FSM



8.2.1 FSM Definition



   BGP MUST maintain a separate FSM for each configured peer. Each BGP
   peer paired in a potential connection, unless configured to remain in
   the idle state, or configured to remain passive, will attempt to con-
   nect to the other.  For the purpose of this discussion, the active or
   connecting side of the TCP connection (the side of a TCP connection
   sending the first TCP SYN packet) is called outgoing.  The passive or
   listening side (the sender of the first SYN/ACK) is called an incom-
   ing connection. (See Section 8.2.1.1 for information on the terms
   active and passive used below.)

   A BGP implementation MUST connect to and listen on TCP port 179 for



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   incoming connections in addition to trying to connect to peers.  For
   each incoming connection, a state machine MUST be instantiated.
   There exists a period in which the identity of the peer on the other
   end of an incoming connection is known, but the BGP identifier is not
   known.  During this time, both an incoming and an outgoing connection
   for the same configured peering may exist. This is referred to as a
   connection collision.  (See Section 6.8.)

   A BGP implementation will have at most one FSM for each configured
   peering plus one FSM for each incoming TCP connection for which the
   peer has not yet been identified. Each FSM corresponds to exactly one
   TCP connection.

   There may be more than one connection between a pair of peers if the
   connections are configured to use a different pair of IP addresses.
   This is referred to as multiple "configured peerings" to the same
   peer.



8.2.1.1 Terms "active" and "passive"



   The terms active and passive have been in the Internet operator's
   vocabulary for almost a decade and have proven useful.  The words
   active and passive have slightly different meanings applied to a TCP
   connection or applied to a peer.  There is only one active side and
   one passive side to any one TCP connection per the definition above
   and the state machine below. When a BGP speaker is configured active,
   it may end up on either the active or passive side of the connection
   that eventually gets established.  Once the TCP connection is com-
   pleted, it doesn't matter which end was active and which end was pas-
   sive.  The only difference is which side of the TCP connection has
   port number 179.


8.2.1.2 FSM and collision detection

   There is one FSM per BGP connection. When the connection collision
   occurs prior to determining what peer a connection is associated
   with,  there may be two connections for one peer.   After the connec-
   tion collision is resolved (see Section 6.8) the FSM for the connec-
   tion that is closed SHOULD be disposed of.







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8.2.1.3  FSM and Optional Session Attributes



      Optional Session Attributes specify either attributes that act
      as flags (TRUE or FALSE) or optional timers.  For optional
      attributes that act as flags, if the optional session attribute
      can be set to TRUE on the system, the corresponding the BGP FSM
      actions must be supported.  For example, if the following options
      can be set in a BGP implementation: AutoStart and
      PassiveTCPEstablishment, then the events 3, 4 and 5 must be
      supported. If an Optional Session attribute cannot be set to
      TRUE, the events supporting that set of options do not have to
      be supported.

      Each of the optional timers (DelayOpenTimer and IdleHoldTimer),
      has a group of attributes that are:

           - flag indicating support,
           - Time set in Timer
           - Timer.

       The two optional timers show this format:

           DelayOpenTimer: DelayOpen, DelayOpenTime, DelayOpenTimer
           IdleHoldTimer:  DampPeerOscillations, IdleHoldTime,
                           IdleHoldTmer

       If the flag indicating support for an optional timer
       (DelayOpen or DampPeerOscillations), cannot be set to TRUE,
       the timers and events supporting that
       option do not have to be supported.



8.2.1.4 FSM Event numbers


   The Event numbers (1-28) utilized in this state machine description
   aid in specifying the behavior of the BGP state machine.  Implementa-
   tions MAY use these numbers to provide network management informa-
   tion. The exact form of a FSM or the FSM events are specific to each
   implementation.








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8.2.1.5 FSM actions that are implementation dependent.


   The BGP FSM specifies at certain points that BGP initialization will
   occur or that BGP resources will be deleted. The initialization of
   the BGP FSM and the associated resources depend on the policy portion
   of the BGP implementation. The details of these actions are outside
   the scope of the FSM document.


8.2.2 Finite State Machine


         Idle state:

           Initially the BGP peer FSM is in the Idle state. (Hereafter
           the BGP peer FSM will be shortened to BGP FSM.)

            In this state BGP FSM refuses all incoming BGP
            connections for this peer.  No resources are allocated to the peer.
            In response to a ManualStart event (Event 1) or an
            AutomaticStart event (Event 3), the local system:
               - initializes all BGP resources for the peer connection,
               - sets ConnectRetryCounter to zero,
               - starts the ConnectRetryTimer with initial value,
               - initiates a TCP connection to the other BGP peer,
               - listens for a connection that may be initiated by
                 the remote BGP peer, and
               - changes its state to Connect.

           The ManualStop event (Event 2) and AutomaticStop (Event 8) event
           are ignored in the Idle state.

           In response to a ManualStart_with_PassiveTcpEstablishment event
           (Event 4) or AutomaticStart_with_PassiveTcpEstablishment event
           (Event 5), the local system:
               - initializes all BGP resources,
               - sets the ConnectRetryCounter to zero,
               - starts the ConnectRetryTimer with initial value,
               - listens for a connection that may be initiated by
                 the remote peer, and
               - changes its state to Active.

           The exact value of the ConnectRetryTimer is a local
           matter, but it SHOULD be sufficiently large to allow TCP
           initialization.

           If the DampPeerOscillations attribute is set to TRUE,



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           the following three additional events may occur
           within Idle state:
               - AutomaticStart_with_DampPeerOscillations (Event6),
               - AutomaticStart_with_DampPeerOscillations_and_
                 PassiveTcpEstablishment (Event7),
               - IdleHoldTimer_Expires (Event 13).

           Upon receiving these 3 events, the local system will
           use these events to prevent peer oscillations.
           The method of preventing persistent peer oscillation is
           outside the scope of this document.

           Any other event (Events 9-12, 15-28) received in the Idle state
           does not cause change in the state of the local system.


         Connect State:

           In this state, BGP FSM is waiting for the TCP connection to
           be completed.


           The start events (Events 1, 3-7) are ignored in connect
           state.

           In response to a ManualStop event [Event 2), the local system:
              - drops the TCP connection,
              - releases all BGP resources,
              - sets ConnectRetryCounter to zero,
              - stops the ConnectRetryTimer and sets ConnectRetryTimer
                to zero, and
              - changes its state to Idle.


           In response to the ConnectRetryTimer_Expires event (Event
           9), the local system:
              - drops the TCP connection,
              - restarts the ConnectRetryTimer,
              - stops the DelayOpenTimer and resets the timer to zero,
              - initiates a TCP connection to the other BGP peer,
              - continues to listen for a connection that may be
                initiated by the remote BGP peer, and
              - stays in Connect state.

           If the DelayOpenTimer_Expires event (Event12) occurs in the
           Connect state, the local system:
              - sends an OPEN message to its peer,
              - sets the HoldTimer to a large value, and



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              - changes its state to OpenSent.

           If the BGP FSM receives a TcpConnection_Valid event
           (Event 14), the TCP connection is processed, and
           the connection remains in the Connect state.

           If the BGP FSM receives a Tcp_CR_Invalid event (Event 15),
           the local system rejects the TCP connection, and the connection
           remains in the Connect state.

           If the TCP connection succeeds (Event 16 or Event 17),
           the local system checks the DelayOpen attribute prior
           to processing.  If the DelayOpen attribute is set to TRUE,
           the local system:
                - stops the ConnectRetryTimer (if running) and sets the
                  ConnectRetryTimer to zero,
                - sets the DelayOpenTimer to the initial value, and
                - stays in the Connect state.
           If the DelayOpen attribute is set to FALSE, the local system:
                - stops the ConnectRetryTimer (if running) and sets the
                  ConnectRetryTimer to zero,
                - completes BGP initialization
                - sends an OPEN message to its peer,
                - sets HoldTimer to a large value, and
                - changes its state to OpenSent.

           A HoldTimer value of 4 minutes is suggested.

           If the TCP connection fails (Event18), the local system
           checks the DelayOpenTimer.  If the DelayOpenTimer is running,
           the local system:
               - restarts the ConnectRetryTimer with initial value,
               - stops the DelayOpenTimer and resets value to zero,
               - continues to listen for a connection that may be
                 initiated by the remote BGP peer, and
               - changes its state to Active.

           If the DelayOpenTimer is not running, the local system:
              - stops the ConnectRetryTimer to zero,
              - drops the TCP connection,
              - releases all BGP resources, and
              - changes its state to Idle.

           If an OPEN message is received while the DelayOpenTimer is
           running (Event 20), the local system:

              - stops the ConnectRetryTimer (if running) and
                sets the ConnectRetryTimer to zero,



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              - completes the BGP initialization,
              - stops and clears the DelayOpenTimer
                (sets the value to zero),
              - sends an OPEN message,
              - sends a KEEPALIVE message,
              - if the HoldTimer initial value is non-zero,
                      - starts the KeepaliveTimer with the initial value and
                      - resets the HoldTimer to the negotiated value,
                else if HoldTimer initial value is zero,
                      - resets the KeepaliveTimer and
                      - resets the HoldTimer value to zero,
              - and changes its state to OpenConfirm.

           If the value of the autonomous system field is the same as the local
           Autonomous System number, set the connection status to an internal
           connection; otherwise it is "external".

           If BGP message header checking detects an error (Event 21) or
           OPEN message checking detects an error (Event 22) (see section
           6.2), the local system:
              - (optionally) If the SendNOTIFICATIONwithoutOPEN attribute
                 is set to TRUE, then the local system first sends
                 a NOTIFICATION message with the appropriate error
                 code, and then

              - stops the ConnectRetryTimer (if running)
                and sets the ConnectRetryTimer to zero,
              - releases all BGP resources,
              - drops the TCP connection,
              - increments the ConnectRetryCounter by 1,
              - (optionally) performs peer oscillation damping
                if the DampPeerOscillations attribute is set to TRUE, and
              - changes its state to Idle.

           If a NOTIFICATION message is received with a version
           error(Event24), the local system checks the DelayOpenTimer.
           If the DelayOpenTimer is running, the local system:
              - stops the ConnectRetryTimer (if running)
                and sets the ConnectRetryTimer to zero,
              - stops and resets the DelayOpenTimer (sets to zero),
              - releases all BGP resources,
              - drops the TCP connection, and
              - changes its state to Idle.

           If the DelayOpenTimer is not running, the local system:
              - stops the ConnectRetryTimer and sets the
                ConnectRetryTimer to zero,
              - releases all BGP resources,



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              - drops the TCP connection,
              - increments the ConnectRetryCounter by 1,
              - performs peer oscillation damping if the
                DampPeerOscillations attribute is set to True, and
              - changes its state to Idle.


          In response to any other events (Events 8,10-11,13,19,23,
          25-28) the local system:
              - if the ConnectRetryTimer is running,
                 stops and resets the ConnectRetryTimer (sets to zero),
              - if the DelayOpenTimer is running,
                 stops and resets the DelayOpenTimer (sets to zero),
              - releases all BGP resources,
              - drops the TCP connection,
              - increments the ConnectRetryCounter by 1,
              - performs peer oscillation damping if the
                DampPeerOscillations attribute is set to True, and
              - changes its state to Idle.


         Active State:

          In this state BGP FSM is trying to acquire a peer by listening
          for and accepting a TCP connection.


          The start events (Event1, 3-7) are ignored in the Active
          state.

          In response to a ManualStop event (Event 2), the local system:
              - If the DelayOpenTimer is running and the
                SendNOTIFICATIONwithoutOPEN session attribute is set,
                  the local system sends a NOTIFICATION with a Cease,
              - releases all BGP resources including
                stopping the DelayOpenTimer
              - drops the TCP connection,
              - sets ConnectRetryCounter to zero,
              - stops the ConnectRetryTimer and sets the
                ConnectRetryTimer to zero, and
              - changes its state to Idle.

          In response to a ConnectRetryTimer_Expires event (Event 9),
          the local system:
              - restarts the ConnectRetryTimer (with initial value),
              - initiates a TCP connection to the other BGP peer,
              - continues to listen for TCP connection that may be
                initiated by remote BGP peer, and



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              - changes its state to Connect.


          If the local system receives a DelayOpenTimer_Expired event
          (Event 12), the local system:
              - sets the ConnectRetryTimer to zero,
              - stops and clears the DelayOpenTimer (set to zero),
              - completes the BGP initialization,
              - sends the OPEN message to its remote peer,
              - sets its hold timer to a large value, and
              - changes its state to OpenSent.

          A HoldTimer value of 4 minutes is also suggested for this
          state transition.

          If the local system receives a TcpConnection_Valid event
          (Event 14), the local system processes the TCP connection
          flags and stays in Active state.


          If the local system receives an Tcp_CR_Invalid event (Event 15):
          the local system rejects the TCP connection and stays in
          the Active State.

          In response to a TCP connection succeeding (Event 16 or Event 17), the
          local system checks the DelayOpen optional attribute prior to
          processing.
              If the DelayOpen attribute is set to TRUE, the local
              system:
                   - stops the ConnectRetryTimer and sets the
                     ConnectRetryTimer to zero,
                   - sets the DelayOpenTimer to the initial value
                     (DelayOpenTime), and
                   - stays in the Active state.
              If the DelayOpen attribute is set to FALSE, the local
              system:
                   - sets the ConnectRetryTimer to zero,
                   - completes the BGP initialization,
                   - sends the OPEN message to its peer,
                   - sets its HoldTimer to a large value, and
                   - changes its state to OpenSent.

          A HoldTimer value of 4 minutes is suggested as a "large value" for
          the HoldTimer.


          If the local system receives a TcpConnectionFails event (Event 18),
          the local system:



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              - restarts ConnectRetryTimer (with initial value),
              - stops and clears the DelayOpenTimer (sets the value to zero),
              - releases all BGP resource,
              - increments ConnectRetryCounter by 1,
              - optionally performs peer oscillation damping if
                the DampPeerOscillations attribute is set to TRUE, and
              - changes its state to Idle.


          If an OPEN message is received and the DelayOpenTimer is
          running (Event 20), the local system:
              - stops ConnectRetryTimer (if running) and sets
                the ConnectRetryTimer to zero,
              - stops and clears DelayOpenTimer (sets to zero),
              - completes the BGP initialization,
              - sends an OPEN message,
              - sends a KEEPALIVE message,
              - if the HoldTimer value is non-zero,
                      - starts the KeepaliveTimer to initial value,
                      - resets the HoldTimer to the negotiated value,
                else if the HoldTimer is zero
                      - resets the KeepaliveTimer (set to zero),
                      - resets the HoldTimer to zero, and
              - changes its state to OpenConfirm.

          If the value of the autonomous system field is the same as
          the local Autonomous System number, set the connection status
          to an internal connection; otherwise it is external.

          If BGP message header checking detects an error (Event 21)
          or OPEN message checking detects an error (Event 22) (see
          section 6.2), the local system:
              - (optionally) sends a NOTIFICATION message with the
                 appropriate error code if the SendNOTIFICATIONwithoutOPEN
                 attribute is set to TRUE,
              - sets the ConnectRetryTimer to zero,
              - releases all BGP resources,
              - drops the TCP connection,
              - increments the ConnectRetryCounter by 1,
              - (optionally) performs peer oscillation damping if the
                DampPeerOscillations attribute is set to TRUE, and
              - changes its state to Idle.

           If a NOTIFICATION message is received with a version
           error (Event 24), the local system checks the DelayOpenTimer.
           If the DelayOpenTimer is running, the local system:
              - stops the ConnectRetryTimer (if running) and
                sets the ConnectRetryTimer to zero,



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              - stops and resets the DelayOpenTimer (sets to zero),
              - releases all BGP resources,
              - drops the TCP connection, and
              - changes its state to Idle.
           If the DelayOpenTimer is not running, the local system:
              - sets the ConnectRetryTimer to zero,
              - releases all BGP resources,
              - drops the TCP connection,
              - increments the ConnectRetryCounter by 1,
              - (optionally) performs peer oscillation damping
                if the DampPeerOscillations attribute is set to TRUE, and
              - changes its state to Idle.

          In response to any other event (Events 8,10-11,13,19,23,25-28),
          the local system:
              - sets the ConnectRetryTimer to zero,
              - releases all BGP resources,
              - drops the TCP connection,
              - increments the ConnectRetryCounter by one,
              - (optionally) performs peer oscillation damping if
                the DampPeerOscillations attribute is set to TRUE, and
              - changes its state to Idle.


         OpenSent:

          In this state BGP FSM waits for an OPEN message from its peer.

          The start events (Event1, 3-7) are ignored in the OpenSent
          state.

          If a ManualStop event (Event 2) is issued in OpenSent
          state, the local system:
              - sends the NOTIFICATION with a cease,
              - sets the ConnectRetryTimer to zero,
              - releases all BGP resources,
              - drops the TCP connection,
              - sets the ConnectRetryCounter to zero, and
              - changes its state to Idle.

          If an AutomaticStop event (Event 8) is issued in OpenSent
          state, the local system:
              - sends the NOTIFICATION with a cease,
              - sets the ConnectRetryTimer to zero,
              - releases all the BGP resources,
              - drops the TCP connection,
              - increments the ConnectRetryCounter by 1,
              - (optionally) performs peer oscillation damping if the



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                DampPeerOscillations attribute is set to TRUE, and
              - changes its state to Idle.

          If the HoldTimer_Expires (Event 10), the local system:
              - sends a NOTIFICATION message with error code Hold
                Timer Expired,
              - sets the ConnectRetryTimer to zero,
              - releases all BGP resources,
              - drops the TCP connection,
              - increments the ConnectRetryCounter,
              - (optionally) performs peer oscillation damping if the
                DampPeerOscillations attribute is set to TRUE, and
              - changes its state to Idle.

           If a TcpConnection_Valid (Event 14) or Tcp_CR_Acked (Event 16)
           is received, or a TcpConnectionConfirmed event (Event 17) is
           received, a second TCP connection may be in progress.  This
           second TCP connection is tracked per Connection Collision
           processing (Section 6.8) until an OPEN message is received.

           A TCP Connection Request for an Invalid port
           (Tcp_CR_Invalid (Event 15)) is ignored.

           If a TcpConnectionFails event (Event18) is received,
           the local system:
              - closes the BGP connection,
              - restarts the ConnectRetryTimer,
              - continues to listen for a connection that may be
                initiated by the remote BGP peer, and
              - changes its state to Active.


           When an OPEN message is received, all fields are checked
           for correctness.  If there are no errors in the OPEN message
           (Event 19), the local system:
              - resets the DelayOpenTimer to zero,
              - sets the BGP ConnectRetryTimer to zero,
              - sends a KEEPALIVE message, and
              - sets a KeepaliveTimer (via the text below)
              - sets the HoldTimer according to the negotiated value
                (see Section 4.2),
              - changes its state to OpenConfirm.

           If the negotiated hold time value is zero, then the HoldTimer and
           KeepaliveTimer are not started. If the value of the Autonomous
           System field is the same as the local Autonomous System number,
           then the connection is an "internal" connection; otherwise, it
           is an "external" connection.  (This will impact UPDATE processing



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           as described below.)

           If the BGP message header checking (Event 21) or OPEN message
           check detects an error (Event 22)(see Section 6.2), the local system:
              - sends a NOTIFICATION message with appropriate error
                code,
              - sets the ConnectRetryTimer to zero,
              - releases all BGP resources,
              - drops the TCP connection,
              - increments the ConnectRetryCounter by 1,
              - (optionally) performs peer oscillation damping if the
                DampPeerOscillations attribute is TRUE, and
              - changes its state to Idle.

           Collision detection mechanisms (Section 6.8) need to be
           applied when a valid BGP OPEN message is received (Event 19 or
           Event 20). Please refer to Section 6.8 for the details of
           the comparison. A CollisionDetectDump event occurs when the
           BGP implementation determines, by a means outside the scope of
           this document, that a connection collision has occurred.

           If a connection in OpenSent state is determined to be the
           connection that must be closed, an OpenCollisionDump (Event 23)
           is signaled to the state machine. If such an event is
           received in OpenSent state, the local system:
              - sends a NOTIFICATION with a Cease
              - sets the ConnectRetryTimer to zero,
              - releases all BGP resources,
              - drops the TCP connection,
              - increments ConnectRetryCounter by 1,
              - (optionally) performs peer oscillation damping if the
                DampPeerOscillations attribute is set to TRUE, and
              - changes its state to Idle.


           If a NOTIFICATION message is received with a version
           error (Event24), the local system:
              - sets the ConnectRetryTimer to zero,
              - releases all BGP resources,
              - drops the TCP connection, and
              - changes its state to Idle.


          In response to any other event (Events 9, 11-13,20,25-28),
          the local system:
              - sends the NOTIFICATION with the Error Code Finite
                state machine error,
              - sets the ConnectRetryTimer to zero,



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              - releases all BGP resources,
              - drops the TCP connection,
              - increments the ConnectRetryCounter by 1,
              - (optionally) performs peer oscillation damping if the
                DampPeerOscillations attribute is set to TRUE, and
              - changes its state to Idle.



         OpenConfirm State:

          In this state BGP waits for a KEEPALIVE or NOTIFICATION
          message.


          Any start event (Event1, 3-7) is ignored in the OpenConfirm
          state.


          In response to a ManualStop event (Event 2) initiated by
          the operator, the local system:
              - sends the NOTIFICATION message with Cease,
              - releases all BGP resources,
              - drops the TCP connection,
              - sets the ConnectRetryCounter to zero,
              - sets the ConnectRetryTimer to zero, and
              - changes its state to Idle.

          In response to the AutomaticStop event initiated by the
          system (Event 8), the local system:
              - sends the NOTIFICATION message with Cease,
              - sets the ConnectRetryTimer to zero,
              - releases all BGP resources,
              - drops the TCP connection,
              - increments the ConnectRetryCounter by 1,
              - (optionally) performs peer oscillation damping
                if the DampPeerOscillations attribute is set to TRUE,
                and
              - changes its state to Idle.

          If the HoldTimer_Expires event (Event 10) occurs before a KEEPALIVE
          message is received, the local system:
              - sends the NOTIFICATION message with the error code,
              - sets the ConnectRetryTimer to zero,
              - releases all BGP resources,
              - drops the TCP connection,
              - increments the ConnectRetryCounter by 1,
              - (optionally) performs peer oscillation damping if



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                the DampPeerOscillations attribute is set to TRUE, and
              - changes its state to Idle.


          If the local system receives a KeepaliveTimer_Expires
          event (Event 11), the system:
              - sends a KEEPALIVE message,
              - restarts the KeepaliveTimer, and
              - remains in OpenConfirmed state.

          In the event of TcpConnection_Valid event (Event 14), or TCP
          connection succeeding (Event 16 or Event 17) while in OpenConfirm,
          the local system needs to track the second connection.

          If a TCP connection is attempted to an invalid port (Event
          15), the local system will ignore the second connection
          attempt.


          If the local system receives a TcpConnectionFails event
          (Event 18) from the  underlying TCP or a NOTIFICATION
          message (Event 25), the local system:
              - sets the ConnectRetryTimer to zero,
              - releases all BGP resources,
              - drops the TCP connection,
              - increments the ConnectRetryCounter by 1,
              - (optionally) performs peer oscillation damping if the
                DampPeerOscillations attribute is set to TRUE, and
              - changes its state to Idle.

          If the local system receives a NOTIFICATION message with a
          version error (NotifMsgVerErr (Event 24)), the local system:
              - sets the ConnectRetryTimer to zero,
              - releases all BGP resources,
              - drops the TCP connection, and
              - changes its state to Idle.


          If the local system receives a valid OPEN message
         (BGPOpen (Event 19)), the collision detect function is
         processed per Section 6.8. If this connection is to be
         dropped due to connection collision, the local system:
              - sends a NOTIFICATION with a Cease,
              - sets the ConnectRetryTimer to zero,
              - releases all BGP resources,
              - drops the TCP connection (send TCP FIN),
              - increments the ConnectRetryCounter by 1,
              - (optionally) performs peer oscillation damping if the



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                DampPeerOscillations attribute is set to TRUE, and
              - changes its state to Idle.


          If an OPEN message is received, all fields are checked for
          correctness.  If the BGP message header checking
          (BGPHeaderErr (Event21)) or OPEN message check detects
          an error (see Section 6.2) (BGPOpenMsgErr(Event22)), the
          local system:
              - sends a NOTIFICATION message with appropriate error
                code,
              - sets the ConnectRetryTimer to zero,
              - releases all BGP resources,
              - drops the TCP connection,
              - increments the ConnectRetryCounter by 1,
              - (optionally) performs peer oscillation damping if the
                DampPeerOscillations attribute is set to TRUE, and
              - changes its state to Idle.


          If during the processing of another OPEN message, the BGP
          implementation determines by a means outside the scope of
          this document that a connection collision has occurred and
          this connection is to be closed, the local system will
          issue an OpenCollisionDump event (Event 23).  When the local
          system receives an OpenCollisionDump event (Event 23), the
          local system:
              - sends a NOTIFICATION with a Cease
              - sets the ConnectRetryTimer to zero,
              - releases all BGP resources
              - drops the TCP connection,
              - increments the ConnectRetryCounter by 1,
              - (optionally) performs peer oscillation damping if the
                DampPeerOscillations attribute is set to TRUE, and
              - changes its state to Idle.


          If the local system receives a KEEPALIVE message
           (KeepAliveMsg (Event 26)), the local system:
              - restarts the HoldTimer and
              - changes its state to Established.

          In response to any other event (Events 9, 12-13, 20, 27-28),
          the local system:
              - sends a NOTIFICATION with a code of Finite State
                Machine Error,
              - sets the ConnectRetryTimer to zero,
              - releases all BGP resources,



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              - drops the TCP connection,
              - increments the ConnectRetryCounter by 1,
              - (optionally) performs peer oscillation damping if the
                DampPeerOscillations attribute is set to TRUE, and
              - changes its state to Idle.


         Established State:

          In the Established state, the BGP FSM can exchange UPDATE,
          NOTIFICATION, and KEEPALIVE messages with its peer.


          Any Start event (Event 1, 3-7) is ignored in the
          Established state.

          In response to a ManualStop event (initiated by an
          operator)(Event2), the local system:
              - sends the NOTIFICATION message with Cease,
              - sets the ConnectRetryTimer to zero,
              - deletes all routes associated with this connection,
              - releases BGP resources,
              - drops the TCP connection,
              - sets ConnectRetryCounter to zero, and
              - changes its state to Idle.

          In response to an AutomaticStop event (Event8), the local system:
              - sends a NOTIFICATION with Cease,
              - sets the ConnectRetryTimer to zero
              - deletes all routes associated with this connection,
              - releases all BGP resources,
              - drops the TCP connection,
              - increments the ConnectRetryCounter by 1,
              - (optionally) performs peer oscillation damping if the
                DampPeerOscillations attribute is set to TRUE, and
              - changes its state to Idle.

          One reason for an AutomaticStop event is: A BGP receives
          UPDATE messages with number of prefixes for a given
          peer so that the total prefixes received exceeds the
          maximum number of prefixes configured.  The local system
         automatically disconnects the peer.


          If the HoldTimer_Expires event occurs (Event10), the
          local system:
              - sends a NOTIFICATION message with Error Code Hold
                Timer Expired,



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              - sets the ConnectRetryTimer to zero,
              - releases all BGP resources,
              - drops the TCP connection,
              - increments the ConnectRetryCounter by 1,
              - (optionally) performs peer oscillation damping if the
                DampPeerOscillations attribute is set to TRUE, and
              - changes its state to Idle.

          If the KeepaliveTimer_Expires event occurs (Event11),
          the local system:
              - sends a KEEPALIVE message, and
              - restarts its KeepaliveTimer unless the negotiated
                HoldTime value is zero.

          Each time the local system sends a KEEPALIVE or UPDATE
          message, it restarts its KeepaliveTimer, unless the
          negotiated HoldTime value is zero.


          A TcpConnection_Valid (Event 14) received for a
          valid port will cause the second connection to be
          tracked.

          An invalid TCP connection (Tcp_CR_Invalid Event
          (Event 15)), will be ignored.

          In response to an indication that the TCP connection
          is successfully established (Event 16
          or Event 17), the second connection SHALL be tracked until
          it sends an OPEN message.

          If a valid OPEN message (BGPOpen (Event 19)) is received,
          and if the CollisionDetectEstablishedState optional
          attribute is TRUE, the OPEN message will be checked
          to see if it collides (Section 6.8) with any other connection.
          If the BGP implementation determines that this connection
          needs to be terminated, it will process an OpenCollisionDump
          event (Event 23).  If this connection needs to be
          terminated, the local system:
              - sends a NOTIFICATION with a Cease,
              - sets the ConnectRetryTimer to zero,
              - deletes all routes associated with this connection,
              - releases all BGP resources,
              - drops the TCP connection,
              - increments ConnectRetryCounter by 1,
              - (optionally) performs peer oscillation damping if the
                DampPeerOscillations is set to TRUE, and
              - changes its state to Idle.



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          If the local system receives a NOTIFICATION message
          (Event24 or Event 25) or a TcpConnectionsFails (Event18)
          from the underlying TCP, it:
              - sets the ConnectRetryTimer to zero,
              - deletes all routes associated with this connection,
              - releases all the BGP resources,
              - drops the TCP connection,
              - increments the ConnectRetryCounter by 1,
              - changes its state to Idle.


          If the local system receives a KEEPALIVE message
          (Event 26), the local system:
              - restarts its HoldTimer, if the negotiated HoldTime
                value is non-zero, and
              - remains in the Established state.


          If the local system receives an UPDATE message (Event27),
          the local system:
              - processes the message,
              - restarts its HoldTimer if the negotiated HoldTime
                value is non-zero, and
              - remains in the Established state.


          If the local system receives an UPDATE message, and the
          UPDATE message error handling procedure (see Section 6.3)
          detects an error (Event28), the local system:
              - sends a NOTIFICATION message with Update error,
              - sets the ConnectRetryTimer to zero,
              - deletes all routes associated with this connection,
              - releases all BGP resources,
              - drops the TCP connection,
              - increments the ConnectRetryCounter by 1,
              - (optionally) performs peer oscillation damping if the
                DampPeerOscillations attribute is set to TRUE, and
              - changes its state to Idle.


          In response to any other event (Events 9, 12-13, 20-22) the
          local system:
              - sends a NOTIFICATION message with Error Code Finite
                State Machine Error,
              - deletes all routes associated with this connection,
              - sets the ConnectRetryTimer to zero,
              - releases all BGP resources,
              - drops the TCP connection,



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              - increments the ConnectRetryCounter by 1,
              - (optionally) performs peer oscillation damping if the
                DampPeerOscillations attribute is set to TRUE, and
              - changes its state to Idle.




9. UPDATE Message Handling


   An UPDATE message may be received only in the Established state.
   Receiving an UPDATE message in any other state is an error.  When an
   UPDATE message is received, each field is checked for validity as
   specified in Section 6.3.

   If an optional non-transitive attribute is unrecognized, it is qui-
   etly ignored. If an optional transitive attribute is unrecognized,
   the Partial bit (the third high-order bit) in the attribute flags
   octet is set to 1, and the attribute is retained for propagation to
   other BGP speakers.

   If an optional attribute is recognized, and has a valid value, then,
   depending on the type of the optional attribute, it is processed
   locally, retained, and updated, if necessary, for possible propaga-
   tion to other BGP speakers.


   If the UPDATE message contains a non-empty WITHDRAWN ROUTES field,
   the previously advertised routes whose destinations (expressed as IP
   prefixes) contained in this field SHALL be removed from the Adj-RIB-
   In.  This BGP speaker SHALL run its Decision Process since the previ-
   ously advertised route is no longer available for use.

   If the UPDATE message contains a feasible route, the Adj-RIB-In will
   be updated with this route as follows: if the NLRI of the new route
   is identical to the one of the route currently stored in the Adj-RIB-
   In, then the new route SHALL replace the older route in the Adj-RIB-
   In, thus implicitly withdrawing the older route from service. Other-
   wise, if the Adj-RIB-In has no route with NLRI identical to the new
   route, the new route SHALL be placed in the Adj-RIB-In.

   Once the BGP speaker updates the Adj-RIB-In, the speaker SHALL run
   its Decision Process.







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9.1 Decision Process


   The Decision Process selects routes for subsequent advertisement by
   applying the policies in the local Policy Information Base (PIB) to
   the routes stored in its Adj-RIBs-In. The output of the Decision Pro-
   cess is the set of routes that will be advertised to peers; the
   selected routes will be stored in the local speaker's Adj-RIBs-Out
   according to policy.

   The BGP Decision Process described here is conceptual, and does not
   have to be implemented precisely as described here, as long as the
   implementations support the described functionality and their exter-
   nally visible behavior is the same.

   The selection process is formalized by defining a function that takes
   the attribute of a given route as an argument and returns either (a)
   a non-negative integer denoting the degree of preference for the
   route, or (b) a value denoting that this route is ineligible to be
   installed in Loc-RIB and will be excluded from the next phase of
   route selection.

   The function that calculates the degree of preference for a given
   route SHALL NOT use as its inputs any of the following: the existence
   of other routes, the non-existence of other routes, or the path
   attributes of other routes. Route selection then consists of individ-
   ual application of the degree of preference function to each feasible
   route, followed by the choice of the one with the highest degree of
   preference.

   The Decision Process operates on routes contained in the Adj-RIBs-In,
   and is responsible for:

      - selection of routes to be used locally by the speaker

      - selection of routes to be advertised to other BGP peers

      - route aggregation and route information reduction

   The Decision Process takes place in three distinct phases, each trig-
   gered by a different event:

      a) Phase 1 is responsible for calculating the degree of preference
      for each route received from a peer.

      b) Phase 2 is invoked on completion of phase 1. It is responsible
      for choosing the best route out of all those available for each
      distinct destination, and for installing each chosen route into



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      the Loc-RIB.

      c) Phase 3 is invoked after the Loc-RIB has been modified. It is
      responsible for disseminating routes in the Loc-RIB to each peer,
      according to the policies contained in the PIB. Route aggregation
      and information reduction can optionally be performed within this
      phase.


9.1.1 Phase 1: Calculation of Degree of Preference


   The Phase 1 decision function is invoked whenever the local BGP
   speaker receives from a peer an UPDATE message that advertises a new
   route, a replacement route, or withdrawn routes.

   The Phase 1 decision function is a separate process which completes
   when it has no further work to do.

   The Phase 1 decision function locks an Adj-RIB-In prior to operating
   on any route contained within it, and unlocks it after operating on
   all new or unfeasible routes contained within it.

   For each newly received or replacement feasible route, the local BGP
   speaker determines a degree of preference as follows:

      If the route is learned from an internal peer, either the value of
      the LOCAL_PREF attribute is taken as the degree of preference, or
      the local system computes the degree of preference of the route
      based on preconfigured policy information. Note that the latter
      (computing the degree of preference based on preconfigured policy
      information) may result in formation of persistent routing loops.

      If the route is learned from an external peer, then the local BGP
      speaker computes the degree of preference based on preconfigured
      policy information. If the return value indicates that the route
      is ineligible, the route MAY NOT serve as an input to the next
      phase of route selection; otherwise the return value MUST be used
      as the LOCAL_PREF value in any IBGP readvertisement.

      The exact nature of this policy information and the computation
      involved is a local matter.


9.1.2 Phase 2: Route Selection


   The Phase 2 decision function is invoked on completion of Phase 1.



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   The Phase 2 function is a separate process which completes when it
   has no further work to do. The Phase 2 process considers all routes
   that are eligible in the Adj-RIBs-In.

   The Phase 2 decision function is blocked from running while the Phase
   3 decision function is in process. The Phase 2 function locks all
   Adj-RIBs-In prior to commencing its function, and unlocks them on
   completion.

   If the NEXT_HOP attribute of a BGP route depicts an address that is
   not resolvable, or it would become unresolvable if the route was
   installed in the routing table the BGP route MUST be excluded from
   the Phase 2 decision function.

   If the AS_PATH attribute of a BGP route contains an AS loop, the BGP
   route should be excluded from the Phase 2 decision function.  AS loop
   detection is done by scanning the full AS path (as specified in the
   AS_PATH attribute), and checking that the autonomous system number of
   the local system does not appear in the AS path.  Operations of a BGP
   speaker that is configured to accept routes with its own autonomous
   system number in the AS path are outside the scope of this document.

   It is critical that BGP speakers within an AS do not make conflicting
   decisions regarding route selection that would cause forwarding loops
   to occur.

   For each set of destinations for which a feasible route exists in the
   Adj-RIBs-In, the local BGP speaker identifies the route that has:

      a) the highest degree of preference of any route to the same set
      of destinations, or

      b) is the only route to that destination, or

      c) is selected as a result of the Phase 2 tie breaking rules spec-
      ified in 9.1.2.2.

   The local speaker SHALL then install that route in the Loc-RIB,
   replacing any route to the same destination that is currently being
   held in the Loc-RIB. When the new BGP route is installed in the Rout-
   ing Table, care must be taken to ensure that existing routes to the
   same destination that are now considered invalid are removed from the
   Routing Table. Whether or not the new BGP route replaces an existing
   non-BGP route in the Routing Table depends on the policy configured
   on the BGP speaker.

   The local speaker MUST determine the immediate next-hop address from
   the NEXT_HOP attribute of the selected route (see Section 5.1.3). If



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   either the immediate next hop or the IGP cost to the NEXT_HOP (where
   the NEXT_HOP is resolved through an IGP route) changes, Phase 2 Route
   Selection MUST be performed again.

   Notice that even though BGP routes do not have to be installed in the
   Routing Table with the immediate next hop(s), implementations MUST
   take care that before any packets are forwarded along a BGP route,
   its associated NEXT_HOP address is resolved to the immediate
   (directly connected) next-hop address and this address (or multiple
   addresses) is finally used for actual packet forwarding.

   Unresolvable routes SHALL be removed from the Loc-RIB and the routing
   table. However, corresponding unresolvable routes SHOULD be kept in
   the Adj-RIBs-In (in case they become resolvable).


9.1.2.1 Route Resolvability Condition


   As indicated in Section 9.1.2, BGP speakers SHOULD exclude unresolv-
   able routes from the Phase 2 decision. This ensures that only valid
   routes are installed in Loc-RIB and the Routing Table.

   The route resolvability condition is defined as follows.

      1. A route Rte1, referencing only the intermediate network
      address, is considered resolvable if the Routing Table contains at
      least one resolvable route Rte2 that matches Rte1's intermediate
      network address and is not recursively resolved (directly or indi-
      rectly) through Rte1. If multiple matching routes are available,
      only the longest matching route SHOULD be considered.

      2. Routes referencing interfaces (with or without intermediate
      addresses) are considered resolvable if the state of the refer-
      enced interface is up and IP processing is enabled on this inter-
      face.

   BGP routes do not refer to interfaces, but can be resolved through
   the routes in the Routing Table that can be of both types (those that
   specify interfaces or those that do not). IGP routes and routes to
   directly connected networks are expected to specify the outbound
   interface. Static routes can specify the outbound interface, or the
   intermediate address, or both.

   Note that a BGP route is considered unresolvable not only in situa-
   tions where the BGP speaker's Routing Table contains no route match-
   ing the BGP route's NEXT_HOP. Mutually recursive routes (routes
   resolving each other or themselves), also fail the resolvability



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   check.

   It is also important that implementations do not consider feasible
   routes that would become unresolvable if they were installed in the
   Routing Table even if their NEXT_HOPs are resolvable using the cur-
   rent contents of the Routing Table (an example of such routes would
   be mutually recursive routes). This check ensures that a BGP speaker
   does not install in the Routing Table routes that will be removed and
   not used by the speaker. Therefore, in addition to local Routing Ta-
   ble stability, this check also improves behavior of the protocol in
   the network.

   Whenever a BGP speaker identifies a route that fails the resolvabil-
   ity check because of mutual recursion, an error message SHOULD be
   logged.


9.1.2.2 Breaking Ties (Phase 2)


   In its Adj-RIBs-In a BGP speaker may have several routes to the same
   destination that have the same degree of preference. The local
   speaker can select only one of these routes for inclusion in the
   associated Loc-RIB. The local speaker considers all routes with the
   same degrees of preference, both those received from internal peers,
   and those received from external peers.

   The following tie-breaking procedure assumes that for each candidate
   route all the BGP speakers within an autonomous system can ascertain
   the cost of a path (interior distance) to the address depicted by the
   NEXT_HOP attribute of the route, and follow the same route selection
   algorithm.

   The tie-breaking algorithm begins by considering all equally prefer-
   able routes to the same destination, and then selects routes to be
   removed from consideration. The algorithm terminates as soon as only
   one route remains in consideration.  The criteria MUST be applied in
   the order specified.

   Several of the criteria are described using pseudo-code. Note that
   the pseudo-code shown was chosen for clarity, not efficiency. It is
   not intended to specify any particular implementation. BGP implemen-
   tations MAY use any algorithm which produces the same results as
   those described here.

      a) Remove from consideration all routes which are not tied for
      having the smallest number of AS numbers present in their AS_PATH
      attributes. Note, that when counting this number, an AS_SET counts



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      as 1, no matter how many ASs are in the set.

      b) Remove from consideration all routes which are not tied for
      having the lowest Origin number in their Origin attribute.

      c) Remove from consideration routes with less-preferred
      MULTI_EXIT_DISC attributes. MULTI_EXIT_DISC is only comparable
      between routes learned from the same neighboring AS (the neighbor-
      ing AS is determined from the AS_PATH attribute). Routes which do
      not have the MULTI_EXIT_DISC attribute are considered to have the
      lowest possible MULTI_EXIT_DISC value.

      This is also described in the following procedure:

            for m = all routes still under consideration
                for n = all routes still under consideration
                    if (neighborAS(m) == neighborAS(n)) and (MED(n) < MED(m))
                        remove route m from consideration

      In the pseudo-code above, MED(n) is a function which returns the
      value of route n's MULTI_EXIT_DISC attribute. If route n has no
      MULTI_EXIT_DISC attribute, the function returns the lowest possi-
      ble MULTI_EXIT_DISC value, i.e. 0.

      Similarly, neighborAS(n) is a function which returns the neighbor
      AS from which the route was received.  If the route is learned via
      IBGP, and the other IBGP speaker didn't originate the route, it is
      the neighbor AS from which the other IBGP speaker learned the
      route.  If the route is learned via IBGP, and the other IBGP
      speaker either (a) originated the route, or (b) created the route
      by aggregation and the AS_PATH attribute of the aggregate route is
      either empty or begins with an AS_SET, it is the local AS.

      If a MULTI_EXIT_DISC attribute is removed before re-advertising a
      route into IBGP, then comparison based on the received EBGP
      MULTI_EXIT_DISC attribute MAY still be performed. If an implemen-
      tation chooses to remove MULTI_EXIT_DISC, then the optional com-
      parison on MULTI_EXIT_DISC if performed at all MUST be performed
      only among EBGP learned routes. The best EBGP learned route may
      then be compared with IBGP learned routes after the removal of the
      MULTI_EXIT_DISC attribute. If MULTI_EXIT_DISC is removed from a
      subset of EBGP learned routes and the selected "best" EBGP learned
      route will not have MULTI_EXIT_DISC removed, then the
      MULTI_EXIT_DISC must be used in the comparison with IBGP learned
      routes. For IBGP learned routes the MULTI_EXIT_DISC MUST be used
      in route comparisons which reach this step in the Decision Pro-
      cess.  Including the MULTI_EXIT_DISC of an EBGP learned route in
      the comparison with an IBGP learned route, then removing the



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      MULTI_EXIT_DISC attribute and advertising the route has been
      proven to cause route loops.

      d) If at least one of the candidate routes was received via EBGP,
      remove from consideration all routes which were received via IBGP.

      e) Remove from consideration any routes with less-preferred inte-
      rior cost.  The interior cost of a route is determined by calcu-
      lating the metric to the NEXT_HOP for the route using the Routing
      Table.  If the NEXT_HOP hop for a route is reachable, but no cost
      can be determined, then this step should be skipped (equivalently,
      consider all routes to have equal costs).

      This is also described in the following procedure.

            for m = all routes still under consideration
                for n = all routes in still under consideration
                    if (cost(n) is lower than cost(m))
                        remove m from consideration

      In the pseudo-code above, cost(n) is a function which returns the
      cost of the path (interior distance) to the address given in the
      NEXT_HOP attribute of the route.

      f) Remove from consideration all routes other than the route that
      was advertised by the BGP speaker whose BGP Identifier has the
      lowest value.

      g) Prefer the route received from the lowest peer address.


9.1.3 Phase 3: Route Dissemination


   The Phase 3 decision function is invoked on completion of Phase 2, or
   when any of the following events occur:

      a) when routes in the Loc-RIB to local destinations have changed

      b) when locally generated routes learned by means outside of BGP
      have changed

      c) when a new BGP speaker - BGP speaker connection has been estab-
      lished

   The Phase 3 function is a separate process which completes when it
   has no further work to do. The Phase 3 Routing Decision function is
   blocked from running while the Phase 2 decision function is in



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   process.

   All routes in the Loc-RIB are processed into Adj-RIBs-Out according
   to configured policy. This policy MAY exclude a route in the Loc-RIB
   from being installed in a particular Adj-RIB-Out. A route SHALL NOT
   be installed in the Adj-Rib-Out unless the destination and NEXT_HOP
   described by this route may be forwarded appropriately by the Routing
   Table. If a route in Loc-RIB is excluded from a particular Adj-RIB-
   Out the previously advertised route in that Adj-RIB-Out MUST be with-
   drawn from service by means of an UPDATE message (see 9.2).

   Route aggregation and information reduction techniques (see 9.2.2.1)
   may optionally be applied.

   Any local policy which results in routes being added to an Adj-RIB-
   Out without also being added to the local BGP speaker's forwarding
   table, is outside the scope of this document.

   When the updating of the Adj-RIBs-Out and the Routing Table is com-
   plete, the local BGP speaker runs the Update-Send process of 9.2.


9.1.4 Overlapping Routes


   A BGP speaker may transmit routes with overlapping Network Layer
   Reachability Information (NLRI) to another BGP speaker. NLRI overlap
   occurs when a set of destinations are identified in non-matching mul-
   tiple routes. Since BGP encodes NLRI using IP prefixes, overlap will
   always exhibit subset relationships.  A route describing a smaller
   set of destinations (a longer prefix) is said to be more specific
   than a route describing a larger set of destinations (a shorter pre-
   fix); similarly, a route describing a larger set of destinations is
   said to be less specific than a route describing a smaller set of
   destinations.

   The precedence relationship effectively decomposes less specific
   routes into two parts:

      - a set of destinations described only by the less specific route,
      and

      - a set of destinations described by the overlap of the less spe-
      cific and the more specific routes


   The set of destinations described by the overlap represents a portion
   of the less specific route that is feasible, but is not currently in



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   use.  If a more specific route is later withdrawn, the set of desti-
   nations described by the overlap will still be reachable using the
   less specific route.

   If a BGP speaker receives overlapping routes, the Decision Process
   MUST consider both routes based on the configured acceptance policy.
   If both a less and a more specific route are accepted, then the Deci-
   sion Process MUST either install in Loc-RIB both the less and the
   more specific routes or it MUST aggregate the two routes and install
   in Loc-RIB the aggregated route, provided that both routes have the
   same value of the NEXT_HOP attribute.

   If a BGP speaker chooses to aggregate, then it SHOULD either include
   all AS used to form the aggregate in an AS_SET or add the
   ATOMIC_AGGREGATE attribute to the route.  This attribute is now pri-
   marily informational.  With the elimination of IP routing protocols
   that do not support classless routing and the elimination of router
   and host implementations that do not support classless routing, there
   is no longer a need to de-aggregate.  Routes SHOULD NOT be de-aggre-
   gated.  A route that carries ATOMIC_AGGREGATE attribute in particular
   MUST NOT be de-aggregated. That is, the NLRI of this route can not be
   made more specific. Forwarding along such a route does not guarantee
   that IP packets will actually traverse only ASs listed in the AS_PATH
   attribute of the route.


9.2 Update-Send Process


   The Update-Send process is responsible for advertising UPDATE mes-
   sages to all peers. For example, it distributes the routes chosen by
   the Decision Process to other BGP speakers which may be located in
   either the same autonomous system or a neighboring autonomous system.

   When a BGP speaker receives an UPDATE message from an internal peer,
   the receiving BGP speaker SHALL NOT re-distribute the routing infor-
   mation contained in that UPDATE message to other internal peers
   (unless the speaker acts as a BGP Route Reflector [RFC2796]).

   As part of Phase 3 of the route selection process, the BGP speaker
   has updated its Adj-RIBs-Out. All newly installed routes and all
   newly unfeasible routes for which there is no replacement route SHALL
   be advertised to its peers by means of an UPDATE message.

   A BGP speaker SHOULD NOT advertise a given feasible BGP route from
   its Adj-RIB-Out if it would produce an UPDATE message containing the
   same BGP route as was previously advertised.




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   Any routes in the Loc-RIB marked as unfeasible SHALL be removed.
   Changes to the reachable destinations within its own autonomous sys-
   tem SHALL also be advertised in an UPDATE message.

   If due to the limits on the maximum size of an UPDATE message (see
   Section 4) a single route doesn't fit into the message, the BGP
   speaker MUST not advertise the route to its peers and MAY choose to
   log an error locally.



9.2.1 Controlling Routing Traffic Overhead


   The BGP protocol constrains the amount of routing traffic (that is,
   UPDATE messages) in order to limit both the link bandwidth needed to
   advertise UPDATE messages and the processing power needed by the
   Decision Process to digest the information contained in the UPDATE
   messages.


9.2.1.1 Frequency of Route Advertisement



   The parameter MinRouteAdvertisementIntervalTimer determines the mini-
   mum amount of time that must elapse between advertisement and/or
   withdrawal of routes to a particular destination by a BGP speaker to
   a peer. This rate limiting procedure applies on a per-destination
   basis, although the value of MinRouteAdvertisementIntervalTimer is
   set on a per BGP peer basis.

   Two UPDATE messages sent by a BGP speaker to a peer that advertise
   feasible routes and/or withdrawal of unfeasible routes to some common
   set of destinations MUST be separated by at least MinRouteAdvertise-
   mentIntervalTimer. Clearly, this can only be achieved precisely by
   keeping a separate timer for each common set of destinations. This
   would be unwarranted overhead.  Any technique which ensures that the
   interval between two UPDATE messages sent from a BGP speaker to a
   peer that advertise feasible routes and/or withdrawal of unfeasible
   routes to some common set of destinations will be at least Min-
   RouteAdvertisementIntervalTimer, and will also ensure a constant
   upper bound on the interval is acceptable.

   Since fast convergence is needed within an autonomous system, either
   (a) the MinRouteAdvertisementIntervalTimer used for internal peers
   SHOULD be shorter than the MinRouteAdvertisementIntervalTimer used
   for external peers, or (b) the procedure describe in this section



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   SHOULD NOT apply for routes sent to internal peers.

   This procedure does not limit the rate of route selection, but only
   the rate of route advertisement. If new routes are selected multiple
   times while awaiting the expiration of MinRouteAdvertisementInterval-
   Timer, the last route selected SHALL be advertised at the end of Min-
   RouteAdvertisementIntervalTimer.


9.2.1.2 Frequency of Route Origination


   The parameter MinASOriginationIntervalTimer determines the minimum
   amount of time that must elapse between successive advertisements of
   UPDATE messages that report changes within the advertising BGP
   speaker's own autonomous systems.


9.2.2 Efficient Organization of Routing Information


   Having selected the routing information which it will advertise, a
   BGP speaker may avail itself of several methods to organize this
   information in an efficient manner.


9.2.2.1 Information Reduction


   Information reduction may imply a reduction in granularity of policy
   control - after information is collapsed, the same policies will
   apply to all destinations and paths in the equivalence class.

   The Decision Process may optionally reduce the amount of information
   that it will place in the Adj-RIBs-Out by any of the following meth-
   ods:

      a)   Network Layer Reachability Information (NLRI):

      Destination IP addresses can be represented as IP address pre-
      fixes. In cases where there is a correspondence between the
      address structure and the systems under control of an autonomous
      system administrator, it will be possible to reduce the size of
      the NLRI carried in the UPDATE messages.

      b)   AS_PATHs:

      AS path information can be represented as ordered AS_SEQUENCEs or



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      unordered AS_SETs. AS_SETs are used in the route aggregation algo-
      rithm described in 9.2.2.2. They reduce the size of the AS_PATH
      information by listing each AS number only once, regardless of how
      many times it may have appeared in multiple AS_PATHs that were
      aggregated.

      An AS_SET implies that the destinations listed in the NLRI can be
      reached through paths that traverse at least some of the con-
      stituent autonomous systems. AS_SETs provide sufficient informa-
      tion to avoid routing information looping; however their use may
      prune potentially feasible paths, since such paths are no longer
      listed individually as in the form of AS_SEQUENCEs. In practice
      this is not likely to be a problem, since once an IP packet
      arrives at the edge of a group of autonomous systems, the BGP
      speaker at that point is likely to have more detailed path infor-
      mation and can distinguish individual paths to destinations.


9.2.2.2 Aggregating Routing Information


   Aggregation is the process of combining the characteristics of sev-
   eral different routes in such a way that a single route can be adver-
   tised.  Aggregation can occur as part of the Decision Process to
   reduce the amount of routing information that will be placed in the
   Adj-RIBs-Out.

   Aggregation reduces the amount of information that a BGP speaker must
   store and exchange with other BGP speakers. Routes can be aggregated
   by applying the following procedure separately to path attributes of
   the same type and to the Network Layer Reachability Information.

   Routes that have different MULTI_EXIT_DISC attribute SHALL NOT be
   aggregated.

   If the aggregated route has an AS_SET as the first element in its
   AS_PATH attribute, then the router that originates the route SHOULD
   NOT advertise the MULTI_EXIT_DISC attribute with this route.

   Path attributes that have different type codes can not be aggregated
   together. Path attributes of the same type code may be aggregated,
   according to the following rules:

      NEXT_HOP:
         When aggregating routes that have different NEXT_HOP attribute,
         the NEXT_HOP attribute of the aggregated route SHALL identify
         an interface on the BGP speaker that performs the aggregation.




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      ORIGIN attribute:
         If at least one route among routes that are aggregated has ORI-
         GIN with the value INCOMPLETE, then the aggregated route MUST
         have the ORIGIN attribute with the value INCOMPLETE.  Other-
         wise, if at least one route among routes that are aggregated
         has ORIGIN with the value EGP, then the aggregated route MUST
         have the ORIGIN attribute with the value EGP. In all other
         cases the value of the ORIGIN attribute of the aggregated route
         is IGP.

      AS_PATH attribute:
         If routes to be aggregated have identical AS_PATH attributes,
         then the aggregated route has the same AS_PATH attribute as
         each individual route.

         For the purpose of aggregating AS_PATH attributes we model each
         AS within the AS_PATH attribute as a tuple <type, value>, where
         "type" identifies a type of the path segment the AS belongs to
         (e.g.  AS_SEQUENCE, AS_SET), and "value" is the AS number. If
         the routes to be aggregated have different AS_PATH attributes,
         then the aggregated AS_PATH attribute SHALL satisfy all of the
         following conditions:

            - all tuples of type AS_SEQUENCE in the aggregated AS_PATH
            SHALL appear in all of the AS_PATH in the initial set of
            routes to be aggregated.

            - all tuples of type AS_SET in the aggregated AS_PATH SHALL
            appear in at least one of the AS_PATH in the initial set
            (they may appear as either AS_SET or AS_SEQUENCE types).

            - for any tuple X of type AS_SEQUENCE in the aggregated
            AS_PATH which precedes tuple Y in the aggregated AS_PATH, X
            precedes Y in each AS_PATH in the initial set which contains
            Y, regardless of the type of Y.

            - No tuple of type AS_SET with the same value SHALL appear
            more than once in the aggregated AS_PATH.

            - Multiple tuples of type AS_SEQUENCE with the same value
            may appear in the aggregated AS_PATH only when adjacent to
            another tuple of the same type and value.

         An implementation may choose any algorithm which conforms to
         these rules. At a minimum a conformant implementation SHALL be
         able to perform the following algorithm that meets all of the
         above conditions:




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            - determine the longest leading sequence of tuples (as
            defined above) common to all the AS_PATH attributes of the
            routes to be aggregated. Make this sequence the leading
            sequence of the aggregated AS_PATH attribute.

            - set the type of the rest of the tuples from the AS_PATH
            attributes of the routes to be aggregated to AS_SET, and
            append them to the aggregated AS_PATH attribute.

            - if the aggregated AS_PATH has more than one tuple with the
            same value (regardless of tuple's type), eliminate all, but
            one such tuple by deleting tuples of the type AS_SET from
            the aggregated AS_PATH attribute.

            - for each pair of adjacent tuples in the aggregated
            AS_PATH, if both tuples have the same type, merge them
            together, as long as doing so will not cause a segment with
            length greater than 255 to be generated.

         Appendix F, Section F.6 presents another algorithm that satis-
         fies the conditions and allows for more complex policy configu-
         rations.

      ATOMIC_AGGREGATE:
         If at least one of the routes to be aggregated has
         ATOMIC_AGGREGATE path attribute, then the aggregated route
         SHALL have this attribute as well.

      AGGREGATOR:
         Any AGGREGATOR attributes from the routes to be aggregated MUST
         NOT be included in the aggregated route. The BGP speaker per-
         forming the route aggregation MAY attach a new AGGREGATOR
         attribute (see Section 5.1.7).


9.3 Route Selection Criteria


   Generally speaking, additional rules for comparing routes among sev-
   eral alternatives are outside the scope of this document. There are
   two exceptions:

      - If the local AS appears in the AS path of the new route being
      considered, then that new route can not be viewed as better than
      any other route (provided that the speaker is configured to accept
      such routes). If such a route were ever used, a routing loop could
      result.




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      - In order to achieve successful distributed operation, only
      routes with a likelihood of stability can be chosen. Thus, an AS
      SHOULD avoid using unstable routes, and it SHOULD NOT make rapid
      spontaneous changes to its choice of route. Quantifying the terms
      "unstable" and "rapid" in the previous sentence will require expe-
      rience, but the principle is clear. Routes that are unstable can
      be "penalized" (e.g., by using the procedures described in
      [RFC2439]).



9.4 Originating BGP routes

   A BGP speaker may originate BGP routes by injecting routing informa-
   tion acquired by some other means (e.g. via an IGP) into BGP. A BGP
   speaker that originates BGP routes assigns the degree of preference
   (e.g., according to local configuration) to these routes by passing
   them through the Decision Process (see Section 9.1). These routes MAY
   also be distributed to other BGP speakers within the local AS as part
   of the update process (see Section 9.2). The decision whether to dis-
   tribute non-BGP acquired routes within an AS via BGP or not depends
   on the environment within the AS (e.g. type of IGP) and SHOULD be
   controlled via configuration.


10 BGP Timers


   BGP employs five timers: ConnectRetryTimer (see Section 8), HoldTimer
   (see Section 4.2), KeepaliveTimer (see Section 8), MinASOrigination-
   IntervalTimer (see Section 9.2.1.2), and MinRouteAdvertisementInter-
   valTimer (see Section 9.2.1.1).

   Two optional timers MAY be supported: DelayOpenTimer, IdleHoldTimer
   by BGP (see section 8). Section 8 describes their use. The full oper-
   ation of these optional timers is outside the scope of this document.

   ConnectRetryTime is a mandatory FSM attribute that stores the initial
   value for the ConnectRetryTimer. The suggested default value for the
   ConnectRetryTime is 120 seconds.

   HoldTime is a mandatory FSM attribute that stores the initial value
   for the HoldTimer. The suggested default value for the HoldTime is 90
   seconds.

   During some portions of the state machine (see Section 8), the Hold-
   Timer is set to a large value. The suggested default for this large
   value is 4 minutes.



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   The KeepaliveTime is a mandatory FSM attribute that stores the ini-
   tial value for the KeepaliveTimer. The suggested default value for
   the KeepaliveTime is 1/3 of the HoldTime.

   The suggested default value for the MinASOriginationIntervalTimer is
   15 seconds.

   The suggested default value for the MinRouteAdvertisementInterval-
   Timer on EBGP connections is 30 seconds.

   The suggested default value for the MinRouteAdvertisementInterval-
   Timer on IBGP connections is 5 seconds.

   An implementation of BGP MUST allow the HoldTimer to be configurable
   on a per peer basis, and MAY allow the other timers to be config-
   urable.

   To minimize the likelihood that the distribution of BGP messages by a
   given BGP speaker will contain peaks, jitter SHOULD be applied to the
   timers associated with MinASOriginationIntervalTimer, KeepaliveTimer,
   MinRouteAdvertisementIntervalTimer, and ConnectRetryTimer.  A given
   BGP speaker MAY apply the same jitter to each of these quantities
   regardless of the destinations to which the updates are being sent;
   that is, jitter need not be configured on a "per peer" basis.

   The suggested default amount of jitter SHALL be determined by multi-
   plying the base value of the appropriate timer by a random factor
   which is uniformly distributed in the range from 0.75 to 1.0. A new
   random value SHOULD be picked each time the timer is set. The range
   of the jitter random value MAY be configurable.

Appendix A. Comparison with RFC1771


   There are numerous editorial changes (too many to list here).

   The following list the technical changes:

      Changes to reflect the usages of such features as TCP MD5
      [RFC2385], BGP Route Reflectors [RFC2796], BGP Confederations
      [RFC3065], and BGP Route Refresh [RFC2918].

      Clarification on the use of the BGP Identifier in the AGGREGATOR
      attribute.

      Procedures for imposing an upper bound on the number of prefixes
      that a BGP speaker would accept from a peer.




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      The ability of a BGP speaker to include more than one instance of
      its own AS in the AS_PATH attribute for the purpose of inter-AS
      traffic engineering.

      Clarifications on the various types of NEXT_HOPs.

      Clarifications to the use of the ATOMIC_AGGREGATE attribute.

      The relationship between the immediate next hop, and the next hop
      as specified in the NEXT_HOP path attribute.

      Clarifications on the tie-breaking procedures.

      Clarifications on the frequency of route advertisements.

      Optional Parameter Type 1 (Authentication Information) has been
      deprecated.

      UPDATE Message Error subcode 7 (AS Routing Loop) has been depre-
      cated.

      OPEN Message Error subcode 5 (Authentication Failure) has been
      deprecated.

      Use of the Marker field for authentication has been deprecated.

      Implementations MUST support TCP MD5 [RFC2385] for authentication.

      Clarification of BGP FSM.


Appendix B. Comparison with RFC1267


   All the changes listed in Appendix A, plus the following.

   BGP-4 is capable of operating in an environment where a set of reach-
   able destinations may be expressed via a single IP prefix.  The con-
   cept of network classes, or subnetting is foreign to BGP-4.  To
   accommodate these capabilities BGP-4 changes semantics and encoding
   associated with the AS_PATH attribute. New text has been added to
   define semantics associated with IP prefixes. These abilities allow
   BGP-4 to support the proposed supernetting scheme [9].

   To simplify configuration this version introduces a new attribute,
   LOCAL_PREF, that facilitates route selection procedures.

   The INTER_AS_METRIC attribute has been renamed to be MULTI_EXIT_DISC.



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   A new attribute, ATOMIC_AGGREGATE, has been introduced to insure that
   certain aggregates are not de-aggregated. Another new attribute,
   AGGREGATOR, can be added to aggregate routes in order to advertise
   which AS and which BGP speaker within that AS caused the aggregation.

   To insure that Hold Timers are symmetric, the Hold Timer is now nego-
   tiated on a per-connection basis. Hold Timers of zero are now sup-
   ported.

Appendix C. Comparison with RFC 1163


   All of the changes listed in Appendices A and B, plus the following.

   To detect and recover from BGP connection collision, a new field (BGP
   Identifier) has been added to the OPEN message. New text (Section
   6.8) has been added to specify the procedure for detecting and recov-
   ering from collision.

   The new document no longer restricts the router that is passed in the
   NEXT_HOP path attribute to be part of the same Autonomous System as
   the BGP Speaker.

   New document optimizes and simplifies the exchange of the information
   about previously reachable routes.


Appendix D. Comparison with RFC 1105


   All of the changes listed in Appendices A, B and C, plus the follow-
   ing.

   Minor changes to the RFC1105 Finite State Machine were necessary to
   accommodate the TCP user interface provided by 4.3 BSD.

   The notion of Up/Down/Horizontal relations present in RFC1105 has
   been removed from the protocol.

   The changes in the message format from RFC1105 are as follows:

      1.  The Hold Time field has been removed from the BGP header and
      added to the OPEN message.

      2.  The version field has been removed from the BGP header and
      added to the OPEN message.

      3.  The Link Type field has been removed from the OPEN message.



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      4.  The OPEN CONFIRM message has been eliminated and replaced with
      implicit confirmation provided by the KEEPALIVE message.

      5.  The format of the UPDATE message has been changed signifi-
      cantly.  New fields were added to the UPDATE message to support
      multiple path attributes.

      6.  The Marker field has been expanded and its role broadened to
      support authentication.

      Note that quite often BGP, as specified in RFC 1105, is referred
      to as BGP-1, BGP, as specified in RFC 1163, is referred to as
      BGP-2, BGP, as specified in RFC1267 is referred to as BGP-3, and
      BGP, as specified in this document is referred to as BGP-4.


Appendix E.  TCP options that may be used with BGP


   If a local system TCP user interface supports TCP PUSH function, then
   each BGP message SHOULD be transmitted with PUSH flag set.  Setting
   PUSH flag forces BGP messages to be transmitted promptly to the
   receiver.

   If a local system TCP user interface supports setting of the DSCP
   field [RFC2474] for TCP connections, then the TCP connection used by
   BGP SHOULD be opened with bits 0-2 of the DSCP field set to 110
   (binary).


Appendix F.  Implementation Recommendations


   This section presents some implementation recommendations.


Appendix F.1 Multiple Networks Per Message


   The BGP protocol allows for multiple address prefixes with the same
   path attributes to be specified in one message. Making use of this
   capability is highly recommended. With one address prefix per message
   there is a substantial increase in overhead in the receiver. Not only
   does the system overhead increase due to the reception of multiple
   messages, but the overhead of scanning the routing table for updates
   to BGP peers and other routing protocols (and sending the associated
   messages) is incurred multiple times as well.




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   One method of building messages containing many address prefixes per
   a path attribute set from a routing table that is not organized on a
   per path attribute set basis is to build many messages as the routing
   table is scanned. As each address prefix is processed, a message for
   the associated set of path attributes is allocated, if it does not
   exist, and the new address prefix is added to it.  If such a message
   exists, the new address prefix is just appended to it. If the message
   lacks the space to hold the new address prefix, it is transmitted, a
   new message is allocated, and the new address prefix is inserted into
   the new message. When the entire routing table has been scanned, all
   allocated messages are sent and their resources released.  Maximum
   compression is achieved when all  the destinations covered by the
   address prefixes share a common set of path attributes making it pos-
   sible to send many address prefixes in one 4096-byte message.

   When peering with a BGP implementation that does not compress multi-
   ple address prefixes into one message, it may be necessary to take
   steps to reduce the overhead from the flood of data received when a
   peer is acquired or a significant network topology change occurs. One
   method of doing this is to limit the rate of updates.  This will
   eliminate the redundant scanning of the routing table to provide
   flash updates for BGP peers and other routing protocols.  A disadvan-
   tage of this approach is that it increases the propagation latency of
   routing information.  By choosing a minimum flash update interval
   that is not much greater than the time it takes to process the multi-
   ple messages this latency should be minimized. A better method would
   be to read all received messages before sending updates.


Appendix F.2 Reducing route flapping


   To avoid excessive route flapping a BGP speaker which needs to with-
   draw a destination and send an update about a more specific or less
   specific route SHOULD combine them into the same UPDATE message.


Appendix F.3 Path attribute ordering


   Implementations which combine update messages as described above in
   6.1 may prefer to see all path attributes presented in a known order.
   This permits them to quickly identify sets of attributes from differ-
   ent update messages which are semantically identical.  To facilitate
   this, it is a useful optimization to order the path attributes
   according to type code.  This optimization is entirely optional.





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Appendix F.4 AS_SET sorting


   Another useful optimization that can be done to simplify this situa-
   tion is to sort the AS numbers found in an AS_SET.  This optimization
   is entirely optional.


Appendix F.5 Control over version negotiation


   Since BGP-4 is capable of carrying aggregated routes which can not be
   properly represented in BGP-3, an implementation which supports BGP-4
   and another BGP version should provide the capability to only speak
   BGP-4 on a per-peer basis.


Appendix F.6 Complex AS_PATH aggregation


   An implementation which chooses to provide a path aggregation algo-
   rithm which retains significant amounts of path information may wish
   to use the following procedure:

      For the purpose of aggregating AS_PATH attributes of two routes,
      we model each AS as a tuple <type, value>, where "type" identifies
      a type of the path segment the AS belongs to (e.g.  AS_SEQUENCE,
      AS_SET), and "value" is the AS number.  Two ASs are said to be the
      same if their corresponding <type, value> tuples are the same.

      The algorithm to aggregate two AS_PATH attributes works as fol-
      lows:

         a) Identify the same ASs (as defined above) within each AS_PATH
         attribute that are in the same relative order within both
         AS_PATH attributes.  Two ASs, X and Y, are said to be in the
         same order if either:
            - X precedes Y in both AS_PATH attributes, or - Y precedes X
            in both AS_PATH attributes.

         b) The aggregated AS_PATH attribute consists of ASs identified
         in (a) in exactly the same order as they appear in the AS_PATH
         attributes to be aggregated. If two consecutive ASs identified
         in (a) do not immediately follow each other in both of the
         AS_PATH attributes to be aggregated, then the intervening ASs
         (ASs that are between the two consecutive ASs that are the
         same) in both attributes are combined into an AS_SET path seg-
         ment that consists of the intervening ASs from both AS_PATH



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         attributes; this segment is then placed in between the two con-
         secutive ASs identified in (a) of the aggregated attribute. If
         two consecutive ASs identified in (a) immediately follow each
         other in one attribute, but do not follow in another, then the
         intervening ASs of the latter are combined into an AS_SET path
         segment; this segment is then placed in between the two consec-
         utive ASs identified in (a) of the aggregated attribute.

         c) For each pair of adjacent tuples in the aggregated AS_PATH,
         if both tuples have the same type, merge them together, as long
         as doing so will not cause a segment with length greater than
         255 to be generated.

      If as a result of the above procedure a given AS number appears
      more than once within the aggregated AS_PATH attribute, all, but
      the last instance (rightmost occurrence) of that AS number SHOULD
      be removed from the aggregated AS_PATH attribute.


Security Considerations


   The authentication mechanism that an implementation of BGP MUST sup-
   port is specified in [RFC2385]. The authentication provided by this
   mechanism could be done on a per peer basis.

   BGP vulnerabilities analysis is discussed in [BGP_VULN].


IANA Considerations


   All new BGP message types, Path Attributes Type codes, Message Header
   Error subcodes, OPEN Message Error subcodes, and UPDATE Message Error
   subcodes MUST only be made using the Standards Action process defined
   in [RFC2434].

   This document defines the following message types: OPEN, UPDATE,
   KEEPALIVE, NOTIFICATION.

   This document defines the following Path Attributes Type codes: ORI-
   GIN, AS_PATH, NEXT_HOP, MULTI_EXIT_DISC, LOCAL_PREF, ATOMIC_AGGRE-
   GATE, AGGREGATOR.

   This document defines the following Message Header Error subcodes:
   Connection Not Synchronized, Bad Message Length, Bad Message Type.

   This document defines the following OPEN Message Error subcodes:



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   Unsupported Version Number, Bad Peer AS,  Bad BGP Identifier, Unsup-
   ported Optional Parameter, Unacceptable Hold Time.

   This document defines the following UPDATE Message Error subcodes:
   Malformed Attribute List, Unrecognized Well-known Attribute, Missing
   Well-known Attribute, Attribute Flags Error, Attribute Length Error,
   Invalid ORIGIN Attribute, Invalid NEXT_HOP Attribute, Optional
   Attribute Error, Invalid Network Field, Malformed AS_PATH.


IPR Notice


   The IETF has been notified of intellectual property rights claimed in
   regard to some or all of the specification contained in this docu-
   ment. For more information consult the online list of claimed rights.

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to per-
   tain to the implementation or use of the technology described in this
   document or the extent to which any license under such rights might
   or might not be available; neither does it represent that it has made
   any effort to identify any such rights.  Information on the IETF's
   procedures with respect to rights in standards-track and standards-
   related documentation can be found in BCP-11.  Copies of claims of
   rights made available for publication and any assurances of licenses
   to be made available, or the result of an attempt made to obtain a
   general license or permission for the use of such proprietary rights
   by implementors or users of this specification can be obtained from
   the IETF Secretariat.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights which may cover technology that may be required to practice
   this standard.  Please address the information to the IETF Executive
   Director.



Full Copyright Notice


   Copyright (C) The Internet Society (2003).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any



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   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this doc-
   ument itself may not be modified in any way, such as by removing the
   copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of develop-
   ing Internet standards in which case the procedures for copyrights
   defined in the Internet Standards process must be followed, or as
   required to translate it into languages other than English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MER-
   CHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."


Normative References

   [RFC791] Postel, J., "Internet Protocol - DARPA Internet Program Pro-
   tocol Specification", RFC791, September 1981.

   [RFC793] Postel, J., "Transmission Control Protocol - DARPA Internet
   Program Protocol Specification", RFC793, September 1981.

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
   Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
   Signature Option", RFC2385, August 1998.

   [RFC2434] Narten, T., Alvestrand, H., "Guidelines for Writing an IANA
   Considerations Section in RFCs", RFC2434, October 1998

   [RFC2474] Nichols, K., et al.,"Definition of the Differentiated Ser-
   vices Field (DS Field) in the IPv4 and IPv6 Headers", RFC2474, Decem-
   ber 1998



Non-normative References

   [RFC904] Mills, D., "Exterior Gateway Protocol Formal Specification",
   RFC904, April 1984.




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   [RFC1092] Rekhter, Y., "EGP and Policy Based Routing in the New
   NSFNET Backbone", RFC1092, February 1989.

   [RFC1093] Braun, H-W., "The NSFNET Routing Architecture", RFC1093,
   February 1989.

   [RFC1772] Rekhter, Y., and P. Gross, "Application of the Border Gate-
   way Protocol in the Internet", RFC1772, March 1995.

   [RFC1518] Rekhter, Y., Li, T., "An Architecture for IP Address Allo-
   cation with CIDR", RFC 1518, September 1993.

   [RFC1519] Fuller, V., Li, T., Yu, J., and Varadhan, K., ""Classless
   Inter-Domain Routing (CIDR): an Address Assignment and Aggregation
   Strategy", RFC1519, September 1993.

   [RFC1997] R. Chandra, P. Traina, T. Li, "BGP Communities Attribute",
   RFC 1997, August 1996.

   [RFC2439] C. Villamizar, R. Chandra, R. Govindan, "BGP Route Flap
   Damping", RFC2439, November 1998.

   [RFC2796] Bates, T., Chandra, R., Chen, E., "BGP Route Reflection -
   An Alternative to Full Mesh IBGP", RFC2796,  April 2000.

   [RFC3392] R. Chandra, J. Scudder, "Capabilities Advertisement with
   BGP-4", RFC2842.

   [RFC2858] T. Bates, R. Chandra, D. Katz, Y. Rekhter, "Multiprotocol
   Extensions for BGP-4", RFC2858.

   [RFC2918] Chen, E., "Route Refresh Capability for BGP-4", RFC2918,
   September 2000.

   [RFC3065] Traina, P, McPherson, D., Scudder, J., "Autonomous System
   Confederations for BGP", RFC3065, February 2001.

   [IS10747] "Information Processing Systems - Telecommunications and
   Information Exchange between Systems - Protocol for Exchange of
   Inter-domain Routeing Information among Intermediate Systems to Sup-
   port Forwarding of ISO 8473 PDUs", ISO/IEC IS10747, 1993

   [BGP_VULN] Murphy, S., "BGP Security Vulnerabilities Analysis",
   draft-ietf-idr-bgp-vuln-00.txt, work in progress







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Editors' Addresses

   Yakov Rekhter
   Juniper Networks
   email:  yakov@juniper.net

   Tony Li
   Procket Networks, Inc.
   email:  tli@procket.com

   Susan Hares

   NextHop Technologies, Inc.
   email: skh@nexthop.com





































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