Network Working Group                                    Y. Rekhter, Ed.
Request for Comments: 4271                                    T. Li, Ed.
Obsoletes: 1771                                            S. Hares, Ed.
Category: Standards Track                                   January 2006


                 A Border Gateway Protocol 4 (BGP-4)

Status of This Memo

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

Copyright Notice

  Copyright (C) The Internet Society (2006).

Abstract

  This document discusses the Border Gateway Protocol (BGP), which is
  an inter-Autonomous System routing protocol.

  The primary function of a BGP speaking system is to exchange network
  reachability information with other BGP systems.  This network
  reachability information includes information on the list of
  Autonomous Systems (ASes) that reachability information traverses.
  This information is sufficient for constructing a graph of AS
  connectivity for this reachability from which routing loops may be
  pruned, and, at the AS level, some policy decisions may be enforced.

  BGP-4 provides a set of mechanisms for supporting Classless Inter-
  Domain Routing (CIDR).  These mechanisms include support for
  advertising a set of destinations as an IP prefix, and eliminating
  the concept of network "class" within BGP.  BGP-4 also introduces
  mechanisms that allow aggregation of routes, including aggregation of
  AS paths.

  This document obsoletes RFC 1771.










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Table of Contents

  1. Introduction ....................................................4
     1.1. Definition of Commonly Used Terms ..........................4
     1.2. Specification of Requirements ..............................6
  2. Acknowledgements ................................................6
  3. Summary of Operation ............................................7
     3.1. Routes: Advertisement and Storage ..........................9
     3.2. Routing Information Base ..................................10
  4. Message Formats ................................................11
     4.1. Message Header Format .....................................12
     4.2. OPEN Message Format .......................................13
     4.3. UPDATE Message Format .....................................14
     4.4. KEEPALIVE Message Format ..................................21
     4.5. NOTIFICATION Message Format ...............................21
  5. Path Attributes ................................................23
     5.1. Path Attribute Usage ......................................25
          5.1.1. ORIGIN .............................................25
          5.1.2. AS_PATH ............................................25
          5.1.3. NEXT_HOP ...........................................26
          5.1.4. MULTI_EXIT_DISC ....................................28
          5.1.5. LOCAL_PREF .........................................29
          5.1.6. ATOMIC_AGGREGATE ...................................29
          5.1.7. AGGREGATOR .........................................30
  6. BGP Error Handling. ............................................30
     6.1. Message Header Error Handling .............................31
     6.2. OPEN Message Error Handling ...............................31
     6.3. UPDATE Message Error Handling .............................32
     6.4. NOTIFICATION Message Error Handling .......................34
     6.5. Hold Timer Expired Error Handling .........................34
     6.6. Finite State Machine Error Handling .......................35
     6.7. Cease .....................................................35
     6.8. BGP Connection Collision Detection ........................35
  7. BGP Version Negotiation ........................................36
  8. BGP Finite State Machine (FSM) .................................37
     8.1. Events for the BGP FSM ....................................38
          8.1.1. Optional Events Linked to Optional Session
                 Attributes .........................................38
          8.1.2. Administrative Events ..............................42
          8.1.3. Timer Events .......................................46
          8.1.4. TCP Connection-Based Events ........................47
          8.1.5. BGP Message-Based Events ...........................49
     8.2. Description of FSM ........................................51
          8.2.1. FSM Definition .....................................51
                 8.2.1.1. Terms "active" and "passive" ..............52
                 8.2.1.2. FSM and Collision Detection ...............52
                 8.2.1.3. FSM and Optional Session Attributes .......52
                 8.2.1.4. FSM Event Numbers .........................53



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                 8.2.1.5. FSM Actions that are Implementation
                          Dependent .................................53
          8.2.2. Finite State Machine ...............................53
  9. UPDATE Message Handling ........................................75
     9.1. Decision Process ..........................................76
          9.1.1. Phase 1: Calculation of Degree of Preference .......77
          9.1.2. Phase 2: Route Selection ...........................77
                 9.1.2.1. Route Resolvability Condition .............79
                 9.1.2.2. Breaking Ties (Phase 2) ...................80
          9.1.3. Phase 3: Route Dissemination .......................82
          9.1.4. Overlapping Routes .................................83
     9.2. Update-Send Process .......................................84
          9.2.1. Controlling Routing Traffic Overhead ...............85
                 9.2.1.1. Frequency of Route Advertisement ..........85
                 9.2.1.2. Frequency of Route Origination ............85
          9.2.2. Efficient Organization of Routing Information ......86
                 9.2.2.1. Information Reduction .....................86
                 9.2.2.2. Aggregating Routing Information ...........87
     9.3. Route Selection Criteria ..................................89
     9.4. Originating BGP routes ....................................89
  10. BGP Timers ....................................................90
  Appendix A.  Comparison with RFC 1771 .............................92
  Appendix B.  Comparison with RFC 1267 .............................93
  Appendix C.  Comparison with RFC 1163 .............................93
  Appendix D.  Comparison with RFC 1105 .............................94
  Appendix E.  TCP Options that May Be Used with BGP ................94
  Appendix F.  Implementation Recommendations .......................95
               Appendix F.1.  Multiple Networks Per Message .........95
               Appendix F.2.  Reducing Route Flapping ...............96
               Appendix F.3.  Path Attribute Ordering ...............96
               Appendix F.4.  AS_SET Sorting ........................96
               Appendix F.5.  Control Over Version Negotiation ......96
               Appendix F.6.  Complex AS_PATH Aggregation ...........96
  Security Considerations ...........................................97
  IANA Considerations ...............................................99
  Normative References .............................................101
  Informative References ...........................................101














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1.  Introduction

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

  The primary function of a BGP speaking system is to exchange network
  reachability information with other BGP systems.  This network
  reachability information includes information on the list of
  Autonomous Systems (ASes) that reachability information traverses.
  This information is sufficient for constructing a graph of AS
  connectivity for this reachability, from which routing loops may be
  pruned and, at the AS level, some policy decisions may be enforced.

  BGP-4 provides a set of mechanisms for supporting Classless Inter-
  Domain Routing (CIDR) [RFC1518, RFC1519].  These mechanisms include
  support for advertising a set of destinations as an IP prefix and
  eliminating the concept of network "class" within BGP.  BGP-4 also
  introduces mechanisms that allow aggregation of routes, including
  aggregation of AS paths.

  Routing information exchanged via BGP supports only the destination-
  based forwarding paradigm, which assumes that a router forwards a
  packet based solely on the destination address carried in the IP
  header of the packet.  This, in turn, reflects the set of policy
  decisions that can (and cannot) be enforced using BGP.  BGP can
  support only those policies conforming to the destination-based
  forwarding paradigm.

1.1.  Definition of Commonly Used Terms

  This section provides definitions for terms that have a specific
  meaning to the BGP protocol and that are used throughout the text.

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

  Adj-RIB-Out
     The Adj-RIBs-Out contains the routes for advertisement to specific
     peers by means of the local speaker's UPDATE messages.

  Autonomous System (AS)
     The classic definition of an Autonomous System is a set of routers
     under a single technical administration, using an interior gateway
     protocol (IGP) and common metrics to determine how to route
     packets within the AS, and using an inter-AS routing protocol to
     determine how to route packets to other ASes.  Since this classic
     definition was developed, it has become common for a single AS to



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     use several IGPs and, sometimes, several sets of metrics within an
     AS.  The use of the term Autonomous System stresses the fact that,
     even when multiple IGPs and metrics are used, the administration
     of an AS appears to other ASes to have a single coherent interior
     routing plan, and presents a consistent picture of the
     destinations that are reachable through it.

  BGP Identifier
     A 4-octet unsigned integer that indicates the BGP Identifier of
     the sender of BGP messages.  A given BGP speaker sets the value of
     its BGP Identifier to an IP address assigned to that BGP speaker.
     The value of the BGP Identifier is determined upon startup and is
     the same for every local interface and BGP peer.

  BGP speaker
     A router that implements BGP.

  EBGP
     External BGP (BGP connection between external peers).

  External peer
     Peer that is in a different Autonomous System than the local
     system.

  Feasible route
     An advertised route that is available for use by the recipient.

  IBGP
     Internal BGP (BGP connection between internal peers).

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

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

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

  NLRI
     Network Layer Reachability Information.

  Route
     A unit of information that pairs a set of destinations with the
     attributes of a path to those destinations.  The set of



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     destinations are systems whose IP addresses are contained in one
     IP address prefix carried in the Network Layer Reachability
     Information (NLRI) field of an UPDATE message.  The path is the
     information reported in the path attributes field of the same
     UPDATE message.

  RIB
     Routing Information Base.

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

1.2.  Specification of Requirements

  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
  "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
  document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Acknowledgements

  This document was originally published as [RFC1267] 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
  Dennis Ferguson to the earlier version of this document.

  We would like to explicitly thank Bob Braden for the review of the
  earlier version (BGP-2) of this document, and for 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
  courtesy.

  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.




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  We would also like to thank Benjamin Abarbanel, Enke Chen, Edward
  Crabbe, Mike Craren, Vincent Gillet, Eric Gray, Jeffrey Haas, Dimitry
  Haskin, Stephen Kent, John Krawczyk, David LeRoy, Dan Massey,
  Jonathan Natale, Dan Pei, Mathew Richardson, John Scudder, John
  Stewart III, Dave Thaler, 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 the support they have given to this
  document.

3.  Summary of Operation

  The Border Gateway Protocol (BGP) is an inter-Autonomous System
  routing 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]).  For more BGP-related
  information, see [RFC1772], [RFC1930], [RFC1997], and [RFC2858].

  The primary function of a BGP speaking system is to exchange network
  reachability information with other BGP systems.  This network
  reachability information includes information on the list of
  Autonomous Systems (ASes) that reachability information traverses.
  This information is sufficient for constructing a graph of AS
  connectivity, from which routing loops may be pruned, and, at the AS
  level, some policy decisions may be enforced.

  In the context of this document, we assume that a BGP speaker
  advertises to its peers only those routes that it uses itself (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
  decisions that can (and cannot) be enforced using BGP.  Note that
  some policies cannot be supported by the destination-based forwarding
  paradigm, and thus require techniques such as source routing (aka
  explicit routing) to be enforced.  Such policies cannot be enforced
  using BGP either.  For example, BGP does not enable one AS to send



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  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
  originating 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 a network "class" within BGP.  BGP-4
  also introduces mechanisms that allow aggregation of routes,
  including 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 ASes.  Since this classic definition was developed,
  it has become common for a single AS to use several IGPs and,
  sometimes, several sets of metrics within an AS.  The use of the term
  Autonomous System stresses the fact that, even when multiple IGPs and
  metrics are used, the administration of an AS appears to other ASes
  to have a single coherent interior routing plan and presents a
  consistent picture of the destinations that are reachable through it.

  BGP uses TCP [RFC793] as its transport protocol.  This eliminates the
  need to implement explicit update fragmentation, retransmission,
  acknowledgement, 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).

  A TCP connection is formed between two systems.  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 a 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
  version of the routes advertised to it by all of its peers for the
  duration of the connection, or (b) make use of the Route Refresh
  extension [RFC2918].





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  KEEPALIVE messages may be sent periodically to ensure that the
  connection is live.  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 as IBGP and EBGP.

  If a particular AS has multiple BGP speakers and is providing transit
  service for other ASes, 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 be, 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 that is
  carried in the Network Layer Reachability Information (NLRI) field of
  an UPDATE message, 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.
  Multiple 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
  advertising it to a peer.






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  BGP provides mechanisms by which a BGP speaker can inform its peers
  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 connection can be closed, which implicitly
        removes all routes the pair of speakers had advertised to each
        other from service.

  Changing the 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 stores routing information learned
        from inbound UPDATE messages that were received from other BGP
        speakers.  Their contents represent routes that are available
        as input to the Decision Process.

     b) Loc-RIB: The Loc-RIB contains the local routing information the
        BGP speaker 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 stores information the local BGP
        speaker 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 contains 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





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  speaker's Decision Process; and the Adj-RIBs-Out organizes the routes
  for advertisement to specific peers (by means of the local speaker's
  UPDATE messages).

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

  Routing information that the BGP speaker uses to forward packets (or
  to construct the forwarding table 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 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, and is not
  specified in this document.  In addition to 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 TCP connections.  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
  message size.  The smallest message that may be sent consists of a
  BGP header without a data portion (19 octets).

  All multi-octet fields are in network byte order.

















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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
  layout of these fields is shown below:

     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, it allows one
        to locate the (Marker field of the) next message in the TCP
        stream.  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.  "padding" of extra data after
        the message is not allowed.  Therefore, 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.



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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
  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
        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





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        Time.  The calculated value indicates the maximum number of
        seconds that may elapse between the receipt of successive
        KEEPALIVE and/or UPDATE messages from the sender.

     BGP Identifier:

        This 4-octet unsigned integer indicates the BGP Identifier of
        the sender.  A given BGP speaker sets the value of its BGP
        Identifier to an IP address that is assigned to that BGP
        speaker.  The value of the BGP Identifier is determined upon
        startup and is the same for every local interface and BGP peer.

     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, in which
        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
        identifies 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 the
  message 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 that describes the relationships of the various
  Autonomous Systems.  By applying rules to be discussed, routing



Rekhter, et al.             Standards Track                    [Page 14]

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  information loops and some other anomalies may be detected and
  removed from inter-AS routing.

  An UPDATE message is used to advertise feasible routes that share
  common path attributes to a peer, or to withdraw multiple unfeasible
  routes from service (see 3.1).  An UPDATE message MAY simultaneously
  advertise 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)       |
                 +---------------------------+





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        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
           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
        specified below.

        A value of 0 indicates that neither the Network Layer
        Reachability Information field nor the Path Attribute 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).



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        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.
        (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
        contained 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, the third and fourth octets of the path attribute contain
        the length of the attribute data in octets.

















Rekhter, et al.             Standards Track                    [Page 17]

RFC 4271                         BGP-4                      January 2006


        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, and their attribute values and uses are as follows:

        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]

              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 length field with the
           following values defined:

              Value      Segment Type

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

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

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

           The path segment value field contains one or more AS
           numbers, each encoded as a 2-octet length field.



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           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 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
           preference 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.




Rekhter, et al.             Standards Track                    [Page 19]

RFC 4271                         BGP-4                      January 2006


     Network Layer Reachability Information:

        This variable length field contains a list of IP address
        prefixes.  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.

        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).




Rekhter, et al.             Standards Track                    [Page 20]

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  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 that are to be withdrawn
  from service.  Each such route is identified by its destination
  (expressed as an IP prefix), which unambiguously identifies the route
  in the context of the BGP speaker - BGP speaker connection to which
  it has been previously advertised.


  An UPDATE message might advertise only routes that are to be
  withdrawn from service, in which case the message 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.

  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 though the WITHDRAWN ROUTES do not 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 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 the 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 it is sent.




Rekhter, et al.             Standards Track                    [Page 21]

RFC 4271                         BGP-4                      January 2006


  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
        NOTIFICATION.  The following Error Codes have been defined:

           Error Code       Symbolic Name               Reference

             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
        information 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.










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     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.
             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 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
  (including 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.



Rekhter, et al.             Standards Track                    [Page 23]

RFC 4271                         BGP-4                      January 2006


  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 to its peers in any updates it transmits.

  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 an unrecognized transitive optional attribute is accepted
  and passed to other BGP peers, then the unrecognized transitive
  optional attribute of that path MUST be passed, along with the path,
  to other BGP peers with the Partial bit in the Attribute Flags octet
  set to 1.  If a path with a 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 UPDATE messages that are out of order.

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







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  The mandatory category refers to an attribute that 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,
  discretionary, 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

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
  identifies 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 it 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, the advertising speaker updates the AS_PATH attribute as
        follows:






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        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 message).  If the act of prepending will cause an
           overflow in the AS_PATH segment (i.e., more than 255 ASes),
           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
           AS_SEQUENCE to the AS_PATH, including its own AS number in
           that segment.

        3) if the AS_PATH is empty, the local system creates a path
           segment of type AS_SEQUENCE, places its own AS into that
           segment, and places that segment into the AS_PATH.

  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
  including 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
  destinations 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



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        locally-originated route to an internal peer, the BGP speaker
        SHOULD use the interface address of the router through which
        the announced network is reachable for the speaker as the
        NEXT_HOP.  If the route is directly connected to the speaker,
        or if 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 its own IP
        address for the NEXT_HOP attribute (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 an
          interface address of the internal peer router (or the
          internal router) through which the announced network is
          reachable for the speaker for the NEXT_HOP attribute,
          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 an IP address of any
          adjacent router (known from the received NEXT_HOP attribute)
          that the speaker itself uses for local route calculation in
          the NEXT_HOP attribute, 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 the IP address of the interface that the
          speaker uses to establish the BGP connection to peer X in the
          NEXT_HOP attribute.

     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



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          attribute of the learned route (the speaker does not modify
          the NEXT_HOP attribute).

        - By default, the BGP speaker SHOULD use the IP address of the
          interface that the speaker uses in the NEXT_HOP attribute 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
  the disabling advertisement of third party NEXT_HOP attributes in
  order 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.

  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
  recursive 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 that 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 that 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
  number, called a metric.  All other factors being equal, the exit
  point with the 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 ASes.

  A BGP speaker MUST implement a mechanism (based on local
  configuration) that allows the MULTI_EXIT_DISC attribute to be
  removed from a route.  If a BGP speaker is configured to remove the



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  MULTI_EXIT_DISC attribute from a route, then this removal MUST be
  done prior to determining the degree of preference of the route and
  prior to 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 prior
  to performing route selection (Decision Process phases 1 and 2).  See
  Section 9.1.2.2 for necessary restrictions on this.

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 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 it
  sends to external peers, except in 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 in the case of BGP Confederations
  [RFC3065].

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 ASes from
  which the aggregate was formed.  In many cases, the network
  administrator can determine if the aggregate can safely be advertised
  without the AS_SET, and without forming 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.





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  A BGP speaker that receives a route with the ATOMIC_AGGREGATE
  attribute SHOULD NOT remove the attribute when propagating the route
  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
  destinations, as specified in the NLRI of the route, while having the
  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 that are formed by aggregation (see Section 9.2.2.2).  A
  BGP speaker that 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
  NOTIFICATION 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 the TCP connection
  has been closed, the associated Adj-RIB-In has been cleared, and 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 destinations of
  the routes marked as invalid.  Before the invalid routes are deleted
  from the system, it 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
  message that is sent to indicate an error is empty.





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6.1.  Message Header Error Handling

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

  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
       minimum 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
  indicated by sending the NOTIFICATION message with the Error Code
  OPEN Message Error.  The Error Subcode elaborates on the specific
  nature of the error.

  If the version number 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-octet unsigned
  integer, which indicates the largest, locally-supported version
  number less than the version the remote BGP peer bid (as indicated in



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  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,
  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
  implementation MUST reject Hold Time values of one or two seconds.
  An implementation MAY reject any proposed Hold Time.  An
  implementation that 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
  recognized, 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
  (Unspecific).

6.3.  UPDATE Message Error Handling

  All errors detected while processing the UPDATE message MUST be
  indicated by sending the NOTIFICATION message with the Error Code
  UPDATE Message 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).





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  If any recognized attribute has an 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).

  If any of the well-known mandatory 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 well-known mandatory 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
  NOTIFICATION message SHOULD NOT be sent, and the 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.






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  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
  message.  If the check determines 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
  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
  validity.  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
  message detects an error in that message, the receiver cannot use a
  NOTIFICATION message to report this error back to the peer.  Any such
  error (e.g., an unrecognized Error Code or Error Subcode) SHOULD be
  noticed, logged locally, and brought to the attention of the
  administration 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, UPDATE, and/or
  NOTIFICATION messages within the period specified in the Hold Time
  field of the OPEN message, then the NOTIFICATION message with the
  Hold Timer Expired Error Code is sent and the BGP connection is
  closed.







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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 the Error Code Finite State Machine Error.

6.7.  Cease

  In the 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 the 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 a locally-configured,
  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) discards
  new address prefixes from the neighbor (while maintaining the 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 the neighbor a NOTIFICATION message
  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 to establish a BGP connection with each
  other simultaneously, then two parallel connections 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, connection collision has occurred.  In the
  event of connection collision, one of the 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
  collision occurs.  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



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  whose BGP Identifier equals the one in the OPEN message, and this
  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
        message).  Comparing BGP Identifiers is done by converting them
        to host byte order and treating them as 4-octet unsigned
        integers.

     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 the 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 the Established state causes
  closing of the newly created connection.

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

  Closing the BGP connection (that results from the collision
  resolution 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
  multiple attempts at opening a BGP connection, starting with the
  highest version number each BGP speaker 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 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
  they exhibit the same externally visible behavior.

  This section specifies the BGP operation in terms of a Finite State
  Machine (FSM).  The section falls into two 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 the current state of the BGP
  FSM.  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
  system:

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



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  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 the "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 8.2.1.3 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 linkage between FSM functionality, events, and the optional
  session attributes are described below.

     Group 1: Automatic Administrative Events (Start/Stop)

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

        Option 1:    AllowAutomaticStart

        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 that
                     is 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.



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                     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 the length of time the BGP
                     peer is held in the Idle state prior to allowing
                     the next automatic restart.  The IdleHoldTimer is
                     the timer that holds 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 connect 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 "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 the BGP connection is
                     using logic that damps BGP peer oscillations in
                     the Idle State.



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        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
                     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 [RFC4272] for details.

        Value:       True or False

     Group 3: TCP processing

        Optional Session Attributes: PassiveTcpEstablishment,
                                     TrackTcpState

        Option 1:    PassiveTcpEstablishment






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        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
                     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 set in the
                     DelayOpenTimer.

        Value:       Time in seconds

        Option 3:    DelayOpenTimer

        Description: The DelayOpenTimer optional session attribute is
                     used to delay the sending of an OPEN message on a




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                     connection.  The DelayOpenTimer_Expires event
                     (Event 12) is described in Section 8.1.3.

        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 (see Section 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 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 indications are
  augmented by optional connection attributes that signal a certain
  type of start or stop mechanism to the BGP FSM.  An example of this
  combination 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 the option to use
  a Passive TCP Establishment.  The Passive TCP establishment signals
  that this BGP FSM will wait for the remote side to start the TCP
  establishment.




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  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 the optional session attributes
  that 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 an FSM error.

  The settings of optional session attributes may be implicit in some
  implementations, and therefore may not be set explicitly by an
  external operator action.  Section 8.2.1.5 describes these implicit
  settings 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.

     Event 1: ManualStart

        Definition: Local system administrator manually starts the peer
                    connection.

        Status:     Mandatory

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

     Event 2: ManualStop

        Definition: Local system administrator manually stops the peer
                    connection.

        Status:     Mandatory

        Optional
        Attribute
        Status:     No interaction with any optional attributes.

     Event 3: AutomaticStart

        Definition: Local system automatically starts the BGP
                    connection.

        Status:     Optional, depending on local system






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        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.

     Event 4: ManualStart_with_PassiveTcpEstablishment

        Definition: Local system administrator manually starts the 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.
                    2) The DampPeerOscillations attribute SHOULD be set
                       to FALSE when this event occurs.

     Event 5: 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.





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     Event 6: 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 determined by 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.

     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
                    determined by 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.

     Event 8: 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.



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        Status:     Optional, depending on local system

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

8.1.3.  Timer Events

     Event 9: ConnectRetryTimer_Expires

        Definition: An event generated when the ConnectRetryTimer
                    expires.

        Status:     Mandatory

     Event 10: HoldTimer_Expires

        Definition: An event generated when the HoldTimer expires.

        Status:     Mandatory

     Event 11: KeepaliveTimer_Expires

        Definition: An event generated when the KeepaliveTimer expires.

        Status:     Mandatory

     Event 12: 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.

     Event 13: 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.





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                    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.

        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

     Event 14: TcpConnection_Valid

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

     Event 15: 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.



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

                    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.

     Event 16: 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,
                    received a TCP SYN/ACK message, and sent a TCP ACK.

        Status:     Mandatory

     Event 17: 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

     Event 18: 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 possibility is that the local peer
                    indicated a timeout in the TCP connection and
                    downed the connection.

        Status:     Mandatory




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8.1.5.  BGP Message-Based Events

     Event 19: 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.

     Event 20: 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.

     Event 21: BGPHeaderErr

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

        Status:     Mandatory

     Event 22: BGPOpenMsgErr

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

        Status:     Mandatory

     Event 23: OpenCollisionDump

        Definition: An event generated administratively when a
                    connection collision has been detected while
                    processing an incoming OPEN message and this



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                    connection has been selected to be disconnected.
                    See Section 6.8 for more information on collision
                    detection.

                    Event 23 is an administrative action generated by
                    implementation logic that determines whether 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
                    the Established state,
                    1) CollisionDetectEstablishedState optional
                       attribute SHOULD be set to TRUE.

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

     Event 24: NotifMsgVerErr

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

        Status:     Mandatory

     Event 25: NotifMsg

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

        Status:     Mandatory

     Event 26: KeepAliveMsg

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

        Status:     Mandatory







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     Event 27: UpdateMsg

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

        Status:     Mandatory

     Event 28: 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 will attempt to connect to the
  other, unless configured to remain in the idle state, or configured
  to remain passive.  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
  incoming 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
  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 outgoing connection
  may exist for the same configured peering.  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.





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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 when applied to a
  TCP connection or 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 as active,
  it may end up on either the active or passive side of the connection
  that eventually gets established.  Once the TCP connection is
  completed, it doesn't matter which end was active and which was
  passive.  The only difference is in 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
  connection collision is resolved (see Section 6.8), the FSM for the
  connection that is closed SHOULD be disposed.

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 BGP FSM actions must be
  supported.  For example, if the following options can be set in a BGP
  implementation: AutoStart and PassiveTcpEstablishment, then 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,
                     IdleHoldTimer





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  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.
  Implementations MAY use these numbers to provide network management
  information.  The exact form of an FSM or the FSM events are specific
  to each implementation.

8.2.1.5.  FSM Actions that are Implementation Dependent

  At certain points, the BGP FSM specifies 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 the 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.




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     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 the 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, the
     following three additional events may occur within the Idle state:

       - AutomaticStart_with_DampPeerOscillations (Event 6),

       - AutomaticStart_with_DampPeerOscillations_and_
         PassiveTcpEstablishment (Event 7),

       - 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 the Connect state.

     In response to a ManualStop event (Event 2), the local system:

       - drops the TCP connection,

       - releases all BGP resources,




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       - 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 the Connect state.

     If the DelayOpenTimer_Expires event (Event 12) occurs in the
     Connect state, the local system:

       - sends an OPEN message to its peer,

       - sets the HoldTimer to a large value, and

       - 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



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       - 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 the HoldTimer to a large value, and

       - changes its state to OpenSent.

     A HoldTimer value of 4 minutes is suggested.

     If the TCP connection fails (Event 18), the local system checks
     the DelayOpenTimer.  If the DelayOpenTimer is running, the local
     system:

       - restarts the ConnectRetryTimer with the initial value,

       - stops the DelayOpenTimer and resets its 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,

       - completes the BGP initialization,




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       - 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 the 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 will be "external".

     If BGP message header checking (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 (Event
     24), the local system checks the DelayOpenTimer.  If the
     DelayOpenTimer is running, the local system:



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       - 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,

       - 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.





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  Active State:

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

     The start events (Events 1, 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 a TCP connection that may be initiated
         by a remote BGP peer, and

       - changes its state to Connect.

     If the local system receives a DelayOpenTimer_Expires 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,




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       - 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 the Active state.

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

     In response to the success of a TCP connection (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:

       - restarts the ConnectRetryTimer (with the initial value),



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       - stops and clears the DelayOpenTimer (sets the value to zero),

       - releases all BGP resource,

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

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

       - stops and clears the 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 will be external.

     If BGP message header checking (Event 21) or OPEN message checking
     detects an error (Event 22) (see Section 6.2), the local system:






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       - (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,

       - 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.





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     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 (Events 1, 3-7) are ignored in the OpenSent
     state.

     If a ManualStop event (Event 2) is issued in the 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 the 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,



Rekhter, et al.             Standards Track                    [Page 63]

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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.

     If the HoldTimer_Expires (Event 10), the local system:

       - sends a NOTIFICATION message with the 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), Tcp_CR_Acked (Event 16), 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 (Event 18) 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.






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     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 as
     described below.)

     If the BGP message header checking (Event 21) or OPEN message
     checking detects an error (Event 22)(see Section 6.2), the local
     system:

       - sends a NOTIFICATION message with the 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





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     CollisionDetectDump event occurs when the BGP implementation
     determines, by means outside the scope of this document, that a
     connection collision has occurred.

     If a connection in the 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
     the 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 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:

       - 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,

       - releases all BGP resources,

       - drops the TCP connection,

       - increments the ConnectRetryCounter by 1,



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       - (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 (Events 1, 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 a 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 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 HoldTimer_Expires event (Event 10) occurs before a
     KEEPALIVE message is received, the local system:




Rekhter, et al.             Standards Track                    [Page 67]

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       - sends the NOTIFICATION message with the Error Code Hold Timer
         Expired,

       - 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 KeepaliveTimer_Expires event (Event
     11), the local system:

       - sends a KEEPALIVE message,

       - restarts the KeepaliveTimer, and

       - remains in the OpenConfirmed state.

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

     If a TCP connection is attempted with 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




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       - 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
         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
     (Event 21)) or OPEN message checking detects an error (see Section
     6.2) (BGPOpenMsgErr (Event 22)), the local system:

       - sends a NOTIFICATION message with the appropriate error code,

       - sets the ConnectRetryTimer to zero,

       - releases all BGP resources,

       - drops the TCP connection,

       - increments the ConnectRetryCounter by 1,




Rekhter, et al.             Standards Track                    [Page 69]

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       - (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,

       - drops the TCP connection,

       - increments the ConnectRetryCounter by 1,




Rekhter, et al.             Standards Track                    [Page 70]

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       - (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 (Events 1, 3-7) is ignored in the Established
     state.

     In response to a ManualStop event (initiated by an operator)
     (Event 2), the local system:

       - sends the NOTIFICATION message with a Cease,

       - sets the ConnectRetryTimer to zero,

       - deletes all routes associated with this connection,

       - releases BGP resources,

       - drops the TCP connection,

       - sets the ConnectRetryCounter to zero, and

        - changes its state to Idle.

     In response to an AutomaticStop event (Event 8), 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 the ConnectRetryCounter by 1,

       - (optionally) performs peer oscillation damping if the
         DampPeerOscillations attribute is set to TRUE, and

       - changes its state to Idle.



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     One reason for an AutomaticStop event is: A BGP receives an UPDATE
     messages with a number of prefixes for a given peer such 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 (Event 10), the local
     system:

       - sends a NOTIFICATION message with the Error Code Hold Timer
         Expired,

       - 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 (Event 11), 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.






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     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 the ConnectRetryCounter by 1,

       - (optionally) performs peer oscillation damping if the
         DampPeerOscillations is set to TRUE, and

       - changes its state to Idle.

     If the local system receives a NOTIFICATION message (Event 24 or
     Event 25) or a TcpConnectionFails (Event 18) from the underlying
     TCP, the local system:

       - 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.











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     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 (Event 27), 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 (Event 28), the local system:

       - sends a NOTIFICATION message with an 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 the Error Code Finite State
         Machine Error,

       - deletes all routes associated with this connection,

       - sets the ConnectRetryTimer to zero,



Rekhter, et al.             Standards Track                    [Page 74]

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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.

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
  quietly 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
  propagation 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) are contained in this field, SHALL be removed from the
  Adj-RIB-In.  This BGP speaker SHALL run its Decision Process because
  the previously 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 the route currently has 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.
  Otherwise, 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
  Process 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, as long as the
  implementations support the described functionality and they exhibit
  the same externally visible behavior.

  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 any of the following as its inputs: the existence
  of other routes, the non-existence of other routes, or the path
  attributes of other routes.  Route selection then consists of the
  individual 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
  triggered 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
        the Loc-RIB.



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     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,f 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
     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 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.
  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.







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  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 if 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
        specified in Section 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
  Routing 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 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
  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.




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  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 that 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
  unresolvable 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 indirectly) 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
        referenced interface is up and if IP processing is enabled on
        this interface.

  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, the
  intermediate address, or both.

  Note that a BGP route is considered unresolvable in a situation where
  the BGP speaker's Routing Table contains no route matching the BGP
  route's NEXT_HOP.  Mutually recursive routes (routes resolving each
  other or themselves) also fail the resolvability 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
  current contents of the Routing Table (an example of such routes



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  would be mutually recursive routes).  This check ensures that a BGP
  speaker does not install routes in the Routing Table that will be
  removed and not used by the speaker.  Therefore, in addition to local
  Routing Table stability, this check also improves behavior of the
  protocol in the network.

  Whenever a BGP speaker identifies a route that fails the
  resolvability 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
  preferable 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
  implementations MAY use any algorithm that produces the same results
  as those described here.

     a) Remove from consideration all routes that 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 as 1, no matter how many ASes are in the set.

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







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     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
        neighboring AS is determined from the AS_PATH attribute).
        Routes that 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 that 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
        possible MULTI_EXIT_DISC value (i.e., 0).

        Similarly, neighborAS(n) is a function that 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
        implementation chooses to remove MULTI_EXIT_DISC, then the
        optional comparison on MULTI_EXIT_DISC, if performed, 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 that reach this step in the Decision Process.
        Including the MULTI_EXIT_DISC of an EBGP-learned route in the
        comparison with an IBGP-learned route, then removing the
        MULTI_EXIT_DISC attribute, and advertising the route has been
        proven to cause route loops.




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

     e) Remove from consideration any routes with less-preferred
        interior cost.  The interior cost of a route is determined by
        calculating 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 that 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 with the lowest BGP
        Identifier 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 connection has been established

  The Phase 3 function is a separate process that 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
  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



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  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 withdrawn from service by means of an UPDATE message (see 9.2).

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

  Any local policy that 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
  complete, 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
  multiple routes.  Because 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 prefix); 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
       specific 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
  use.  If a more specific route is later withdrawn, the set of
  destinations 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
  Decision Process MUST install, in Loc-RIB, either both the less and



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  the more specific routes or 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 ASes used to form the aggregate in an AS_SET, or add the
  ATOMIC_AGGREGATE attribute to the route.  This attribute is now
  primarily 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-aggregated.  In particular, a route that carries the
  ATOMIC_AGGREGATE attribute MUST NOT be de-aggregated.  That is, the
  NLRI of this route cannot be more specific.  Forwarding along such a
  route does not guarantee that IP packets will actually traverse only
  ASes listed in the AS_PATH attribute of the route.

9.2.  Update-Send Process

  The Update-Send process is responsible for advertising UPDATE
  messages 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
  information 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.

  Any routes in the Loc-RIB marked as unfeasible SHALL be removed.
  Changes to the reachable destinations within its own autonomous
  system 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.




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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
  minimum amount of time that must elapse between an 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
  MinRouteAdvertisementIntervalTimer.  This can only be achieved by
  keeping a separate timer for each common set of destinations.  This
  would be unwarranted overhead.  Any technique that 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
  MinRouteAdvertisementIntervalTimer, and will also ensure that 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
  SHOULD NOT apply to 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
  MinRouteAdvertisementIntervalTimer, the last route selected SHALL be
  advertised at the end of MinRouteAdvertisementIntervalTimer.

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.




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9.2.2.  Efficient Organization of Routing Information

  Having selected the routing information 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
  methods:

     a) Network Layer Reachability Information (NLRI):

        Destination IP addresses can be represented as IP address
        prefixes.  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 unordered AS_SETs.  AS_SETs are used in the route
        aggregation algorithm described in Section 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
        constituent autonomous systems.  AS_SETs provide sufficient
        information to avoid routing information looping; however,
        their use may prune potentially feasible paths because such
        paths are no longer listed individually in the form of
        AS_SEQUENCEs.  In practice, this is not likely to be a problem
        because once an IP packet arrives at the edge of a group of
        autonomous systems, the BGP speaker is likely to have more
        detailed path information and can distinguish individual paths
        from destinations.







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9.2.2.2.  Aggregating Routing Information

  Aggregation is the process of combining the characteristics of
  several different routes in such a way that a single route can be
  advertised.  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 attributes 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 cannot 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
        attributes, the NEXT_HOP attribute of the aggregated route
        SHALL identify an interface on the BGP speaker that performs
        the aggregation.

     ORIGIN attribute:
        If at least one route among routes that are aggregated has
        ORIGIN with the value INCOMPLETE, then the aggregated route
        MUST have the ORIGIN attribute with the value INCOMPLETE.
        Otherwise, 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



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        belongs to (e.g., AS_SEQUENCE, AS_SET), and "value" identifies
        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_PATHs 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_PATHs 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 that 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:

          - 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 a length
            greater than 255 to be generated.



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        Appendix F, Section F.6 presents another algorithm that
        satisfies the conditions and allows for more complex policy
        configurations.

     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
        performing the route aggregation MAY attach a new AGGREGATOR
        attribute (see Section 5.1.7).

9.3.  Route Selection Criteria

  Generally, additional rules for comparing routes among several
  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 cannot 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.

     - In order to achieve a 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" (from the previous sentence)
       will require experience, 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
  information 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 of whether to distribute non-BGP acquired routes within an
  AS via BGP depends on the environment within the AS (e.g., type of
  IGP) and SHOULD be controlled via configuration.



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10.  BGP Timers

  BGP employs five timers: ConnectRetryTimer (see Section 8), HoldTimer
  (see Section 4.2), KeepaliveTimer (see Section 8),
  MinASOriginationIntervalTimer (see Section 9.2.1.2), and
  MinRouteAdvertisementIntervalTimer (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
  operation 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
  HoldTimer is set to a large value.  The suggested default for this
  large value is 4 minutes.

  The KeepaliveTime is a mandatory FSM attribute that stores the
  initial 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
  MinRouteAdvertisementIntervalTimer on EBGP connections is 30 seconds.

  The suggested default value for the
  MinRouteAdvertisementIntervalTimer 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
  configurable.

  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.



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  The suggested default amount of jitter SHALL be determined by
  multiplying 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's random value MAY be configurable.














































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Appendix A.  Comparison with RFC 1771

  There are numerous editorial changes in comparison to [RFC1771] (too
  many to list here).

  The following list the technical changes:

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

     Clarification of 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.

     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.

     Clarification of the various types of NEXT_HOPs.

     Clarification of 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.

     Clarification of the tie-breaking procedures.

     Clarification of the frequency of route advertisements.

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

     UPDATE Message Error subcode 7 (AS Routing Loop) has been
     deprecated.

     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.





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Appendix B.  Comparison with RFC 1267

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

  BGP-4 is capable of operating in an environment where a set of
  reachable destinations may be expressed via a single IP prefix.  The
  concept of network classes, or subnetting, is foreign to BGP-4.  To
  accommodate these capabilities, BGP-4 changes the 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
  [RFC1518, RFC1519].

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

  The INTER_AS_METRIC attribute has been renamed MULTI_EXIT_DISC.

  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 to advertise which AS
  and which BGP speaker within that AS caused the aggregation.

  To ensure that Hold Timers are symmetric, the Hold Timer is now
  negotiated on a per-connection basis.  Hold Timers of zero are now
  supported.

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
  recovering 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.

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









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Appendix D.  Comparison with RFC 1105

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

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

  The notion of Up/Down/Horizontal relations presented in RFC 1105 has
  been removed from the protocol.

  The changes in the message format from RFC 1105 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.

     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
        significantly.  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 [RFC1163], is referred to as BGP-2;
  BGP, as specified in RFC 1267 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 the TCP PUSH function,
  then each BGP message SHOULD be transmitted with PUSH flag set.
  Setting PUSH flag forces BGP messages to be transmitted to the
  receiver promptly.

  If a local system TCP user interface supports setting 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).

  An implementation MUST support the TCP MD5 option [RFC2385].



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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.  Using 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.

  One method of building messages that contain many address prefixes
  per 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 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 are
  released.  Maximum compression is achieved when all destinations
  covered by the address prefixes share a common set of path
  attributes, making it possible to send many address prefixes in one
  4096-byte message.

  When peering with a BGP implementation that does not compress
  multiple 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 when 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
  disadvantage 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 multiple messages, this latency should be minimized.  A better
  method would be to read all received messages before sending updates.








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Appendix F.2.  Reducing Route Flapping

  To avoid excessive route flapping, a BGP speaker that needs to
  withdraw 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 that combine update messages (as described above in
  Section 6.1) may prefer to see all path attributes presented in a
  known order.  This permits them to quickly identify sets of
  attributes from different update messages that 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.

Appendix F.4.  AS_SET Sorting

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

Appendix F.5.  Control Over Version Negotiation

  Because BGP-4 is capable of carrying aggregated routes that cannot be
  properly represented in BGP-3, an implementation that 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 that chooses to provide a path aggregation
  algorithm retaining 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 ASes 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
     follows:

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



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             - 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 ASes identified
           in (a), in exactly the same order as they appear in the
           AS_PATH attributes to be aggregated.  If two consecutive
           ASes identified in (a) do not immediately follow each other
           in both of the AS_PATH attributes to be aggregated, then the
           intervening ASes (ASes that are between the two consecutive
           ASes that are the same) in both attributes are combined into
           an AS_SET path segment that consists of the intervening ASes
           from both AS_PATH attributes.  This segment is then placed
           between the two consecutive ASes identified in (a) of the
           aggregated attribute.  If two consecutive ASes identified in
           (a) immediately follow each other in one attribute, but do
           not follow in another, then the intervening ASes of the
           latter are combined into an AS_SET path segment.  This
           segment is then placed between the two consecutive ASes
           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 if
           doing so will not cause a segment of a 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

  A BGP implementation MUST support the authentication mechanism
  specified in RFC 2385 [RFC2385].  The authentication provided by this
  mechanism could be done on a per-peer basis.

  BGP makes use of TCP for reliable transport of its traffic between
  peer routers.  To provide connection-oriented integrity and data
  origin authentication on a point-to-point basis, BGP specifies use of
  the mechanism defined in RFC 2385.  These services are intended to
  detect and reject active wiretapping attacks against the inter-router
  TCP connections.  Absent the use of mechanisms that effect these
  security services, attackers can disrupt these TCP connections and/or
  masquerade as a legitimate peer router.  Because the mechanism
  defined in the RFC does not provide peer-entity authentication, these
  connections may be subject to some forms of replay attacks that will
  not be detected at the TCP layer.  Such attacks might result in
  delivery (from TCP) of "broken" or "spoofed" BGP messages.



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  The mechanism defined in RFC 2385 augments the normal TCP checksum
  with a 16-byte message authentication code (MAC) that is computed
  over the same data as the TCP checksum.  This MAC is based on a one-
  way hash function (MD5) and use of a secret key.  The key is shared
  between peer routers and is used to generate MAC values that are not
  readily computed by an attacker who does not have access to the key.
  A compliant implementation must support this mechanism, and must
  allow a network administrator to activate it on a per-peer basis.

  RFC 2385 does not specify a means of managing (e.g., generating,
  distributing, and replacing) the keys used to compute the MAC.  RFC
  3562 [RFC3562] (an informational document) provides some guidance in
  this area, and provides rationale to support this guidance.  It notes
  that a distinct key should be used for communication with each
  protected peer.  If the same key is used for multiple peers, the
  offered security services may be degraded, e.g., due to an increased
  risk of compromise at one router that adversely affects other
  routers.

  The keys used for MAC computation should be changed periodically, to
  minimize the impact of a key compromise or successful cryptanalytic
  attack.  RFC 3562 suggests a crypto period (the interval during which
  a key is employed) of, at most, 90 days.  More frequent key changes
  reduce the likelihood that replay attacks (as described above) will
  be feasible.  However, absent a standard mechanism for effecting such
  changes in a coordinated fashion between peers, one cannot assume
  that BGP-4 implementations complying with this RFC will support
  frequent key changes.

  Obviously, each should key also be chosen to be difficult for an
  attacker to guess.  The techniques specified in RFC 1750 for random
  number generation provide a guide for generation of values that could
  be used as keys.  RFC 2385 calls for implementations to support keys
  "composed of a string of printable ASCII of 80 bytes or less."  RFC
  3562 suggests keys used in this context be 12 to 24 bytes of random
  (pseudo-random) bits.  This is fairly consistent with suggestions for
  analogous MAC algorithms, which typically employ keys in the range of
  16 to 20 bytes.  To provide enough random bits at the low end of this
  range, RFC 3562 also observes that a typical ACSII text string would
  have to be close to the upper bound for the key length specified in
  RFC 2385.

  BGP vulnerabilities analysis is discussed in [RFC4272].








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IANA Considerations

  All the BGP messages contain an 8-bit message type, for which IANA
  has created and is maintaining a registry entitled "BGP Message
  Types".  This document defines the following message types:

        Name             Value       Definition
        ----             -----       ----------
        OPEN             1           See Section 4.2
        UPDATE           2           See Section 4.3
        NOTIFICATION     3           See Section 4.5
        KEEPALIVE        4           See Section 4.4

  Future assignments are to be made using either the Standards Action
  process defined in [RFC2434], or the Early IANA Allocation process
  defined in [RFC4020].  Assignments consist of a name and the value.

  The BGP UPDATE messages may carry one or more Path Attributes, where
  each Attribute contains an 8-bit Attribute Type Code.  IANA is
  already maintaining such a registry, entitled "BGP Path Attributes".
  This document defines the following Path Attributes Type Codes:

       Name               Value       Definition
       ----               -----       ----------
       ORIGIN              1          See Section 5.1.1
       AS_PATH             2          See Section 5.1.2
       NEXT_HOP            3          See Section 5.1.3
       MULTI_EXIT_DISC     4          See Section 5.1.4
       LOCAL_PREF          5          See Section 5.1.5
       ATOMIC_AGGREGATE    6          See Section 5.1.6
       AGGREGATOR          7          See Section 5.1.7

  Future assignments are to be made using either the Standards Action
  process defined in [RFC2434], or the Early IANA Allocation process
  defined in [RFC4020].  Assignments consist of a name and the value.

  The BGP NOTIFICATION message carries an 8-bit Error Code, for which
  IANA has created and is maintaining a registry entitled "BGP Error
  Codes".  This document defines the following Error Codes:

        Name                       Value      Definition
        ------------               -----      ----------
        Message Header Error       1          Section 6.1
        OPEN Message Error         2          Section 6.2
        UPDATE Message Error       3          Section 6.3
        Hold Timer Expired         4          Section 6.5
        Finite State Machine Error 5          Section 6.6
        Cease                      6          Section 6.7



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  Future assignments are to be made using either the Standards Action
  process defined in [RFC2434], or the Early IANA Allocation process
  defined in [RFC4020].  Assignments consist of a name and the value.

  The BGP NOTIFICATION message carries an 8-bit Error Subcode, where
  each Subcode has to be defined within the context of a particular
  Error Code, and thus has to be unique only within that context.

  IANA has created and is maintaining a set of registries, "Error
  Subcodes", with a separate registry for each BGP Error Code.  Future
  assignments are to be made using either the Standards Action process
  defined in [RFC2434], or the Early IANA Allocation process defined in
  [RFC4020].  Assignments consist of a name and the value.

  This document defines the following Message Header Error subcodes:

        Name                         Value        Definition
        --------------------         -----        ----------
        Connection Not Synchronized   1           See Section 6.1
        Bad Message Length            2           See Section 6.1
        Bad Message Type              3           See Section 6.1

  This document defines the following OPEN Message Error subcodes:

        Name                         Value        Definition
        --------------------         -----        ----------
        Unsupported Version Number     1          See Section 6.2
        Bad Peer AS                    2          See Section 6.2
        Bad BGP Identifier             3          See Section 6.2
        Unsupported Optional Parameter 4          See Section 6.2
        [Deprecated]                   5          See Appendix A
        Unacceptable Hold Time         6          See Section 6.2

   This document defines the following UPDATE Message Error subcodes:

        Name                             Value    Definition
        --------------------              ---     ----------
        Malformed Attribute List           1      See Section 6.3
        Unrecognized Well-known Attribute  2      See Section 6.3
        Missing Well-known Attribute       3      See Section 6.3
        Attribute Flags Error              4      See Section 6.3
        Attribute Length Error             5      See Section 6.3
        Invalid ORIGIN Attribute           6      See Section 6.3
        [Deprecated]                       7      See Appendix A
        Invalid NEXT_HOP Attribute         8      See Section 6.3
        Optional Attribute Error           9      See Section 6.3
        Invalid Network Field             10      See Section 6.3
        Malformed AS_PATH                 11      See Section 6.3



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Normative References

  [RFC791]  Postel, J., "Internet Protocol", STD 5, RFC 791, September
            1981.

  [RFC793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
            793, 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", RFC 2385, August 1998.

  [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 2434,
            October 1998.

Informative References

  [RFC904]  Mills, D., "Exterior Gateway Protocol formal
            specification", RFC 904, April 1984.

  [RFC1092] Rekhter, J., "EGP and policy based routing in the new
            NSFNET backbone", RFC 1092, February 1989.

  [RFC1093] Braun, H., "NSFNET routing architecture", RFC 1093,
            February 1989.

  [RFC1105] Lougheed, K. and Y. Rekhter, "Border Gateway Protocol
            (BGP)", RFC 1105, June 1989.

  [RFC1163] Lougheed, K. and Y. Rekhter, "Border Gateway Protocol
            (BGP)", RFC 1163, June 1990.

  [RFC1267] Lougheed, K. and Y. Rekhter, "Border Gateway Protocol 3
            (BGP-3)", RFC 1267, October 1991.

  [RFC1771] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-
            4)", RFC 1771, March 1995.

  [RFC1772] Rekhter, Y. and P. Gross, "Application of the Border
            Gateway Protocol in the Internet", RFC 1772, March 1995.

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





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RFC 4271                         BGP-4                      January 2006


  [RFC1519] Fuller, V., Li, T., Yu, J., and K. Varadhan, "Classless
            Inter-Domain Routing (CIDR): an Address Assignment and
            Aggregation Strategy", RFC 1519, September 1993.

  [RFC1930] Hawkinson, J. and T. Bates, "Guidelines for creation,
            selection, and registration of an Autonomous System (AS)",
            BCP 6, RFC 1930, March 1996.

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

  [RFC2439] Villamizar, C., Chandra, R., and R. Govindan, "BGP Route
            Flap Damping", RFC 2439, November 1998.

  [RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
            "Definition of the Differentiated Services Field (DS Field)
            in the IPv4 and IPv6 Headers", RFC 2474, December 1998.

  [RFC2796] Bates, T., Chandra, R., and E. Chen, "BGP Route Reflection
            - An Alternative to Full Mesh IBGP", RFC 2796, April 2000.

  [RFC2858] Bates, T., Rekhter, Y., Chandra, R., and D. Katz,
            "Multiprotocol Extensions for BGP-4", RFC 2858, June 2000.

  [RFC3392] Chandra, R. and J. Scudder, "Capabilities Advertisement
            with BGP-4", RFC 3392, November 2002.

  [RFC2918] Chen, E., "Route Refresh Capability for BGP-4", RFC 2918,
            September 2000.

  [RFC3065] Traina, P., McPherson, D., and J. Scudder, "Autonomous
            System Confederations for BGP", RFC 3065, February 2001.

  [RFC3562] Leech, M., "Key Management Considerations for the TCP MD5
            Signature Option", RFC 3562, July 2003.

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

  [RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis", RFC
            4272, January 2006

  [RFC4020] Kompella, K. and A. Zinin, "Early IANA Allocation of
            Standards Track Code Points", BCP 100, RFC 4020, February
            2005.



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RFC 4271                         BGP-4                      January 2006


Editors' Addresses

  Yakov Rekhter
  Juniper Networks

  EMail: [email protected]


  Tony Li

  EMail: [email protected]


  Susan Hares
  NextHop Technologies, Inc.
  825 Victors Way
  Ann Arbor, MI 48108

  Phone: (734)222-1610
  EMail: [email protected]































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RFC 4271                         BGP-4                      January 2006


Full Copyright Statement

  Copyright (C) The Internet Society (2006).

  This document is subject to the rights, licenses and restrictions
  contained in BCP 78, and except as set forth therein, the authors
  retain all their rights.

  This document and the information contained herein are provided on an
  "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
  OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
  ENGINEERING TASK FORCE DISCLAIM 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 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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  this document or the extent to which any license under such rights
  might or might not be available; nor does it represent that it has
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Acknowledgement

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  Administrative Support Activity (IASA).







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