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rfc:rfc4271

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.

Rekhter, et al. Standards Track [Page 1] RFC 4271 BGP-4 January 2006

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

Rekhter, et al. Standards Track [Page 2] RFC 4271 BGP-4 January 2006

                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

Rekhter, et al. Standards Track [Page 3] RFC 4271 BGP-4 January 2006

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

Rekhter, et al. Standards Track [Page 4] RFC 4271 BGP-4 January 2006

    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

Rekhter, et al. Standards Track [Page 5] RFC 4271 BGP-4 January 2006

    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.

Rekhter, et al. Standards Track [Page 6] RFC 4271 BGP-4 January 2006

 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

Rekhter, et al. Standards Track [Page 7] RFC 4271 BGP-4 January 2006

 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].

Rekhter, et al. Standards Track [Page 8] RFC 4271 BGP-4 January 2006

 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.

Rekhter, et al. Standards Track [Page 9] RFC 4271 BGP-4 January 2006

 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

Rekhter, et al. Standards Track [Page 10] RFC 4271 BGP-4 January 2006

 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.

Rekhter, et al. Standards Track [Page 11] RFC 4271 BGP-4 January 2006

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.

Rekhter, et al. Standards Track [Page 12] RFC 4271 BGP-4 January 2006

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

Rekhter, et al. Standards Track [Page 13] RFC 4271 BGP-4 January 2006

       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] RFC 4271 BGP-4 January 2006

 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)       |
                +---------------------------+

Rekhter, et al. Standards Track [Page 15] RFC 4271 BGP-4 January 2006

       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).

Rekhter, et al. Standards Track [Page 16] RFC 4271 BGP-4 January 2006

       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.

Rekhter, et al. Standards Track [Page 18] RFC 4271 BGP-4 January 2006

          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] RFC 4271 BGP-4 January 2006

 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.

Rekhter, et al. Standards Track [Page 22] RFC 4271 BGP-4 January 2006

    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.

Rekhter, et al. Standards Track [Page 24] RFC 4271 BGP-4 January 2006

 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:

Rekhter, et al. Standards Track [Page 25] RFC 4271 BGP-4 January 2006

       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

Rekhter, et al. Standards Track [Page 26] RFC 4271 BGP-4 January 2006

       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:
  1. 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.
  1. 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.
  1. 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.
  1. 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"):
  1. 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

Rekhter, et al. Standards Track [Page 27] RFC 4271 BGP-4 January 2006

         attribute of the learned route (the speaker does not modify
         the NEXT_HOP attribute).
  1. 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

Rekhter, et al. Standards Track [Page 28] RFC 4271 BGP-4 January 2006

 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.

Rekhter, et al. Standards Track [Page 29] RFC 4271 BGP-4 January 2006

 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.

Rekhter, et al. Standards Track [Page 30] RFC 4271 BGP-4 January 2006

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:
  1. if the Length field of the message header is less than 19 or

greater than 4096, or

  1. if the Length field of an OPEN message is less than the minimum

length of the OPEN message, or

  1. if the Length field of an UPDATE message is less than the

minimum length of the UPDATE message, or

  1. if the Length field of a KEEPALIVE message is not equal to 19,

or

  1. 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

Rekhter, et al. Standards Track [Page 31] RFC 4271 BGP-4 January 2006

 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).

Rekhter, et al. Standards Track [Page 32] RFC 4271 BGP-4 January 2006

 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.

Rekhter, et al. Standards Track [Page 33] RFC 4271 BGP-4 January 2006

 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.

Rekhter, et al. Standards Track [Page 34] RFC 4271 BGP-4 January 2006

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

Rekhter, et al. Standards Track [Page 35] RFC 4271 BGP-4 January 2006

 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.

Rekhter, et al. Standards Track [Page 45] RFC 4271 BGP-4 January 2006

       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.

Rekhter, et al. Standards Track [Page 46] RFC 4271 BGP-4 January 2006

                   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.

Rekhter, et al. Standards Track [Page 47] RFC 4271 BGP-4 January 2006

                   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

Rekhter, et al. Standards Track [Page 48] RFC 4271 BGP-4 January 2006

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

Rekhter, et al. Standards Track [Page 49] RFC 4271 BGP-4 January 2006

                   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

Rekhter, et al. Standards Track [Page 50] RFC 4271 BGP-4 January 2006

    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.

Rekhter, et al. Standards Track [Page 51] RFC 4271 BGP-4 January 2006

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:
  1. flag indicating support,
  2. Time set in Timer
  3. Timer.
 The two optional timers show this format:
    DelayOpenTimer: DelayOpen, DelayOpenTime, DelayOpenTimer
    IdleHoldTimer:  DampPeerOscillations, IdleHoldTime,
                    IdleHoldTimer

Rekhter, et al. Standards Track [Page 52] RFC 4271 BGP-4 January 2006

 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:
  1. initializes all BGP resources for the peer connection,
  1. sets ConnectRetryCounter to zero,
  1. starts the ConnectRetryTimer with the initial value,
  1. initiates a TCP connection to the other BGP peer,
  1. listens for a connection that may be initiated by the remote

BGP peer, and

  1. changes its state to Connect.
    The ManualStop event (Event 2) and AutomaticStop (Event 8) event
    are ignored in the Idle state.

Rekhter, et al. Standards Track [Page 53] RFC 4271 BGP-4 January 2006

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

peer, and

  1. 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:
  1. AutomaticStart_with_DampPeerOscillations (Event 6),
  1. AutomaticStart_with_DampPeerOscillations_and_

PassiveTcpEstablishment (Event 7),

  1. 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:
  1. drops the TCP connection,
  1. releases all BGP resources,

Rekhter, et al. Standards Track [Page 54] RFC 4271 BGP-4 January 2006

  1. sets ConnectRetryCounter to zero,
  1. stops the ConnectRetryTimer and sets ConnectRetryTimer to

zero, and

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

the remote BGP peer, and

  1. stays in the Connect state.
    If the DelayOpenTimer_Expires event (Event 12) occurs in the
    Connect state, the local system:
  1. sends an OPEN message to its peer,
  1. sets the HoldTimer to a large value, and
  1. 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:
  1. stops the ConnectRetryTimer (if running) and sets the

ConnectRetryTimer to zero,

  1. sets the DelayOpenTimer to the initial value, and

Rekhter, et al. Standards Track [Page 55] RFC 4271 BGP-4 January 2006

  1. stays in the Connect state.
    If the DelayOpen attribute is set to FALSE, the local system:
  1. stops the ConnectRetryTimer (if running) and sets the

ConnectRetryTimer to zero,

  1. completes BGP initialization
  1. sends an OPEN message to its peer,
  1. sets the HoldTimer to a large value, and
  1. 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:
  1. restarts the ConnectRetryTimer with the initial value,
  1. stops the DelayOpenTimer and resets its value to zero,
  1. continues to listen for a connection that may be initiated by

the remote BGP peer, and

  1. changes its state to Active.
    If the DelayOpenTimer is not running, the local system:
  1. stops the ConnectRetryTimer to zero,
  1. drops the TCP connection,
  1. releases all BGP resources, and
  1. changes its state to Idle.
    If an OPEN message is received while the DelayOpenTimer is running
    (Event 20), the local system:
  1. stops the ConnectRetryTimer (if running) and sets the

ConnectRetryTimer to zero,

  1. completes the BGP initialization,

Rekhter, et al. Standards Track [Page 56] RFC 4271 BGP-4 January 2006

  1. stops and clears the DelayOpenTimer (sets the value to zero),
  1. sends an OPEN message,
  1. sends a KEEPALIVE message,
  1. if the HoldTimer initial value is non-zero,
  1. starts the KeepaliveTimer with the initial value and
  1. resets the HoldTimer to the negotiated value,
        else, if the HoldTimer initial value is zero,
  1. resets the KeepaliveTimer and
  1. resets the HoldTimer value to zero,
  1. 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:
  1. (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
  1. stops the ConnectRetryTimer (if running) and sets the

ConnectRetryTimer to zero,

  1. releases all BGP resources,
  1. drops the TCP connection,
  1. increments the ConnectRetryCounter by 1,
  1. (optionally) performs peer oscillation damping if the

DampPeerOscillations attribute is set to TRUE, and

  1. 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:

Rekhter, et al. Standards Track [Page 57] RFC 4271 BGP-4 January 2006

  1. stops the ConnectRetryTimer (if running) and sets the

ConnectRetryTimer to zero,

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

zero,

  1. releases all BGP resources,
  1. drops the TCP connection,
  1. increments the ConnectRetryCounter by 1,
  1. performs peer oscillation damping if the DampPeerOscillations

attribute is set to True, and

  1. changes its state to Idle.
    In response to any other events (Events 8, 10-11, 13, 19, 23,
    25-28), the local system:
  1. if the ConnectRetryTimer is running, stops and resets the

ConnectRetryTimer (sets to zero),

  1. if the DelayOpenTimer is running, stops and resets the

DelayOpenTimer (sets to zero),

  1. releases all BGP resources,
  1. drops the TCP connection,
  1. increments the ConnectRetryCounter by 1,
  1. performs peer oscillation damping if the DampPeerOscillations

attribute is set to True, and

  1. changes its state to Idle.

Rekhter, et al. Standards Track [Page 58] RFC 4271 BGP-4 January 2006

 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:
  1. If the DelayOpenTimer is running and the

SendNOTIFICATIONwithoutOPEN session attribute is set, the

        local system sends a NOTIFICATION with a Cease,
  1. releases all BGP resources including stopping the

DelayOpenTimer

  1. drops the TCP connection,
  1. sets ConnectRetryCounter to zero,
  1. stops the ConnectRetryTimer and sets the ConnectRetryTimer to

zero, and

  1. changes its state to Idle.
    In response to a ConnectRetryTimer_Expires event (Event 9), the
    local system:
  1. restarts the ConnectRetryTimer (with initial value),
  1. initiates a TCP connection to the other BGP peer,
  1. continues to listen for a TCP connection that may be initiated

by a remote BGP peer, and

  1. changes its state to Connect.
    If the local system receives a DelayOpenTimer_Expires event (Event
    12), the local system:
  1. sets the ConnectRetryTimer to zero,
  1. stops and clears the DelayOpenTimer (set to zero),
  1. completes the BGP initialization,
  1. sends the OPEN message to its remote peer,

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  1. sets its hold timer to a large value, and
  1. 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:
  1. stops the ConnectRetryTimer and sets the ConnectRetryTimer

to zero,

  1. sets the DelayOpenTimer to the initial value

(DelayOpenTime), and

  1. stays in the Active state.
      If the DelayOpen attribute is set to FALSE, the local system:
  1. sets the ConnectRetryTimer to zero,
  1. completes the BGP initialization,
  1. sends the OPEN message to its peer,
  1. sets its HoldTimer to a large value, and
  1. 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:
  1. restarts the ConnectRetryTimer (with the initial value),

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  1. stops and clears the DelayOpenTimer (sets the value to zero),
  1. releases all BGP resource,
  1. increments the ConnectRetryCounter by 1,
  1. optionally performs peer oscillation damping if the

DampPeerOscillations attribute is set to TRUE, and

  1. changes its state to Idle.
    If an OPEN message is received and the DelayOpenTimer is running
    (Event 20), the local system:
  1. stops the ConnectRetryTimer (if running) and sets the

ConnectRetryTimer to zero,

  1. stops and clears the DelayOpenTimer (sets to zero),
  1. completes the BGP initialization,
  1. sends an OPEN message,
  1. sends a KEEPALIVE message,
  1. if the HoldTimer value is non-zero,
  1. starts the KeepaliveTimer to initial value,
  1. resets the HoldTimer to the negotiated value,
        else if the HoldTimer is zero
  1. resets the KeepaliveTimer (set to zero),
  1. resets the HoldTimer to zero, and
  1. 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:

Rekhter, et al. Standards Track [Page 61] RFC 4271 BGP-4 January 2006

  1. (optionally) sends a NOTIFICATION message with the appropriate

error code if the SendNOTIFICATIONwithoutOPEN attribute is set

        to TRUE,
  1. sets the ConnectRetryTimer to zero,
  1. releases all BGP resources,
  1. drops the TCP connection,
  1. increments the ConnectRetryCounter by 1,
  1. (optionally) performs peer oscillation damping if the

DampPeerOscillations attribute is set to TRUE, and

  1. 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:
  1. stops the ConnectRetryTimer (if running) and sets the

ConnectRetryTimer to zero,

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

DampPeerOscillations attribute is set to TRUE, and

  1. 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:
  1. sets the ConnectRetryTimer to zero,
  1. releases all BGP resources,
  1. drops the TCP connection,
  1. increments the ConnectRetryCounter by one,
  1. (optionally) performs peer oscillation damping if the

DampPeerOscillations attribute is set to TRUE, and

  1. 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:
  1. sends the NOTIFICATION with a Cease,
  1. sets the ConnectRetryTimer to zero,
  1. releases all BGP resources,
  1. drops the TCP connection,
  1. sets the ConnectRetryCounter to zero, and
  1. changes its state to Idle.
    If an AutomaticStop event (Event 8) is issued in the OpenSent
    state, the local system:
  1. sends the NOTIFICATION with a Cease,
  1. sets the ConnectRetryTimer to zero,
  1. releases all the BGP resources,
  1. drops the TCP connection,

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  1. increments the ConnectRetryCounter by 1,
  1. (optionally) performs peer oscillation damping if the

DampPeerOscillations attribute is set to TRUE, and

  1. changes its state to Idle.
    If the HoldTimer_Expires (Event 10), the local system:
  1. sends a NOTIFICATION message with the error code Hold Timer

Expired,

  1. sets the ConnectRetryTimer to zero,
  1. releases all BGP resources,
  1. drops the TCP connection,
  1. increments the ConnectRetryCounter,
  1. (optionally) performs peer oscillation damping if the

DampPeerOscillations attribute is set to TRUE, and

  1. 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:
  1. closes the BGP connection,
  1. restarts the ConnectRetryTimer,
  1. continues to listen for a connection that may be initiated by

the remote BGP peer, and

  1. 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:
  1. resets the DelayOpenTimer to zero,
  1. sets the BGP ConnectRetryTimer to zero,
  1. sends a KEEPALIVE message, and
  1. sets a KeepaliveTimer (via the text below)
  1. sets the HoldTimer according to the negotiated value (see

Section 4.2),

  1. 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:
  1. sends a NOTIFICATION message with the appropriate error code,
  1. sets the ConnectRetryTimer to zero,
  1. releases all BGP resources,
  1. drops the TCP connection,
  1. increments the ConnectRetryCounter by 1,
  1. (optionally) performs peer oscillation damping if the

DampPeerOscillations attribute is TRUE, and

  1. 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:
  1. sends a NOTIFICATION with a Cease,
  1. sets the ConnectRetryTimer to zero,
  1. releases all BGP resources,
  1. drops the TCP connection,
  1. increments the ConnectRetryCounter by 1,
  1. (optionally) performs peer oscillation damping if the

DampPeerOscillations attribute is set to TRUE, and

  1. changes its state to Idle.
    If a NOTIFICATION message is received with a version error (Event
    24), the local system:
  1. sets the ConnectRetryTimer to zero,
  1. releases all BGP resources,
  1. drops the TCP connection, and
  1. changes its state to Idle.
    In response to any other event (Events 9, 11-13, 20, 25-28), the
    local system:
  1. sends the NOTIFICATION with the Error Code Finite State

Machine Error,

  1. sets the ConnectRetryTimer to zero,
  1. releases all BGP resources,
  1. drops the TCP connection,
  1. increments the ConnectRetryCounter by 1,

Rekhter, et al. Standards Track [Page 66] RFC 4271 BGP-4 January 2006

  1. (optionally) performs peer oscillation damping if the

DampPeerOscillations attribute is set to TRUE, and

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

DampPeerOscillations attribute is set to TRUE, and

  1. 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] RFC 4271 BGP-4 January 2006

  1. sends the NOTIFICATION message with the Error Code Hold Timer

Expired,

  1. sets the ConnectRetryTimer to zero,
  1. releases all BGP resources,
  1. drops the TCP connection,
  1. increments the ConnectRetryCounter by 1,
  1. (optionally) performs peer oscillation damping if the

DampPeerOscillations attribute is set to TRUE, and

  1. changes its state to Idle.
    If the local system receives a KeepaliveTimer_Expires event (Event
    11), the local system:
  1. sends a KEEPALIVE message,
  1. restarts the KeepaliveTimer, and
  1. 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:
  1. sets the ConnectRetryTimer to zero,
  1. releases all BGP resources,
  1. drops the TCP connection,
  1. increments the ConnectRetryCounter by 1,
  1. (optionally) performs peer oscillation damping if the

DampPeerOscillations attribute is set to TRUE, and

Rekhter, et al. Standards Track [Page 68] RFC 4271 BGP-4 January 2006

  1. changes its state to Idle.
    If the local system receives a NOTIFICATION message with a version
    error (NotifMsgVerErr (Event 24)), the local system:
  1. sets the ConnectRetryTimer to zero,
  1. releases all BGP resources,
  1. drops the TCP connection, and
  1. 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:
  1. sends a NOTIFICATION with a Cease,
  1. sets the ConnectRetryTimer to zero,
  1. releases all BGP resources,
  1. drops the TCP connection (send TCP FIN),
  1. increments the ConnectRetryCounter by 1,
  1. (optionally) performs peer oscillation damping if the

DampPeerOscillations attribute is set to TRUE, and

  1. 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:
  1. sends a NOTIFICATION message with the appropriate error code,
  1. sets the ConnectRetryTimer to zero,
  1. releases all BGP resources,
  1. drops the TCP connection,
  1. increments the ConnectRetryCounter by 1,

Rekhter, et al. Standards Track [Page 69] RFC 4271 BGP-4 January 2006

  1. (optionally) performs peer oscillation damping if the

DampPeerOscillations attribute is set to TRUE, and

  1. 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:
  1. sends a NOTIFICATION with a Cease,
  1. sets the ConnectRetryTimer to zero,
  1. releases all BGP resources
  1. drops the TCP connection,
  1. increments the ConnectRetryCounter by 1,
  1. (optionally) performs peer oscillation damping if the

DampPeerOscillations attribute is set to TRUE, and

  1. changes its state to Idle.
    If the local system receives a KEEPALIVE message (KeepAliveMsg
    (Event 26)), the local system:
  1. restarts the HoldTimer and
  1. changes its state to Established.
    In response to any other event (Events 9, 12-13, 20, 27-28), the
    local system:
  1. sends a NOTIFICATION with a code of Finite State Machine

Error,

  1. sets the ConnectRetryTimer to zero,
  1. releases all BGP resources,
  1. drops the TCP connection,
  1. increments the ConnectRetryCounter by 1,

Rekhter, et al. Standards Track [Page 70] RFC 4271 BGP-4 January 2006

  1. (optionally) performs peer oscillation damping if the

DampPeerOscillations attribute is set to TRUE, and

  1. 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:
  1. sends the NOTIFICATION message with a Cease,
  1. sets the ConnectRetryTimer to zero,
  1. deletes all routes associated with this connection,
  1. releases BGP resources,
  1. drops the TCP connection,
  1. sets the ConnectRetryCounter to zero, and
  1. changes its state to Idle.
    In response to an AutomaticStop event (Event 8), the local system:
  1. sends a NOTIFICATION with a Cease,
  1. sets the ConnectRetryTimer to zero
  1. deletes all routes associated with this connection,
  1. releases all BGP resources,
  1. drops the TCP connection,
  1. increments the ConnectRetryCounter by 1,
  1. (optionally) performs peer oscillation damping if the

DampPeerOscillations attribute is set to TRUE, and

  1. changes its state to Idle.

Rekhter, et al. Standards Track [Page 71] RFC 4271 BGP-4 January 2006

    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:
  1. sends a NOTIFICATION message with the Error Code Hold Timer

Expired,

  1. sets the ConnectRetryTimer to zero,
  1. releases all BGP resources,
  1. drops the TCP connection,
  1. increments the ConnectRetryCounter by 1,
  1. (optionally) performs peer oscillation damping if the

DampPeerOscillations attribute is set to TRUE, and

  1. changes its state to Idle.
    If the KeepaliveTimer_Expires event occurs (Event 11), the local
    system:
  1. sends a KEEPALIVE message, and
  1. 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.

Rekhter, et al. Standards Track [Page 72] RFC 4271 BGP-4 January 2006

    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:
  1. sends a NOTIFICATION with a Cease,
  1. sets the ConnectRetryTimer to zero,
  1. deletes all routes associated with this connection,
  1. releases all BGP resources,
  1. drops the TCP connection,
  1. increments the ConnectRetryCounter by 1,
  1. (optionally) performs peer oscillation damping if the

DampPeerOscillations is set to TRUE, and

  1. 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:
  1. sets the ConnectRetryTimer to zero,
  1. deletes all routes associated with this connection,
  1. releases all the BGP resources,
  1. drops the TCP connection,
  1. increments the ConnectRetryCounter by 1,
  1. changes its state to Idle.

Rekhter, et al. Standards Track [Page 73] RFC 4271 BGP-4 January 2006

    If the local system receives a KEEPALIVE message (Event 26), the
    local system:
  1. restarts its HoldTimer, if the negotiated HoldTime value is

non-zero, and

  1. remains in the Established state.
    If the local system receives an UPDATE message (Event 27), the
    local system:
  1. processes the message,
  1. restarts its HoldTimer, if the negotiated HoldTime value is

non-zero, and

  1. 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:
  1. sends a NOTIFICATION message with an Update error,
  1. sets the ConnectRetryTimer to zero,
  1. deletes all routes associated with this connection,
  1. releases all BGP resources,
  1. drops the TCP connection,
  1. increments the ConnectRetryCounter by 1,
  1. (optionally) performs peer oscillation damping if the

DampPeerOscillations attribute is set to TRUE, and

  1. changes its state to Idle.
    In response to any other event (Events 9, 12-13, 20-22), the local
    system:
  1. sends a NOTIFICATION message with the Error Code Finite State

Machine Error,

  1. deletes all routes associated with this connection,
  1. sets the ConnectRetryTimer to zero,

Rekhter, et al. Standards Track [Page 74] RFC 4271 BGP-4 January 2006

  1. releases all BGP resources,
  1. drops the TCP connection,
  1. increments the ConnectRetryCounter by 1,
  1. (optionally) performs peer oscillation damping if the

DampPeerOscillations attribute is set to TRUE, and

  1. 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.

Rekhter, et al. Standards Track [Page 75] RFC 4271 BGP-4 January 2006

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:
  1. selection of routes to be used locally by the speaker
  1. selection of routes to be advertised to other BGP peers
  1. 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.

Rekhter, et al. Standards Track [Page 76] RFC 4271 BGP-4 January 2006

    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.

Rekhter, et al. Standards Track [Page 77] RFC 4271 BGP-4 January 2006

 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.

Rekhter, et al. Standards Track [Page 78] RFC 4271 BGP-4 January 2006

 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

Rekhter, et al. Standards Track [Page 79] RFC 4271 BGP-4 January 2006

 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.

Rekhter, et al. Standards Track [Page 81] RFC 4271 BGP-4 January 2006

    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

Rekhter, et al. Standards Track [Page 82] RFC 4271 BGP-4 January 2006

 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:
  1. a set of destinations described only by the less specific route,

and

  1. 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

Rekhter, et al. Standards Track [Page 83] RFC 4271 BGP-4 January 2006

 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

Rekhter, et al. Standards Track [Page 87] RFC 4271 BGP-4 January 2006

       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:
  1. 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.
  1. 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).
  1. 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.
  1. No tuple of type AS_SET with the same value SHALL appear

more than once in the aggregated AS_PATH.

  1. 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:
  1. 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.
  1. 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.
  1. 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.
  1. 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:
  1. 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.
  1. 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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  1. X precedes Y in both AS_PATH attributes, or
  2. 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].

Rekhter, et al. Standards Track [Page 98] RFC 4271 BGP-4 January 2006

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

Rekhter, et al. Standards Track [Page 99] RFC 4271 BGP-4 January 2006

 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

Rekhter, et al. Standards Track [Page 100] RFC 4271 BGP-4 January 2006

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.

Rekhter, et al. Standards Track [Page 101] 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.

Rekhter, et al. Standards Track [Page 102] RFC 4271 BGP-4 January 2006

Editors' Addresses

 Yakov Rekhter
 Juniper Networks
 EMail: yakov@juniper.net
 Tony Li
 EMail: tony.li@tony.li
 Susan Hares
 NextHop Technologies, Inc.
 825 Victors Way
 Ann Arbor, MI 48108
 Phone: (734)222-1610
 EMail: skh@nexthop.com

Rekhter, et al. Standards Track [Page 103] 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,
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 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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Acknowledgement

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 Administrative Support Activity (IASA).

Rekhter, et al. Standards Track [Page 104]

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