File: rfc4271.html

package info (click to toggle)
doc-rfc 20230121-1
links: PTS, VCS
area: non-free
in suites: bookworm, forky, sid, trixie
size: 1,609,944 kB
file content (5821 lines) | stat: -rw-r--r-- 259,360 bytes
parent folder | download | duplicates (2)
<pre>Network Working Group                                    Y. Rekhter, Ed.
Request for Comments: 4271                                    T. Li, Ed.
Obsoletes: <a href="./rfc1771">1771</a>                                            S. Hares, Ed.
Category: Standards Track                                   January 2006


                  <span class="h1">A Border Gateway Protocol 4 (BGP-4)</span>

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 <a href="./rfc1771">RFC 1771</a>.










<span class="grey">Rekhter, et al.             Standards Track                     [Page 1]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-2" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


Table of Contents

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



<span class="grey">Rekhter, et al.             Standards Track                     [Page 2]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-3" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


                  8.2.1.5. FSM Actions that are Implementation
                           Dependent .................................<a href="#page-53">53</a>
           <a href="#section-8.2.2">8.2.2</a>. Finite State Machine ...............................<a href="#page-53">53</a>
   <a href="#section-9">9</a>. UPDATE Message Handling ........................................<a href="#page-75">75</a>
      <a href="#section-9.1">9.1</a>. Decision Process ..........................................<a href="#page-76">76</a>
           <a href="#section-9.1.1">9.1.1</a>. Phase 1: Calculation of Degree of Preference .......<a href="#page-77">77</a>
           <a href="#section-9.1.2">9.1.2</a>. Phase 2: Route Selection ...........................<a href="#page-77">77</a>
                  <a href="#section-9.1.2.1">9.1.2.1</a>. Route Resolvability Condition .............<a href="#page-79">79</a>
                  <a href="#section-9.1.2.2">9.1.2.2</a>. Breaking Ties (Phase 2) ...................<a href="#page-80">80</a>
           <a href="#section-9.1.3">9.1.3</a>. Phase 3: Route Dissemination .......................<a href="#page-82">82</a>
           <a href="#section-9.1.4">9.1.4</a>. Overlapping Routes .................................<a href="#page-83">83</a>
      <a href="#section-9.2">9.2</a>. Update-Send Process .......................................<a href="#page-84">84</a>
           <a href="#section-9.2.1">9.2.1</a>. Controlling Routing Traffic Overhead ...............<a href="#page-85">85</a>
                  <a href="#section-9.2.1.1">9.2.1.1</a>. Frequency of Route Advertisement ..........<a href="#page-85">85</a>
                  <a href="#section-9.2.1.2">9.2.1.2</a>. Frequency of Route Origination ............<a href="#page-85">85</a>
           <a href="#section-9.2.2">9.2.2</a>. Efficient Organization of Routing Information ......<a href="#page-86">86</a>
                  <a href="#section-9.2.2.1">9.2.2.1</a>. Information Reduction .....................<a href="#page-86">86</a>
                  <a href="#section-9.2.2.2">9.2.2.2</a>. Aggregating Routing Information ...........<a href="#page-87">87</a>
      <a href="#section-9.3">9.3</a>. Route Selection Criteria ..................................<a href="#page-89">89</a>
      <a href="#section-9.4">9.4</a>. Originating BGP routes ....................................<a href="#page-89">89</a>
   <a href="#section-10">10</a>. BGP Timers ....................................................<a href="#page-90">90</a>
   <a href="#appendix-A">Appendix A</a>.  Comparison with <a href="./rfc1771">RFC 1771</a> .............................<a href="#page-92">92</a>
   <a href="#appendix-B">Appendix B</a>.  Comparison with <a href="./rfc1267">RFC 1267</a> .............................<a href="#page-93">93</a>
   <a href="#appendix-C">Appendix C</a>.  Comparison with <a href="./rfc1163">RFC 1163</a> .............................<a href="#page-93">93</a>
   <a href="#appendix-D">Appendix D</a>.  Comparison with <a href="./rfc1105">RFC 1105</a> .............................<a href="#page-94">94</a>
   <a href="#appendix-E">Appendix E</a>.  TCP Options that May Be Used with BGP ................<a href="#page-94">94</a>
   <a href="#appendix-F">Appendix F</a>.  Implementation Recommendations .......................<a href="#page-95">95</a>
                <a href="#appendix-F.1">Appendix F.1</a>.  Multiple Networks Per Message .........<a href="#page-95">95</a>
                <a href="#appendix-F.2">Appendix F.2</a>.  Reducing Route Flapping ...............<a href="#page-96">96</a>
                <a href="#appendix-F.3">Appendix F.3</a>.  Path Attribute Ordering ...............<a href="#page-96">96</a>
                <a href="#appendix-F.4">Appendix F.4</a>.  AS_SET Sorting ........................<a href="#page-96">96</a>
                <a href="#appendix-F.5">Appendix F.5</a>.  Control Over Version Negotiation ......<a href="#page-96">96</a>
                <a href="#appendix-F.6">Appendix F.6</a>.  Complex AS_PATH Aggregation ...........<a href="#page-96">96</a>
   Security Considerations ...........................................<a href="#page-97">97</a>
   IANA Considerations ...............................................<a href="#page-99">99</a>
   Normative References .............................................<a href="#page-101">101</a>
   Informative References ...........................................<a href="#page-101">101</a>














<span class="grey">Rekhter, et al.             Standards Track                     [Page 3]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-4" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Introduction</span>

   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, <a href="./rfc1519">RFC1519</a>].  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.

<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a>.  Definition of Commonly Used Terms</span>

   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



<span class="grey">Rekhter, et al.             Standards Track                     [Page 4]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-5" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


      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



<span class="grey">Rekhter, et al.             Standards Track                     [Page 5]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-6" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


      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.

<span class="h3"><a class="selflink" id="section-1.2" href="#section-1.2">1.2</a>.  Specification of Requirements</span>

   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 <a href="./rfc2119">RFC 2119</a> [<a href="./rfc2119" title="&quot;Key words for use in RFCs to Indicate Requirement Levels&quot;">RFC2119</a>].

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Acknowledgements</span>

   This document was originally published as [<a href="./rfc1267" title="&quot;Border Gateway Protocol 3 (BGP-3)&quot;">RFC1267</a>] 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
   [<a href="#ref-IS10747" title="&quot;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&quot;">IS10747</a>], 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.




<span class="grey">Rekhter, et al.             Standards Track                     [Page 6]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-7" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


   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.

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  Summary of Operation</span>

   The Border Gateway Protocol (BGP) is an inter-Autonomous System
   routing protocol.  It is built on experience gained with EGP (as
   defined in [<a href="./rfc904" title="&quot;Exterior Gateway Protocol formal specification&quot;">RFC904</a>]) and EGP usage in the NSFNET Backbone (as
   described in [<a href="./rfc1092" title="&quot;EGP and policy based routing in the new NSFNET backbone&quot;">RFC1092</a>] and [<a href="./rfc1093" title="&quot;NSFNET routing architecture&quot;">RFC1093</a>]).  For more BGP-related
   information, see [<a href="./rfc1772" title="&quot;Application of the Border Gateway Protocol in the Internet&quot;">RFC1772</a>], [<a href="./rfc1930" title="&quot;Guidelines for creation, selection, and registration of an Autonomous System (AS)&quot;">RFC1930</a>], [<a href="./rfc1997" title="&quot;BGP Communities Attribute&quot;">RFC1997</a>], and [<a href="./rfc2858" title="&quot;Multiprotocol Extensions for BGP-4&quot;">RFC2858</a>].

   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 [<a href="./rfc791" title="&quot;Internet Protocol&quot;">RFC791</a>].

   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



<span class="grey">Rekhter, et al.             Standards Track                     [Page 7]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-8" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


   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, <a href="./rfc1519">RFC1519</a>].  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 [<a href="./rfc793" title="&quot;Transmission Control Protocol&quot;">RFC793</a>] 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 [<a href="./rfc2918" title="&quot;Route Refresh Capability for BGP-4&quot;">RFC2918</a>].





<span class="grey">Rekhter, et al.             Standards Track                     [Page 8]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-9" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


   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.

<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>.  Routes: Advertisement and Storage</span>

   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
   <a href="#section-3.2">Section 3.2</a>.

   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.






<span class="grey">Rekhter, et al.             Standards Track                     [Page 9]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-10" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


   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.

<span class="h3"><a class="selflink" id="section-3.2" href="#section-3.2">3.2</a>.  Routing Information Base</span>

   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





<span class="grey">Rekhter, et al.             Standards Track                    [Page 10]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-11" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


   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 <a href="#section-5.1.3">Section 5.1.3</a>).

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  Message Formats</span>

   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.

















<span class="grey">Rekhter, et al.             Standards Track                    [Page 11]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-12" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>.  Message Header Format</span>

   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

         [<a id="ref-RFC2918">RFC2918</a>] defines one more type code.



<span class="grey">Rekhter, et al.             Standards Track                    [Page 12]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-13" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>.  OPEN Message Format</span>

   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





<span class="grey">Rekhter, et al.             Standards Track                    [Page 13]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-14" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


         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 &lt;Parameter Type, Parameter
         Length, Parameter Value&gt; 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.

         [<a id="ref-RFC3392">RFC3392</a>] defines the Capabilities Optional Parameter.

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

<span class="h3"><a class="selflink" id="section-4.3" href="#section-4.3">4.3</a>.  UPDATE Message Format</span>

   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



<span class="grey">Rekhter, et al.             Standards Track                    [Page 14]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-15" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


   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 &lt;length, prefix&gt;, whose fields are described below:

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





<span class="grey">Rekhter, et al.             Standards Track                    [Page 15]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-16" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


         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
         &lt;attribute type, attribute length, attribute value&gt; 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).



<span class="grey">Rekhter, et al.             Standards Track                    [Page 16]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-17" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


         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 <a href="#section-5">Section 5</a> 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 <a href="#section-5">Section</a>
         <a href="#section-5">5</a>.

         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.

















<span class="grey">Rekhter, et al.             Standards Track                    [Page 17]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-18" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


         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 [<a href="./rfc904" title="&quot;Exterior Gateway Protocol formal specification&quot;">RFC904</a>]

               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 &lt;path segment type, path segment
            length, path segment value&gt;.

            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.



<span class="grey">Rekhter, et al.             Standards Track                    [Page 18]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-19" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


            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.




<span class="grey">Rekhter, et al.             Standards Track                    [Page 19]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-20" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


      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 &lt;length, prefix&gt;, 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).




<span class="grey">Rekhter, et al.             Standards Track                    [Page 20]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-21" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


   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.

<span class="h3"><a class="selflink" id="section-4.4" href="#section-4.4">4.4</a>.  KEEPALIVE Message Format</span>

   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.

<span class="h3"><a class="selflink" id="section-4.5" href="#section-4.5">4.5</a>.  NOTIFICATION Message Format</span>

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




<span class="grey">Rekhter, et al.             Standards Track                    [Page 21]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-22" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


   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             <a href="#section-6.1">Section 6.1</a>

              2         OPEN Message Error               <a href="#section-6.2">Section 6.2</a>

              3         UPDATE Message Error             <a href="#section-6.3">Section 6.3</a>

              4         Hold Timer Expired               <a href="#section-6.5">Section 6.5</a>

              5         Finite State Machine Error       <a href="#section-6.6">Section 6.6</a>

              6         Cease                            <a href="#section-6.7">Section 6.7</a>

      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.










<span class="grey">Rekhter, et al.             Standards Track                    [Page 22]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-23" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


      OPEN Message Error subcodes:

               1 - Unsupported Version Number.
               2 - Bad Peer AS.
               3 - Bad BGP Identifier.
               4 - Unsupported Optional Parameter.
               5 - [Deprecated - see <a href="#appendix-A">Appendix A</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 <a href="#appendix-A">Appendix A</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 <a href="#section-6">Section 6</a> 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).

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  Path Attributes</span>

   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.



<span class="grey">Rekhter, et al.             Standards Track                    [Page 23]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-24" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


   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.







<span class="grey">Rekhter, et al.             Standards Track                    [Page 24]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-25" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


   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 <a href="#section-5.1.5">Section 5.1.5</a>       required
         ATOMIC_AGGREGATE   see <a href="#section-5.1.6">Section 5.1.6</a> and 9.1.4
         AGGREGATOR         discretionary           discretionary

<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>.  Path Attribute Usage</span>

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

<span class="h4"><a class="selflink" id="section-5.1.1" href="#section-5.1.1">5.1.1</a>.  ORIGIN</span>

   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.

<span class="h4"><a class="selflink" id="section-5.1.2" href="#section-5.1.2">5.1.2</a>.  AS_PATH</span>

   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:






<span class="grey">Rekhter, et al.             Standards Track                    [Page 25]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-26" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


         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.

<span class="h4"><a class="selflink" id="section-5.1.3" href="#section-5.1.3">5.1.3</a>.  NEXT_HOP</span>

   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



<span class="grey">Rekhter, et al.             Standards Track                    [Page 26]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-27" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


         locally-originated route to an internal peer, the BGP speaker
         SHOULD use the interface address of the router through which
         the announced network is reachable for the speaker as the
         NEXT_HOP.  If the route is directly connected to the speaker,
         or if the interface address of the router through which the
         announced network is reachable for the speaker is the internal
         peer's address, then the BGP speaker SHOULD use its own IP
         address for the NEXT_HOP attribute (the address of the
         interface that is used to reach the peer).

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

         - If the route being announced was learned from an internal
           peer or is locally originated, the BGP speaker can use an
           interface address of the internal peer router (or the
           internal router) through which the announced network is
           reachable for the speaker for the NEXT_HOP attribute,
           provided that peer X shares a common subnet with this
           address.  This is a form of "third party" NEXT_HOP attribute.

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

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

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

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

         - The speaker MAY be configured to propagate the NEXT_HOP
           attribute.  In this case, when advertising a route that the
           speaker learned from one of its peers, the NEXT_HOP attribute
           of the advertised route is exactly the same as the NEXT_HOP



<span class="grey">Rekhter, et al.             Standards Track                    [Page 27]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-28" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


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

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

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

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

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

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

<span class="h4"><a class="selflink" id="section-5.1.4" href="#section-5.1.4">5.1.4</a>.  MULTI_EXIT_DISC</span>

   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



<span class="grey">Rekhter, et al.             Standards Track                    [Page 28]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-29" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


   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
   <a href="#section-9.1.2.2">Section 9.1.2.2</a> for necessary restrictions on this.

<span class="h4"><a class="selflink" id="section-5.1.5" href="#section-5.1.5">5.1.5</a>.  LOCAL_PREF</span>

   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 <a href="#section-9.1.1">Section 9.1.1</a>).

   A BGP speaker MUST NOT include this attribute in UPDATE messages it
   sends to external peers, except in the case of BGP Confederations
   [<a href="./rfc3065" title="&quot;Autonomous System Confederations for BGP&quot;">RFC3065</a>].  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
   [<a href="./rfc3065" title="&quot;Autonomous System Confederations for BGP&quot;">RFC3065</a>].

<span class="h4"><a class="selflink" id="section-5.1.6" href="#section-5.1.6">5.1.6</a>.  ATOMIC_AGGREGATE</span>

   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.





<span class="grey">Rekhter, et al.             Standards Track                    [Page 29]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-30" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


   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.

<span class="h4"><a class="selflink" id="section-5.1.7" href="#section-5.1.7">5.1.7</a>.  AGGREGATOR</span>

   AGGREGATOR is an optional transitive attribute, which MAY be included
   in updates that are formed by aggregation (see <a href="#section-9.2.2.2">Section 9.2.2.2</a>).  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.

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  BGP Error Handling.</span>

   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.





<span class="grey">Rekhter, et al.             Standards Track                    [Page 30]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-31" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


<span class="h3"><a class="selflink" id="section-6.1" href="#section-6.1">6.1</a>.  Message Header Error Handling</span>

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

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

   If at least one of the following is true:

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

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

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

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

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

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

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

<span class="h3"><a class="selflink" id="section-6.2" href="#section-6.2">6.2</a>.  OPEN Message Error Handling</span>

   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



<span class="grey">Rekhter, et al.             Standards Track                    [Page 31]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-32" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


   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).

<span class="h3"><a class="selflink" id="section-6.3" href="#section-6.3">6.3</a>.  UPDATE Message Error Handling</span>

   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).





<span class="grey">Rekhter, et al.             Standards Track                    [Page 32]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-33" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


   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.






<span class="grey">Rekhter, et al.             Standards Track                    [Page 33]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-34" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


   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.

<span class="h3"><a class="selflink" id="section-6.4" href="#section-6.4">6.4</a>.  NOTIFICATION Message Error Handling</span>

   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.

<span class="h3"><a class="selflink" id="section-6.5" href="#section-6.5">6.5</a>.  Hold Timer Expired Error Handling</span>

   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.







<span class="grey">Rekhter, et al.             Standards Track                    [Page 34]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-35" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


<span class="h3"><a class="selflink" id="section-6.6" href="#section-6.6">6.6</a>.  Finite State Machine Error Handling</span>

   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.

<span class="h3"><a class="selflink" id="section-6.7" href="#section-6.7">6.7</a>.  Cease</span>

   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.

<span class="h3"><a class="selflink" id="section-6.8" href="#section-6.8">6.8</a>.  BGP Connection Collision Detection</span>

   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



<span class="grey">Rekhter, et al.             Standards Track                    [Page 35]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-36" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


   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.

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  BGP Version Negotiation</span>

   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.




<span class="grey">Rekhter, et al.             Standards Track                    [Page 36]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-37" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>.  BGP Finite State Machine (FSM)</span>

   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 (<a href="#section-8.1">Section 8.1</a>)
      2) Description of the FSM (<a href="#section-8.2">Section 8.2</a>)

   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 <a href="#section-10">Section</a>
   <a href="#section-10">10</a>.  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



<span class="grey">Rekhter, et al.             Standards Track                    [Page 37]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-38" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


   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 <a href="#section-8.1.1">Section 8.1.1</a> for an explanation of the interaction
   between these optional attributes and the events signaled to the
   state machine.  <a href="#section-8.2.1.3">Section 8.2.1.3</a> also provides a short overview of the
   different types of optional attributes (flags or timers).

<span class="h3"><a class="selflink" id="section-8.1" href="#section-8.1">8.1</a>.  Events for the BGP FSM</span>

<span class="h4"><a class="selflink" id="section-8.1.1" href="#section-8.1.1">8.1.1</a>.  Optional Events Linked to Optional Session Attributes</span>

   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.



<span class="grey">Rekhter, et al.             Standards Track                    [Page 38]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-39" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


                      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.



<span class="grey">Rekhter, et al.             Standards Track                    [Page 39]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-40" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


         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
                      <a href="#section-8.1.3">Section 8.1.3</a>.

         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 [<a href="./rfc4272" title="&quot;BGP Security Vulnerabilities Analysis&quot;">RFC4272</a>] for details.

         Value:       True or False

      Group 3: TCP processing

         Optional Session Attributes: PassiveTcpEstablishment,
                                      TrackTcpState

         Option 1:    PassiveTcpEstablishment






<span class="grey">Rekhter, et al.             Standards Track                    [Page 40]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-41" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


         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




<span class="grey">Rekhter, et al.             Standards Track                    [Page 41]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-42" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


                      connection.  The DelayOpenTimer_Expires event
                      (Event 12) is described in <a href="#section-8.1.3">Section 8.1.3</a>.

         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 <a href="#section-6.8">Section 6.8</a>)
                      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.

<span class="h4"><a class="selflink" id="section-8.1.2" href="#section-8.1.2">8.1.2</a>.  Administrative Events</span>

   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.




<span class="grey">Rekhter, et al.             Standards Track                    [Page 42]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-43" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


   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.  <a href="#section-8.2.1.5">Section 8.2.1.5</a> 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






<span class="grey">Rekhter, et al.             Standards Track                    [Page 43]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-44" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


         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.





<span class="grey">Rekhter, et al.             Standards Track                    [Page 44]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-45" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


      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.



<span class="grey">Rekhter, et al.             Standards Track                    [Page 45]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-46" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


         Status:     Optional, depending on local system

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

<span class="h4"><a class="selflink" id="section-8.1.3" href="#section-8.1.3">8.1.3</a>.  Timer Events</span>

      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.





<span class="grey">Rekhter, et al.             Standards Track                    [Page 46]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-47" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


                     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.

<span class="h4"><a class="selflink" id="section-8.1.4" href="#section-8.1.4">8.1.4</a>.  TCP Connection-Based Events</span>

      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.



<span class="grey">Rekhter, et al.             Standards Track                    [Page 47]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-48" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


                     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




<span class="grey">Rekhter, et al.             Standards Track                    [Page 48]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-49" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


<span class="h4"><a class="selflink" id="section-8.1.5" href="#section-8.1.5">8.1.5</a>.  BGP Message-Based Events</span>

      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



<span class="grey">Rekhter, et al.             Standards Track                    [Page 49]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-50" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


                     connection has been selected to be disconnected.
                     See <a href="#section-6.8">Section 6.8</a> 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
                     <a href="#section-6.8">Section 6.8</a>.  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







<span class="grey">Rekhter, et al.             Standards Track                    [Page 50]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-51" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


      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

<span class="h3"><a class="selflink" id="section-8.2" href="#section-8.2">8.2</a>.  Description of FSM</span>

<span class="h4"><a class="selflink" id="section-8.2.1" href="#section-8.2.1">8.2.1</a>.  FSM Definition</span>

   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 <a href="#section-8.2.1.1">Section 8.2.1.1</a> 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 <a href="#section-6.8">Section 6.8</a>).

   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.





<span class="grey">Rekhter, et al.             Standards Track                    [Page 51]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-52" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


<span class="h5"><a class="selflink" id="section-8.2.1.1" href="#section-8.2.1.1">8.2.1.1</a>.  Terms "active" and "passive"</span>

   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.

<span class="h5"><a class="selflink" id="section-8.2.1.2" href="#section-8.2.1.2">8.2.1.2</a>.  FSM and Collision Detection</span>

   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 <a href="#section-6.8">Section 6.8</a>), the FSM for the
   connection that is closed SHOULD be disposed.

<span class="h5"><a class="selflink" id="section-8.2.1.3" href="#section-8.2.1.3">8.2.1.3</a>.  FSM and Optional Session Attributes</span>

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

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

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

   The two optional timers show this format:

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





<span class="grey">Rekhter, et al.             Standards Track                    [Page 52]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-53" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


   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.

<span class="h5"><a class="selflink" id="section-8.2.1.4" href="#section-8.2.1.4">8.2.1.4</a>.  FSM Event Numbers</span>

   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.

<span class="h5"><a class="selflink" id="section-8.2.1.5" href="#section-8.2.1.5">8.2.1.5</a>.  FSM Actions that are Implementation Dependent</span>

   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.

<span class="h4"><a class="selflink" id="section-8.2.2" href="#section-8.2.2">8.2.2</a>.  Finite State Machine</span>

   Idle state:

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

      In this state, BGP FSM refuses all incoming BGP connections for
      this peer.  No resources are allocated to the peer.  In response
      to a ManualStart event (Event 1) or an AutomaticStart event (Event
      3), the local system:

        - initializes all BGP resources for the peer connection,

        - sets ConnectRetryCounter to zero,

        - starts the ConnectRetryTimer with the initial value,

        - initiates a TCP connection to the other BGP peer,

        - listens for a connection that may be initiated by the remote
          BGP peer, and

        - changes its state to Connect.

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




<span class="grey">Rekhter, et al.             Standards Track                    [Page 53]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-54" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


      In response to a ManualStart_with_PassiveTcpEstablishment event
      (Event 4) or AutomaticStart_with_PassiveTcpEstablishment event
      (Event 5), the local system:

        - initializes all BGP resources,

        - sets the ConnectRetryCounter to zero,

        - starts the ConnectRetryTimer with the initial value,

        - listens for a connection that may be initiated by the remote
          peer, and

        - changes its state to Active.

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

      If the DampPeerOscillations attribute is set to TRUE, the
      following three additional events may occur within the Idle state:

        - AutomaticStart_with_DampPeerOscillations (Event 6),

        - AutomaticStart_with_DampPeerOscillations_and_
          PassiveTcpEstablishment (Event 7),

        - IdleHoldTimer_Expires (Event 13).

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

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

   Connect State:

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

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

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

        - drops the TCP connection,

        - releases all BGP resources,




<span class="grey">Rekhter, et al.             Standards Track                    [Page 54]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-55" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


        - sets ConnectRetryCounter to zero,

        - stops the ConnectRetryTimer and sets ConnectRetryTimer to
          zero, and

        - changes its state to Idle.

      In response to the ConnectRetryTimer_Expires event (Event 9), the
      local system:

        - drops the TCP connection,

        - restarts the ConnectRetryTimer,

        - stops the DelayOpenTimer and resets the timer to zero,

        - initiates a TCP connection to the other BGP peer,

        - continues to listen for a connection that may be initiated by
          the remote BGP peer, and

        - stays in the Connect state.

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

        - sends an OPEN message to its peer,

        - sets the HoldTimer to a large value, and

        - changes its state to OpenSent.

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

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

      If the TCP connection succeeds (Event 16 or Event 17), the local
      system checks the DelayOpen attribute prior to processing.  If the
      DelayOpen attribute is set to TRUE, the local system:

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

        - sets the DelayOpenTimer to the initial value, and



<span class="grey">Rekhter, et al.             Standards Track                    [Page 55]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-56" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


        - stays in the Connect state.

      If the DelayOpen attribute is set to FALSE, the local system:

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

        - completes BGP initialization

        - sends an OPEN message to its peer,

        - sets the HoldTimer to a large value, and

        - changes its state to OpenSent.

      A HoldTimer value of 4 minutes is suggested.

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

        - restarts the ConnectRetryTimer with the initial value,

        - stops the DelayOpenTimer and resets its value to zero,

        - continues to listen for a connection that may be initiated by
          the remote BGP peer, and

        - changes its state to Active.

      If the DelayOpenTimer is not running, the local system:

        - stops the ConnectRetryTimer to zero,

        - drops the TCP connection,

        - releases all BGP resources, and

        - changes its state to Idle.

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

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

        - completes the BGP initialization,




<span class="grey">Rekhter, et al.             Standards Track                    [Page 56]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-57" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


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

        - sends an OPEN message,

        - sends a KEEPALIVE message,

        - if the HoldTimer initial value is non-zero,

            - starts the KeepaliveTimer with the initial value and

            - resets the HoldTimer to the negotiated value,

          else, if the HoldTimer initial value is zero,

            - resets the KeepaliveTimer and

            - resets the HoldTimer value to zero,

        - and changes its state to OpenConfirm.

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

      If BGP message header checking (Event 21) or OPEN message checking
      detects an error (Event 22) (see <a href="#section-6.2">Section 6.2</a>), the local system:

        - (optionally) If the SendNOTIFICATIONwithoutOPEN attribute is
          set to TRUE, then the local system first sends a NOTIFICATION
          message with the appropriate error code, and then

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

        - releases all BGP resources,

        - drops the TCP connection,

        - increments the ConnectRetryCounter by 1,

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

        - changes its state to Idle.

      If a NOTIFICATION message is received with a version error (Event
      24), the local system checks the DelayOpenTimer.  If the
      DelayOpenTimer is running, the local system:



<span class="grey">Rekhter, et al.             Standards Track                    [Page 57]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-58" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


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

        - stops and resets the DelayOpenTimer (sets to zero),

        - releases all BGP resources,

        - drops the TCP connection, and

        - changes its state to Idle.

      If the DelayOpenTimer is not running, the local system:

        - stops the ConnectRetryTimer and sets the ConnectRetryTimer to
          zero,

        - releases all BGP resources,

        - drops the TCP connection,

        - increments the ConnectRetryCounter by 1,

        - performs peer oscillation damping if the DampPeerOscillations
          attribute is set to True, and

        - changes its state to Idle.

      In response to any other events (Events 8, 10-11, 13, 19, 23,
      25-28), the local system:

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

        - if the DelayOpenTimer is running, stops and resets the
          DelayOpenTimer (sets to zero),

        - releases all BGP resources,

        - drops the TCP connection,

        - increments the ConnectRetryCounter by 1,

        - performs peer oscillation damping if the DampPeerOscillations
          attribute is set to True, and

        - changes its state to Idle.





<span class="grey">Rekhter, et al.             Standards Track                    [Page 58]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-59" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


   Active State:

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

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

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

        - If the DelayOpenTimer is running and the
          SendNOTIFICATIONwithoutOPEN session attribute is set, the
          local system sends a NOTIFICATION with a Cease,

        - releases all BGP resources including stopping the
          DelayOpenTimer

        - drops the TCP connection,

        - sets ConnectRetryCounter to zero,

        - stops the ConnectRetryTimer and sets the ConnectRetryTimer to
          zero, and

        - changes its state to Idle.

      In response to a ConnectRetryTimer_Expires event (Event 9), the
      local system:

        - restarts the ConnectRetryTimer (with initial value),

        - initiates a TCP connection to the other BGP peer,

        - continues to listen for a TCP connection that may be initiated
          by a remote BGP peer, and

        - changes its state to Connect.

      If the local system receives a DelayOpenTimer_Expires event (Event
      12), the local system:

        - sets the ConnectRetryTimer to zero,

        - stops and clears the DelayOpenTimer (set to zero),

        - completes the BGP initialization,

        - sends the OPEN message to its remote peer,




<span class="grey">Rekhter, et al.             Standards Track                    [Page 59]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-60" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


        - sets its hold timer to a large value, and

        - changes its state to OpenSent.

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

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

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

      In response to the success of a TCP connection (Event 16 or Event
      17), the local system checks the DelayOpen optional attribute
      prior to processing.

        If the DelayOpen attribute is set to TRUE, the local system:

          - stops the ConnectRetryTimer and sets the ConnectRetryTimer
            to zero,

          - sets the DelayOpenTimer to the initial value
            (DelayOpenTime), and

          - stays in the Active state.

        If the DelayOpen attribute is set to FALSE, the local system:

          - sets the ConnectRetryTimer to zero,

          - completes the BGP initialization,

          - sends the OPEN message to its peer,

          - sets its HoldTimer to a large value, and

          - changes its state to OpenSent.

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

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

        - restarts the ConnectRetryTimer (with the initial value),



<span class="grey">Rekhter, et al.             Standards Track                    [Page 60]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-61" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


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

        - releases all BGP resource,

        - increments the ConnectRetryCounter by 1,

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

        - changes its state to Idle.

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

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

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

        - completes the BGP initialization,

        - sends an OPEN message,

        - sends a KEEPALIVE message,

        - if the HoldTimer value is non-zero,

            - starts the KeepaliveTimer to initial value,

            - resets the HoldTimer to the negotiated value,

          else if the HoldTimer is zero

            - resets the KeepaliveTimer (set to zero),

            - resets the HoldTimer to zero, and

        - changes its state to OpenConfirm.

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

      If BGP message header checking (Event 21) or OPEN message checking
      detects an error (Event 22) (see <a href="#section-6.2">Section 6.2</a>), the local system:






<span class="grey">Rekhter, et al.             Standards Track                    [Page 61]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-62" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


        - (optionally) sends a NOTIFICATION message with the appropriate
          error code if the SendNOTIFICATIONwithoutOPEN attribute is set
          to TRUE,

        - sets the ConnectRetryTimer to zero,

        - releases all BGP resources,

        - drops the TCP connection,

        - increments the ConnectRetryCounter by 1,

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

        - changes its state to Idle.

      If a NOTIFICATION message is received with a version error (Event
      24), the local system checks the DelayOpenTimer.  If the
      DelayOpenTimer is running, the local system:

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

        - stops and resets the DelayOpenTimer (sets to zero),

        - releases all BGP resources,

        - drops the TCP connection, and

        - changes its state to Idle.

      If the DelayOpenTimer is not running, the local system:

        - sets the ConnectRetryTimer to zero,

        - releases all BGP resources,

        - drops the TCP connection,

        - increments the ConnectRetryCounter by 1,

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

        - changes its state to Idle.





<span class="grey">Rekhter, et al.             Standards Track                    [Page 62]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-63" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


      In response to any other event (Events 8, 10-11, 13, 19, 23,
      25-28), the local system:

        - sets the ConnectRetryTimer to zero,

        - releases all BGP resources,

        - drops the TCP connection,

        - increments the ConnectRetryCounter by one,

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

        - changes its state to Idle.

   OpenSent:

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

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

      If a ManualStop event (Event 2) is issued in the OpenSent state,
      the local system:

        - sends the NOTIFICATION with a Cease,

        - sets the ConnectRetryTimer to zero,

        - releases all BGP resources,

        - drops the TCP connection,

        - sets the ConnectRetryCounter to zero, and

        - changes its state to Idle.

      If an AutomaticStop event (Event 8) is issued in the OpenSent
      state, the local system:

        - sends the NOTIFICATION with a Cease,

        - sets the ConnectRetryTimer to zero,

        - releases all the BGP resources,

        - drops the TCP connection,



<span class="grey">Rekhter, et al.             Standards Track                    [Page 63]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-64" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


        - increments the ConnectRetryCounter by 1,

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

        - changes its state to Idle.

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

        - sends a NOTIFICATION message with the error code Hold Timer
          Expired,

        - sets the ConnectRetryTimer to zero,

        - releases all BGP resources,

        - drops the TCP connection,

        - increments the ConnectRetryCounter,

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

        - changes its state to Idle.

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

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

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

        - closes the BGP connection,

        - restarts the ConnectRetryTimer,

        - continues to listen for a connection that may be initiated by
          the remote BGP peer, and

        - changes its state to Active.






<span class="grey">Rekhter, et al.             Standards Track                    [Page 64]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-65" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


      When an OPEN message is received, all fields are checked for
      correctness.  If there are no errors in the OPEN message (Event
      19), the local system:

        - resets the DelayOpenTimer to zero,

        - sets the BGP ConnectRetryTimer to zero,

        - sends a KEEPALIVE message, and

        - sets a KeepaliveTimer (via the text below)

        - sets the HoldTimer according to the negotiated value (see
          <a href="#section-4.2">Section 4.2</a>),

        - 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 <a href="#section-6.2">Section 6.2</a>), the local
      system:

        - sends a NOTIFICATION message with the appropriate error code,

        - sets the ConnectRetryTimer to zero,

        - releases all BGP resources,

        - drops the TCP connection,

        - increments the ConnectRetryCounter by 1,

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

        - changes its state to Idle.

      Collision detection mechanisms (<a href="#section-6.8">Section 6.8</a>) need to be applied
      when a valid BGP OPEN message is received (Event 19 or Event 20).
      Please refer to <a href="#section-6.8">Section 6.8</a> for the details of the comparison.  A





<span class="grey">Rekhter, et al.             Standards Track                    [Page 65]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-66" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


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

      If a connection in the OpenSent state is determined to be the
      connection that must be closed, an OpenCollisionDump (Event 23) is
      signaled to the state machine.  If such an event is received in
      the OpenSent state, the local system:

        - sends a NOTIFICATION with a Cease,

        - sets the ConnectRetryTimer to zero,

        - releases all BGP resources,

        - drops the TCP connection,

        - increments the ConnectRetryCounter by 1,

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

        - changes its state to Idle.

      If a NOTIFICATION message is received with a version error (Event
      24), the local system:

        - sets the ConnectRetryTimer to zero,

        - releases all BGP resources,

        - drops the TCP connection, and

        - changes its state to Idle.

      In response to any other event (Events 9, 11-13, 20, 25-28), the
      local system:

        - sends the NOTIFICATION with the Error Code Finite State
          Machine Error,

        - sets the ConnectRetryTimer to zero,

        - releases all BGP resources,

        - drops the TCP connection,

        - increments the ConnectRetryCounter by 1,



<span class="grey">Rekhter, et al.             Standards Track                    [Page 66]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-67" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


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

        - changes its state to Idle.

   OpenConfirm State:

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

      Any start event (Events 1, 3-7) is ignored in the OpenConfirm
      state.

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

        - sends the NOTIFICATION message with a Cease,

        - releases all BGP resources,

        - drops the TCP connection,

        - sets the ConnectRetryCounter to zero,

        - sets the ConnectRetryTimer to zero, and

        - changes its state to Idle.

      In response to the AutomaticStop event initiated by the system
      (Event 8), the local system:

        - sends the NOTIFICATION message with a Cease,

        - sets the ConnectRetryTimer to zero,

        - releases all BGP resources,

        - drops the TCP connection,

        - increments the ConnectRetryCounter by 1,

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

        - changes its state to Idle.

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




<span class="grey">Rekhter, et al.             Standards Track                    [Page 67]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-68" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


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

        - sets the ConnectRetryTimer to zero,

        - releases all BGP resources,

        - drops the TCP connection,

        - increments the ConnectRetryCounter by 1,

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

        - changes its state to Idle.

      If the local system receives a KeepaliveTimer_Expires event (Event
      11), the local system:

        - sends a KEEPALIVE message,

        - restarts the KeepaliveTimer, and

        - remains in the OpenConfirmed state.

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

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

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

        - sets the ConnectRetryTimer to zero,

        - releases all BGP resources,

        - drops the TCP connection,

        - increments the ConnectRetryCounter by 1,

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




<span class="grey">Rekhter, et al.             Standards Track                    [Page 68]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-69" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


        - changes its state to Idle.

      If the local system receives a NOTIFICATION message with a version
      error (NotifMsgVerErr (Event 24)), the local system:

        - sets the ConnectRetryTimer to zero,

        - releases all BGP resources,

        - drops the TCP connection, and

        - changes its state to Idle.

      If the local system receives a valid OPEN message (BGPOpen (Event
      19)), the collision detect function is processed per <a href="#section-6.8">Section 6.8</a>.
      If this connection is to be dropped due to connection collision,
      the local system:

        - sends a NOTIFICATION with a Cease,

        - sets the ConnectRetryTimer to zero,

        - releases all BGP resources,

        - drops the TCP connection (send TCP FIN),

        - increments the ConnectRetryCounter by 1,

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

        - changes its state to Idle.

      If an OPEN message is received, all fields are checked for
      correctness.  If the BGP message header checking (BGPHeaderErr
      (Event 21)) or OPEN message checking detects an error (see <a href="#section-6.2">Section</a>
      <a href="#section-6.2">6.2</a>) (BGPOpenMsgErr (Event 22)), the local system:

        - sends a NOTIFICATION message with the appropriate error code,

        - sets the ConnectRetryTimer to zero,

        - releases all BGP resources,

        - drops the TCP connection,

        - increments the ConnectRetryCounter by 1,




<span class="grey">Rekhter, et al.             Standards Track                    [Page 69]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-70" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


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

        - changes its state to Idle.

      If, during the processing of another OPEN message, the BGP
      implementation determines, by a means outside the scope of this
      document, that a connection collision has occurred and this
      connection is to be closed, the local system will issue an
      OpenCollisionDump event (Event 23).  When the local system
      receives an OpenCollisionDump event (Event 23), the local system:

        - sends a NOTIFICATION with a Cease,

        - sets the ConnectRetryTimer to zero,

        - releases all BGP resources

        - drops the TCP connection,

        - increments the ConnectRetryCounter by 1,

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

        - changes its state to Idle.

      If the local system receives a KEEPALIVE message (KeepAliveMsg
      (Event 26)), the local system:

        - restarts the HoldTimer and

        - changes its state to Established.

      In response to any other event (Events 9, 12-13, 20, 27-28), the
      local system:

        - sends a NOTIFICATION with a code of Finite State Machine
          Error,

        - sets the ConnectRetryTimer to zero,

        - releases all BGP resources,

        - drops the TCP connection,

        - increments the ConnectRetryCounter by 1,




<span class="grey">Rekhter, et al.             Standards Track                    [Page 70]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-71" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


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

        - changes its state to Idle.

   Established State:

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

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

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

        - sends the NOTIFICATION message with a Cease,

        - sets the ConnectRetryTimer to zero,

        - deletes all routes associated with this connection,

        - releases BGP resources,

        - drops the TCP connection,

        - sets the ConnectRetryCounter to zero, and

         - changes its state to Idle.

      In response to an AutomaticStop event (Event 8), the local system:

        - sends a NOTIFICATION with a Cease,

        - sets the ConnectRetryTimer to zero

        - deletes all routes associated with this connection,

        - releases all BGP resources,

        - drops the TCP connection,

        - increments the ConnectRetryCounter by 1,

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

        - changes its state to Idle.



<span class="grey">Rekhter, et al.             Standards Track                    [Page 71]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-72" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


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

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

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

        - sets the ConnectRetryTimer to zero,

        - releases all BGP resources,

        - drops the TCP connection,

        - increments the ConnectRetryCounter by 1,

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

        - changes its state to Idle.

      If the KeepaliveTimer_Expires event occurs (Event 11), the local
      system:

        - sends a KEEPALIVE message, and

        - restarts its KeepaliveTimer, unless the negotiated HoldTime
          value is zero.

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

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

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

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






<span class="grey">Rekhter, et al.             Standards Track                    [Page 72]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-73" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


      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 (<a href="#section-6.8">Section</a>
      <a href="#section-6.8">6.8</a>) with any other connection.  If the BGP implementation
      determines that this connection needs to be terminated, it will
      process an OpenCollisionDump event (Event 23).  If this connection
      needs to be terminated, the local system:

        - sends a NOTIFICATION with a Cease,

        - sets the ConnectRetryTimer to zero,

        - deletes all routes associated with this connection,

        - releases all BGP resources,

        - drops the TCP connection,

        - increments the ConnectRetryCounter by 1,

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

        - changes its state to Idle.

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

        - sets the ConnectRetryTimer to zero,

        - deletes all routes associated with this connection,

        - releases all the BGP resources,

        - drops the TCP connection,

        - increments the ConnectRetryCounter by 1,

        - changes its state to Idle.











<span class="grey">Rekhter, et al.             Standards Track                    [Page 73]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-74" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


      If the local system receives a KEEPALIVE message (Event 26), the
      local system:

        - restarts its HoldTimer, if the negotiated HoldTime value is
          non-zero, and

        - remains in the Established state.

      If the local system receives an UPDATE message (Event 27), the
      local system:

        - processes the message,

        - restarts its HoldTimer, if the negotiated HoldTime value is
          non-zero, and

        - remains in the Established state.

      If the local system receives an UPDATE message, and the UPDATE
      message error handling procedure (see <a href="#section-6.3">Section 6.3</a>) detects an
      error (Event 28), the local system:

        - sends a NOTIFICATION message with an Update error,

        - sets the ConnectRetryTimer to zero,

        - deletes all routes associated with this connection,

        - releases all BGP resources,

        - drops the TCP connection,

        - increments the ConnectRetryCounter by 1,

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

        - changes its state to Idle.

      In response to any other event (Events 9, 12-13, 20-22), the local
      system:

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

        - deletes all routes associated with this connection,

        - sets the ConnectRetryTimer to zero,



<span class="grey">Rekhter, et al.             Standards Track                    [Page 74]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-75" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


        - releases all BGP resources,

        - drops the TCP connection,

        - increments the ConnectRetryCounter by 1,

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

        - changes its state to Idle.

<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>.  UPDATE Message Handling</span>

   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 <a href="#section-6.3">Section 6.3</a>.

   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.






<span class="grey">Rekhter, et al.             Standards Track                    [Page 75]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-76" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


<span class="h3"><a class="selflink" id="section-9.1" href="#section-9.1">9.1</a>.  Decision Process</span>

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

   The BGP Decision Process described here is conceptual, and does not
   have to be implemented precisely as described, as long as the
   implementations support the described functionality and they exhibit
   the same externally visible behavior.

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

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

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

      - selection of routes to be used locally by the speaker

      - selection of routes to be advertised to other BGP peers

      - route aggregation and route information reduction

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

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

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



<span class="grey">Rekhter, et al.             Standards Track                    [Page 76]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-77" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


      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.

<span class="h4"><a class="selflink" id="section-9.1.1" href="#section-9.1.1">9.1.1</a>.  Phase 1: Calculation of Degree of Preference</span>

   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.

<span class="h4"><a class="selflink" id="section-9.1.2" href="#section-9.1.2">9.1.2</a>.  Phase 2: Route Selection</span>

   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.







<span class="grey">Rekhter, et al.             Standards Track                    [Page 77]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-78" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


   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 <a href="#section-9.1.2.2">Section 9.1.2.2</a>.

   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 <a href="#section-5.1.3">Section 5.1.3</a>).  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.




<span class="grey">Rekhter, et al.             Standards Track                    [Page 78]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-79" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


   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).

<span class="h5"><a class="selflink" id="section-9.1.2.1" href="#section-9.1.2.1">9.1.2.1</a>.  Route Resolvability Condition</span>

   As indicated in <a href="#section-9.1.2">Section 9.1.2</a>, 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



<span class="grey">Rekhter, et al.             Standards Track                    [Page 79]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-80" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


   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.

<span class="h5"><a class="selflink" id="section-9.1.2.2" href="#section-9.1.2.2">9.1.2.2</a>.  Breaking Ties (Phase 2)</span>

   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.







<span class="grey">Rekhter, et al.             Standards Track                    [Page 80]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-81" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


      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) &lt; 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.




<span class="grey">Rekhter, et al.             Standards Track                    [Page 81]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-82" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


      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.

<span class="h4"><a class="selflink" id="section-9.1.3" href="#section-9.1.3">9.1.3</a>.  Phase 3: Route Dissemination</span>

   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



<span class="grey">Rekhter, et al.             Standards Track                    [Page 82]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-83" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


   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 <a href="#section-9.2.2.1">Section</a>
   <a href="#section-9.2.2.1">9.2.2.1</a>) 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.

<span class="h4"><a class="selflink" id="section-9.1.4" href="#section-9.1.4">9.1.4</a>.  Overlapping Routes</span>

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

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

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

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

   The set of destinations described by the overlap represents a portion
   of the less specific route that is feasible, but is not currently in
   use.  If a more specific route is later withdrawn, the set of
   destinations described by the overlap will still be reachable using
   the less specific route.

   If a BGP speaker receives overlapping routes, the Decision Process
   MUST consider both routes based on the configured acceptance policy.
   If both a less and a more specific route are accepted, then the
   Decision Process MUST install, in Loc-RIB, either both the less and



<span class="grey">Rekhter, et al.             Standards Track                    [Page 83]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-84" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


   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.

<span class="h3"><a class="selflink" id="section-9.2" href="#section-9.2">9.2</a>.  Update-Send Process</span>

   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 [<a href="./rfc2796" title="&quot;BGP Route Reflection - An Alternative to Full Mesh IBGP&quot;">RFC2796</a>]).

   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
   <a href="#section-4">Section 4</a>), 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.




<span class="grey">Rekhter, et al.             Standards Track                    [Page 84]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-85" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


<span class="h4"><a class="selflink" id="section-9.2.1" href="#section-9.2.1">9.2.1</a>.  Controlling Routing Traffic Overhead</span>

   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.

<span class="h5"><a class="selflink" id="section-9.2.1.1" href="#section-9.2.1.1">9.2.1.1</a>.  Frequency of Route Advertisement</span>

   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.

<span class="h5"><a class="selflink" id="section-9.2.1.2" href="#section-9.2.1.2">9.2.1.2</a>.  Frequency of Route Origination</span>

   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.




<span class="grey">Rekhter, et al.             Standards Track                    [Page 85]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-86" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


<span class="h4"><a class="selflink" id="section-9.2.2" href="#section-9.2.2">9.2.2</a>.  Efficient Organization of Routing Information</span>

   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.

<span class="h5"><a class="selflink" id="section-9.2.2.1" href="#section-9.2.2.1">9.2.2.1</a>.  Information Reduction</span>

   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 <a href="#section-9.2.2.2">Section 9.2.2.2</a>.  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.







<span class="grey">Rekhter, et al.             Standards Track                    [Page 86]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-87" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


<span class="h5"><a class="selflink" id="section-9.2.2.2" href="#section-9.2.2.2">9.2.2.2</a>.  Aggregating Routing Information</span>

   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 &lt;type, value&gt;,
         where "type" identifies a type of the path segment the AS



<span class="grey">Rekhter, et al.             Standards Track                    [Page 87]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-88" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


         belongs to (e.g., AS_SEQUENCE, AS_SET), and "value" identifies
         the AS number.  If the routes to be aggregated have different
         AS_PATH attributes, then the aggregated AS_PATH attribute SHALL
         satisfy all of the following conditions:

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

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

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

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

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

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

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

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

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

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



<span class="grey">Rekhter, et al.             Standards Track                    [Page 88]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-89" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


         <a href="#appendix-F">Appendix F</a>, 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 <a href="#section-5.1.7">Section 5.1.7</a>).

<span class="h3"><a class="selflink" id="section-9.3" href="#section-9.3">9.3</a>.  Route Selection Criteria</span>

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

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

      - In order to achieve a successful distributed operation, only
        routes with a likelihood of stability can be chosen.  Thus, an
        AS SHOULD avoid using unstable routes, and it SHOULD NOT make
        rapid, spontaneous changes to its choice of route.  Quantifying
        the terms "unstable" and "rapid" (from the previous sentence)
        will require experience, but the principle is clear.  Routes
        that are unstable can be "penalized" (e.g., by using the
        procedures described in [<a href="./rfc2439" title="&quot;BGP Route Flap Damping&quot;">RFC2439</a>]).

<span class="h3"><a class="selflink" id="section-9.4" href="#section-9.4">9.4</a>.  Originating BGP routes</span>

   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 <a href="#section-9.1">Section 9.1</a>).
   These routes MAY also be distributed to other BGP speakers within the
   local AS as part of the update process (see <a href="#section-9.2">Section 9.2</a>).  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.



<span class="grey">Rekhter, et al.             Standards Track                    [Page 89]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-90" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


<span class="h2"><a class="selflink" id="section-10" href="#section-10">10</a>.  BGP Timers</span>

   BGP employs five timers: ConnectRetryTimer (see <a href="#section-8">Section 8</a>), HoldTimer
   (see <a href="#section-4.2">Section 4.2</a>), KeepaliveTimer (see <a href="#section-8">Section 8</a>),
   MinASOriginationIntervalTimer (see <a href="#section-9.2.1.2">Section 9.2.1.2</a>), and
   MinRouteAdvertisementIntervalTimer (see <a href="#section-9.2.1.1">Section 9.2.1.1</a>).

   Two optional timers MAY be supported: DelayOpenTimer, IdleHoldTimer
   by BGP (see <a href="#section-8">Section 8</a>).  <a href="#section-8">Section 8</a> 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 <a href="#section-8">Section 8</a>), 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.



<span class="grey">Rekhter, et al.             Standards Track                    [Page 90]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-91" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


   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.














































<span class="grey">Rekhter, et al.             Standards Track                    [Page 91]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-92" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


<span class="h2"><a class="selflink" id="appendix-A" href="#appendix-A">Appendix A</a>.  Comparison with <a href="./rfc1771">RFC 1771</a></span>

   There are numerous editorial changes in comparison to [<a href="./rfc1771" title="&quot;A Border Gateway Protocol 4 (BGP- 4)&quot;">RFC1771</a>] (too
   many to list here).

   The following list the technical changes:

      Changes to reflect the usage of features such as TCP MD5
      [<a href="./rfc2385" title="&quot;Protection of BGP Sessions via the TCP MD5 Signature Option&quot;">RFC2385</a>], BGP Route Reflectors [<a href="./rfc2796" title="&quot;BGP Route Reflection - An Alternative to Full Mesh IBGP&quot;">RFC2796</a>], BGP Confederations
      [<a href="./rfc3065" title="&quot;Autonomous System Confederations for BGP&quot;">RFC3065</a>], and BGP Route Refresh [<a href="./rfc2918" title="&quot;Route Refresh Capability for BGP-4&quot;">RFC2918</a>].

      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 [<a href="./rfc2385" title="&quot;Protection of BGP Sessions via the TCP MD5 Signature Option&quot;">RFC2385</a>] for authentication.

      Clarification of BGP FSM.





<span class="grey">Rekhter, et al.             Standards Track                    [Page 92]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-93" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


<span class="h2"><a class="selflink" id="appendix-B" href="#appendix-B">Appendix B</a>.  Comparison with <a href="./rfc1267">RFC 1267</a></span>

   All the changes listed in <a href="#appendix-A">Appendix A</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, <a href="./rfc1519">RFC1519</a>].

   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.

<span class="h2"><a class="selflink" id="appendix-C" href="#appendix-C">Appendix C</a>.  Comparison with <a href="./rfc1163">RFC 1163</a></span>

   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 (<a href="#section-6.8">Section</a>
   <a href="#section-6.8">6.8</a>) 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.









<span class="grey">Rekhter, et al.             Standards Track                    [Page 93]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-94" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


<span class="h2"><a class="selflink" id="appendix-D" href="#appendix-D">Appendix D</a>.  Comparison with <a href="./rfc1105">RFC 1105</a></span>

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

   Minor changes to the [<a href="./rfc1105" title="&quot;Border Gateway Protocol (BGP)&quot;">RFC1105</a>] 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 <a href="./rfc1105">RFC 1105</a> has
   been removed from the protocol.

   The changes in the message format from <a href="./rfc1105">RFC 1105</a> 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 <a href="./rfc1105">RFC 1105</a>, is referred to
   as BGP-1; BGP, as specified in [<a href="./rfc1163" title="&quot;Border Gateway Protocol (BGP)&quot;">RFC1163</a>], is referred to as BGP-2;
   BGP, as specified in <a href="./rfc1267">RFC 1267</a> is referred to as BGP-3; and BGP, as
   specified in this document is referred to as BGP-4.

<span class="h2"><a class="selflink" id="appendix-E" href="#appendix-E">Appendix E</a>.  TCP Options that May Be Used with BGP</span>

   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
   [<a href="./rfc2474" title="&quot;Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers&quot;">RFC2474</a>] 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 [<a href="./rfc2385" title="&quot;Protection of BGP Sessions via the TCP MD5 Signature Option&quot;">RFC2385</a>].



<span class="grey">Rekhter, et al.             Standards Track                    [Page 94]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-95" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


<span class="h2"><a class="selflink" id="appendix-F" href="#appendix-F">Appendix F</a>.  Implementation Recommendations</span>

   This section presents some implementation recommendations.

<span class="h3"><a class="selflink" id="appendix-F.1" href="#appendix-F.1">Appendix F.1</a>.  Multiple Networks Per Message</span>

   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.








<span class="grey">Rekhter, et al.             Standards Track                    [Page 95]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-96" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


<span class="h3"><a class="selflink" id="appendix-F.2" href="#appendix-F.2">Appendix F.2</a>.  Reducing Route Flapping</span>

   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.

<span class="h3"><a class="selflink" id="appendix-F.3" href="#appendix-F.3">Appendix F.3</a>.  Path Attribute Ordering</span>

   Implementations that combine update messages (as described above in
   <a href="#section-6.1">Section 6.1</a>) 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.

<span class="h3"><a class="selflink" id="appendix-F.4" href="#appendix-F.4">Appendix F.4</a>.  AS_SET Sorting</span>

   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.

<span class="h3"><a class="selflink" id="appendix-F.5" href="#appendix-F.5">Appendix F.5</a>.  Control Over Version Negotiation</span>

   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.

<span class="h3"><a class="selflink" id="appendix-F.6" href="#appendix-F.6">Appendix F.6</a>.  Complex AS_PATH Aggregation</span>

   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 &lt;type, value&gt;, 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 &lt;type, value&gt; 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:



<span class="grey">Rekhter, et al.             Standards Track                    [Page 96]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-97" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


              - X precedes Y in both AS_PATH attributes, or
              - Y precedes X in both AS_PATH attributes.

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

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

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

Security Considerations

   A BGP implementation MUST support the authentication mechanism
   specified in <a href="./rfc2385">RFC 2385</a> [<a href="./rfc2385" title="&quot;Protection of BGP Sessions via the TCP MD5 Signature Option&quot;">RFC2385</a>].  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 <a href="./rfc2385">RFC 2385</a>.  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.



<span class="grey">Rekhter, et al.             Standards Track                    [Page 97]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-98" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


   The mechanism defined in <a href="./rfc2385">RFC 2385</a> 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.

   <a href="./rfc2385">RFC 2385</a> does not specify a means of managing (e.g., generating,
   distributing, and replacing) the keys used to compute the MAC.  <a href="./rfc3562">RFC</a>
   <a href="./rfc3562">3562</a> [<a href="./rfc3562" title="&quot;Key Management Considerations for the TCP MD5 Signature Option&quot;">RFC3562</a>] (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.  <a href="./rfc3562">RFC 3562</a> 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 <a href="./rfc1750">RFC 1750</a> for random
   number generation provide a guide for generation of values that could
   be used as keys.  <a href="./rfc2385">RFC 2385</a> calls for implementations to support keys
   "composed of a string of printable ASCII of 80 bytes or less."  <a href="./rfc3562">RFC</a>
   <a href="./rfc3562">3562</a> 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, <a href="./rfc3562">RFC 3562</a> also observes that a typical ACSII text string would
   have to be close to the upper bound for the key length specified in
   <a href="./rfc2385">RFC 2385</a>.

   BGP vulnerabilities analysis is discussed in [<a href="./rfc4272" title="&quot;BGP Security Vulnerabilities Analysis&quot;">RFC4272</a>].








<span class="grey">Rekhter, et al.             Standards Track                    [Page 98]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-99" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


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 <a href="#section-4.2">Section 4.2</a>
         UPDATE           2           See <a href="#section-4.3">Section 4.3</a>
         NOTIFICATION     3           See <a href="#section-4.5">Section 4.5</a>
         KEEPALIVE        4           See <a href="#section-4.4">Section 4.4</a>

   Future assignments are to be made using either the Standards Action
   process defined in [<a href="./rfc2434" title="">RFC2434</a>], or the Early IANA Allocation process
   defined in [<a href="./rfc4020" title="&quot;Early IANA Allocation of Standards Track Code Points&quot;">RFC4020</a>].  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 <a href="#section-5.1.1">Section 5.1.1</a>
        AS_PATH             2          See <a href="#section-5.1.2">Section 5.1.2</a>
        NEXT_HOP            3          See <a href="#section-5.1.3">Section 5.1.3</a>
        MULTI_EXIT_DISC     4          See <a href="#section-5.1.4">Section 5.1.4</a>
        LOCAL_PREF          5          See <a href="#section-5.1.5">Section 5.1.5</a>
        ATOMIC_AGGREGATE    6          See <a href="#section-5.1.6">Section 5.1.6</a>
        AGGREGATOR          7          See <a href="#section-5.1.7">Section 5.1.7</a>

   Future assignments are to be made using either the Standards Action
   process defined in [<a href="./rfc2434" title="">RFC2434</a>], or the Early IANA Allocation process
   defined in [<a href="./rfc4020" title="&quot;Early IANA Allocation of Standards Track Code Points&quot;">RFC4020</a>].  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          <a href="#section-6.1">Section 6.1</a>
         OPEN Message Error         2          <a href="#section-6.2">Section 6.2</a>
         UPDATE Message Error       3          <a href="#section-6.3">Section 6.3</a>
         Hold Timer Expired         4          <a href="#section-6.5">Section 6.5</a>
         Finite State Machine Error 5          <a href="#section-6.6">Section 6.6</a>
         Cease                      6          <a href="#section-6.7">Section 6.7</a>



<span class="grey">Rekhter, et al.             Standards Track                    [Page 99]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-100" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


   Future assignments are to be made using either the Standards Action
   process defined in [<a href="./rfc2434" title="">RFC2434</a>], or the Early IANA Allocation process
   defined in [<a href="./rfc4020" title="&quot;Early IANA Allocation of Standards Track Code Points&quot;">RFC4020</a>].  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 [<a href="./rfc2434" title="">RFC2434</a>], or the Early IANA Allocation process defined in
   [<a href="./rfc4020" title="&quot;Early IANA Allocation of Standards Track Code Points&quot;">RFC4020</a>].  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 <a href="#section-6.1">Section 6.1</a>
         Bad Message Length            2           See <a href="#section-6.1">Section 6.1</a>
         Bad Message Type              3           See <a href="#section-6.1">Section 6.1</a>

   This document defines the following OPEN Message Error subcodes:

         Name                         Value        Definition
         --------------------         -----        ----------
         Unsupported Version Number     1          See <a href="#section-6.2">Section 6.2</a>
         Bad Peer AS                    2          See <a href="#section-6.2">Section 6.2</a>
         Bad BGP Identifier             3          See <a href="#section-6.2">Section 6.2</a>
         Unsupported Optional Parameter 4          See <a href="#section-6.2">Section 6.2</a>
         [Deprecated]                   5          See <a href="#appendix-A">Appendix A</a>
         Unacceptable Hold Time         6          See <a href="#section-6.2">Section 6.2</a>

    This document defines the following UPDATE Message Error subcodes:

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



<span class="grey">Rekhter, et al.             Standards Track                   [Page 100]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-101" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


Normative References

   [<a id="ref-RFC791">RFC791</a>]  Postel, J., "Internet Protocol", STD 5, <a href="./rfc791">RFC 791</a>, September
             1981.

   [<a id="ref-RFC793">RFC793</a>]  Postel, J., "Transmission Control Protocol", STD 7, <a href="./rfc793">RFC</a>
             <a href="./rfc793">793</a>, September 1981.

   [<a id="ref-RFC2119">RFC2119</a>] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997.

   [<a id="ref-RFC2385">RFC2385</a>] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
             Signature Option", <a href="./rfc2385">RFC 2385</a>, August 1998.

   [<a id="ref-RFC2434">RFC2434</a>] Narten, T. and H. Alvestrand, "Guidelines for Writing an
             IANA Considerations Section in RFCs", <a href="https://www.rfc-editor.org/bcp/bcp26">BCP 26</a>, <a href="./rfc2434">RFC 2434</a>,
             October 1998.

Informative References

   [<a id="ref-RFC904">RFC904</a>]  Mills, D., "Exterior Gateway Protocol formal
             specification", <a href="./rfc904">RFC 904</a>, April 1984.

   [<a id="ref-RFC1092">RFC1092</a>] Rekhter, J., "EGP and policy based routing in the new
             NSFNET backbone", <a href="./rfc1092">RFC 1092</a>, February 1989.

   [<a id="ref-RFC1093">RFC1093</a>] Braun, H., "NSFNET routing architecture", <a href="./rfc1093">RFC 1093</a>,
             February 1989.

   [<a id="ref-RFC1105">RFC1105</a>] Lougheed, K. and Y. Rekhter, "Border Gateway Protocol
             (BGP)", <a href="./rfc1105">RFC 1105</a>, June 1989.

   [<a id="ref-RFC1163">RFC1163</a>] Lougheed, K. and Y. Rekhter, "Border Gateway Protocol
             (BGP)", <a href="./rfc1163">RFC 1163</a>, June 1990.

   [<a id="ref-RFC1267">RFC1267</a>] Lougheed, K. and Y. Rekhter, "Border Gateway Protocol 3
             (BGP-3)", <a href="./rfc1267">RFC 1267</a>, October 1991.

   [<a id="ref-RFC1771">RFC1771</a>] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-
             4)", <a href="./rfc1771">RFC 1771</a>, March 1995.

   [<a id="ref-RFC1772">RFC1772</a>] Rekhter, Y. and P. Gross, "Application of the Border
             Gateway Protocol in the Internet", <a href="./rfc1772">RFC 1772</a>, March 1995.

   [<a id="ref-RFC1518">RFC1518</a>] Rekhter, Y. and T. Li, "An Architecture for IP Address
             Allocation with CIDR", <a href="./rfc1518">RFC 1518</a>, September 1993.





<span class="grey">Rekhter, et al.             Standards Track                   [Page 101]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-102" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


   [<a id="ref-RFC1519">RFC1519</a>] Fuller, V., Li, T., Yu, J., and K. Varadhan, "Classless
             Inter-Domain Routing (CIDR): an Address Assignment and
             Aggregation Strategy", <a href="./rfc1519">RFC 1519</a>, September 1993.

   [<a id="ref-RFC1930">RFC1930</a>] Hawkinson, J. and T. Bates, "Guidelines for creation,
             selection, and registration of an Autonomous System (AS)",
             <a href="https://www.rfc-editor.org/bcp/bcp6">BCP 6</a>, <a href="./rfc1930">RFC 1930</a>, March 1996.

   [<a id="ref-RFC1997">RFC1997</a>] Chandra, R., Traina, P., and T. Li, "BGP Communities
             Attribute", <a href="./rfc1997">RFC 1997</a>, August 1996.

   [<a id="ref-RFC2439">RFC2439</a>] Villamizar, C., Chandra, R., and R. Govindan, "BGP Route
             Flap Damping", <a href="./rfc2439">RFC 2439</a>, November 1998.

   [<a id="ref-RFC2474">RFC2474</a>] Nichols, K., Blake, S., Baker, F., and D. Black,
             "Definition of the Differentiated Services Field (DS Field)
             in the IPv4 and IPv6 Headers", <a href="./rfc2474">RFC 2474</a>, December 1998.

   [<a id="ref-RFC2796">RFC2796</a>] Bates, T., Chandra, R., and E. Chen, "BGP Route Reflection
             - An Alternative to Full Mesh IBGP", <a href="./rfc2796">RFC 2796</a>, April 2000.

   [<a id="ref-RFC2858">RFC2858</a>] Bates, T., Rekhter, Y., Chandra, R., and D. Katz,
             "Multiprotocol Extensions for BGP-4", <a href="./rfc2858">RFC 2858</a>, June 2000.

   [<a id="ref-RFC3392">RFC3392</a>] Chandra, R. and J. Scudder, "Capabilities Advertisement
             with BGP-4", <a href="./rfc3392">RFC 3392</a>, November 2002.

   [<a id="ref-RFC2918">RFC2918</a>] Chen, E., "Route Refresh Capability for BGP-4", <a href="./rfc2918">RFC 2918</a>,
             September 2000.

   [<a id="ref-RFC3065">RFC3065</a>] Traina, P., McPherson, D., and J. Scudder, "Autonomous
             System Confederations for BGP", <a href="./rfc3065">RFC 3065</a>, February 2001.

   [<a id="ref-RFC3562">RFC3562</a>] Leech, M., "Key Management Considerations for the TCP MD5
             Signature Option", <a href="./rfc3562">RFC 3562</a>, July 2003.

   [<a id="ref-IS10747">IS10747</a>] "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.

   [<a id="ref-RFC4272">RFC4272</a>] Murphy, S., "BGP Security Vulnerabilities Analysis", <a href="./rfc4272">RFC</a>
             <a href="./rfc4272">4272</a>, January 2006

   [<a id="ref-RFC4020">RFC4020</a>] Kompella, K. and A. Zinin, "Early IANA Allocation of
             Standards Track Code Points", <a href="https://www.rfc-editor.org/bcp/bcp100">BCP 100</a>, <a href="./rfc4020">RFC 4020</a>, February
             2005.



<span class="grey">Rekhter, et al.             Standards Track                   [Page 102]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-103" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


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































<span class="grey">Rekhter, et al.             Standards Track                   [Page 103]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-104" ></span>
<span class="grey"><a href="./rfc4271">RFC 4271</a>                         BGP-4                      January 2006</span>


Full Copyright Statement

   Copyright (C) The Internet Society (2006).

   This document is subject to the rights, licenses and restrictions
   contained in <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a>, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and <a href="https://www.rfc-editor.org/bcp/bcp79">BCP 79</a>.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   <a href="http://www.ietf.org/ipr">http://www.ietf.org/ipr</a>.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.

Acknowledgement

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).







Rekhter, et al.             Standards Track                   [Page 104]
</pre>