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rfc:rfc1771

Network Working Group Y. Rekhter Request for Comments: 1771 T.J. Watson Research Center, IBM Corp. Obsoletes: 1654 T. Li Category: Standards Track cisco Systems

                                                               Editors
                                                            March 1995
                A Border Gateway Protocol 4 (BGP-4)

Status of this Memo

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

Abstract

 This document, together with its companion document, "Application of
 the Border Gateway Protocol in the Internet", define an inter-
 autonomous system routing protocol for the Internet.

1. Acknowledgements

 This document was originally published as RFC 1267 in October 1991,
 jointly authored by Kirk Lougheed (cisco Systems) and Yakov Rekhter
 (IBM).
 We would like to express our thanks to Guy Almes (ANS), Len Bosack
 (cisco Systems), and Jeffrey C. Honig (Cornell University) for their
 contributions to the earlier version of this document.
 We like to explicitly thank Bob Braden (ISI) for the review of the
 earlier version of this document as well as his constructive and
 valuable comments.
 We would also like to thank Bob Hinden, Director for Routing of the
 Internet Engineering Steering Group, and the team of reviewers he
 assembled to review the previous 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.

Rekhter & Li [Page 1] RFC 1771 BGP-4 March 1995

 This updated version of the document is the product of the IETF IDR
 Working Group with Yakov Rekhter and Tony Li as editors. Certain
 sections of the document borrowed heavily from IDRP [7], which is the
 OSI counterpart of BGP. For this credit should be given to the ANSI
 X3S3.3 group chaired by Lyman Chapin (BBN) and to Charles Kunzinger
 (IBM Corp.) who was the IDRP editor within that group.  We would also
 like to thank Mike Craren (Proteon, Inc.), Dimitry Haskin (Bay
 Networks, Inc.), John Krawczyk (Bay Networks, Inc.), and Paul Traina
 (cisco Systems) for their insightful comments.
 We would like to specially acknowledge numerous contributions by
 Dennis Ferguson (MCI).
 The work of Yakov Rekhter was supported in part by the National
 Science Foundation under Grant Number NCR-9219216.

2. Introduction

 The Border Gateway Protocol (BGP) is an inter-Autonomous System
 routing protocol.  It is built on experience gained with EGP as
 defined in RFC 904 [1] and EGP usage in the NSFNET Backbone as
 described in RFC 1092 [2] and RFC 1093 [3].
 The primary function of a BGP speaking system is to exchange network
 reachability information with other BGP systems.  This network
 reachability information includes information on the list of
 Autonomous Systems (ASs) that reachability information traverses.
 This information is sufficient to construct a graph of AS
 connectivity from which routing loops may be pruned and some policy
 decisions at the AS level may be enforced.
 BGP-4 provides a new set of mechanisms for supporting classless
 interdomain routing.  These mechanisms include support for
 advertising an IP prefix and eliminates the concept of network
 "class" within BGP.  BGP-4 also introduces mechanisms which allow
 aggregation of routes, including aggregation of AS paths.  These
 changes provide support for the proposed supernetting scheme [8, 9].
 To characterize the set of policy decisions that can be enforced
 using BGP, one must focus on the rule that a BGP speaker advertise to
 its peers (other BGP speakers which it communicates with) in
 neighboring ASs only those routes that it itself uses.  This rule
 reflects the "hop-by-hop" routing paradigm generally used throughout
 the current Internet.  Note that some policies cannot be supported by
 the "hop-by-hop" routing paradigm and thus require techniques such as
 source routing to enforce.  For example, BGP does not enable one AS
 to send traffic to a neighboring AS intending that the traffic take a
 different route from that taken by traffic originating in the

Rekhter & Li [Page 2] RFC 1771 BGP-4 March 1995

 neighboring AS.  On the other hand, BGP can support any policy
 conforming to the "hop-by-hop" routing paradigm.  Since the current
 Internet uses only the "hop-by-hop" routing paradigm and since BGP
 can support any policy that conforms to that paradigm, BGP is highly
 applicable as an inter-AS routing protocol for the current Internet.
 A more complete discussion of what policies can and cannot be
 enforced with BGP is outside the scope of this document (but refer to
 the companion document discussing BGP usage [5]).
 BGP runs over a reliable transport protocol.  This eliminates the
 need to implement explicit update fragmentation, retransmission,
 acknowledgement, and sequencing.  Any authentication scheme used by
 the transport protocol may be used in addition to BGP's own
 authentication mechanisms.  The error notification mechanism used in
 BGP assumes that the transport protocol supports a "graceful" close,
 i.e., that all outstanding data will be delivered before the
 connection is closed.
 BGP uses TCP [4] as its transport protocol.  TCP meets BGP's
 transport requirements and is present in virtually all commercial
 routers and hosts.  In the following descriptions the phrase
 "transport protocol connection" can be understood to refer to a TCP
 connection.  BGP uses TCP port 179 for establishing its connections.
 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
 and common metrics to route packets within the AS, and using an
 exterior gateway protocol to route packets to other ASs.  Since this
 classic definition was developed, it has become common for a single
 AS to use several interior gateway protocols and sometimes several
 sets of metrics within an AS.  The use of the term Autonomous System
 here stresses the fact that, even when multiple IGPs and metrics are
 used, the administration of an AS appears to other ASs to have a
 single coherent interior routing plan and presents a consistent
 picture of what destinations are reachable through it.
 The planned use of BGP in the Internet environment, including such
 issues as topology, the interaction between BGP and IGPs, and the
 enforcement of routing policy rules is presented in a companion
 document [5].  This document is the first of a series of documents
 planned to explore various aspects of BGP application.  Please send
 comments to the BGP mailing list (bgp@ans.net).

Rekhter & Li [Page 3] RFC 1771 BGP-4 March 1995

3. Summary of Operation

 Two systems form a transport protocol connection between one another.
 They exchange messages to open and confirm the connection parameters.
 The initial data flow is the entire BGP routing table.  Incremental
 updates are sent as the routing tables change.  BGP does not require
 periodic refresh of the entire BGP routing table.  Therefore, a BGP
 speaker must retain the current version of the entire BGP routing
 tables of all of its peers for the duration of the connection.
 KeepAlive messages are sent periodically to ensure the liveness of
 the connection.  Notification messages are sent in response to errors
 or special conditions.  If a connection encounters an error
 condition, a notification message is sent and the connection is
 closed.
 The hosts executing the Border Gateway Protocol need not be routers.
 A non-routing host could exchange routing information with routers
 via EGP or even an interior routing protocol.  That non-routing host
 could then use BGP to exchange routing information with a border
 router in another Autonomous System.  The implications and
 applications of this architecture are for further study.
 If a particular AS has multiple BGP speakers and is providing transit
 service for other ASs, then care must be taken to ensure a consistent
 view of routing within the AS.  A consistent view of the interior
 routes of the AS is provided by the interior routing protocol.  A
 consistent view of the routes exterior to the AS can be provided by
 having all BGP speakers within the AS maintain direct BGP connections
 with each other.  Using a common set of policies, the BGP speakers
 arrive at an agreement as to which border routers will serve as
 exit/entry points for particular destinations outside the AS.  This
 information is communicated to the AS's internal routers, possibly
 via the interior routing protocol.  Care must be taken to ensure that
 the interior routers have all been updated with transit information
 before the BGP speakers announce to other ASs that transit service is
 being provided.
 Connections between BGP speakers of different ASs are referred to as
 "external" links.  BGP connections between BGP speakers within the
 same AS are referred to as "internal" links.  Similarly, a peer in a
 different AS is referred to as an external peer, while a peer in the
 same AS may be described as an internal peer.

Rekhter & Li [Page 4] RFC 1771 BGP-4 March 1995

3.1 Routes: Advertisement and Storage

 For purposes of this protocol a route is defined as a unit of
 information that pairs a destination with the attributes of a path to
 that destination:
  1. Routes are advertised between a pair of BGP speakers in UPDATE

messages: the destination is the systems whose IP addresses are

    reported in the Network Layer Reachability Information (NLRI)
    field, and the the path is the information reported in the path
    attributes fields of the same UPDATE message.
  1. Routes are stored in the Routing Information Bases (RIBs):

namely, the Adj-RIBs-In, the Loc-RIB, and the Adj-RIBs-Out. Routes

    that will be advertised to other BGP speakers must be present in
    the Adj-RIB-Out; routes that will be used by the local BGP speaker
    must be present in the Loc-RIB, and the next hop for each of these
    routes must be present in the local BGP speaker's forwarding
    information base; and routes that are received from other BGP
    speakers are present in the Adj-RIBs-In.
 If a BGP speaker chooses to advertise the route, it may add to or
 modify the path attributes of the route before advertising it to a
 peer.
 BGP provides mechanisms by which a BGP speaker can inform its peer
 that a previously advertised route is no longer available for use.
 There are three methods by which a given BGP speaker can indicate
 that a route has been withdrawn from service:
    a) the IP prefix that expresses destinations 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 Network Layer Reachability
    Information can be advertised, or
    c) the BGP speaker - BGP speaker connection can be closed, which
    implicitly removes from service all routes which the pair of
    speakers had advertised to each other.

Rekhter & Li [Page 5] RFC 1771 BGP-4 March 1995

3.2 Routing Information Bases

 The Routing Information Base (RIB) within a BGP speaker consists of
 three distinct parts:
    a) Adj-RIBs-In: The Adj-RIBs-In store routing information that has
    been learned from inbound UPDATE messages. Their contents
    represent routes that are available as an input to the Decision
    Process.
    b) Loc-RIB: The Loc-RIB contains the local routing information
    that the BGP speaker has selected by applying its local policies
    to the routing information contained in its Adj-RIBs-In.
    c) Adj-RIBs-Out: The Adj-RIBs-Out store the information that the
    local BGP speaker has selected for advertisement to its peers. The
    routing information stored in the Adj-RIBs-Out will be carried in
    the local BGP speaker's UPDATE messages and advertised to its
    peers.
 In summary, the Adj-RIBs-In contain unprocessed routing information
 that has been advertised to the local BGP speaker by its peers; the
 Loc-RIB contains the routes that have been selected by the local BGP
 speaker's Decision Process; and the Adj-RIBs-Out organize the routes
 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.

4. Message Formats

 This section describes message formats used by BGP.
 Messages are sent over a reliable transport protocol connection.  A
 message is processed only after it is entirely received.  The maximum
 message size is 4096 octets.  All implementations are required to
 support this maximum message size.  The smallest message that may be
 sent consists of a BGP header without a data portion, or 19 octets.

Rekhter & Li [Page 6] RFC 1771 BGP-4 March 1995

4.1 Message Header Format

 Each message has a fixed-size header.  There may or may not be a data
 portion following the header, depending on the message type.  The
 layout of these fields is shown below:
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                                                               +
    |                                                               |
    +                                                               +
    |                           Marker                              |
    +                                                               +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          Length               |      Type     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Marker:
       This 16-octet field contains a value that the receiver of the
       message can predict.  If the Type of the message is OPEN, or if
       the OPEN message carries no Authentication Information (as an
       Optional Parameter), then the Marker must be all ones.
       Otherwise, the value of the marker can be predicted by some a
       computation specified as part of the authentication mechanism
       (which is specified as part of the Authentication Information)
       used.  The Marker can be used to detect loss of synchronization
       between a pair of BGP peers, and to authenticate incoming BGP
       messages.
    Length:
       This 2-octet unsigned integer indicates the total length of the
       message, including the header, in octets.  Thus, e.g., it
       allows one to locate in the transport-level stream the (Marker
       field of the) next message.  The value of the Length field must
       always be at least 19 and no greater than 4096, and may be
       further constrained, depending on the message type.  No
       "padding" of extra data after the message is allowed, so the
       Length field must have the smallest value required given the
       rest of the message.

Rekhter & Li [Page 7] RFC 1771 BGP-4 March 1995

    Type:
       This 1-octet unsigned integer indicates the type code of the
       message.  The following type codes are defined:
                                  1 - OPEN
                                  2 - UPDATE
                                  3 - NOTIFICATION
                                  4 - KEEPALIVE

4.2 OPEN Message Format

 After a transport protocol 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.
 Once the OPEN is confirmed, UPDATE, KEEPALIVE, and NOTIFICATION
 messages may be exchanged.
 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                     |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    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.

Rekhter & Li [Page 8] RFC 1771 BGP-4 March 1995

    Hold Time:
       This 2-octet unsigned integer indicates the number of seconds
       that the sender proposes for the value of the Hold Timer.  Upon
       receipt of an OPEN message, a BGP speaker MUST calculate the
       value of the Hold Timer by using the smaller of its configured
       Hold Time and the Hold Time received in the OPEN message.  The
       Hold Time MUST be either zero or at least three seconds.  An
       implementation may reject connections on the basis of the Hold
       Time.  The calculated value indicates the maximum number of
       seconds that may elapse between the receipt of successive
       KEEPALIVE, and/or UPDATE messages by the sender.
    BGP Identifier:
       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 assigned to that BGP speaker.  The
       value of the BGP Identifier is determined on startup and is the
       same for every local interface and every BGP peer.
    Optional Parameters Length:
       This 1-octet unsigned integer indicates the total length of the
       Optional Parameters field in octets. If the value of this field
       is zero, no Optional Parameters are present.
    Optional Parameters:
       This field may contain a list of optional parameters, where
       each parameter is encoded as a <Parameter Type, Parameter
       Length, Parameter Value> triplet.
        0                   1
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...
       |  Parm. Type   | Parm. Length  |  Parameter Value (variable)
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...
       Parameter Type is a one octet field that unambiguously
       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.

Rekhter & Li [Page 9] RFC 1771 BGP-4 March 1995

       This document defines the following Optional Parameters:
       a) Authentication Information (Parameter Type 1):
          This optional parameter may be used to authenticate a BGP
          peer. The Parameter Value field contains a 1-octet
          Authentication Code followed by a variable length
          Authentication Data.
              0 1 2 3 4 5 6 7 8
              +-+-+-+-+-+-+-+-+
              |  Auth. Code   |
              +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              |                                                     |
              |              Authentication Data                    |
              |                                                     |
              +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             Authentication Code:
                This 1-octet unsigned integer indicates the
                authentication mechanism being used.  Whenever an
                authentication mechanism is specified for use within
                BGP, three things must be included in the
                specification:
  1. the value of the Authentication Code which indicates

use of the mechanism,

  1. the form and meaning of the Authentication Data, and
  2. the algorithm for computing values of Marker fields.
                Note that a separate authentication mechanism may be
                used in establishing the transport level connection.
             Authentication Data:
                The form and meaning of this field is a variable-
                length field depend on the Authentication Code.
       The minimum length of the OPEN message is 29 octets (including
       message header).

Rekhter & Li [Page 10] RFC 1771 BGP-4 March 1995

4.3 UPDATE Message Format

 UPDATE messages are used to transfer routing information between BGP
 peers.  The information in the UPDATE packet can be used to construct
 a graph describing the relationships of the various Autonomous
 Systems.  By applying rules to be discussed, routing information
 loops and some other anomalies may be detected and removed from
 inter-AS routing.
 An UPDATE message is used to advertise a single feasible route 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 can optionally
 include the other fields as shown below:
    +-----------------------------------------------------+
    |   Unfeasible Routes Length (2 octets)               |
    +-----------------------------------------------------+
    |  Withdrawn Routes (variable)                        |
    +-----------------------------------------------------+
    |   Total Path Attribute Length (2 octets)            |
    +-----------------------------------------------------+
    |    Path Attributes (variable)                       |
    +-----------------------------------------------------+
    |   Network Layer Reachability Information (variable) |
    +-----------------------------------------------------+
    Unfeasible Routes Length:
       This 2-octets unsigned integer indicates the total length of
       the Withdrawn Routes field in octets.  Its value must allow the
       length of the Network Layer Reachability Information field to
       be determined as specified below.
       A value of 0 indicates that no routes are being withdrawn from
       service, and that the WITHDRAWN ROUTES field is not present in
       this UPDATE message.
    Withdrawn Routes:
       This is a variable length field that contains a list of IP
       address prefixes for the routes that are being withdrawn from
       service.  Each IP address prefix is encoded as a 2-tuple of the
       form <length, prefix>, whose fields are described below:

Rekhter & Li [Page 11] RFC 1771 BGP-4 March 1995

                +---------------------------+
                |   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 IP address prefixes followed by
          enough trailing bits 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 must allow the
       length of the Network Layer Reachability field to be determined
       as specified below.
       A value of 0 indicates that no Network Layer Reachability
       Information field is present in this UPDATE message.
    Path Attributes:
       A variable length sequence of path attributes is present in
       every UPDATE.  Each path attribute is a triple <attribute type,
       attribute length, attribute value> of variable length.
       Attribute Type is a two-octet field that consists of the
       Attribute Flags octet followed by the Attribute Type Code
       octet.
              0                   1
              0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             |  Attr. Flags  |Attr. Type Code|
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Rekhter & Li [Page 12] RFC 1771 BGP-4 March 1995

       The high-order bit (bit 0) of the Attribute Flags octet is the
       Optional bit.  It defines whether the attribute is optional (if
       set to 1) or well-known (if set to 0).
       The second high-order bit (bit 1) of the Attribute Flags octet
       is the Transitive bit.  It defines whether an optional
       attribute is transitive (if set to 1) or non-transitive (if set
       to 0).  For well-known attributes, the Transitive bit must be
       set to 1.  (See Section 5 for a discussion of transitive
       attributes.)
       The third high-order bit (bit 2) of the Attribute Flags octet
       is the Partial bit.  It defines whether the information
       contained in the optional transitive attribute is partial (if
       set to 1) or complete (if set to 0).  For well-known attributes
       and for optional non-transitive attributes the Partial bit must
       be set to 0.
       The fourth high-order bit (bit 3) of the Attribute Flags octet
       is the Extended Length bit.  It defines whether the Attribute
       Length is one octet (if set to 0) or two octets (if set to 1).
       Extended Length may be used only if the length of the attribute
       value is greater than 255 octets.
       The lower-order four bits of the Attribute Flags octet are .
       unused. They must be zero (and must be ignored when received).
       The Attribute Type Code octet contains the Attribute Type Code.
       Currently defined Attribute Type Codes are discussed in Section
       5.
       If the Extended Length bit of the Attribute Flags octet is set
       to 0, the third octet of the Path Attribute contains the length
       of the attribute data in octets.
       If the Extended Length bit of the Attribute Flags octet is set
       to 1, then the third and the fourth octets of the path
       attribute contain the length of the attribute data in octets.
       The remaining octets of the Path Attribute represent the
       attribute value and are interpreted according to the Attribute
       Flags and the Attribute Type Code. The supported Attribute Type
       Codes, their attribute values and uses are the following:

Rekhter & Li [Page 13] RFC 1771 BGP-4 March 1995

       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 EGP
                2         INCOMPLETE - Network Layer Reachability
                             Information learned by some other means
          Its usage is defined in 5.1.1
       b) AS_PATH (Type Code 2):
          AS_PATH is a well-known mandatory attribute that is composed
          of a sequence of AS path segments. Each AS path segment is
          represented by a triple <path segment type, path segment
          length, path segment value>.

Rekhter & Li [Page 14] RFC 1771 BGP-4 March 1995

          The path segment type is a 1-octet long field with the
          following values defined:
                Value      Segment Type
                1         AS_SET: unordered set of ASs a route in the
                             UPDATE message has traversed
                2         AS_SEQUENCE: ordered set of ASs a route in
                             the UPDATE message has traversed
          The path segment length is a 1-octet long field containing
          the number of ASs in the path segment value field.
          The path segment value field contains one or more AS
          numbers, each encoded as a 2-octets long field.
          Usage of this attribute is defined in 5.1.2.
       c)   NEXT_HOP (Type Code 3):
          This is a well-known mandatory attribute that defines the IP
          address of the border router that should be used as the next
          hop to the destinations listed in the Network Layer
          Reachability 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 non-negative integer. The value of this attribute may
          be used by a BGP speaker's decision process to discriminate
          among multiple exit points to a neighboring autonomous
          system.
          Its usage is defined in 5.1.4.
       e) LOCAL_PREF (Type Code 5):
          LOCAL_PREF is a well-known discretionary attribute that is a
          four octet non-negative integer. It is used by a BGP speaker
          to inform other BGP speakers in its own autonomous system of
          the originating speaker's degree of preference for an
          advertised route. Usage of this attribute is described in
          5.1.5.

Rekhter & Li [Page 15] RFC 1771 BGP-4 March 1995

       f) ATOMIC_AGGREGATE (Type Code 6)
          ATOMIC_AGGREGATE is a well-known discretionary attribute of
          length 0. It is used by a BGP speaker to inform other BGP
          speakers that the local system selected a less specific
          route without selecting a more specific route which is
          included in it. Usage of this attribute is described 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).  Usage of this attribute is described
          in 5.1.7
    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 -
          Unfeasible Routes Length
       where UPDATE message Length is the value encoded in the fixed-
       size BGP header, Total Path Attribute Length and Unfeasible
       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
       Unfeasible Routes Length field.
       Reachability information is encoded as one or more 2-tuples of
       the form <length, prefix>, whose fields are described below:
                +---------------------------+
                |   Length (1 octet)        |
                +---------------------------+
                |   Prefix (variable)       |
                +---------------------------+

Rekhter & Li [Page 16] RFC 1771 BGP-4 March 1995

       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 IP address prefixes 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 Unfeasible Routes Length + 2
 octets for the Total Path Attribute Length (the value of Unfeasible
 Routes Length is 0  and the value of Total Path Attribute Length is
 0).
 An UPDATE message can advertise at most one route, which may be
 described by several path attributes. All path attributes contained
 in a given UPDATE messages apply to the destinations carried in the
 Network Layer Reachability Information field of the UPDATE message.
 An UPDATE message can list multiple routes 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 been advertised.
 An UPDATE message may advertise only routes to be withdrawn from
 service, in which case it will not include path attributes or Network
 Layer Reachability Information. Conversely, it may advertise only a
 feasible route, in which case the WITHDRAWN ROUTES field need not be
 present.

4.4 KEEPALIVE Message Format

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

Rekhter & Li [Page 17] RFC 1771 BGP-4 March 1995

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

4.5 NOTIFICATION Message Format

 A NOTIFICATION message is sent when an error condition is detected.
 The BGP connection is closed immediately after sending it.
 In addition to the fixed-size BGP header, the NOTIFICATION message
 contains the following fields:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Error code    | Error subcode |           Data                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Error Code:
       This 1-octet unsigned integer indicates the type of
       NOTIFICATION.  The following Error Codes have been defined:
          Error Code       Symbolic Name               Reference
            1         Message Header Error             Section 6.1
            2         OPEN Message Error               Section 6.2
            3         UPDATE Message Error             Section 6.3
            4         Hold Timer Expired               Section 6.5
            5         Finite State Machine Error       Section 6.6
            6         Cease                            Section 6.7
    Error subcode:
       This 1-octet unsigned integer provides more specific
       information about the nature of the reported error.  Each Error
       Code may have one or more Error Subcodes associated with it.

Rekhter & Li [Page 18] RFC 1771 BGP-4 March 1995

       If no appropriate Error Subcode is defined, then a zero
       (Unspecific) value is used for the Error Subcode field.
       Message Header Error subcodes:
                             1  - Connection Not Synchronized.
                             2  - Bad Message Length.
                             3  - Bad Message Type.
       OPEN Message Error subcodes:
                             1  - Unsupported Version Number.
                             2  - Bad Peer AS.
                             3  - Bad BGP Identifier. '
       4  - Unsupported Optional Parameter.
                             5  - Authentication Failure.
                                         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 - AS Routing Loop.
                             8 - Invalid NEXT_HOP Attribute.
                             9 - Optional Attribute Error.
                            10 - Invalid Network Field.
                            11 - Malformed AS_PATH.
    Data:
       This variable-length field is used to diagnose the reason for
       the NOTIFICATION.  The contents of the Data field depend upon
       the Error Code and Error Subcode.  See Section 6 below for more
       details.
       Note that the length of the Data field can be determined from
       the message Length field by the formula:
                Message Length = 21 + Data Length
 The minimum length of the NOTIFICATION message is 21 octets
 (including message header).

Rekhter & Li [Page 19] RFC 1771 BGP-4 March 1995

5. Path Attributes

 This section discusses the path attributes of the UPDATE message.
 Path attributes fall into four separate categories:
             1. Well-known mandatory.
             2. Well-known discretionary.
             3. Optional transitive.
             4. Optional non-transitive.
 Well-known attributes must be recognized by all BGP implementations.
 Some of these attributes are mandatory and must be included in every
 UPDATE message.  Others are discretionary and may or may not be sent
 in a particular UPDATE message.
 All well-known attributes must be passed along (after proper
 updating, if necessary) to other BGP peers.
 In addition to well-known attributes, each path may contain one or
 more optional attributes.  It is not required or expected that all
 BGP implementations support all optional attributes.  The handling of
 an unrecognized optional attribute is determined by the setting of
 the Transitive bit in the attribute flags octet.  Paths with
 unrecognized transitive optional attributes should be accepted. If a
 path with unrecognized transitive optional attribute is accepted and
 passed along to other BGP peers, then the unrecognized transitive
 optional attribute of that path must be passed along with the path to
 other BGP peers with the Partial bit in the Attribute Flags octet set
 to 1. If a path with recognized transitive optional attribute is
 accepted and passed along to other BGP peers and the Partial bit in
 the Attribute Flags octet is set to 1 by some previous AS, it is not
 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 AS 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 ASs in the path.

Rekhter & Li [Page 20] RFC 1771 BGP-4 March 1995

 The sender of an UPDATE message should order path attributes within
 the UPDATE message in ascending order of attribute type.  The
 receiver of an UPDATE message must be prepared to handle path
 attributes within the UPDATE message that are out of order.
 The same attribute cannot appear more than once within the Path
 Attributes field of a particular UPDATE message.

5.1 Path Attribute Usage

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

5.1.1 ORIGIN

 ORIGIN is a well-known mandatory attribute.  The ORIGIN attribute
 shall be generated by the autonomous system that originates the
 associated routing information. It shall be included in the UPDATE
 messages of all BGP speakers that choose to propagate this
 information to other BGP speakers.

5.1.2 AS_PATH

 AS_PATH is a well-known mandatory attribute. This attribute
 identifies the autonomous systems through which routing information
 carried in this UPDATE message has passed. The components of this
 list can be AS_SETs or AS_SEQUENCEs.
 When a BGP speaker propagates a route which it has learned from
 another BGP speaker's UPDATE message, it shall modify 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 another BGP
    speaker located in its own autonomous system, the advertising
    speaker shall not modify the AS_PATH attribute associated with the
    route.
    b) When a given BGP speaker advertises the route to a BGP speaker
    located in a neighboring autonomous system, then the advertising
    speaker shall update the AS_PATH attribute as follows:
       1) if the first path segment of the AS_PATH is of type
       AS_SEQUENCE, the local system shall prepend its own AS number
       as the last element of the sequence (put it in the leftmost
       position).

Rekhter & Li [Page 21] RFC 1771 BGP-4 March 1995

       2) if the first path segment of the AS_PATH is of type AS_SET,
       the local system shall prepend a new path segment of type
       AS_SEQUENCE to the AS_PATH, including its own AS number in that
       segment.
    When a BGP speaker originates a route then:
       a) the originating speaker shall include its own AS number in
       the AS_PATH attribute of all UPDATE messages sent to BGP
       speakers located in neighboring autonomous systems. (In this
       case, the AS number of the originating speaker's autonomous
       system will be the only entry in the AS_PATH attribute).
       b) the originating speaker shall include an empty AS_PATH
       attribute in all UPDATE messages sent to BGP speakers located
       in its own autonomous system. (An empty AS_PATH attribute is
       one whose length field contains the value zero).

5.1.3 NEXT_HOP

 The NEXT_HOP path attribute defines the IP address of the border
 router that should be used as the next hop to the destinations listed
 in the UPDATE message.  If a border router belongs to the same AS as
 its peer, then the peer is an internal border router. Otherwise, it
 is an external border router.  A BGP speaker can advertise any
 internal border router as the next hop provided that the interface
 associated with the IP address of this border router (as specified in
 the NEXT_HOP path attribute) shares a common subnet with both the
 local and remote BGP speakers. A BGP speaker can advertise any
 external border router as the next hop, provided that the IP address
 of this border router was learned from one of the BGP speaker's
 peers, and the interface associated with the IP address of this
 border router (as specified in the NEXT_HOP path attribute) shares a
 common subnet with the local and remote BGP speakers.  A BGP speaker
 needs to be able to support disabling advertisement of external
 border routers.
 A BGP speaker must never advertise an address of a peer to that peer
 as a NEXT_HOP, for a route that the speaker is originating.  A BGP
 speaker must never install a route with itself as the next hop.
 When a BGP speaker advertises the route to a BGP speaker located in
 its own autonomous system, the advertising speaker shall not modify
 the NEXT_HOP attribute associated with the route.  When a BGP speaker
 receives the route via an internal link, it may forward packets to
 the NEXT_HOP address if the address contained in the attribute is on
 a common subnet with the local and remote BGP speakers.

Rekhter & Li [Page 22] RFC 1771 BGP-4 March 1995

5.1.4 MULTI_EXIT_DISC

 The MULTI_EXIT_DISC attribute may 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 which is called a metric.  All other factors
 being equal, the exit or entry point with lower metric should be
 preferred.  If received over external links, the MULTI_EXIT_DISC
 attribute may be propagated over internal links to other BGP speakers
 within the same AS.  The MULTI_EXIT_DISC attribute is never
 propagated to other BGP speakers in neighboring AS's.

5.1.5 LOCAL_PREF

 LOCAL_PREF is a well-known discretionary attribute that shall be
 included in all UPDATE messages that a given BGP speaker sends to the
 other BGP speakers located in its own autonomous system. A BGP
 speaker shall calculate the degree of preference for each external
 route and include the degree of preference when advertising a route
 to its internal peers. The higher degree of preference should be
 preferred. A BGP speaker shall use the degree of preference learned
 via LOCAL_PREF in its decision process (see section 9.1.1).
 A BGP speaker shall not include this attribute in UPDATE messages
 that it sends to BGP speakers located in a neighboring autonomous
 system. If it is contained in an UPDATE message that is received from
 a BGP speaker which is not located in the same autonomous system as
 the receiving speaker, then this attribute shall be ignored by the
 receiving speaker.

5.1.6 ATOMIC_AGGREGATE

 ATOMIC_AGGREGATE is a well-known discretionary attribute.  If a BGP
 speaker, when presented with a set of overlapping routes from one of
 its peers (see 9.1.4), selects the less specific route without
 selecting the more specific one, then the local system shall attach
 the ATOMIC_AGGREGATE attribute to the route when propagating it to
 other BGP speakers (if that attribute is not already present in the
 received less specific route). A BGP speaker that receives a route
 with the ATOMIC_AGGREGATE attribute shall not remove the attribute
 from the route when propagating it to other speakers. A BGP speaker
 that receives a route with the ATOMIC_AGGREGATE attribute shall 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
 cognizant of the fact that the actual path to destinations, as
 specified in the NLRI of the route, while having the loop-free
 property, may traverse ASs that are not listed in the AS_PATH

Rekhter & Li [Page 23] RFC 1771 BGP-4 March 1995

 attribute.

5.1.7 AGGREGATOR

 AGGREGATOR is an optional transitive attribute which may be included
 in updates which are formed by aggregation (see Section 9.2.4.2).  A
 BGP speaker which performs route aggregation may add the AGGREGATOR
 attribute which shall contain its own AS number and IP address.

6. BGP Error Handling.

 This section describes actions to be taken when errors are detected
 while processing BGP messages.
 When any of the conditions described here are detected, a
 NOTIFICATION message with the indicated Error Code, Error Subcode,
 and Data fields is sent, and the BGP connection is closed.  If no
 Error Subcode is specified, then a zero must be used.
 The phrase "the BGP connection is closed" means that the transport
 protocol connection has been closed and that all resources for that
 BGP connection have been deallocated.  Routing table entries
 associated with the remote peer are marked as invalid.  The fact that
 the routes have become invalid is passed to other BGP peers before
 the 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.

6.1 Message Header error handling.

 All errors detected while processing the Message Header are indicated
 by sending the NOTIFICATION message with 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 message type is OPEN.  The expected value of the Marker
 field for all other types of BGP messages determined based on the
 presence of the Authentication Information Optional Parameter in the
 BGP OPEN message and the actual authentication mechanism (if the
 Authentication Information in the BGP OPEN message is present). If
 the Marker field of the message header is not the expected one, then
 a synchronization error has occurred and the Error Subcode is set to
 Connection Not Synchronized.

Rekhter & Li [Page 24] RFC 1771 BGP-4 March 1995

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

6.2 OPEN message error handling.

 All errors detected while processing the OPEN message are indicated
 by sending the NOTIFICATION message with Error Code OPEN Message
 Error.  The Error Subcode elaborates on the specific nature of the
 error.
 If the version number contained in the Version field of the received
 OPEN message is not supported, then the Error Subcode is 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 the
 received OPEN message).
 If the Autonomous System field of the OPEN message is unacceptable,
 then the Error Subcode is 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 which accepts a Hold Time MUST use the negotiated
 value for the Hold Time.
 If the BGP Identifier field of the OPEN message is syntactically
 incorrect, then the Error Subcode is set to Bad BGP Identifier.
 Syntactic correctness means that the BGP Identifier field represents
 a valid IP host address.
 If one of the Optional Parameters in the OPEN message is not
 recognized, then the Error Subcode is set to Unsupported Optional
 Parameters.

Rekhter & Li [Page 25] RFC 1771 BGP-4 March 1995

 If the OPEN message carries Authentication Information (as an
 Optional Parameter), then the corresponding authentication procedure
 is invoked.  If the authentication procedure (based on Authentication
 Code and Authentication Data) fails, then the Error Subcode is set to
 Authentication Failure.

6.3 UPDATE message error handling.

 All errors detected while processing the UPDATE message are indicated
 by sending the NOTIFICATION message with 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 Unfeasible Routes Length or Total Attribute
 Length is too large (i.e., if Unfeasible Routes Length + Total
 Attribute Length + 23 exceeds the message Length), then the Error
 Subcode is set to Malformed Attribute List.
 If any recognized attribute has Attribute Flags that conflict with
 the Attribute Type Code, then the Error Subcode is set to Attribute
 Flags Error.  The Data field contains the erroneous attribute (type,
 length and value).
 If any recognized attribute has Attribute Length that conflicts with
 the expected length (based on the attribute type code), then the
 Error Subcode is set to Attribute Length Error.  The Data field
 contains the erroneous attribute (type, length and value).
 If any of the mandatory well-known attributes are not present, then
 the Error Subcode is set to Missing Well-known Attribute.  The Data
 field contains the Attribute Type Code of the missing well-known
 attribute.
 If any of the mandatory well-known attributes are not recognized,
 then the Error Subcode is set to Unrecognized Well-known Attribute.
 The Data field contains the unrecognized attribute (type, length and
 value).
 If the ORIGIN attribute has an undefined value, then the Error
 Subcode is set to Invalid Origin Attribute.  The Data field contains
 the unrecognized attribute (type, length and value).
 If the NEXT_HOP attribute field is syntactically incorrect, then the
 Error Subcode is set to Invalid NEXT_HOP Attribute.  The Data field
 contains the incorrect attribute (type, length and value).  Syntactic
 correctness means that the NEXT_HOP attribute represents a valid IP
 host address.  Semantic correctness applies only to the external BGP

Rekhter & Li [Page 26] RFC 1771 BGP-4 March 1995

 links. It means that the interface associated with the IP address, as
 specified in the NEXT_HOP attribute, shares a common subnet with the
 receiving BGP speaker and is not the IP address of the receiving BGP
 speaker.  If the NEXT_HOP attribute is semantically incorrect, the
 error should be logged, and the the route should be ignored.  In this
 case, no NOTIFICATION message should be sent.
 The AS_PATH attribute is checked for syntactic correctness.  If the
 path is syntactically incorrect, then the Error Subcode is set to
 Malformed AS_PATH.
 If an optional attribute is recognized, then the value of this
 attribute is checked.  If an error is detected, the attribute is
 discarded, and the Error Subcode is set to Optional Attribute Error.
 The Data field contains the attribute (type, length and value).
 If any attribute appears more than once in the UPDATE message, then
 the Error Subcode is 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 is set to Invalid Network Field.

6.4 NOTIFICATION message error handling.

 If a peer sends a NOTIFICATION message, and there is an error in that
 message, there is unfortunately no means of reporting this error via
 a subsequent NOTIFICATION message.  Any such error, such as an
 unrecognized Error Code or Error Subcode, should be noticed, logged
 locally, and brought to the attention of the administration of the
 peer.  The means to do this, however, lies outside the scope of this
 document.

6.5 Hold Timer Expired error handling.

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

6.6 Finite State Machine error handling.

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

Rekhter & Li [Page 27] RFC 1771 BGP-4 March 1995

6.7 Cease.

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

6.8 Connection collision detection.

 If a pair of BGP speakers try simultaneously to establish a TCP
 connection to each other, then two parallel connections between this
 pair of speakers might well be formed.  We refer to this situation as
 connection collision.  Clearly, one of these connections must be
 closed.
 Based on the value of the BGP Identifier a convention is established
 for detecting which BGP connection is to be preserved when a
 collision does occur. The convention is to compare the BGP
 Identifiers of the peers involved in the collision and to retain only
 the connection initiated by the BGP speaker with the higher-valued
 BGP Identifier.
 Upon receipt of an OPEN message, the local system must examine all of
 its connections that are in the OpenConfirm state.  A BGP speaker may
 also examine connections in an OpenSent state if it knows the BGP
 Identifier of the peer by means outside of the protocol.  If among
 these connections there is a connection to a remote BGP speaker whose
 BGP Identifier equals the one in the OPEN message, 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).
    2. If the value of the local BGP Identifier is less than the
    remote one, the local system closes BGP connection that already
    exists (the one that is already in the OpenConfirm state), and
    accepts BGP connection initiated by the remote system.
    3. Otherwise, the local system closes newly created BGP connection
    (the one associated with the newly received OPEN message), and
    continues to use the existing one (the one that is already in the
    OpenConfirm state).
    Comparing BGP Identifiers is done by treating them as (4-octet
    long) unsigned integers.

Rekhter & Li [Page 28] RFC 1771 BGP-4 March 1995

    A connection collision with an existing BGP connection that is in
    Established states causes unconditional closing of the newly
    created connection. Note that a connection collision cannot be
    detected with connections that are in Idle, or Connect, or Active
    states.
    Closing the BGP connection (that results from the collision
    resolution procedure) is accomplished by sending the NOTIFICATION
    message with the Error Code Cease.

7. BGP Version Negotiation.

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

8. BGP Finite State machine.

 This section specifies BGP operation in terms of a Finite State
 Machine (FSM).  Following is a brief summary and overview of BGP
 operations by state as determined by this FSM.  A condensed version
 of the BGP FSM is found in Appendix 1.
    Initially BGP is in the Idle state.
    Idle state:
       In this state BGP refuses all incoming BGP connections.  No
       resources are allocated to the peer.  In response to the Start
       event (initiated by either system or operator) the local system
       initializes all BGP resources, starts the ConnectRetry timer,
       initiates a transport connection to other BGP peer, while
       listening for connection that may be initiated by the remote
       BGP peer, and changes its state to Connect.  The exact value of
       the ConnectRetry timer is a local matter, but should be
       sufficiently large to allow TCP initialization.
       If a BGP speaker detects an error, it shuts down the connection
       and changes its state to Idle. Getting out of the Idle state

Rekhter & Li [Page 29] RFC 1771 BGP-4 March 1995

       requires generation of the Start event.  If such an event is
       generated automatically, then persistent BGP errors may result
       in persistent flapping of the speaker.  To avoid such a
       condition it is recommended that Start events should not be
       generated immediately for a peer that was previously
       transitioned to Idle due to an error. For a peer that was
       previously transitioned to Idle due to an error, the time
       between consecutive generation of Start events, if such events
       are generated automatically, shall exponentially increase. The
       value of the initial timer shall be 60 seconds. The time shall
       be doubled for each consecutive retry.
       Any other event received in the Idle state is ignored.
    Connect state:
       In this state BGP is waiting for the transport protocol
       connection to be completed.
       If the transport protocol connection succeeds, the local system
       clears the ConnectRetry timer, completes initialization, sends
       an OPEN message to its peer, and changes its state to OpenSent.
       If the transport protocol connect fails (e.g., retransmission
       timeout), the local system restarts the ConnectRetry timer,
       continues to listen for a connection that may be initiated by
       the remote BGP peer, and changes its state to Active state.
       In response to the ConnectRetry timer expired event, the local
       system restarts the ConnectRetry timer, initiates a transport
       connection to other BGP peer, continues to listen for a
       connection that may be initiated by the remote BGP peer, and
       stays in the Connect state.
       Start event is ignored in the Active state.
       In response to any other event (initiated by either system or
       operator), the local system releases all BGP resources
       associated with this connection and changes its state to Idle.
    Active state:
       In this state BGP is trying to acquire a peer by initiating a
       transport protocol connection.
       If the transport protocol connection succeeds, the local system
       clears the ConnectRetry timer, completes initialization, sends
       an OPEN message to its peer, sets its Hold Timer to a large

Rekhter & Li [Page 30] RFC 1771 BGP-4 March 1995

       value, and changes its state to OpenSent.  A Hold Timer value
       of 4 minutes is suggested.
       In response to the ConnectRetry timer expired event, the local
       system restarts the ConnectRetry timer, initiates a transport
       connection to other BGP peer, continues to listen for a
       connection that may be initiated by the remote BGP peer, and
       changes its state to Connect.
       If the local system detects that a remote peer is trying to
       establish BGP connection to it, and the IP address of the
       remote peer is not an expected one, the local system restarts
       the ConnectRetry timer, rejects the attempted connection,
       continues to listen for a connection that may be initiated by
       the remote BGP peer, and stays in the Active state.
       Start event is ignored in the Active state.
       In response to any other event (initiated by either system or
       operator), the local system releases all BGP resources
       associated with this connection and changes its state to Idle.
    OpenSent state:
       In this state BGP waits for an OPEN message from its peer.
       When an OPEN message is received, all fields are checked for
       correctness.  If the BGP message header checking or OPEN
       message checking detects an error (see Section 6.2), or a
       connection collision (see Section 6.8) the local system sends a
       NOTIFICATION message and changes its state to Idle.
       If there are no errors in the OPEN message, BGP sends a
       KEEPALIVE message and sets a KeepAlive timer.  The Hold Timer,
       which was originally set to a large value (see above), is
       replaced with the negotiated Hold Time value (see section 4.2).
       If the negotiated Hold Time value is zero, then the Hold Time
       timer and KeepAlive timers 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 "external".  (This will effect UPDATE
       processing as described below.)  Finally, the state is changed
       to OpenConfirm.
       If a disconnect notification is received from the underlying
       transport protocol, the local system closes the BGP connection,
       restarts the ConnectRetry timer, while continue listening for
       connection that may be initiated by the remote BGP peer, and
       goes into the Active state.

Rekhter & Li [Page 31] RFC 1771 BGP-4 March 1995

       If the Hold Timer expires, the local system sends NOTIFICATION
       message with error code Hold Timer Expired and changes its
       state to Idle.
       In response to the Stop event (initiated by either system or
       operator) the local system sends NOTIFICATION message with
       Error Code Cease and changes its state to Idle.
       Start event is ignored in the OpenSent state.
       In response to any other event the local system sends
       NOTIFICATION message with Error Code Finite State Machine Error
       and changes its state to Idle.
       Whenever BGP changes its state from OpenSent to Idle, it closes
       the BGP (and transport-level) connection and releases all
       resources associated with that connection.
    OpenConfirm state:
       In this state BGP waits for a KEEPALIVE or NOTIFICATION
       message.
       If the local system receives a KEEPALIVE message, it changes
       its state to Established.
       If the Hold Timer expires before a KEEPALIVE message is
       received, the local system sends NOTIFICATION message with
       error code Hold Timer Expired and changes its state to Idle.
       If the local system receives a NOTIFICATION message, it changes
       its state to Idle.
       If the KeepAlive timer expires, the local system sends a
       KEEPALIVE message and restarts its KeepAlive timer.
       If a disconnect notification is received from the underlying
       transport protocol, the local system changes its state to Idle.
       In response to the Stop event (initiated by either system or
       operator) the local system sends NOTIFICATION message with
       Error Code Cease and changes its state to Idle.
       Start event is ignored in the OpenConfirm state.
       In response to any other event the local system sends
       NOTIFICATION message with Error Code Finite State Machine Error
       and changes its state to Idle.

Rekhter & Li [Page 32] RFC 1771 BGP-4 March 1995

       Whenever BGP changes its state from OpenConfirm to Idle, it
       closes the BGP (and transport-level) connection and releases
       all resources associated with that connection.
    Established state:
       In the Established state BGP can exchange UPDATE, NOTIFICATION,
       and KEEPALIVE messages with its peer.
       If the local system receives an UPDATE or KEEPALIVE message, it
       restarts its Hold Timer, if the negotiated Hold Time value is
       non-zero.
       If the local system receives a NOTIFICATION message, it changes
       its state to Idle.
       If the local system receives an UPDATE message and the UPDATE
       message error handling procedure (see Section 6.3) detects an
       error, the local system sends a NOTIFICATION message and
       changes its state to Idle.
       If a disconnect notification is received from the underlying
       transport protocol, the local system changes its state to Idle.
       If the Hold Timer expires, the local system sends a
       NOTIFICATION message with Error Code Hold Timer Expired and
       changes its state to Idle.
       If the KeepAlive timer expires, the local system sends a
       KEEPALIVE message and restarts its KeepAlive timer.
       Each time the local system sends a KEEPALIVE or UPDATE message,
       it restarts its KeepAlive timer, unless the negotiated Hold
       Time value is zero.
       In response to the Stop event (initiated by either system or
       operator), the local system sends a NOTIFICATION message with
       Error Code Cease and changes its state to Idle.
       Start event is ignored in the Established state.
       In response to any other event, the local system sends
       NOTIFICATION message with Error Code Finite State Machine Error
       and changes its state to Idle.
       Whenever BGP changes its state from Established to Idle, it
       closes the BGP (and transport-level) connection, releases all
       resources associated with that connection, and deletes all

Rekhter & Li [Page 33] RFC 1771 BGP-4 March 1995

       routes derived from that connection.

9. UPDATE Message Handling

 An UPDATE message may be received only in the Established state.
 When an UPDATE message is received, each field is checked for
 validity as specified in Section 6.3.
 If an optional non-transitive attribute is unrecognized, it is
 quietly ignored.  If an optional transitive attribute is
 unrecognized, the Partial bit (the third high-order bit) in the
 attribute flags octet is set to 1, and the attribute is retained for
 propagation to other BGP speakers.
 If an optional attribute is recognized, and has a valid value, then,
 depending on the type of the optional attribute, it is processed
 locally, retained, and updated, if necessary, for possible
 propagation to other BGP speakers.
 If the UPDATE message contains a non-empty WITHDRAWN ROUTES field,
 the previously advertised routes whose destinations (expressed as IP
 prefixes) contained in this field shall be removed from the Adj-RIB-
 In.  This BGP speaker shall run its Decision Process since the
 previously advertised route is not longer available for use.
 If the UPDATE message contains a feasible route, it shall be placed
 in the appropriate Adj-RIB-In, and the following additional actions
 shall be taken:
 i) If its Network Layer Reachability Information (NLRI) is identical
 to the one of a route currently stored in the Adj-RIB-In, then the
 new route shall replace the older route in the Adj-RIB-In, thus
 implicitly withdrawing the older route from service. The BGP speaker
 shall run its Decision Process since the older route is no longer
 available for use.
 ii) If the new route is an overlapping route that is included (see
 9.1.4) in an earlier route contained in the Adj-RIB-In, the BGP
 speaker shall run its Decision Process since the more specific route
 has implicitly made a portion of the less specific route unavailable
 for use.
 iii) If the new route has identical path attributes to an earlier
 route contained in the Adj-RIB-In, and is more specific (see 9.1.4)
 than the earlier route, no further actions are necessary.
 iv) If the new route has NLRI that is not present in any of the
 routes currently stored in the Adj-RIB-In, then the new route shall

Rekhter & Li [Page 34] RFC 1771 BGP-4 March 1995

 be placed in the Adj-RIB-In. The BGP speaker shall run its Decision
 Process.
 v) If the new route is an overlapping route that is less specific
 (see 9.1.4) than an earlier route contained in the Adj-RIB-In, the
 BGP speaker shall run its Decision Process on the set of destinations
 described only by the less specific route.

9.1 Decision Process

 The Decision Process selects routes for subsequent advertisement by
 applying the policies in the local Policy Information Base (PIB) to
 the routes stored in its Adj-RIB-In. The output of the Decision
 Process is the set of routes that will be advertised to all peers;
 the selected routes will be stored in the local speaker's Adj-RIB-
 Out.
 The selection process is formalized by defining a function that takes
 the attribute of a given route as an argument and returns a non-
 negative integer denoting the degree of preference for the route.
 The function that calculates the degree of preference for a given
 route shall not use as its inputs any of the following:  the
 existence of other routes, the non-existence of other routes, or the
 path attributes of other routes. Route selection then consists of
 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 each Adj-RIB-In,
 and is responsible for:
  1. selection of routes to be advertised to BGP speakers located in

the local speaker's autonomous system

  1. selection of routes to be advertised to BGP speakers located in

neighboring autonomous systems

  1. route aggregation and route information reduction
 The Decision Process takes place in three distinct phases, each
 triggered by a different event:
    a) Phase 1 is responsible for calculating the degree of preference
    for each route received from a BGP speaker located in a
    neighboring autonomous system, and for advertising to the other
    BGP speakers in the local autonomous system the routes that have
    the highest degree of preference for each distinct destination.

Rekhter & Li [Page 35] RFC 1771 BGP-4 March 1995

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

9.1.1 Phase 1: Calculation of Degree of Preference

 The Phase 1 decision function shall be invoked whenever the local BGP
 speaker receives an UPDATE message from a peer located in a
 neighboring autonomous system that advertises a new route, a
 replacement route, or a withdrawn route.
 The Phase 1 decision function is a separate process which completes
 when it has no further work to do.
 The Phase 1 decision function shall lock an Adj-RIB-In prior to
 operating on any route contained within it, and shall unlock it after
 operating on all new or unfeasible routes contained within it.
 For each newly received or replacement feasible route, the local BGP
 speaker shall determine a degree of preference. If the route is
 learned from a BGP speaker in the local autonomous system, either the
 value of the LOCAL_PREF attribute shall be taken as the degree of
 preference, or the local system shall compute the degree of
 preference of the route based on preconfigured policy information. If
 the route is learned from a BGP speaker in a neighboring autonomous
 system, then the degree of preference shall be computed based on
 preconfigured policy information.  The exact nature of this policy
 information and the computation involved is a local matter.  The
 local speaker shall then run the internal update process of 9.2.1 to
 select and advertise the most preferable route.

9.1.2 Phase 2: Route Selection

 The Phase 2 decision function shall be 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 shall consider all
 routes that are present in the Adj-RIBs-In, including those received
 from BGP speakers located in its own autonomous system and those
 received from BGP speakers located in neighboring autonomous systems.

Rekhter & Li [Page 36] RFC 1771 BGP-4 March 1995

 The Phase 2 decision function shall be blocked from running while the
 Phase 3 decision function is in process. The Phase 2 function shall
 lock all Adj-RIBs-In prior to commencing its function, and shall
 unlock them on completion.
 If the NEXT_HOP attribute of a BGP route depicts an address to which
 the local BGP speaker doesn't have a route in its Loc-RIB, the BGP
 route SHOULD be excluded from the Phase 2 decision function.
 For each set of destinations for which a feasible route exists in the
 Adj-RIBs-In, the local BGP speaker shall identify 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 9.1.2.1.
 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. The local speaker MUST determine the immediate
 next hop to the address depicted by the NEXT_HOP attribute of the
 selected route by performing a lookup in the IGP and selecting one of
 the possible paths in the IGP.  This immediate next hop MUST be used
 when installing the selected route in the Loc-RIB.  If the route to
 the address depicted by the NEXT_HOP attribute changes such that the
 immediate next hop changes, route selection should be recalculated as
 specified above.
 Unfeasible routes shall be removed from the Loc-RIB, and
 corresponding unfeasible routes shall then be removed from the Adj-
 RIBs-In.

9.1.2.1 Breaking Ties (Phase 2)

 In its Adj-RIBs-In a BGP speaker may have several routes to the same
 destination that have the same degree of preference. The local
 speaker can select only one of these routes for inclusion in the
 associated Loc-RIB. The local speaker considers all equally
 preferable routes, both those received from BGP speakers located in
 neighboring autonomous systems, and those received from other BGP
 speakers located in the local speaker's autonomous system.
 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

Rekhter & Li [Page 37] RFC 1771 BGP-4 March 1995

 NEXT_HOP attribute of the route.  Ties shall be broken according to
 the following algorithm:
    a) If the local system is configured to take into account
    MULTI_EXIT_DISC, and the candidate routes differ in their
    MULTI_EXIT_DISC attribute, select the route that has the lowest
    value of the MULTI_EXIT_DISC attribute.
    b) Otherwise, select the route that has the lowest cost (interior
    distance) to the entity depicted by the NEXT_HOP attribute of the
    route.  If there are several routes with the same cost, then the
    tie-breaking shall be broken as follows:
  1. if at least one of the candidate routes was advertised by the

BGP speaker in a neighboring autonomous system, select the

       route that was advertised by the BGP speaker in a neighboring
       autonomous system whose BGP Identifier has the lowest value
       among all other BGP speakers in neighboring autonomous systems;
  1. otherwise, select the route that was advertised by the BGP

speaker whose BGP Identifier has the lowest value.

9.1.3 Phase 3: Route Dissemination

 The Phase 3 decision function shall be invoked on completion of Phase
 2, or when any of the following events occur:
    a) when routes in a Loc-RIB to local destinations have changed
    b) when locally generated routes learned by means outside of BGP
    have changed
    c) when a new BGP speaker - BGP speaker connection has been
    established
 The Phase 3 function is a separate process which completes when it
 has no further work to do. The Phase 3 Routing Decision function
 shall be blocked from running while the Phase 2 decision function is
 in process.
 All routes in the Loc-RIB shall be processed into a corresponding
 entry in the associated Adj-RIBs-Out. Route aggregation and
 information reduction techniques (see 9.2.4.1) may optionally be
 applied.
 For the benefit of future support of inter-AS multicast capabilities,
 a BGP speaker that participates in inter-AS multicast routing shall
 advertise a route it receives from one of its external peers and if

Rekhter & Li [Page 38] RFC 1771 BGP-4 March 1995

 it installs it in its Loc-RIB, it shall advertise it back to the peer
 from which the route was received. For a BGP speaker that does not
 participate in inter-AS multicast routing such an advertisement is
 optional. When doing such an advertisement, the NEXT_HOP attribute
 should be set to the address of the peer. An implementation may also
 optimize such an advertisement by truncating information in the
 AS_PATH attribute to include only its own AS number and that of the
 peer that advertised the route (such truncation requires the ORIGIN
 attribute to be set to INCOMPLETE).  In addition an implementation is
 not required to pass optional or discretionary path attributes with
 such an advertisement.
 When the updating of the Adj-RIBs-Out and the Forwarding Information
 Base (FIB) is complete, the local BGP speaker shall run the external
 update process of 9.2.2.

9.1.4 Overlapping Routes

 A BGP speaker may transmit routes with overlapping Network Layer
 Reachability Information (NLRI) to another BGP speaker. NLRI overlap
 occurs when a set of destinations are identified in non-matching
 multiple routes. Since BGP encodes NLRI using IP prefixes, overlap
 will always exhibit subset relationships.  A route describing a
 smaller set of destinations (a longer prefix) is said to be more
 specific than a route describing a larger set of destinations (a
 shorted prefix); similarly, a route describing a larger set of
 destinations (a shorter prefix) is said to be less specific than a
 route describing a smaller set of destinations (a longer prefix).
 The precedence relationship effectively decomposes less specific
 routes into two parts:
  1. a set of destinations described only by the less specific

route, and

  1. a set of destinations described by the overlap of the less

specific and the more specific routes

 When overlapping routes are present in the same Adj-RIB-In, the more
 specific route shall take precedence, in order from more specific to
 least specific.
 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.

Rekhter & Li [Page 39] RFC 1771 BGP-4 March 1995

 If a BGP speaker receives overlapping routes, the Decision Process
 shall take into account the semantics of the overlapping routes. In
 particular, if a BGP speaker accepts the less specific route while
 rejecting the more specific route from the same peer, then the
 destinations represented by the overlap may not forward along the ASs
 listed in the AS_PATH attribute of that route. Therefore, a BGP
 speaker has the following choices:
    a)   Install both the less and the more specific routes
    b)   Install the more specific route only
    c)   Install the non-overlapping part of the less specific
               route only (that implies de-aggregation)
    d)   Aggregate the two routes and install the aggregated route
    e)   Install the less specific route only
    f)   Install neither route
 If a BGP speaker chooses e), then it should add ATOMIC_AGGREGATE
 attribute to the route. A route that carries ATOMIC_AGGREGATE
 attribute can not be de-aggregated. That is, the NLRI of this route
 can not be made more specific.  Forwarding along such a route does
 not guarantee that IP packets will actually traverse only ASs listed
 in the AS_PATH attribute of the route.  If a BGP speaker chooses a),
 it must not advertise the more general route without the more
 specific route.

9.2 Update-Send Process

 The Update-Send process is responsible for advertising UPDATE
 messages to all peers. For example, it distributes the routes chosen
 by the Decision Process to other BGP speakers which may be located in
 either the same autonomous system or a neighboring autonomous system.
 rules for information exchange between BGP speakers located in
 different autonomous systems are given in 9.2.2; rules for
 information exchange between BGP speakers located in the same
 autonomous system are given in 9.2.1.
 Distribution of routing information between a set of BGP speakers,
 all of which are located in the same autonomous system, is referred
 to as internal distribution.

Rekhter & Li [Page 40] RFC 1771 BGP-4 March 1995

9.2.1 Internal Updates

 The Internal update process is concerned with the distribution of
 routing information to BGP speakers located in the local speaker's
 autonomous system.
 When a BGP speaker receives an UPDATE message from another BGP
 speaker located in its own autonomous system, the receiving BGP
 speaker shall not re-distribute the routing information contained in
 that UPDATE message to other BGP speakers located in its own
 autonomous system.
 When a BGP speaker receives a new route from a BGP speaker in a
 neighboring autonomous system, it shall advertise that route to all
 other BGP speakers in its autonomous system by means of an UPDATE
 message if any of the following conditions occur:
    1) the degree of preference assigned to the newly received route
    by the local BGP speaker is higher than the degree of preference
    that the local speaker has assigned to other routes that have been
    received from BGP speakers in neighboring autonomous systems, or
    2) there are no other routes that have been received from BGP
    speakers in neighboring autonomous systems, or
    3) the newly received route is selected as a result of breaking a
    tie between several routes which have the highest degree of
    preference, and the same destination (the tie-breaking procedure
    is specified in 9.2.1.1).
 When a BGP speaker receives an UPDATE message with a non-empty
 WITHDRAWN ROUTES field, it shall remove from its Adj-RIB-In all
 routes whose destinations was carried in this field (as IP prefixes).
 The speaker shall take the following additional steps:
    1) if the corresponding feasible route had not been previously
    advertised, then no further action is necessary
    2) if the corresponding feasible route had been previously
    advertised, then:
       i) if a new route is selected for advertisement that has the
       same Network Layer Reachability Information as the unfeasible
       routes, then the local BGP speaker shall advertise the
       replacement route
       ii) if a replacement route is not available for advertisement,
       then the BGP speaker shall include the destinations  of the

Rekhter & Li [Page 41] RFC 1771 BGP-4 March 1995

       unfeasible route (in form of IP prefixes) in the WITHDRAWN
       ROUTES field of an UPDATE message, and shall send this message
       to each peer to whom it had previously advertised the
       corresponding feasible route.
 All feasible routes which are advertised shall be placed in the
 appropriate Adj-RIBs-Out, and all unfeasible routes which are
 advertised shall be removed from the Adj-RIBs-Out.

9.2.1.1 Breaking Ties (Internal Updates)

 If a local BGP speaker has connections to several BGP speakers in
 neighboring autonomous systems, there will be multiple Adj-RIBs-In
 associated with these peers. These Adj-RIBs-In might contain several
 equally preferable routes to the same destination, all of which were
 advertised by BGP speakers located in neighboring autonomous systems.
 The local BGP speaker shall select one of these routes according to
 the following rules:
    a) If the candidate route differ only in their NEXT_HOP and
    MULTI_EXIT_DISC attributes, and the local system is configured to
    take into account MULTI_EXIT_DISC attribute, select the routes
    that has the lowest value of the MULTI_EXIT_DISC attribute.
    b) If the local system can ascertain the cost of a path to the
    entity depicted by the NEXT_HOP attribute of the candidate route,
    select the route with the lowest cost.
    c) In all other cases, select the route that was advertised by the
    BGP speaker whose BGP Identifier has the lowest value.

9.2.2 External Updates

 The external update process is concerned with the distribution of
 routing information to BGP speakers located in neighboring autonomous
 systems. As part of Phase 3 route selection process, the BGP speaker
 has updated its Adj-RIBs-Out and its Forwarding Table. All newly
 installed routes and all newly unfeasible routes for which there is
 no replacement route shall be advertised to BGP speakers located in
 neighboring autonomous systems by means of UPDATE message.
 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.

Rekhter & Li [Page 42] RFC 1771 BGP-4 March 1995

9.2.3 Controlling Routing Traffic Overhead

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

9.2.3.1 Frequency of Route Advertisement

 The parameter MinRouteAdvertisementInterval determines the minimum
 amount of time that must elapse between advertisement of routes to a
 particular destination from a single BGP speaker. This rate limiting
 procedure applies on a per-destination basis, although the value of
 MinRouteAdvertisementInterval is set on a per BGP peer basis.
 Two UPDATE messages sent from a single BGP speaker that advertise
 feasible routes to some common set of destinations received from BGP
 speakers in neighboring autonomous systems must be separated by at
 least MinRouteAdvertisementInterval. Clearly, this can only be
 achieved precisely by keeping a separate timer for each common set of
 destinations. This would be unwarranted overhead. Any technique which
 ensures that the interval between two UPDATE messages sent from a
 single BGP speaker that advertise feasible routes to some common set
 of destinations received from BGP speakers in neighboring autonomous
 systems will be at least MinRouteAdvertisementInterval, and will also
 ensure a constant upper bound on the interval is acceptable.
 Since fast convergence is needed within an autonomous system, this
 procedure does not apply for routes receives from other BGP speakers
 in the same autonomous system. To avoid long-lived black holes, the
 procedure does not apply to the explicit withdrawal of unfeasible
 routes (that is, routes whose destinations (expressed as IP prefixes)
 are listed in the WITHDRAWN ROUTES field of an UPDATE message).
 This procedure does not limit the rate of route selection, but only
 the rate of route advertisement. If new routes are selected multiple
 times while awaiting the expiration of MinRouteAdvertisementInterval,
 the last route selected shall be advertised at the end of
 MinRouteAdvertisementInterval.

9.2.3.2 Frequency of Route Origination

 The parameter MinASOriginationInterval 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.

Rekhter & Li [Page 43] RFC 1771 BGP-4 March 1995

9.2.3.3 Jitter

 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 MinASOriginationInterval, Keepalive, and
 MinRouteAdvertisementInterval. A given BGP speaker shall apply the
 same jitter to each of these quantities regardless of the
 destinations to which the updates are being sent; that is, jitter
 will not be applied on a "per peer" basis.
 The amount of jitter to be introduced 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.

9.2.4 Efficient Organization of Routing Information

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

9.2.4.1 Information Reduction

 Information reduction may imply a reduction in granularity of policy
 control - after information is collapsed, the same policies will
 apply to all destinations and paths in the equivalence class.
 The Decision Process may optionally reduce the amount of information
 that it will place in the Adj-RIBs-Out by any of the following
 methods:
    a)   Network Layer Reachability Information (NLRI):
    Destination IP addresses can be represented as IP address
    prefixes.  In cases where there is a correspondence between the
    address structure and the systems under control of an autonomous
    system administrator, it will be possible to reduce the size of
    the NLRI carried in the UPDATE messages.
    b)   AS_PATHs:
    AS path information can be represented as ordered AS_SEQUENCEs or
    unordered AS_SETs. AS_SETs are used in the route aggregation
    algorithm described in 9.2.4.2. They reduce the size of the
    AS_PATH information by listing each AS number only once,
    regardless of how many times it may have appeared in multiple
    AS_PATHs that were aggregated.

Rekhter & Li [Page 44] RFC 1771 BGP-4 March 1995

    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, since such paths are no
    longer listed individually as in the form of AS_SEQUENCEs.  In
    practice this is not likely to be a problem, since once an IP
    packet arrives at the edge of a group of autonomous systems, the
    BGP speaker at that point is likely to have more detailed path
    information and can distinguish individual paths to destinations.

9.2.4.2 Aggregating Routing Information

 Aggregation is the process of combining the characteristics of
 several different routes in such a way that a single route can be
 advertised.  Aggregation can occur as part of the decision  process
 to reduce the amount of routing information that will be placed in
 the Adj-RIBs-Out.
 Aggregation reduces the amount of information that a BGP speaker must
 store and exchange with other BGP speakers. Routes can be aggregated
 by applying the following procedure separately to path attributes of
 like type and to the Network Layer Reachability Information.
 Routes that have the following attributes shall not be aggregated
 unless the corresponding attributes of each route are identical:
 MULTI_EXIT_DISC, NEXT_HOP.
 Path attributes that have different type codes can not be aggregated
 together. Path of the same type code may be aggregated, according to
 the following rules:
    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 case the value of the ORIGIN attribute of the aggregated
    route is INTERNAL.
    AS_PATH attribute: If routes to be aggregated have identical
    AS_PATH attributes, then the aggregated route has the same AS_PATH
    attribute as each individual route.
    For the purpose of aggregating AS_PATH attributes we model each AS
    within the AS_PATH attribute as a tuple <type, value>, where
    "type" identifies a type of the path segment the AS belongs to

Rekhter & Li [Page 45] RFC 1771 BGP-4 March 1995

    (e.g. AS_SEQUENCE, AS_SET), and "value" is the AS number.  If the
    routes to be aggregated have different AS_PATH attributes, then
    the aggregated AS_PATH attribute shall satisfy all of the
    following conditions:
  1. all tuples of the type AS_SEQUENCE in the aggregated AS_PATH

shall appear in all of the AS_PATH in the initial set of routes

       to be aggregated.
  1. all tuples of the type AS_SET in the aggregated AS_PATH shall

appear in at least one of the AS_PATH in the initial set (they

       may appear as either AS_SET or AS_SEQUENCE types).
  1. for any tuple X of the type AS_SEQUENCE in the aggregated

AS_PATH which precedes tuple Y in the aggregated AS_PATH, X

       precedes Y in each AS_PATH in the initial set which contains Y,
       regardless of the type of Y.
  1. No tuple with the same value shall appear more than once in

the aggregated AS_PATH, regardless of the tuple's type.

    An implementation may choose any algorithm which conforms to these
    rules.  At a minimum a conformant implementation shall be able to
    perform the following algorithm that meets all of the above
    conditions:
  1. determine the longest leading sequence of tuples (as defined

above) common to all the AS_PATH attributes of the routes to be

       aggregated. Make this sequence the leading sequence of the
       aggregated AS_PATH attribute.
  1. set the type of the rest of the tuples from the AS_PATH

attributes of the routes to be aggregated to AS_SET, and append

       them to the aggregated AS_PATH attribute.
  1. if the aggregated AS_PATH has more than one tuple with the

same value (regardless of tuple's type), eliminate all, but one

       such tuple by deleting tuples of the type AS_SET from the
       aggregated AS_PATH attribute.
    Appendix 6, section 6.8 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.

Rekhter & Li [Page 46] RFC 1771 BGP-4 March 1995

    AGGREGATOR: All AGGREGATOR attributes of all routes to be
    aggregated should be ignored.

9.3 Route Selection Criteria

 Generally speaking, additional rules for comparing routes among
 several alternatives are outside the scope of this document.  There
 are two exceptions:
  1. If the local AS appears in the AS path of the new route being

considered, then that new route cannot be viewed as better than

    any other route.  If such a route were ever used, a routing loop
    would result.
  1. In order to achieve successful distributed operation, only

routes with a likelihood of stability can be chosen. Thus, an AS

    must avoid using unstable routes, and it must not make rapid
    spontaneous changes to its choice of route.  Quantifying the terms
    "unstable" and "rapid" in the previous sentence will require
    experience, but the principle is clear.

9.4 Originating BGP routes

 A BGP speaker may originate BGP routes by injecting routing
 information acquired by some other means (e.g. via an IGP) into BGP.
 A BGP speaker that originates BGP routes shall assign the degree of
 preference to these routes by passing them through the Decision
 Process (see Section 9.1).  These routes may also be distributed to
 other BGP speakers within the local AS as part of the Internal update
 process (see Section 9.2.1). The decision whether to distribute non-
 BGP acquired routes within an AS via BGP or not depends on the
 environment within the AS (e.g. type of IGP) and should be controlled
 via configuration.

Rekhter & Li [Page 47] RFC 1771 BGP-4 March 1995

Appendix 1. BGP FSM State Transitions and Actions.

 This Appendix discusses the transitions between states in the BGP FSM
 in response to BGP events.  The following is the list of these states
 and events when the negotiated Hold Time value is non-zero.
     BGP States:
              1 - Idle
              2 - Connect
              3 - Active
              4 - OpenSent
              5 - OpenConfirm
              6 - Established
     BGP Events:
              1 - BGP Start
              2 - BGP Stop
              3 - BGP Transport connection open
              4 - BGP Transport connection closed
              5 - BGP Transport connection open failed
              6 - BGP Transport fatal error
              7 - ConnectRetry timer expired
              8 - Hold Timer expired
              9 - KeepAlive timer expired
             10 - Receive OPEN message
             11 - Receive KEEPALIVE message
             12 - Receive UPDATE messages
             13 - Receive NOTIFICATION message

Rekhter & Li [Page 48] RFC 1771 BGP-4 March 1995

 The following table describes the state transitions of the BGP FSM
 and the actions triggered by these transitions.
  Event                Actions               Message Sent   Next State
  --------------------------------------------------------------------
  Idle (1)
   1            Initialize resources            none             2
                Start ConnectRetry timer
                Initiate a transport connection
   others               none                    none             1
  Connect(2)
   1                    none                    none             2
   3            Complete initialization         OPEN             4
                Clear ConnectRetry timer
   5            Restart ConnectRetry timer      none             3
   7            Restart ConnectRetry timer      none             2
                Initiate a transport connection
   others       Release resources               none             1
  Active (3)
   1                    none                    none             3
   3            Complete initialization         OPEN             4
                Clear ConnectRetry timer
   5            Close connection                                 3
                Restart ConnectRetry timer
   7            Restart ConnectRetry timer      none             2
                Initiate a transport connection
   others       Release resources               none             1
  OpenSent(4)
   1                    none                    none             4
   4            Close transport connection      none             3
                Restart ConnectRetry timer
   6            Release resources               none             1
  10            Process OPEN is OK            KEEPALIVE          5
                Process OPEN failed           NOTIFICATION       1
  others        Close transport connection    NOTIFICATION       1
                Release resources

Rekhter & Li [Page 49] RFC 1771 BGP-4 March 1995

  OpenConfirm (5)
   1                   none                     none             5
   4            Release resources               none             1
   6            Release resources               none             1
   9            Restart KeepAlive timer       KEEPALIVE          5
  11            Complete initialization         none             6
                Restart Hold Timer
  13            Close transport connection                       1
                Release resources
  others        Close transport connection    NOTIFICATION       1
                Release resources
  Established (6)
   1                   none                     none             6
   4            Release resources               none             1
   6            Release resources               none             1
   9            Restart KeepAlive timer       KEEPALIVE          6
  11            Restart Hold Timer            KEEPALIVE          6
  12            Process UPDATE is OK          UPDATE             6
                Process UPDATE failed         NOTIFICATION       1
  13            Close transport connection                       1
                Release resources
  others        Close transport connection    NOTIFICATION       1
                Release resources
 ---------------------------------------------------------------------

Rekhter & Li [Page 50] RFC 1771 BGP-4 March 1995

    The following is a condensed version of the above state transition
    table.
 Events| Idle | Connect | Active | OpenSent | OpenConfirm | Estab
       | (1)  |   (2)   |  (3)   |    (4)   |     (5)     |   (6)
       |--------------------------------------------------------------
  1    |  2   |    2    |   3    |     4    |      5      |    6
       |      |         |        |          |             |
  2    |  1   |    1    |   1    |     1    |      1      |    1
       |      |         |        |          |             |
  3    |  1   |    4    |   4    |     1    |      1      |    1
       |      |         |        |          |             |
  4    |  1   |    1    |   1    |     3    |      1      |    1
       |      |         |        |          |             |
  5    |  1   |    3    |   3    |     1    |      1      |    1
       |      |         |        |          |             |
  6    |  1   |    1    |   1    |     1    |      1      |    1
       |      |         |        |          |             |
  7    |  1   |    2    |   2    |     1    |      1      |    1
       |      |         |        |          |             |
  8    |  1   |    1    |   1    |     1    |      1      |    1
       |      |         |        |          |             |
  9    |  1   |    1    |   1    |     1    |      5      |    6
       |      |         |        |          |             |
 10    |  1   |    1    |   1    |  1 or 5  |      1      |    1
       |      |         |        |          |             |
 11    |  1   |    1    |   1    |     1    |      6      |    6
       |      |         |        |          |             |
 12    |  1   |    1    |   1    |     1    |      1      | 1 or 6
       |      |         |        |          |             |
 13    |  1   |    1    |   1    |     1    |      1      |    1
       |      |         |        |          |             |
       ---------------------------------------------------------------

Appendix 2. Comparison with RFC1267

 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 semantics and encoding
 associated with the AS_PATH attribute. New text has been added to
 define semantics associated with IP prefixes.  These abilities allow
 BGP-4 to support the proposed supernetting scheme [9].
 To simplify configuration this version introduces a new attribute,
 LOCAL_PREF, that facilitates route selection procedures.

Rekhter & Li [Page 51] RFC 1771 BGP-4 March 1995

 The INTER_AS_METRIC attribute has been renamed to be 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 in order to advertise
 which AS and which BGP speaker within that AS caused the aggregation.
 To insure that Hold Timers are symmetric, the Hold Time is now
 negotiated on a per-connection basis.  Hold Times of zero are now
 supported.

Appendix 3. Comparison with RFC 1163

 All of the changes listed in Appendix 2, plus the following.
 To detect and recover from BGP connection collision, a new field (BGP
 Identifier) has been added to the OPEN message. New text (Section
 6.8) has been added to specify the procedure for detecting and
 recovering from collision.
 The new document no longer restricts the border router that is passed
 in the NEXT_HOP path attribute to be part of the same Autonomous
 System as the BGP Speaker.
 New document optimizes and simplifies the exchange of the information
 about previously reachable routes.

Appendix 4. Comparison with RFC 1105

 All of the changes listed in Appendices 2 and 3, plus the following.
 Minor changes to the RFC1105 Finite State Machine were necessary to
 accommodate the TCP user interface provided by 4.3 BSD.
 The notion of Up/Down/Horizontal relations present in RFC1105 has
 been removed from the protocol.
 The changes in the message format from RFC1105 are as follows:
    1.  The Hold Time field has been removed from the BGP header and
    added to the OPEN message.
    2.  The version field has been removed from the BGP header and
    added to the OPEN message.
    3.  The Link Type field has been removed from the OPEN message.
    4.  The OPEN CONFIRM message has been eliminated and replaced with
    implicit confirmation provided by the KEEPALIVE message.

Rekhter & Li [Page 52] RFC 1771 BGP-4 March 1995

    5.  The format of the UPDATE message has been changed
    significantly.  New fields were added to the UPDATE message to
    support multiple path attributes.
    6.  The Marker field has been expanded and its role broadened to
    support authentication.
    Note that quite often BGP, as specified in RFC 1105, is referred
    to as BGP-1, BGP, as specified in RFC 1163, is referred to as
    BGP-2, BGP, as specified in RFC1267 is referred to as BGP-3, and
    BGP, as specified in this document is referred to as BGP-4.

Appendix 5. TCP options that may be used with BGP

 If a local system TCP user interface supports TCP PUSH function, then
 each BGP message should be transmitted with PUSH flag set.  Setting
 PUSH flag forces BGP messages to be transmitted promptly to the
 receiver.
 If a local system TCP user interface supports setting precedence for
 TCP connection, then the BGP transport connection should be opened
 with precedence set to Internetwork Control (110) value (see also
 [6]).

Appendix 6. Implementation Recommendations

 This section presents some implementation recommendations.

6.1 Multiple Networks Per Message

 The BGP protocol allows for multiple address prefixes with the same
 AS path and next-hop gateway to be specified in one message. Making
 use of this capability is highly recommended. With one address prefix
 per message there is a substantial increase in overhead in the
 receiver. Not only does the system overhead increase due to the
 reception of multiple messages, but the overhead of scanning the
 routing table for updates to BGP peers and other routing protocols
 (and sending the associated messages) is incurred multiple times as
 well. One method of building messages containing many address
 prefixes per AS path and gateway from a routing table that is not
 organized per AS path is to build many messages as the routing table
 is scanned. As each address prefix is processed, a message for the
 associated AS path and gateway is allocated, if it does not exist,
 and the new address prefix is added to it.  If such a message exists,
 the new address prefix is just appended to it. If the message lacks
 the space to hold the new address prefix, it is transmitted, a new
 message is allocated, and the new address prefix is inserted into the
 new message. When the entire routing table has been scanned, all

Rekhter & Li [Page 53] RFC 1771 BGP-4 March 1995

 allocated messages are sent and their resources released.  Maximum
 compression is achieved when all  the destinations covered by the
 address prefixes share a gateway and common 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 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.

6.2 Processing Messages on a Stream Protocol

 BGP uses TCP as a transport mechanism.  Due to the stream nature of
 TCP, all the data for received messages does not necessarily arrive
 at the same time. This can make it difficult to process the data as
 messages, especially on systems such as BSD Unix where it is not
 possible to determine how much data has been received but not yet
 processed.
 One method that can be used in this situation is to first try to read
 just the message header. For the KEEPALIVE message type, this is a
 complete message; for other message types, the header should first be
 verified, in particular the total length. If all checks are
 successful, the specified length, minus the size of the message
 header is the amount of data left to read. An implementation that
 would "hang" the routing information process while trying to read
 from a peer could set up a message buffer (4096 bytes) per peer and
 fill it with data as available until a complete message has been
 received.

6.3 Reducing route flapping

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

Rekhter & Li [Page 54] RFC 1771 BGP-4 March 1995

6.4 BGP Timers

 BGP employs five timers: ConnectRetry, Hold Time, KeepAlive,
 MinASOriginationInterval, and MinRouteAdvertisementInterval The
 suggested value for the ConnectRetry timer is 120 seconds.  The
 suggested value for the Hold Time is 90 seconds.  The suggested value
 for the KeepAlive timer is 30 seconds.  The suggested value for the
 MinASOriginationInterval is 15 seconds.  The suggested value for the
 MinRouteAdvertisementInterval is 30 seconds.
 An implementation of BGP MUST allow these timers to be configurable.

6.5 Path attribute ordering

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

6.6 AS_SET sorting

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

6.7 Control over version negotiation

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

6.8 Complex AS_PATH aggregation

 An implementation which chooses to provide a path aggregation
 algorithm which retains significant amounts of path information may
 wish to use the following procedure:
    For the purpose of aggregating AS_PATH attributes of two routes,
    we model each AS as a tuple <type, value>, where "type" identifies
    a type of the path segment the AS belongs to (e.g.  AS_SEQUENCE,
    AS_SET), and "value" is the AS number.  Two ASs are said to be the
    same if their corresponding <type, value> tuples are the same.

Rekhter & Li [Page 55] RFC 1771 BGP-4 March 1995

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

in both AS_PATH attributes.

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

References

 [1] Mills, D., "Exterior Gateway Protocol Formal Specification", RFC
     904, BBN, April 1984.
 [2] Rekhter, Y., "EGP and Policy Based Routing in the New NSFNET
     Backbone", RFC 1092, T.J. Watson Research Center, February 1989.
 [3] Braun, H-W., "The NSFNET Routing Architecture", RFC 1093,
     MERIT/NSFNET Project, February 1989.
 [4] Postel, J., "Transmission Control Protocol - DARPA Internet
     Program Protocol Specification", STD 7, RFC 793, DARPA, September
     1981.

Rekhter & Li [Page 56] RFC 1771 BGP-4 March 1995

 [5] Rekhter, Y., and P. Gross, "Application of the Border Gateway
     Protocol in the Internet", RFC 1772, T.J. Watson Research Center,
     IBM Corp., MCI, March 1995.
 [6] Postel, J., "Internet Protocol - DARPA Internet Program Protocol
     Specification", STD 5, RFC 791, DARPA, September 1981.
 [7] "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
 [8] Fuller, V., Li, T., Yu, J., and K. Varadhan, "Classless Inter-
     Domain Routing (CIDR): an Address Assignment and Aggregation
     Strategy", RFC 1519, BARRNet, cisco, MERIT, OARnet, September
     1993
 [9] Rekhter, Y., Li, T., "An Architecture for IP Address Allocation
     with CIDR", RFC 1518, T.J. Watson Research Center, cisco,
     September 1993

Security Considerations

 Security issues are not discussed in this document.

Editors' Addresses

 Yakov Rekhter
 T.J. Watson Research Center IBM Corporation
 P.O. Box 704, Office H3-D40
 Yorktown Heights, NY 10598
 Phone:  +1 914 784 7361
 EMail:  yakov@watson.ibm.com
 Tony Li
 cisco Systems, Inc.
 170 W. Tasman Dr.
 San Jose, CA 95134
 EMail: tli@cisco.com

Rekhter & Li [Page 57]

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