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rfc:rfc1267

Network Working Group K. Lougheed Request for Comments: 1267 cisco Systems Obsoletes RFCs: 1105, 1163 Y. Rekhter

                                T.J. Watson Research Center, IBM Corp.
                                                          October 1991
                A Border Gateway Protocol 3 (BGP-3)

Status of this Memo

 This memo, together with its companion document, "Application of the
 Border Gateway Protocol in the Internet", define an inter-autonomous
 system routing protocol for the Internet.  This RFC specifies an IAB
 standards track protocol for the Internet community, and requests
 discussion and suggestions for improvements.  Please refer to the
 current edition of the "IAB Official Protocol Standards" for the
 standardization state and status of this protocol.  Distribution of
 this memo is unlimited.

1. Acknowledgements

 We would like to express our thanks to Guy Almes (Rice University),
 Len Bosack (cisco Systems), Jeffrey C. Honig (Cornell Theory Center)
 and all members of the Interconnectivity Working Group of the
 Internet Engineering Task Force, chaired by Guy Almes, for their
 contributions to this document.
 We like to explicitly thank Bob Braden (ISI) for the review 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 earlier versions 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.

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 full path of

Lougheed & Rekhter [Page 1] RFC 1267 BGP-3 October 1991

 Autonomous Systems (ASs) that traffic must transit to reach these
 networks.  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.
 To characterize the set of policy decisions that can be enforced
 using BGP, one must focus on the rule that an AS advertize to its
 neighbor 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 neighbor AS intending that that traffic take a
 different route from that taken by traffic originating in the
 neighbor 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 memo 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

Lougheed & Rekhter [Page 2] RFC 1267 BGP-3 October 1991

 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 networks are reachable through it.  From the
 standpoint of exterior routing, an AS can be viewed as monolithic:
 reachability to networks directly connected to the AS must be
 equivalent from all border gateways of the AS.
 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 (iwg@rice.edu).

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 networks 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

Lougheed & Rekhter [Page 3] RFC 1267 BGP-3 October 1991

 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.

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.
 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 Authentication Code used in the OPEN message of the connection
    is zero, 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 used.  The Marker can be used
    to detect loss of synchronization between a pair of BGP peers, and
    to authenticate incoming BGP messages.

Lougheed & Rekhter [Page 4] RFC 1267 BGP-3 October 1991

 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.
 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:

Lougheed & Rekhter [Page 5] RFC 1267 BGP-3 October 1991

   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                        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Auth. Code   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  |                       Authentication Data                     |
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Version:
    This 1-octet unsigned integer indicates the protocol version
    number of the message.  The current BGP version number is 3.
 My Autonomous System:
    This 2-octet unsigned integer indicates the Autonomous System
    number of the sender.
 Hold Time:
    This 2-octet unsigned integer indicates the maximum number of
    seconds that may elapse between the receipt of successive
    KEEPALIVE and/or UPDATE and/or NOTIFICATION messages.
 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 the IP address of one of its interfaces.
    The value of the BGP Identifier is determined on startup
    and is the same for every local interface and every BGP peer.
 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:

Lougheed & Rekhter [Page 6] RFC 1267 BGP-3 October 1991

  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.

Only one authentication mechanism is specified as part of this

    memo:
       - its Authentication Code is zero,
       - its Authentication Data must be empty (of zero length), and
       - the Marker fields of all messages must be all ones.
    The semantics of non-zero Authentication Codes lies outside the
    scope of this memo.
    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.  If the value of Authentication
    Code field is zero, the Authentication Data field must have zero
    length.  The semantics of the non-zero length Authentication Data
    field is outside the scope of this memo.
    Note that the length of the Authentication Data field can be
    determined from the message Length field by the formula:
       Message Length = 29 + Authentication Data Length
    The minimum length of the OPEN message is 29 octets (including
    message header).

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.
 In addition to the fixed-size BGP header, the UPDATE message contains
 the following fields (note that all fields may have arbitrary
 alignment):

Lougheed & Rekhter [Page 7] RFC 1267 BGP-3 October 1991

   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
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Total Path Attributes Length |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  /                      Path Attributes                          /
  /                                                               /
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                       Network 1                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  /                                                               /
  /                                                               /
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                       Network n                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 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 (non-
    negative integer) number of Network fields to be determined as
    specified below.
 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|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    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.)

Lougheed & Rekhter [Page 8] RFC 1267 BGP-3 October 1991

    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 meaning and handling of Path Attributes is discussed in
    Section 5.
 Network:
    Each 4-octet Internet network number indicates one network whose
    Inter-Autonomous System routing is described by the Path
    Attributes.  Subnets and host addresses are specifically not
    allowed.  The total number of Network fields in the UPDATE message
    can be determined by the formula:
       Message Length = 19 + Total Path Attribute Length + 4 * #Nets
    The message Length field of the message header and the Path
    Attributes Length field of the UPDATE message must be such that
    the formula results in a non-negative integer number of Network
    fields.

Lougheed & Rekhter [Page 9] RFC 1267 BGP-3 October 1991

 The minimum length of the UPDATE message is 37 octets (including
 message header).

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 time
 (as advertised in the OPEN message) to expire.  A reasonable maximum
 time between KEEPALIVE messages would be one third of the Hold Time
 interval.
 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

Lougheed & Rekhter [Page 10] RFC 1267 BGP-3 October 1991

 Error subcode:
    This 1-octet unsigned integer provides more specific information
    about the nature of the reported error.  Each Error Code may have
    one or more Error Subcodes associated with it.  If no appropriate
    Error Subcode is defined, then a zero (Unspecific) value is used
    for the Error Subcode field.
    Message Header Error subcodes:
                    1  - Connection Not Synchronized.
                    2  - Bad Message Length.
                    3  - Bad Message Type.
    OPEN Message Error subcodes:
                    1  - Unsupported Version Number.
                    2  - Bad Peer AS.
                    3  - Bad BGP Identifier.
                    4  - Unsupported Authentication Code.
                    5  - Authentication Failure.
    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.
 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

Lougheed & Rekhter [Page 11] RFC 1267 BGP-3 October 1991

 The minimum length of the NOTIFICATION message is 21 octets
 (including message header).

5. Path Attributes

 This section discusses the path attributes of the UPDATE message.
 Path attributes fall into four separate categories:
          1. Well-known mandatory.
          2. Well-known discretionary.
          3. Optional transitive.
          4. Optional non-transitive.
 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.  Which well-known attributes are
 mandatory or discretionary is noted in the table below.
 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 INTER-AS
 METRIC attribute gives an example.)  All optional attributes (both

Lougheed & Rekhter [Page 12] RFC 1267 BGP-3 October 1991

 transitive and non-transitive) may be updated (if appropriate) by ASs
 in the path.
 The sender of an UPDATE message should order path attributes within
 the UPDATE message in ascending order of attribute type.  The
 receiver of an UPDATE message must be prepared to handle path
 attributes within the UPDATE message that are out of order.
 The same attribute cannot appear more than once within the Path
 Attributes field of a particular UPDATE message.
 Following table specifies attribute type code, attribute length, and
 attribute category for path attributes defined in this document:
 Attribute Name     Type Code    Length     Attribute category
    ORIGIN              1          1        well-known, mandatory
    AS_PATH             2       variable    well-known, mandatory
    NEXT_HOP            3          4        well-known, mandatory
    UNREACHABLE         4          0        well-known, discretionary
    INTER-AS METRIC     5          2        optional, non-transitive
 ORIGIN:
    The ORIGIN path attribute defines the origin of the path
    information.  The data octet can assume the following values:
       Value    Meaning
         0       IGP - network(s) are interior to the originating AS
         1       EGP - network(s) learned via EGP
         2       INCOMPLETE - network(s) learned by some other means
 AS_PATH:
    The AS_PATH attribute enumerates the ASs that must be traversed to
    reach the networks listed in the UPDATE message.  Since an AS
    identifier is 2 octets, the length of an AS_PATH attribute is
    twice the number of ASs in the path.  Rules for constructing an
    AS_PATH attribute are discussed in Section 9.
    If a previously advertised route has become unreachable, then
    the AS_PATH path attribute of the unreachable route may be
    truncated when passed in the UPDATE message. Truncation is
    achieved by constructing the AS_PATH path attribute that consists
    of only the autonomous system of the sender of the UPDATE message.
    To make the truncated AS_PATH semantically correct, the sender
    also sends the ORIGIN path attribute with the value INCOMPLETE.
    Note that truncation may be done only over external BGP links.

Lougheed & Rekhter [Page 13] RFC 1267 BGP-3 October 1991

 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 networks listed
    in the UPDATE message.  If this border router belongs to the same
    AS as the BGP peer that advertises it, it is called an internal
    border router. If this border router belongs to a different AS
    than the one that the BGP peer that advertises it, it is called 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.
    The NEXT_HOP path attribute has meaning only on external BGP
    links.  However, presence of the NEXT_HOP path attribute in the
    UPDATE message received via an internal BGP link does not
    constitute an error.
 UNREACHABLE:
    The UNREACHABLE attribute is used to notify a BGP peer that some
    of the previously advertised routes have become unreachable.
 INTER-AS METRIC:
    The INTER-AS METRIC attribute may be used on external (inter-AS)
    links to discriminate between multiple exit or entry points to the
    same neighboring AS.  The value of the INTER-AS METRIC attribute
    is a 2-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 INTER-
    AS METRIC attribute may be propagated over internal links to other
    BGP speaker within the same AS.  The INTER-AS METRIC attribute is
    never propagated to other BGP speakers in neighboring AS's.
    If a previously advertised route has become unreachable, then
    the INTER-AS METRIC path attribute may be omitted from the UPDATE
    message.

Lougheed & Rekhter [Page 14] RFC 1267 BGP-3 October 1991

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
 Authentication Code in the BGP OPEN message and the actual
 authentication mechanism (if the Authentication Code in the BGP OPEN
 message is non-zero). 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.
 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

Lougheed & Rekhter [Page 15] RFC 1267 BGP-3 October 1991

 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 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 the Authentication Code of the OPEN message is not recognized,
 then the Error Subcode is set to Unsupported Authentication Code.  If
 the Authentication Code is zero, then the Authentication Data must be
 of zero length.  Otherwise, the Error Subcode is set to
 Authentication Failure.
 If the Authentication Code is non-zero, 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 Total Attribute Length is too large (i.e., if
 Total Attribute Length + 21 exceeds the message Length), or if the
 (non-negative integer) Number of Network fields cannot be computed as

Lougheed & Rekhter [Page 16] RFC 1267 BGP-3 October 1991

 in Section 4.3, 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 or semantically
 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 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.
 The AS route specified by the AS_PATH attribute is checked for AS
 loops.  AS loop detection is done by scanning the full AS route (as
 specified in the AS_PATH attribute) and checking that each AS occurs
 at most once.  If a loop is detected, then the Error Subcode is set
 to AS Routing Loop.  The Data field contains the incorrect attribute
 (type, length and value).
 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.

Lougheed & Rekhter [Page 17] RFC 1267 BGP-3 October 1991

 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.
 Each Network field in the UPDATE message is checked for syntactic
 validity.  If the Network field is syntactically incorrect, or
 contains a subnet or a host address, 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.

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

Lougheed & Rekhter [Page 18] RFC 1267 BGP-3 October 1991

 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 OpenSent state.  If among them 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 OpenSent
        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 OpenSent state).
        Comparing BGP Identifiers is done by treating them as
        (4-octet long) unsigned integers.
        A connection collision with existing BGP connections that
        are either in OpenConfirm or 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

Lougheed & Rekhter [Page 19] RFC 1267 BGP-3 October 1991

 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 BGP neighbor.  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.
       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

Lougheed & Rekhter [Page 20] RFC 1267 BGP-3 October 1991

       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 BGP neighbor 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
       value, and changes its state to OpenSent.
       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 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

Lougheed & Rekhter [Page 21] RFC 1267 BGP-3 October 1991

       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 an arbitrary large value (see
       above), is replaced with the value indicated in the OPEN
       message.  If the value of the Autonomous System field is the
       same as our own, then the connection is "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.
       If the hold time 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.

Lougheed & Rekhter [Page 22] RFC 1267 BGP-3 October 1991

       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.
       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 Holdtime timer.
       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 Holdtime 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

Lougheed & Rekhter [Page 23] RFC 1267 BGP-3 October 1991

       KEEPALIVE message and restarts its KeepAlive timer.
       Each time the local system sends a KEEPALIVE or UPDATE message,
       it restarts its KeepAlive timer.
       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
       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 network and the path attributes associated with a route to
 that network are correct, then the route is compared with other
 routes to the same network.
 When a BGP speaker receives a new route from a peer over external BGP
 link, it shall advertise that route to other BGP speakers in its
 autonomous system by means of an UPDATE message if either of the
 following conditions occur:
    a) the newly received route is considered to be better
       than the other routes to the same network (as listed

Lougheed & Rekhter [Page 24] RFC 1267 BGP-3 October 1991

       in the UPDATE message) that have been received over
       external BGP links, or
    b) there are no other acceptable routes to the network
       (as listed in the UPDATE message) that have been
       received over external BGP links.
 When a BGP speaker receives an unreachable route from a BGP peer over
 external BGP link, it shall advertise that route to all other BGP
 speakers in its autonomous system, indicating that it has become
 unreachable, if the following condition occur:
    a) a corresponding acceptable route to the same destination
       was considered to be the best one among all routes to that
       destination that have been received over external BGP links
       (that is the local system has been advertising the
       route to all other BGP speakers in its autonomous system
       before it received the UPDATE message that reported it
       as unreachable).
 Whenever a BGP speaker selects a new route (among all the routes
 received from external and internal BGP peers), or determines that
 the reachable destinations within its own autonomous system have
 changed, it shall generate an UPDATE message and forward it to each
 of its external peers (peers connected via external BGP links).
 If a route in the UPDATE was received over an internal link, it is
 not propagated over any other internal link.  This restriction is due
 to the fact that all BGP speakers within a single AS form a
 completely connected graph (see above).
 If the UPDATE message is propagated over an external link, then the
 local AS number is prepended to the AS_PATH attribute, and the
 NEXT_HOP attribute is updated with an IP address of the router that
 should be used as a next hop to the network.  If the UPDATE message
 is propagated over an internal link, then the AS_PATH attribute and
 the NEXT_HOP attribute are passed unmodified.
 Generally speaking, the 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

Lougheed & Rekhter [Page 25] RFC 1267 BGP-3 October 1991

      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.

10. Detection of Inter-AS Policy Contradictions

 Since BGP requires no central authority for coordinating routing
 policies among ASs, and since routing policies are not exchanged via
 the protocol itself, it is possible for a group of ASs to have a set
 of routing policies that cannot simultaneously be satisfied.  This
 may cause an indefinite oscillation of the routes in this group of
 ASs.
 To help detect such a situation, all BGP speakers must observe the
 following rule.  If a route to a destination that is currently used
 by the local system is determined to be unreachable (e.g., as a
 result of receiving an UPDATE message for this route with the
 UNREACHABLE attribute), then, before switching to another route, this
 local system must advertize this route as unreachable to all the BGP
 neighbors to which it previously advertized this route.
 This rule will allow other ASs to distinguish between two different
 situations:
  1. The local system has chosen to use a new route because the old

route become unreachable.

  1. The local system has chosen to use a new route because it

preferred it over the old route. The old route is still

      viable.
 In the former case, an UPDATE message with the UNREACHABLE attribute
 will be received for the old route.  In the latter case it will not.
 In some cases, this may allow a BGP speaker to detect the fact that
 its policies, taken together with the policies of some other AS,
 cannot simultaneously be satisfied.  For example, consider the
 following situation involving AS A and its neighbor AS B.  B
 advertises a route with a path of the form <B,...>, where A is not
 present in the path.  A then decides to use this path, and advertises
 <A,B,...> to all its neighbors.  B later advertises <B,...,A,...>
 back to A, without ever declaring its previous path <B,...> to be
 unreachable.  Evidently, A prefers routes via B and B prefers routes
 via A.  The combined policies of A and B, taken together, cannot be
 satisfied.  Such an event should be noticed, logged locally, and
 brought to the attention of AS A's administration.  The means to do

Lougheed & Rekhter [Page 26] RFC 1267 BGP-3 October 1991

 this, however, lies outside the scope of this document.  Also outside
 the document is a more complete procedure for detecting such
 contradictions of policy.
 While the above rules provide a mechanism to detect a set of routing
 policies that cannot be satisfied simultaneously, the protocol itself
 does not provide any mechanism for suppressing the route oscillation
 that may result from these unsatisfiable policies.  The reason for
 doing this is that routing policies are viewed as external to the
 protocol and as determined by the local AS administrator.

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.
  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 - Holdtime timer expired
           9 - KeepAlive timer expired
          10 - Receive OPEN message
          11 - Receive KEEPALIVE message
          12 - Receive UPDATE messages
          13 - Receive NOTIFICATION message
 The following table describes the state transitions of the BGP FSM
 and the actions triggered by these transitions.

Lougheed & Rekhter [Page 27] RFC 1267 BGP-3 October 1991

  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
  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 Holdtime timer
  13            Close transport connection                       1
                Release resources
  others        Close transport connection    NOTIFICATION       1
                Release resources

Lougheed & Rekhter [Page 28] RFC 1267 BGP-3 October 1991

  Established (6)
   1                   none                     none             6
   4            Release resources               none             1
   6            Release resources               none             1
   9            Restart KeepAlive timer       KEEPALIVE          6
  11            Restart Holdtime 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
 ---------------------------------------------------------------------
 The following is a condensed version of the above state transition
 table.

Lougheed & Rekhter [Page 29] RFC 1267 BGP-3 October 1991

Events| Idle | Active | Connect | 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 RFC 1163

 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 3. Comparison with RFC 1105

 All of the changes listed in Appendix 2, plus the following.

Lougheed & Rekhter [Page 30] RFC 1267 BGP-3 October 1991

 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.
    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, and
 BGP, as specified in this document is referred to as BGP-3.

Appendix 4. 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]).

Lougheed & Rekhter [Page 31] RFC 1267 BGP-3 October 1991

Appendix 5. Implementation Recommendations

 This section presents some implementation recommendations.

5.1 Multiple Networks Per Message

 The BGP protocol allows for multiple networks 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 network 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 flash
 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 networks 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 network is
 processed, a message for the associated AS path and gateway is
 allocated, if it does not exist, and the new network is added to it.
 If such a message exists, the new network is just appended to it. If
 the message lacks the space to hold the new network, it is
 transmitted, a new message is allocated, and the new network is
 inserted into the new message. When the entire routing table has been
 scanned, all allocated messages are sent and their resources
 released.  Maximum compression is achieved when all networks share a
 gateway and common path attributes, making it possible to send many
 networks in one 4096-byte message.
 When peering with a BGP implementation that does not compress
 multiple networks 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 flash 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.

5.2 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

Lougheed & Rekhter [Page 32] RFC 1267 BGP-3 October 1991

 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.

5.3 Processing Update Messages

 In BGP, all UPDATE messages are incremental. Once a particular
 network is listed in an Update message as being reachable through an
 AS path and gateway, that piece of information is expected to be
 retained indefinitely.
 In order for a route to a network to be removed, it must be
 explicitly listed in an Update message as being unreachable or with
 new routing information to replace the old. Note that a BGP peer will
 only advertise one route to a given network, so any announcement of
 that network by a particular peer replaces any previous information
 about that network received from the same peer.
 One useful optimization is that unreachable networks need not be
 advertised with their original attributes.  Instead, all unreachable
 networks could be sent in a single message, perhaps with an AS path
 consisting of the local AS only and with an origin set to INCOMPLETE.
 This approach has the obvious advantage of low overhead; if all
 routes are stable, only KEEPALIVE messages will be sent. There is no
 periodic flood of route information.
 However, this means that a consistent view of routing information
 between BGP peers is only possible over the course of a single
 transport connection, since there is no mechanism for a complete
 update. This requirement is accommodated by specifying that BGP peers
 must transition to the Idle state upon the failure of a transport
 connection.

5.4 BGP Timers

    BGP employs three timers: ConnectRetry, Holdtime, and KeepAlive.
    Suggested value for the ConnectRetry timer is 120 seconds.
    Suggested value for the Holdtime timer is 90 seconds.

Lougheed & Rekhter [Page 33] RFC 1267 BGP-3 October 1991

    Suggested value for the KeepAlive timer is 30 seconds.
    An implementation of BGP shall allow any of these timers to be
    configurable.

5.5 Frequency of Route Selection

 An implementation of BGP shall allow a border router to set up the
 minimum amount of time that must elapse between selection and
 subsequent advertisement of better routes received by a given BGP
 speaker from BGP speakers located in adjacent ASs.
 Since fast convergence is needed within an AS, deferring selection
 does not apply to selection of better routes chosen as a result of
 UPDATEs from BGP speakers located in the advertising speaker's own
 AS.  To avoid long-lived black holes, it does not apply to
 advertisement of previously selected routes which have become
 unreachable. In both of these situations, the local BGP speaker must
 select and advertise such routes immediately.
 If a BGP speaker received better routes from BGP speakers in adjacent
 ASs, but have not yet advertised them because the time has not yet
 elapsed, the reception of any routes from other BGP speakers in its
 own AS shall trigger a new route selection process that will be based
 on both updates from BGP speakers in the same AS and in adjacent ASs.

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", RFC 793, DARPA, September 1981.
 [5] Rekhter, Y., and P. Gross, "Application of the Border Gateway
     Protocol in the Internet", RFC 1268, T.J. Watson Research Center,
     IBM Corp., ANS, October 1991.
 [6] Postel, J., "Internet Protocol - DARPA Internet Program Protocol
     Specification", RFC 791, DARPA, September 1981.

Lougheed & Rekhter [Page 34] RFC 1267 BGP-3 October 1991

Security Considerations

 Security issues are not discussed in this memo.

Authors' Addresses

 Kirk Lougheed
 cisco Systems, Inc.
 1525 O'Brien Drive
 Menlo Park, CA 94025
 Phone:  (415) 326-1941
 Email:  LOUGHEED@CISCO.COM
 Yakov Rekhter
 T.J. Watson Research Center IBM Corporation
 P.O. Box 218
 Yorktown Heights, NY 10598
 Phone:  (914) 945-3896
 Email:  YAKOV@WATSON.IBM.COM

Lougheed & Rekhter [Page 35]

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