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RFC 4271

 
 
 

A Border Gateway Protocol 4 (BGP-4)

Part 2 of 4, p. 11 to 36
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4.  Message Formats

   This section describes message formats used by BGP.

   BGP messages are sent over TCP connections.  A message is processed
   only after it is entirely received.  The maximum message size is 4096
   octets.  All implementations are required to support this maximum
   message size.  The smallest message that may be sent consists of a
   BGP header without a data portion (19 octets).

   All multi-octet fields are in network byte order.

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4.1.  Message Header Format

   Each message has a fixed-size header.  There may or may not be a data
   portion following the header, depending on the message type.  The
   layout of these fields is shown below:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                                                               +
      |                                                               |
      +                                                               +
      |                           Marker                              |
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Length               |      Type     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Marker:

         This 16-octet field is included for compatibility; it MUST be
         set to all ones.

      Length:

         This 2-octet unsigned integer indicates the total length of the
         message, including the header in octets.  Thus, it allows one
         to locate the (Marker field of the) next message in the TCP
         stream.  The value of the Length field MUST always be at least
         19 and no greater than 4096, and MAY be further constrained,
         depending on the message type.  "padding" of extra data after
         the message is not allowed.  Therefore, the Length field MUST
         have the smallest value required, given the rest of the
         message.

      Type:

         This 1-octet unsigned integer indicates the type code of the
         message.  This document defines the following type codes:

                              1 - OPEN
                              2 - UPDATE
                              3 - NOTIFICATION
                              4 - KEEPALIVE

         [RFC2918] defines one more type code.

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4.2.  OPEN Message Format

   After a TCP connection is established, the first message sent by each
   side is an OPEN message.  If the OPEN message is acceptable, a
   KEEPALIVE message confirming the OPEN is sent back.

   In addition to the fixed-size BGP header, the OPEN message contains
   the following fields:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+
       |    Version    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     My Autonomous System      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Hold Time           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         BGP Identifier                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Opt Parm Len  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |             Optional Parameters (variable)                    |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Version:

         This 1-octet unsigned integer indicates the protocol version
         number of the message.  The current BGP version number is 4.

      My Autonomous System:

         This 2-octet unsigned integer indicates the Autonomous System
         number of the sender.

      Hold Time:

         This 2-octet unsigned integer indicates the number of seconds
         the sender proposes for the value of the Hold Timer.  Upon
         receipt of an OPEN message, a BGP speaker MUST calculate the
         value of the Hold Timer by using the smaller of its configured
         Hold Time and the Hold Time received in the OPEN message.  The
         Hold Time MUST be either zero or at least three seconds.  An
         implementation MAY reject connections on the basis of the Hold

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

      BGP Identifier:

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

      Optional Parameters Length:

         This 1-octet unsigned integer indicates the total length of the
         Optional Parameters field in octets.  If the value of this
         field is zero, no Optional Parameters are present.

      Optional Parameters:

         This field contains a list of optional parameters, in which
         each parameter is encoded as a <Parameter Type, Parameter
         Length, Parameter Value> triplet.

         0                   1
         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...
         |  Parm. Type   | Parm. Length  |  Parameter Value (variable)
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...

         Parameter Type is a one octet field that unambiguously
         identifies individual parameters.  Parameter Length is a one
         octet field that contains the length of the Parameter Value
         field in octets.  Parameter Value is a variable length field
         that is interpreted according to the value of the Parameter
         Type field.

         [RFC3392] defines the Capabilities Optional Parameter.

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

4.3.  UPDATE Message Format

   UPDATE messages are used to transfer routing information between BGP
   peers.  The information in the UPDATE message can be used to
   construct a graph that describes the relationships of the various
   Autonomous Systems.  By applying rules to be discussed, routing

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

   An UPDATE message is used to advertise feasible routes that share
   common path attributes to a peer, or to withdraw multiple unfeasible
   routes from service (see 3.1).  An UPDATE message MAY simultaneously
   advertise a feasible route and withdraw multiple unfeasible routes
   from service.  The UPDATE message always includes the fixed-size BGP
   header, and also includes the other fields, as shown below (note,
   some of the shown fields may not be present in every UPDATE message):

      +-----------------------------------------------------+
      |   Withdrawn Routes Length (2 octets)                |
      +-----------------------------------------------------+
      |   Withdrawn Routes (variable)                       |
      +-----------------------------------------------------+
      |   Total Path Attribute Length (2 octets)            |
      +-----------------------------------------------------+
      |   Path Attributes (variable)                        |
      +-----------------------------------------------------+
      |   Network Layer Reachability Information (variable) |
      +-----------------------------------------------------+

      Withdrawn Routes Length:

         This 2-octets unsigned integer indicates the total length of
         the Withdrawn Routes field in octets.  Its value allows the
         length of the Network Layer Reachability Information field to
         be determined, as specified below.

         A value of 0 indicates that no routes are being withdrawn from
         service, and that the WITHDRAWN ROUTES field is not present in
         this UPDATE message.

      Withdrawn Routes:

         This is a variable-length field that contains a list of IP
         address prefixes for the routes that are being withdrawn from
         service.  Each IP address prefix is encoded as a 2-tuple of the
         form <length, prefix>, whose fields are described below:

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

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         The use and the meaning of these fields are as follows:

         a) Length:

            The Length field indicates the length in bits of the IP
            address prefix.  A length of zero indicates a prefix that
            matches all IP addresses (with prefix, itself, of zero
            octets).

         b) Prefix:

            The Prefix field contains an IP address prefix, followed by
            the minimum number of trailing bits needed to make the end
            of the field fall on an octet boundary.  Note that the value
            of trailing bits is irrelevant.

      Total Path Attribute Length:

         This 2-octet unsigned integer indicates the total length of the
         Path Attributes field in octets.  Its value allows the length
         of the Network Layer Reachability field to be determined as
         specified below.

         A value of 0 indicates that neither the Network Layer
         Reachability Information field nor the Path Attribute field is
         present in this UPDATE message.

      Path Attributes:

         A variable-length sequence of path attributes is present in
         every UPDATE message, except for an UPDATE message that carries
         only the withdrawn routes.  Each path attribute is a triple
         <attribute type, attribute length, attribute value> of variable
         length.

         Attribute Type is a two-octet field that consists of the
         Attribute Flags octet, followed by the Attribute Type Code
         octet.

               0                   1
               0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               |  Attr. Flags  |Attr. Type Code|
               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         The high-order bit (bit 0) of the Attribute Flags octet is the
         Optional bit.  It defines whether the attribute is optional (if
         set to 1) or well-known (if set to 0).

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         The second high-order bit (bit 1) of the Attribute Flags octet
         is the Transitive bit.  It defines whether an optional
         attribute is transitive (if set to 1) or non-transitive (if set
         to 0).

         For well-known attributes, the Transitive bit MUST be set to 1.
         (See Section 5 for a discussion of transitive attributes.)

         The third high-order bit (bit 2) of the Attribute Flags octet
         is the Partial bit.  It defines whether the information
         contained in the optional transitive attribute is partial (if
         set to 1) or complete (if set to 0).  For well-known attributes
         and for optional non-transitive attributes, the Partial bit
         MUST be set to 0.

         The fourth high-order bit (bit 3) of the Attribute Flags octet
         is the Extended Length bit.  It defines whether the Attribute
         Length is one octet (if set to 0) or two octets (if set to 1).

         The lower-order four bits of the Attribute Flags octet are
         unused.  They MUST be zero when sent and MUST be ignored when
         received.

         The Attribute Type Code octet contains the Attribute Type Code.
         Currently defined Attribute Type Codes are discussed in Section
         5.

         If the Extended Length bit of the Attribute Flags octet is set
         to 0, the third octet of the Path Attribute contains the length
         of the attribute data in octets.

         If the Extended Length bit of the Attribute Flags octet is set
         to 1, the third and fourth octets of the path attribute contain
         the length of the attribute data in octets.

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         The remaining octets of the Path Attribute represent the
         attribute value and are interpreted according to the Attribute
         Flags and the Attribute Type Code.  The supported Attribute
         Type Codes, and their attribute values and uses are as follows:

         a) ORIGIN (Type Code 1):

            ORIGIN is a well-known mandatory attribute that defines the
            origin of the path information.  The data octet can assume
            the following values:

               Value      Meaning

               0         IGP - Network Layer Reachability Information
                            is interior to the originating AS

               1         EGP - Network Layer Reachability Information
                            learned via the EGP protocol [RFC904]

               2         INCOMPLETE - Network Layer Reachability
                            Information learned by some other means

            Usage of this attribute is defined in 5.1.1.

         b) AS_PATH (Type Code 2):

            AS_PATH is a well-known mandatory attribute that is composed
            of a sequence of AS path segments.  Each AS path segment is
            represented by a triple <path segment type, path segment
            length, path segment value>.

            The path segment type is a 1-octet length field with the
            following values defined:

               Value      Segment Type

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

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

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

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

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            Usage of this attribute is defined in 5.1.2.

         c) NEXT_HOP (Type Code 3):

            This is a well-known mandatory attribute that defines the
            (unicast) IP address of the router that SHOULD be used as
            the next hop to the destinations listed in the Network Layer
            Reachability Information field of the UPDATE message.

            Usage of this attribute is defined in 5.1.3.

         d) MULTI_EXIT_DISC (Type Code 4):

            This is an optional non-transitive attribute that is a
            four-octet unsigned integer.  The value of this attribute
            MAY be used by a BGP speaker's Decision Process to
            discriminate among multiple entry points to a neighboring
            autonomous system.

            Usage of this attribute is defined in 5.1.4.

         e) LOCAL_PREF (Type Code 5):

            LOCAL_PREF is a well-known attribute that is a four-octet
            unsigned integer.  A BGP speaker uses it to inform its other
            internal peers of the advertising speaker's degree of
            preference for an advertised route.

            Usage of this attribute is defined in 5.1.5.

         f) ATOMIC_AGGREGATE (Type Code 6)

            ATOMIC_AGGREGATE is a well-known discretionary attribute of
            length 0.

            Usage of this attribute is defined in 5.1.6.

         g) AGGREGATOR (Type Code 7)

            AGGREGATOR is an optional transitive attribute of length 6.
            The attribute contains the last AS number that formed the
            aggregate route (encoded as 2 octets), followed by the IP
            address of the BGP speaker that formed the aggregate route
            (encoded as 4 octets).  This SHOULD be the same address as
            the one used for the BGP Identifier of the speaker.

            Usage of this attribute is defined in 5.1.7.

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      Network Layer Reachability Information:

         This variable length field contains a list of IP address
         prefixes.  The length, in octets, of the Network Layer
         Reachability Information is not encoded explicitly, but can be
         calculated as:

               UPDATE message Length - 23 - Total Path Attributes Length
               - Withdrawn Routes Length

         where UPDATE message Length is the value encoded in the fixed-
         size BGP header, Total Path Attribute Length, and Withdrawn
         Routes Length are the values encoded in the variable part of
         the UPDATE message, and 23 is a combined length of the fixed-
         size BGP header, the Total Path Attribute Length field, and the
         Withdrawn Routes Length field.

         Reachability information is encoded as one or more 2-tuples of
         the form <length, prefix>, whose fields are described below:

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

         The use and the meaning of these fields are as follows:

         a) Length:

            The Length field indicates the length in bits of the IP
            address prefix.  A length of zero indicates a prefix that
            matches all IP addresses (with prefix, itself, of zero
            octets).

         b) Prefix:

            The Prefix field contains an IP address prefix, followed by
            enough trailing bits to make the end of the field fall on an
            octet boundary.  Note that the value of the trailing bits is
            irrelevant.

   The minimum length of the UPDATE message is 23 octets -- 19 octets
   for the fixed header + 2 octets for the Withdrawn Routes Length + 2
   octets for the Total Path Attribute Length (the value of Withdrawn
   Routes Length is 0 and the value of Total Path Attribute Length is
   0).

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   An UPDATE message can advertise, at most, one set of path attributes,
   but multiple destinations, provided that the destinations share these
   attributes.  All path attributes contained in a given UPDATE message
   apply to all destinations carried in the NLRI field of the UPDATE
   message.


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


   An UPDATE message might advertise only routes that are to be
   withdrawn from service, in which case the message will not include
   path attributes or Network Layer Reachability Information.
   Conversely, it may advertise only a feasible route, in which case the
   WITHDRAWN ROUTES field need not be present.

   An UPDATE message SHOULD NOT include the same address prefix in the
   WITHDRAWN ROUTES and Network Layer Reachability Information fields.
   However, a BGP speaker MUST be able to process UPDATE messages in
   this form.  A BGP speaker SHOULD treat an UPDATE message of this form
   as though the WITHDRAWN ROUTES do not contain the address prefix.

4.4.  KEEPALIVE Message Format

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

   If the negotiated Hold Time interval is zero, then periodic KEEPALIVE
   messages MUST NOT be sent.

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

4.5.  NOTIFICATION Message Format

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

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   In addition to the fixed-size BGP header, the NOTIFICATION message
   contains the following fields:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Error code    | Error subcode |   Data (variable)             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Error Code:

         This 1-octet unsigned integer indicates the type of
         NOTIFICATION.  The following Error Codes have been defined:

            Error Code       Symbolic Name               Reference

              1         Message Header Error             Section 6.1

              2         OPEN Message Error               Section 6.2

              3         UPDATE Message Error             Section 6.3

              4         Hold Timer Expired               Section 6.5

              5         Finite State Machine Error       Section 6.6

              6         Cease                            Section 6.7

      Error subcode:

         This 1-octet unsigned integer provides more specific
         information about the nature of the reported error.  Each Error
         Code may have one or more Error Subcodes associated with it.
         If no appropriate Error Subcode is defined, then a zero
         (Unspecific) value is used for the Error Subcode field.

      Message Header Error subcodes:

               1 - Connection Not Synchronized.
               2 - Bad Message Length.
               3 - Bad Message Type.

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      OPEN Message Error subcodes:

               1 - Unsupported Version Number.
               2 - Bad Peer AS.
               3 - Bad BGP Identifier.
               4 - Unsupported Optional Parameter.
               5 - [Deprecated - see Appendix A].
               6 - Unacceptable Hold Time.

      UPDATE Message Error subcodes:

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

      Data:

         This variable-length field is used to diagnose the reason for
         the NOTIFICATION.  The contents of the Data field depend upon
         the Error Code and Error Subcode.  See Section 6 for more
         details.

         Note that the length of the Data field can be determined from
         the message Length field by the formula:

                  Message Length = 21 + Data Length

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

5.  Path Attributes

   This section discusses the path attributes of the UPDATE message.

   Path attributes fall into four separate categories:

         1. Well-known mandatory.
         2. Well-known discretionary.
         3. Optional transitive.
         4. Optional non-transitive.

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   BGP implementations MUST recognize all well-known attributes.  Some
   of these attributes are mandatory and MUST be included in every
   UPDATE message that contains NLRI.  Others are discretionary and MAY
   or MAY NOT be sent in a particular UPDATE message.

   Once a BGP peer has updated any well-known attributes, it MUST pass
   these attributes to its peers in any updates it transmits.

   In addition to well-known attributes, each path MAY contain one or
   more optional attributes.  It is not required or expected that all
   BGP implementations support all optional attributes.  The handling of
   an unrecognized optional attribute is determined by the setting of
   the Transitive bit in the attribute flags octet.  Paths with
   unrecognized transitive optional attributes SHOULD be accepted.  If a
   path with an unrecognized transitive optional attribute is accepted
   and passed to other BGP peers, then the unrecognized transitive
   optional attribute of that path MUST be passed, along with the path,
   to other BGP peers with the Partial bit in the Attribute Flags octet
   set to 1.  If a path with a recognized, transitive optional attribute
   is accepted and passed along to other BGP peers and the Partial bit
   in the Attribute Flags octet is set to 1 by some previous AS, it MUST
   NOT be set back to 0 by the current AS.  Unrecognized non-transitive
   optional attributes MUST be quietly ignored and not passed along to
   other BGP peers.

   New, transitive optional attributes MAY be attached to the path by
   the originator or by any other BGP speaker in the path.  If they are
   not attached by the originator, the Partial bit in the Attribute
   Flags octet is set to 1.  The rules for attaching new non-transitive
   optional attributes will depend on the nature of the specific
   attribute.  The documentation of each new non-transitive optional
   attribute will be expected to include such rules (the description of
   the MULTI_EXIT_DISC attribute gives an example).  All optional
   attributes (both transitive and non-transitive), MAY be updated (if
   appropriate) by BGP speakers in the path.

   The sender of an UPDATE message SHOULD order path attributes within
   the UPDATE message in ascending order of attribute type.  The
   receiver of an UPDATE message MUST be prepared to handle path
   attributes within UPDATE messages that are out of order.

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

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   The mandatory category refers to an attribute that MUST be present in
   both IBGP and EBGP exchanges if NLRI are contained in the UPDATE
   message.  Attributes classified as optional for the purpose of the
   protocol extension mechanism may be purely discretionary,
   discretionary, required, or disallowed in certain contexts.

        attribute           EBGP                    IBGP
         ORIGIN             mandatory               mandatory
         AS_PATH            mandatory               mandatory
         NEXT_HOP           mandatory               mandatory
         MULTI_EXIT_DISC    discretionary           discretionary
         LOCAL_PREF         see Section 5.1.5       required
         ATOMIC_AGGREGATE   see Section 5.1.6 and 9.1.4
         AGGREGATOR         discretionary           discretionary

5.1.  Path Attribute Usage

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

5.1.1.  ORIGIN

   ORIGIN is a well-known mandatory attribute.  The ORIGIN attribute is
   generated by the speaker that originates the associated routing
   information.  Its value SHOULD NOT be changed by any other speaker.

5.1.2.  AS_PATH

   AS_PATH is a well-known mandatory attribute.  This attribute
   identifies the autonomous systems through which routing information
   carried in this UPDATE message has passed.  The components of this
   list can be AS_SETs or AS_SEQUENCEs.

   When a BGP speaker propagates a route it learned from another BGP
   speaker's UPDATE message, it modifies the route's AS_PATH attribute
   based on the location of the BGP speaker to which the route will be
   sent:

      a) When a given BGP speaker advertises the route to an internal
         peer, the advertising speaker SHALL NOT modify the AS_PATH
         attribute associated with the route.

      b) When a given BGP speaker advertises the route to an external
         peer, the advertising speaker updates the AS_PATH attribute as
         follows:

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         1) if the first path segment of the AS_PATH is of type
            AS_SEQUENCE, the local system prepends its own AS number as
            the last element of the sequence (put it in the leftmost
            position with respect to the position of octets in the
            protocol message).  If the act of prepending will cause an
            overflow in the AS_PATH segment (i.e., more than 255 ASes),
            it SHOULD prepend a new segment of type AS_SEQUENCE and
            prepend its own AS number to this new segment.

         2) if the first path segment of the AS_PATH is of type AS_SET,
            the local system prepends a new path segment of type
            AS_SEQUENCE to the AS_PATH, including its own AS number in
            that segment.

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

   When a BGP speaker originates a route then:

      a) the originating speaker includes its own AS number in a path
         segment, of type AS_SEQUENCE, in the AS_PATH attribute of all
         UPDATE messages sent to an external peer.  In this case, the AS
         number of the originating speaker's autonomous system will be
         the only entry the path segment, and this path segment will be
         the only segment in the AS_PATH attribute.

      b) the originating speaker includes an empty AS_PATH attribute in
         all UPDATE messages sent to internal peers.  (An empty AS_PATH
         attribute is one whose length field contains the value zero).

   Whenever the modification of the AS_PATH attribute calls for
   including or prepending the AS number of the local system, the local
   system MAY include/prepend more than one instance of its own AS
   number in the AS_PATH attribute.  This is controlled via local
   configuration.

5.1.3.  NEXT_HOP

   The NEXT_HOP is a well-known mandatory attribute that defines the IP
   address of the router that SHOULD be used as the next hop to the
   destinations listed in the UPDATE message.  The NEXT_HOP attribute is
   calculated as follows:

      1) When sending a message to an internal peer, if the route is not
         locally originated, the BGP speaker SHOULD NOT modify the
         NEXT_HOP attribute unless it has been explicitly configured to
         announce its own IP address as the NEXT_HOP.  When announcing a

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

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

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

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

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

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

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

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

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

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

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

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

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

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

5.1.4.  MULTI_EXIT_DISC

   The MULTI_EXIT_DISC is an optional non-transitive attribute that is
   intended to be used on external (inter-AS) links to discriminate
   among multiple exit or entry points to the same neighboring AS.  The
   value of the MULTI_EXIT_DISC attribute is a four-octet unsigned
   number, called a metric.  All other factors being equal, the exit
   point with the lower metric SHOULD be preferred.  If received over
   EBGP, the MULTI_EXIT_DISC attribute MAY be propagated over IBGP to
   other BGP speakers within the same AS (see also 9.1.2.2).  The
   MULTI_EXIT_DISC attribute received from a neighboring AS MUST NOT be
   propagated to other neighboring ASes.

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

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

   An implementation MAY also (based on local configuration) alter the
   value of the MULTI_EXIT_DISC attribute received over EBGP.  If a BGP
   speaker is configured to alter the value of the MULTI_EXIT_DISC
   attribute received over EBGP, then altering the value MUST be done
   prior to determining the degree of preference of the route and prior
   to performing route selection (Decision Process phases 1 and 2).  See
   Section 9.1.2.2 for necessary restrictions on this.

5.1.5.  LOCAL_PREF

   LOCAL_PREF is a well-known attribute that SHALL be included in all
   UPDATE messages that a given BGP speaker sends to other internal
   peers.  A BGP speaker SHALL calculate the degree of preference for
   each external route based on the locally-configured policy, and
   include the degree of preference when advertising a route to its
   internal peers.  The higher degree of preference MUST be preferred.
   A BGP speaker uses the degree of preference learned via LOCAL_PREF in
   its Decision Process (see Section 9.1.1).

   A BGP speaker MUST NOT include this attribute in UPDATE messages it
   sends to external peers, except in the case of BGP Confederations
   [RFC3065].  If it is contained in an UPDATE message that is received
   from an external peer, then this attribute MUST be ignored by the
   receiving speaker, except in the case of BGP Confederations
   [RFC3065].

5.1.6.  ATOMIC_AGGREGATE

   ATOMIC_AGGREGATE is a well-known discretionary attribute.

   When a BGP speaker aggregates several routes for the purpose of
   advertisement to a particular peer, the AS_PATH of the aggregated
   route normally includes an AS_SET formed from the set of ASes from
   which the aggregate was formed.  In many cases, the network
   administrator can determine if the aggregate can safely be advertised
   without the AS_SET, and without forming route loops.

   If an aggregate excludes at least some of the AS numbers present in
   the AS_PATH of the routes that are aggregated as a result of dropping
   the AS_SET, the aggregated route, when advertised to the peer, SHOULD
   include the ATOMIC_AGGREGATE attribute.

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   A BGP speaker that receives a route with the ATOMIC_AGGREGATE
   attribute SHOULD NOT remove the attribute when propagating the route
   to other speakers.

   A BGP speaker that receives a route with the ATOMIC_AGGREGATE
   attribute MUST NOT make any NLRI of that route more specific (as
   defined in 9.1.4) when advertising this route to other BGP speakers.

   A BGP speaker that receives a route with the ATOMIC_AGGREGATE
   attribute needs to be aware of the fact that the actual path to
   destinations, as specified in the NLRI of the route, while having the
   loop-free property, may not be the path specified in the AS_PATH
   attribute of the route.

5.1.7.  AGGREGATOR

   AGGREGATOR is an optional transitive attribute, which MAY be included
   in updates that are formed by aggregation (see Section 9.2.2.2).  A
   BGP speaker that performs route aggregation MAY add the AGGREGATOR
   attribute, which SHALL contain its own AS number and IP address.  The
   IP address SHOULD be the same as the BGP Identifier of the speaker.

6.  BGP Error Handling.

   This section describes actions to be taken when errors are detected
   while processing BGP messages.

   When any of the conditions described here are detected, a
   NOTIFICATION message, with the indicated Error Code, Error Subcode,
   and Data fields, is sent, and the BGP connection is closed (unless it
   is explicitly stated that no NOTIFICATION message is to be sent and
   the BGP connection is not to be closed).  If no Error Subcode is
   specified, then a zero MUST be used.

   The phrase "the BGP connection is closed" means the TCP connection
   has been closed, the associated Adj-RIB-In has been cleared, and all
   resources for that BGP connection have been deallocated.  Entries in
   the Loc-RIB associated with the remote peer are marked as invalid.
   The local system recalculates its best routes for the destinations of
   the routes marked as invalid.  Before the invalid routes are deleted
   from the system, it advertises, to its peers, either withdraws for
   the routes marked as invalid, or the new best routes before the
   invalid routes are deleted from the system.

   Unless specified explicitly, the Data field of the NOTIFICATION
   message that is sent to indicate an error is empty.

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6.1.  Message Header Error Handling

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

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

   If at least one of the following is true:

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

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

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

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

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

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

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

6.2.  OPEN Message Error Handling

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

   If the version number in the Version field of the received OPEN
   message is not supported, then the Error Subcode MUST be set to
   Unsupported Version Number.  The Data field is a 2-octet unsigned
   integer, which indicates the largest, locally-supported version
   number less than the version the remote BGP peer bid (as indicated in

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   the received OPEN message), or if the smallest, locally-supported
   version number is greater than the version the remote BGP peer bid,
   then the smallest, locally-supported version number.

   If the Autonomous System field of the OPEN message is unacceptable,
   then the Error Subcode MUST be set to Bad Peer AS.  The determination
   of acceptable Autonomous System numbers is outside the scope of this
   protocol.

   If the Hold Time field of the OPEN message is unacceptable, then the
   Error Subcode MUST be set to Unacceptable Hold Time.  An
   implementation MUST reject Hold Time values of one or two seconds.
   An implementation MAY reject any proposed Hold Time.  An
   implementation that accepts a Hold Time MUST use the negotiated value
   for the Hold Time.

   If the BGP Identifier field of the OPEN message is syntactically
   incorrect, then the Error Subcode MUST be set to Bad BGP Identifier.
   Syntactic correctness means that the BGP Identifier field represents
   a valid unicast IP host address.

   If one of the Optional Parameters in the OPEN message is not
   recognized, then the Error Subcode MUST be set to Unsupported
   Optional Parameters.

   If one of the Optional Parameters in the OPEN message is recognized,
   but is malformed, then the Error Subcode MUST be set to 0
   (Unspecific).

6.3.  UPDATE Message Error Handling

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

   Error checking of an UPDATE message begins by examining the path
   attributes.  If the Withdrawn Routes Length or Total Attribute Length
   is too large (i.e., if Withdrawn Routes Length + Total Attribute
   Length + 23 exceeds the message Length), then the Error Subcode MUST
   be set to Malformed Attribute List.

   If any recognized attribute has Attribute Flags that conflict with
   the Attribute Type Code, then the Error Subcode MUST be set to
   Attribute Flags Error.  The Data field MUST contain the erroneous
   attribute (type, length, and value).

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   If any recognized attribute has an Attribute Length that conflicts
   with the expected length (based on the attribute type code), then the
   Error Subcode MUST be set to Attribute Length Error.  The Data field
   MUST contain the erroneous attribute (type, length, and value).

   If any of the well-known mandatory attributes are not present, then
   the Error Subcode MUST be set to Missing Well-known Attribute.  The
   Data field MUST contain the Attribute Type Code of the missing,
   well-known attribute.

   If any of the well-known mandatory attributes are not recognized,
   then the Error Subcode MUST be set to Unrecognized Well-known
   Attribute.  The Data field MUST contain the unrecognized attribute
   (type, length, and value).

   If the ORIGIN attribute has an undefined value, then the Error Sub-
   code MUST be set to Invalid Origin Attribute.  The Data field MUST
   contain the unrecognized attribute (type, length, and value).

   If the NEXT_HOP attribute field is syntactically incorrect, then the
   Error Subcode MUST be set to Invalid NEXT_HOP Attribute.  The Data
   field MUST contain the incorrect attribute (type, length, and value).
   Syntactic correctness means that the NEXT_HOP attribute represents a
   valid IP host address.

   The IP address in the NEXT_HOP MUST meet the following criteria to be
   considered semantically correct:

      a) It MUST NOT be the IP address of the receiving speaker.

      b) In the case of an EBGP, where the sender and receiver are one
         IP hop away from each other, either the IP address in the
         NEXT_HOP MUST be the sender's IP address that is used to
         establish the BGP connection, or the interface associated with
         the NEXT_HOP IP address MUST share a common subnet with the
         receiving BGP speaker.

   If the NEXT_HOP attribute is semantically incorrect, the error SHOULD
   be logged, and the route SHOULD be ignored.  In this case, a
   NOTIFICATION message SHOULD NOT be sent, and the connection SHOULD
   NOT be closed.

   The AS_PATH attribute is checked for syntactic correctness.  If the
   path is syntactically incorrect, then the Error Subcode MUST be set
   to Malformed AS_PATH.

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   If the UPDATE message is received from an external peer, the local
   system MAY check whether the leftmost (with respect to the position
   of octets in the protocol message) AS in the AS_PATH attribute is
   equal to the autonomous system number of the peer that sent the
   message.  If the check determines this is not the case, the Error
   Subcode MUST be set to Malformed AS_PATH.

   If an optional attribute is recognized, then the value of this
   attribute MUST be checked.  If an error is detected, the attribute
   MUST be discarded, and the Error Subcode MUST be set to Optional
   Attribute Error.  The Data field MUST contain the attribute (type,
   length, and value).

   If any attribute appears more than once in the UPDATE message, then
   the Error Subcode MUST be set to Malformed Attribute List.

   The NLRI field in the UPDATE message is checked for syntactic
   validity.  If the field is syntactically incorrect, then the Error
   Subcode MUST be set to Invalid Network Field.

   If a prefix in the NLRI field is semantically incorrect (e.g., an
   unexpected multicast IP address), an error SHOULD be logged locally,
   and the prefix SHOULD be ignored.

   An UPDATE message that contains correct path attributes, but no NLRI,
   SHALL be treated as a valid UPDATE message.

6.4.  NOTIFICATION Message Error Handling

   If a peer sends a NOTIFICATION message, and the receiver of the
   message detects an error in that message, the receiver cannot use a
   NOTIFICATION message to report this error back to the peer.  Any such
   error (e.g., an unrecognized Error Code or Error Subcode) SHOULD be
   noticed, logged locally, and brought to the attention of the
   administration of the peer.  The means to do this, however, lies
   outside the scope of this document.

6.5.  Hold Timer Expired Error Handling

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

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6.6.  Finite State Machine Error Handling

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

6.7.  Cease

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

   A BGP speaker MAY support the ability to impose a locally-configured,
   upper bound on the number of address prefixes the speaker is willing
   to accept from a neighbor.  When the upper bound is reached, the
   speaker, under control of local configuration, either (a) discards
   new address prefixes from the neighbor (while maintaining the BGP
   connection with the neighbor), or (b) terminates the BGP connection
   with the neighbor.  If the BGP speaker decides to terminate its BGP
   connection with a neighbor because the number of address prefixes
   received from the neighbor exceeds the locally-configured, upper
   bound, then the speaker MUST send the neighbor a NOTIFICATION message
   with the Error Code Cease.  The speaker MAY also log this locally.

6.8.  BGP Connection Collision Detection

   If a pair of BGP speakers try to establish a BGP connection with each
   other simultaneously, then two parallel connections well be formed.
   If the source IP address used by one of these connections is the same
   as the destination IP address used by the other, and the destination
   IP address used by the first connection is the same as the source IP
   address used by the other, connection collision has occurred.  In the
   event of connection collision, one of the connections MUST be closed.

   Based on the value of the BGP Identifier, a convention is established
   for detecting which BGP connection is to be preserved when a
   collision occurs.  The convention is to compare the BGP Identifiers
   of the peers involved in the collision and to retain only the
   connection initiated by the BGP speaker with the higher-valued BGP
   Identifier.

   Upon receipt of an OPEN message, the local system MUST examine all of
   its connections that are in the OpenConfirm state.  A BGP speaker MAY
   also examine connections in an OpenSent state if it knows the BGP
   Identifier of the peer by means outside of the protocol.  If, among
   these connections, there is a connection to a remote BGP speaker

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   whose BGP Identifier equals the one in the OPEN message, and this
   connection collides with the connection over which the OPEN message
   is received, then the local system performs the following collision
   resolution procedure:

      1) The BGP Identifier of the local system is compared to the BGP
         Identifier of the remote system (as specified in the OPEN
         message).  Comparing BGP Identifiers is done by converting them
         to host byte order and treating them as 4-octet unsigned
         integers.

      2) If the value of the local BGP Identifier is less than the
         remote one, the local system closes the BGP connection that
         already exists (the one that is already in the OpenConfirm
         state), and accepts the BGP connection initiated by the remote
         system.

      3) Otherwise, the local system closes the newly created BGP
         connection (the one associated with the newly received OPEN
         message), and continues to use the existing one (the one that
         is already in the OpenConfirm state).

   Unless allowed via configuration, a connection collision with an
   existing BGP connection that is in the Established state causes
   closing of the newly created connection.

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

   Closing the BGP connection (that results from the collision
   resolution procedure) is accomplished by sending the NOTIFICATION
   message with the Error Code Cease.



(page 36 continued on part 3)

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