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RFC 1771

 
 
 

A Border Gateway Protocol 4 (BGP-4)

Part 2 of 3, p. 6 to 34
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prevText      Top       Page 6 
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.

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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 contains a value that the receiver of the
         message can predict.  If the Type of the message is OPEN, or if
         the OPEN message carries no Authentication Information (as an
         Optional Parameter), then the Marker must be all ones.
         Otherwise, the value of the marker can be predicted by some a
         computation specified as part of the authentication mechanism
         (which is specified as part of the Authentication Information)
         used.  The Marker can be used to detect loss of synchronization
         between a pair of BGP peers, and to authenticate incoming BGP
         messages.

      Length:

         This 2-octet unsigned integer indicates the total length of the
         message, including the header, in octets.  Thus, e.g., it
         allows one to locate in the transport-level stream the (Marker
         field of the) next message.  The value of the Length field must
         always be at least 19 and no greater than 4096, and may be
         further constrained, depending on the message type.  No
         "padding" of extra data after the message is allowed, so the
         Length field must have the smallest value required given the
         rest of the message.

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      Type:

         This 1-octet unsigned integer indicates the type code of the
         message.  The following type codes are defined:

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

4.2 OPEN Message Format

   After a transport protocol connection is established, the first
   message sent by each side is an OPEN message.  If the OPEN message is
   acceptable, a KEEPALIVE message confirming the OPEN is sent back.
   Once the OPEN is confirmed, UPDATE, KEEPALIVE, and NOTIFICATION
   messages may be exchanged.

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

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

      Version:

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

      My Autonomous System:

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

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      Hold Time:

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

      BGP Identifier:

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

      Optional Parameters Length:

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

      Optional Parameters:

         This field may contain a list of optional parameters, where
         each parameter is encoded as a <Parameter Type, Parameter
         Length, Parameter Value> triplet.

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

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

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         This document defines the following Optional Parameters:

         a) Authentication Information (Parameter Type 1):

            This optional parameter may be used to authenticate a BGP
            peer. The Parameter Value field contains a 1-octet
            Authentication Code followed by a variable length
            Authentication Data.

                0 1 2 3 4 5 6 7 8
                +-+-+-+-+-+-+-+-+
                |  Auth. Code   |
                +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                |                                                     |
                |              Authentication Data                    |
                |                                                     |
                +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Authentication Code:

                  This 1-octet unsigned integer indicates the
                  authentication mechanism being used.  Whenever an
                  authentication mechanism is specified for use within
                  BGP, three things must be included in the
                  specification:

                  - the value of the Authentication Code which indicates
                  use of the mechanism,
                  - the form and meaning of the Authentication Data, and
                  - the algorithm for computing values of Marker fields.

                  Note that a separate authentication mechanism may be
                  used in establishing the transport level connection.

               Authentication Data:

                  The form and meaning of this field is a variable-
                  length field depend on the Authentication Code.

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

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4.3 UPDATE Message Format

   UPDATE messages are used to transfer routing information between BGP
   peers.  The information in the UPDATE packet can be used to construct
   a graph describing the relationships of the various Autonomous
   Systems.  By applying rules to be discussed, routing information
   loops and some other anomalies may be detected and removed from
   inter-AS routing.

   An UPDATE message is used to advertise a single feasible route to a
   peer, or to withdraw multiple unfeasible routes from service (see
   3.1). An UPDATE message may simultaneously advertise a feasible route
   and withdraw multiple unfeasible routes from service.  The UPDATE
   message always includes the fixed-size BGP header, and can optionally
   include the other fields as shown below:

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

      Unfeasible Routes Length:

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

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

      Withdrawn Routes:

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

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

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

         a) Length:

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

         b) Prefix:

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

      Total Path Attribute Length:

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

         A value of 0 indicates that no Network Layer Reachability
         Information field is present in this UPDATE message.

      Path Attributes:

         A variable length sequence of path attributes is present in
         every UPDATE.  Each path attribute is a triple <attribute type,
         attribute length, attribute value> of variable length.

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

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

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

         The second high-order bit (bit 1) of the Attribute Flags octet
         is the Transitive bit.  It defines whether an optional
         attribute is transitive (if set to 1) or non-transitive (if set
         to 0).  For well-known attributes, the Transitive bit must be
         set to 1.  (See Section 5 for a discussion of transitive
         attributes.)

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

         The fourth high-order bit (bit 3) of the Attribute Flags octet
         is the Extended Length bit.  It defines whether the Attribute
         Length is one octet (if set to 0) or two octets (if set to 1).
         Extended Length may be used only if the length of the attribute
         value is greater than 255 octets.

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

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

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

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

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

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         a)   ORIGIN (Type Code 1):

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

                  Value      Meaning

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

                  1         EGP - Network Layer Reachability Information
                               learned via EGP

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

            Its usage is defined in 5.1.1

         b) AS_PATH (Type Code 2):

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

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

                  Value      Segment Type

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

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

            The path segment length is a 1-octet long field containing
            the number of ASs in the path segment value field.

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

            Usage of this attribute is defined in 5.1.2.

         c)   NEXT_HOP (Type Code 3):

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

            Usage of this attribute is defined in 5.1.3.

         d) MULTI_EXIT_DISC (Type Code 4):

            This is an optional non-transitive attribute that is a four
            octet non-negative integer. The value of this attribute may
            be used by a BGP speaker's decision process to discriminate
            among multiple exit points to a neighboring autonomous
            system.

            Its usage is defined in 5.1.4.

         e) LOCAL_PREF (Type Code 5):

            LOCAL_PREF is a well-known discretionary attribute that is a
            four octet non-negative integer. It is used by a BGP speaker
            to inform other BGP speakers in its own autonomous system of
            the originating speaker's degree of preference for an
            advertised route. Usage of this attribute is described in
            5.1.5.

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         f) ATOMIC_AGGREGATE (Type Code 6)

            ATOMIC_AGGREGATE is a well-known discretionary attribute of
            length 0. It is used by a BGP speaker to inform other BGP
            speakers that the local system selected a less specific
            route without selecting a more specific route which is
            included in it. Usage of this attribute is described in
            5.1.6.

         g) AGGREGATOR (Type Code 7)

            AGGREGATOR is an optional transitive attribute of length 6.
            The attribute contains the last AS number that formed the
            aggregate route (encoded as 2 octets), followed by the IP
            address of the BGP speaker that formed the aggregate route
            (encoded as 4 octets).  Usage of this attribute is described
            in 5.1.7

      Network Layer Reachability Information:

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

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

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

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

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

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

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

   An UPDATE message can advertise at most one route, which may be
   described by several path attributes. All path attributes contained
   in a given UPDATE messages apply to the destinations carried in the
   Network Layer Reachability Information field of the UPDATE message.

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

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

4.4 KEEPALIVE Message Format

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

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   messages as a function of the Hold Time interval.

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

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

4.5 NOTIFICATION Message Format

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

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

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

      Error Code:

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

            Error Code       Symbolic Name               Reference

              1         Message Header Error             Section 6.1

              2         OPEN Message Error               Section 6.2

              3         UPDATE Message Error             Section 6.3

              4         Hold Timer Expired               Section 6.5

              5         Finite State Machine Error       Section 6.6

              6         Cease                            Section 6.7

      Error subcode:

         This 1-octet unsigned integer provides more specific
         information about the nature of the reported error.  Each Error
         Code may have one or more Error Subcodes associated with it.

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         If no appropriate Error Subcode is defined, then a zero
         (Unspecific) value is used for the Error Subcode field.

         Message Header Error subcodes:

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

         OPEN Message Error subcodes:

                               1  - Unsupported Version Number.
                               2  - Bad Peer AS.
                               3  - Bad BGP Identifier. '
         4  - Unsupported Optional Parameter.
                               5  - Authentication Failure.
                                           6  - Unacceptable Hold Time.

         UPDATE Message Error subcodes:

                               1 - Malformed Attribute List.
                               2 - Unrecognized Well-known Attribute.
                               3 - Missing Well-known Attribute.
                               4 - Attribute Flags Error.
                               5 - Attribute Length Error.
                               6 - Invalid ORIGIN Attribute
                               7 - AS Routing Loop.
                               8 - Invalid NEXT_HOP Attribute.
                               9 - Optional Attribute Error.
                              10 - Invalid Network Field.
                              11 - Malformed AS_PATH.

      Data:

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

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

                  Message Length = 21 + Data Length

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

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5.  Path Attributes

   This section discusses the path attributes of the UPDATE message.

   Path attributes fall into four separate categories:

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

   Well-known attributes must be recognized by all BGP implementations.
   Some of these attributes are mandatory and must be included in every
   UPDATE message.  Others are discretionary and may or may not be sent
   in a particular UPDATE message.

   All well-known attributes must be passed along (after proper
   updating, if necessary) to other BGP peers.

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

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

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

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

5.1 Path Attribute Usage

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

5.1.1 ORIGIN

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

5.1.2   AS_PATH

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

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

      a) When a given BGP speaker advertises the route to another BGP
      speaker located in its own autonomous system, the advertising
      speaker shall not modify the AS_PATH attribute associated with the
      route.

      b) When a given BGP speaker advertises the route to a BGP speaker
      located in a neighboring autonomous system, then the advertising
      speaker shall update the AS_PATH attribute as follows:

         1) if the first path segment of the AS_PATH is of type
         AS_SEQUENCE, the local system shall prepend its own AS number
         as the last element of the sequence (put it in the leftmost
         position).

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

      When a BGP speaker originates a route then:

         a) the originating speaker shall include its own AS number in
         the AS_PATH attribute of all UPDATE messages sent to BGP
         speakers located in neighboring autonomous systems. (In this
         case, the AS number of the originating speaker's autonomous
         system will be the only entry in the AS_PATH attribute).

         b) the originating speaker shall include an empty AS_PATH
         attribute in all UPDATE messages sent to BGP speakers located
         in its own autonomous system. (An empty AS_PATH attribute is
         one whose length field contains the value zero).

5.1.3 NEXT_HOP

   The NEXT_HOP path attribute defines the IP address of the border
   router that should be used as the next hop to the destinations listed
   in the UPDATE message.  If a border router belongs to the same AS as
   its peer, then the peer is an internal border router. Otherwise, it
   is an external border router.  A BGP speaker can advertise any
   internal border router as the next hop provided that the interface
   associated with the IP address of this border router (as specified in
   the NEXT_HOP path attribute) shares a common subnet with both the
   local and remote BGP speakers. A BGP speaker can advertise any
   external border router as the next hop, provided that the IP address
   of this border router was learned from one of the BGP speaker's
   peers, and the interface associated with the IP address of this
   border router (as specified in the NEXT_HOP path attribute) shares a
   common subnet with the local and remote BGP speakers.  A BGP speaker
   needs to be able to support disabling advertisement of external
   border routers.

   A BGP speaker must never advertise an address of a peer to that peer
   as a NEXT_HOP, for a route that the speaker is originating.  A BGP
   speaker must never install a route with itself as the next hop.

   When a BGP speaker advertises the route to a BGP speaker located in
   its own autonomous system, the advertising speaker shall not modify
   the NEXT_HOP attribute associated with the route.  When a BGP speaker
   receives the route via an internal link, it may forward packets to
   the NEXT_HOP address if the address contained in the attribute is on
   a common subnet with the local and remote BGP speakers.

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5.1.4   MULTI_EXIT_DISC

   The MULTI_EXIT_DISC attribute may be used on external (inter-AS)
   links to discriminate among multiple exit or entry points to the same
   neighboring AS.  The value of the MULTI_EXIT_DISC attribute is a four
   octet unsigned number which is called a metric.  All other factors
   being equal, the exit or entry point with lower metric should be
   preferred.  If received over external links, the MULTI_EXIT_DISC
   attribute may be propagated over internal links to other BGP speakers
   within the same AS.  The MULTI_EXIT_DISC attribute is never
   propagated to other BGP speakers in neighboring AS's.

5.1.5   LOCAL_PREF

   LOCAL_PREF is a well-known discretionary attribute that shall be
   included in all UPDATE messages that a given BGP speaker sends to the
   other BGP speakers located in its own autonomous system. A BGP
   speaker shall calculate the degree of preference for each external
   route and include the degree of preference when advertising a route
   to its internal peers. The higher degree of preference should be
   preferred. A BGP speaker shall use the degree of preference learned
   via LOCAL_PREF in its decision process (see section 9.1.1).

   A BGP speaker shall not include this attribute in UPDATE messages
   that it sends to BGP speakers located in a neighboring autonomous
   system. If it is contained in an UPDATE message that is received from
   a BGP speaker which is not located in the same autonomous system as
   the receiving speaker, then this attribute shall be ignored by the
   receiving speaker.

5.1.6   ATOMIC_AGGREGATE

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

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   attribute.

5.1.7   AGGREGATOR

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

6.  BGP Error Handling.

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

   When any of the conditions described here are detected, a
   NOTIFICATION message with the indicated Error Code, Error Subcode,
   and Data fields is sent, and the BGP connection is closed.  If no
   Error Subcode is specified, then a zero must be used.

   The phrase "the BGP connection is closed" means that the transport
   protocol connection has been closed and that all resources for that
   BGP connection have been deallocated.  Routing table entries
   associated with the remote peer are marked as invalid.  The fact that
   the routes have become invalid is passed to other BGP peers before
   the routes are deleted from the system.

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

6.1 Message Header error handling.

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

   The expected value of the Marker field of the message header is all
   ones if the message type is OPEN.  The expected value of the Marker
   field for all other types of BGP messages determined based on the
   presence of the Authentication Information Optional Parameter in the
   BGP OPEN message and the actual authentication mechanism (if the
   Authentication Information in the BGP OPEN message is present). If
   the Marker field of the message header is not the expected one, then
   a synchronization error has occurred and the Error Subcode is set to
   Connection Not Synchronized.

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   If the Length field of the message header is less than 19 or greater
   than 4096, or if the Length field of an OPEN message is less  than
   the minimum length of the OPEN message, or if the Length field of an
   UPDATE message is less than the minimum length of the UPDATE message,
   or if the Length field of a KEEPALIVE message is not equal to 19, or
   if the Length field of a NOTIFICATION message is less than the
   minimum length of the NOTIFICATION message, then the Error Subcode is
   set to Bad Message Length.  The Data field contains the erroneous
   Length field.

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

6.2 OPEN message error handling.

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

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

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

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

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

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

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

6.3 UPDATE message error handling.

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

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

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

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

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

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

   If the ORIGIN attribute has an undefined value, then the Error
   Subcode is set to Invalid Origin Attribute.  The Data field contains
   the unrecognized attribute (type, length and value).

   If the NEXT_HOP attribute field is syntactically incorrect, then the
   Error Subcode is set to Invalid NEXT_HOP Attribute.  The Data field
   contains the incorrect attribute (type, length and value).  Syntactic
   correctness means that the NEXT_HOP attribute represents a valid IP
   host address.  Semantic correctness applies only to the external BGP

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   links. It means that the interface associated with the IP address, as
   specified in the NEXT_HOP attribute, shares a common subnet with the
   receiving BGP speaker and is not the IP address of the receiving BGP
   speaker.  If the NEXT_HOP attribute is semantically incorrect, the
   error should be logged, and the the route should be ignored.  In this
   case, no NOTIFICATION message should be sent.

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

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

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

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

6.4 NOTIFICATION message error handling.

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

6.5 Hold Timer Expired error handling.

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

6.6 Finite State Machine error handling.

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

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6.7 Cease.

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

6.8 Connection collision detection.

   If a pair of BGP speakers try simultaneously to establish a TCP
   connection to each other, then two parallel connections between this
   pair of speakers might well be formed.  We refer to this situation as
   connection collision.  Clearly, one of these connections must be
   closed.

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

   Upon receipt of an OPEN message, the local system must examine all of
   its connections that are in the OpenConfirm state.  A BGP speaker may
   also examine connections in an OpenSent state if it knows the BGP
   Identifier of the peer by means outside of the protocol.  If among
   these connections there is a connection to a remote BGP speaker whose
   BGP Identifier equals the one in the OPEN message, then the local
   system performs the following collision resolution procedure:

      1. The BGP Identifier of the local system is compared to the BGP
      Identifier of the remote system (as specified in the OPEN
      message).

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

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

      Comparing BGP Identifiers is done by treating them as (4-octet
      long) unsigned integers.

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      A connection collision with an existing BGP connection that is in
      Established states causes unconditional closing of the newly
      created connection. Note that a connection collision cannot be
      detected with connections that are in Idle, or Connect, or Active
      states.

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

7.  BGP Version Negotiation.

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

8.  BGP Finite State machine.

   This section specifies BGP operation in terms of a Finite State
   Machine (FSM).  Following is a brief summary and overview of BGP
   operations by state as determined by this FSM.  A condensed version
   of the BGP FSM is found in Appendix 1.

      Initially BGP is in the Idle state.

      Idle state:

         In this state BGP refuses all incoming BGP connections.  No
         resources are allocated to the peer.  In response to the Start
         event (initiated by either system or operator) the local system
         initializes all BGP resources, starts the ConnectRetry timer,
         initiates a transport connection to other BGP peer, while
         listening for connection that may be initiated by the remote
         BGP peer, and changes its state to Connect.  The exact value of
         the ConnectRetry timer is a local matter, but should be
         sufficiently large to allow TCP initialization.

         If a BGP speaker detects an error, it shuts down the connection
         and changes its state to Idle. Getting out of the Idle state

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         requires generation of the Start event.  If such an event is
         generated automatically, then persistent BGP errors may result
         in persistent flapping of the speaker.  To avoid such a
         condition it is recommended that Start events should not be
         generated immediately for a peer that was previously
         transitioned to Idle due to an error. For a peer that was
         previously transitioned to Idle due to an error, the time
         between consecutive generation of Start events, if such events
         are generated automatically, shall exponentially increase. The
         value of the initial timer shall be 60 seconds. The time shall
         be doubled for each consecutive retry.

         Any other event received in the Idle state is ignored.

      Connect state:

         In this state BGP is waiting for the transport protocol
         connection to be completed.

         If the transport protocol connection succeeds, the local system
         clears the ConnectRetry timer, completes initialization, sends
         an OPEN message to its peer, and changes its state to OpenSent.

         If the transport protocol connect fails (e.g., retransmission
         timeout), the local system restarts the ConnectRetry timer,
         continues to listen for a connection that may be initiated by
         the remote BGP peer, and changes its state to Active state.

         In response to the ConnectRetry timer expired event, the local
         system restarts the ConnectRetry timer, initiates a transport
         connection to other BGP peer, continues to listen for a
         connection that may be initiated by the remote BGP peer, and
         stays in the Connect state.

         Start event is ignored in the Active state.

         In response to any other event (initiated by either system or
         operator), the local system releases all BGP resources
         associated with this connection and changes its state to Idle.

      Active state:

         In this state BGP is trying to acquire a peer by initiating a
         transport protocol connection.

         If the transport protocol connection succeeds, the local system
         clears the ConnectRetry timer, completes initialization, sends
         an OPEN message to its peer, sets its Hold Timer to a large

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         value, and changes its state to OpenSent.  A Hold Timer value
         of 4 minutes is suggested.

         In response to the ConnectRetry timer expired event, the local
         system restarts the ConnectRetry timer, initiates a transport
         connection to other BGP peer, continues to listen for a
         connection that may be initiated by the remote BGP peer, and
         changes its state to Connect.

         If the local system detects that a remote peer is trying to
         establish BGP connection to it, and the IP address of the
         remote peer is not an expected one, the local system restarts
         the ConnectRetry timer, rejects the attempted connection,
         continues to listen for a connection that may be initiated by
         the remote BGP peer, and stays in the Active state.

         Start event is ignored in the Active state.

         In response to any other event (initiated by either system or
         operator), the local system releases all BGP resources
         associated with this connection and changes its state to Idle.

      OpenSent state:

         In this state BGP waits for an OPEN message from its peer.
         When an OPEN message is received, all fields are checked for
         correctness.  If the BGP message header checking or OPEN
         message checking detects an error (see Section 6.2), or a
         connection collision (see Section 6.8) the local system sends a
         NOTIFICATION message and changes its state to Idle.

         If there are no errors in the OPEN message, BGP sends a
         KEEPALIVE message and sets a KeepAlive timer.  The Hold Timer,
         which was originally set to a large value (see above), is
         replaced with the negotiated Hold Time value (see section 4.2).
         If the negotiated Hold Time value is zero, then the Hold Time
         timer and KeepAlive timers are not started.  If the value of
         the Autonomous System field is the same as the local Autonomous
         System number, then the connection is an "internal" connection;
         otherwise, it is "external".  (This will effect UPDATE
         processing as described below.)  Finally, the state is changed
         to OpenConfirm.

         If a disconnect notification is received from the underlying
         transport protocol, the local system closes the BGP connection,
         restarts the ConnectRetry timer, while continue listening for
         connection that may be initiated by the remote BGP peer, and
         goes into the Active state.

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         If the Hold Timer expires, the local system sends NOTIFICATION
         message with error code Hold Timer Expired and changes its
         state to Idle.

         In response to the Stop event (initiated by either system or
         operator) the local system sends NOTIFICATION message with
         Error Code Cease and changes its state to Idle.

         Start event is ignored in the OpenSent state.

         In response to any other event the local system sends
         NOTIFICATION message with Error Code Finite State Machine Error
         and changes its state to Idle.

         Whenever BGP changes its state from OpenSent to Idle, it closes
         the BGP (and transport-level) connection and releases all
         resources associated with that connection.

      OpenConfirm state:

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

         If the local system receives a KEEPALIVE message, it changes
         its state to Established.

         If the Hold Timer expires before a KEEPALIVE message is
         received, the local system sends NOTIFICATION message with
         error code Hold Timer Expired and changes its state to Idle.

         If the local system receives a NOTIFICATION message, it changes
         its state to Idle.

         If the KeepAlive timer expires, the local system sends a
         KEEPALIVE message and restarts its KeepAlive timer.

         If a disconnect notification is received from the underlying
         transport protocol, the local system changes its state to Idle.

         In response to the Stop event (initiated by either system or
         operator) the local system sends NOTIFICATION message with
         Error Code Cease and changes its state to Idle.

         Start event is ignored in the OpenConfirm state.

         In response to any other event the local system sends
         NOTIFICATION message with Error Code Finite State Machine Error
         and changes its state to Idle.

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         Whenever BGP changes its state from OpenConfirm to Idle, it
         closes the BGP (and transport-level) connection and releases
         all resources associated with that connection.

      Established state:

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

         If the local system receives an UPDATE or KEEPALIVE message, it
         restarts its Hold Timer, if the negotiated Hold Time value is
         non-zero.

         If the local system receives a NOTIFICATION message, it changes
         its state to Idle.

         If the local system receives an UPDATE message and the UPDATE
         message error handling procedure (see Section 6.3) detects an
         error, the local system sends a NOTIFICATION message and
         changes its state to Idle.

         If a disconnect notification is received from the underlying
         transport protocol, the local system changes its state to Idle.

         If the Hold Timer expires, the local system sends a
         NOTIFICATION message with Error Code Hold Timer Expired and
         changes its state to Idle.

         If the KeepAlive timer expires, the local system sends a
         KEEPALIVE message and restarts its KeepAlive timer.

         Each time the local system sends a KEEPALIVE or UPDATE message,
         it restarts its KeepAlive timer, unless the negotiated Hold
         Time value is zero.

         In response to the Stop event (initiated by either system or
         operator), the local system sends a NOTIFICATION message with
         Error Code Cease and changes its state to Idle.

         Start event is ignored in the Established state.

         In response to any other event, the local system sends
         NOTIFICATION message with Error Code Finite State Machine Error
         and changes its state to Idle.

         Whenever BGP changes its state from Established to Idle, it
         closes the BGP (and transport-level) connection, releases all
         resources associated with that connection, and deletes all

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         routes derived from that connection.



(page 34 continued on part 3)

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