draft-ietf-idr-bgp4-20.txt

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




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RFC DRAFT                                                     April 2003


                  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 long field with the fol-
            lowing 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 (not the number of octets) in the path
            segment value field.

            The path segment value field contains one or more AS num-
            bers, 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
            (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



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RFC DRAFT                                                     April 2003


            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 other
            internal peers of the advertising speaker's degree of pref-
            erence 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.

      Network Layer Reachability Information:

         This variable length field contains a list of IP address pre-
         fixes. 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.




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RFC DRAFT                                                     April 2003


         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).

   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 to be withdrawn from ser-
   vice.  Each such route is identified by its destination (expressed as
   an IP prefix), which unambiguously identifies the route in the con-
   text of the BGP speaker - BGP speaker connection to which it has been
   previously advertised.

   An UPDATE message might 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.



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RFC DRAFT                                                     April 2003


   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 if
   the WITHDRAWN ROUTES doesn't 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 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 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 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 (variable)             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


      Error Code:

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

            Error Code       Symbolic Name               Reference



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RFC DRAFT                                                     April 2003


              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 informa-
         tion about the nature of the reported error.  Each Error Code
         may have one or more Error Subcodes associated with it. If no
         appropriate Error Subcode is defined, then a zero (Unspecific)
         value is used for the Error Subcode field.

         Message Header Error subcodes:

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

         OPEN Message Error subcodes:

                               1 - Unsupported Version Number.
                               2 - Bad Peer AS.
                               3 - Bad BGP Identifier.
                               4 - Unsupported 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.



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RFC DRAFT                                                     April 2003


                              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 (includ-
   ing message header).


5. Path Attributes