draft-ietf-idr-bgp4-13.txt

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                  +---------------------------+
                  |   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 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:



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RFC DRAFT                                                 September 2001


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

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




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


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

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

         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



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RFC DRAFT                                                 September 2001


         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:

         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

                  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>.

            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



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RFC DRAFT                                                 September 2001


            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 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 non-negative 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.

            Its usage is defined in 5.1.4.

         e) LOCAL_PREF (Type Code 5):

            LOCAL_PREF is a well-known mandatory attribute that is a
            four octet non-negative integer. A BGP speaker uses it to
            inform other internal peers of the advertising speaker's
            degree of preference for an advertised route. Usage of this
            attribute is described in 5.1.5.

         f) ATOMIC_AGGREGATE (Type Code 6)

            ATOMIC_AGGREGATE is a well-known discretionary attribute of
            length 0. A BGP speaker uses it 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



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RFC DRAFT                                                 September 2001


            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 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 -
            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 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



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RFC DRAFT                                                 September 2001


            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
   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 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 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.







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