4.1 Message Header Format (original) (raw)

Connected: An Internet Encyclopedia
4.1 Message Header Format

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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.

  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

Next: 4.2 OPEN Message Format

Connected: An Internet Encyclopedia
4.1 Message Header Format