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.
      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.

         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).