4.2. OPEN Message Format

After a TCP 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.

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 (variable)                    |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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 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 from 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 that is assigned to that BGP speaker. The value of the BGP Identifier is determined upon startup and is the same for every local interface and 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 contains a list of optional parameters, in which 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.

[RFC3392] defines the Capabilities Optional Parameter.

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

4.2. OPEN Message Format​

4.2. OPEN Message Format