As soon as two BGP peers establish a TCP connection, these are sent by the OPEN Message which declares the autonomous system number and establish other operating parameters. In addition to the standard header an OPEN message contains a value for the hold timer which is used to specify the maximum number of seconds which may elapse between the receipts of two successive messages. Here the version field identifies the protocol version which is used. This format is for the version 4. Each autonomous system is assigned with a unique number. The field AUTONOMOUS SYSTEMS NUM gives the autonomous system number of the sender's system. The HOLD time field specifies a maximum time which receiver should wait for a message from the sender. Free Virtual Machines - Home Cloud Computing Virtualization
Here the receiver is required to implement the timer using this value. The timer is reset every time whenever the message arrives. If the timer expires, the receiver assumes that the sender is no longer available and stops forwarding the datagrams along the routes which are learned from the sender. Field BGP IDENTIFIER which contains a 32 bit integer value which uniquely identifies the sender. If the machine has the multiple peers or the multiple autonomous systems by which the machine must use the same identifier in all the communications. This protocol specifies that the identifier which is an IP address. Thus a router chooses one of the IP addresses to use with all the BGP Peers. The last field of the OPEN message is optional.
If the present field which is the PARM, the LAN specifies the length measured in the octets and the field which is labeled as the optional parameter contains a list of parameters. So, if there are no parameters, the value of the PARM, LAN is zero and the message ends with no further data. So, when it accepts the incoming OPEN message, a machine speaking and the BGP responds by sending the KEEPALIVE message. Here each side must send in the OPEN and receiver KEEPALIVE message before it can exchange the routing information.