common header specifies this to be a BGP OPEN message, then the
Version field will be found in the message but not in the BGP common
header.
to establish a connection between BGP peers. The NOTIFICATION
message type is used to advertise errors. The UPDATE message type is
used to advertise topology updates and changes, and the KEEP-
ALIVE message type is sent to keep a session active when no UPDATE
messages are exchanged during the established hold time.