Each session starts with an session-message LOGON and ends with session-message LOGOUT. Session-messages are also known as “admin messages”. Main purpose is session management and flow control as in TCP
To maintain the session, I think FIX uses not only the session messages — each app message also has session-related tags like (10)checksum, (9)bodyLength and (34)sequence numbers — used to maintain a session between client and server.
retrans-request — is a message to ask for a resend of a missing message
heartbeat messages — between 2 parties