SenderCompID(49) and TargetCompID(56)  are used to identify sending and receiving firms, respectively. They are required in the standard message header for all messages exchanged between FIX Client and FIX Server. SenderCompID(49) and TargetCompID(56) values shall be assigned from FIX Server’s side. FIX Client establishes the FIX session by sending a Logon (A) request, and the FIX Server responds with Logon (A) response.

TargetSubID(57) FIX tag may optionally be specified to identify specific message originator (firm, desk, trader, etc.) in third-party transactions and specify individual or unit intended to receive message, respectively.

If a Logon (A) message sent by FIX Client is not accepted for some reason, then a Logout(5) message with Text(58) tag detailing the problem shall be sent. If the reason of logon failure is that the sequence number of the logon message was less than expected, tag 58 of the Logout message shall contain a fixed-format text which contains the expected sequence number. The format of the logout text shall be specified to the Fix Client.

The FIX session lifetime may vary based on the nature of final destinations, regions and exchange times. Start and stop times of Fix Server and Client, as well as the sequence number reset time, shall be agreed upon during the setup process.  FIX Client establishes a new FIX session by sending Logon (A) message with MsgSeqNum(34)=1. The FIX session lifetime is not ended by Client connectivity loss or even Logouts.

Resend Request (2) message can be used by both FIX Client and Server to recover FIX messages when a sequence number gap is detected during current session.

Recommended Heartbeat interval value is 30 seconds (HeartBtInt(108) in Logon message).

The fix server shall not suspend/cancel orders on Client disconnect because of connection loss.