​Establishing a connection

1. Client sends server → Logon <A> message
2. Is client ResetSeqNumFlag <141> set to Y?
   • Yes, reset sequence numbers and proceed:
     • Server resets the next expected client sequence number
     • Server resets its own sequence number
     • Server responds with ← Logon <A> message with reset sequence number
     • Client sends server → Heartbeat <0>
   • No, negotiate sequence numbers on both sides:
     1. Is client MsgSeqNum <34> value what the server is expecting?
        • No, value is greater than expected:
          • Server responds with ← Logon <A> message
          • Server sends client ← Resend Request <2>
          • Client sends server → Sequence Reset <4> (GapFillFlag <123> = Y)
        • No, value is less than expected:
          • Server disconnects
          • After checking what happened, client re-sends Logon <A> message with ResetSeqNumFlag <141> field set to Y
        • Otherwise:
          • server sends client ← Heartbeat <0>
     2. Is server MsgSeqNum <34> value what the client is expecting?
        • No, value is greater than expected:
          • Server responds with ← Logon <A> message
          • Client sends server → Resend Request <2>
          • If the server is unable to replay the messages due to technical reasons, it may send a Business Message Reject <j> message with:
            • BusinessRejectReason <380> set to 0 (Other)
            • Text <58> set to "Unable to process ResendRequest. Please contact support."
          • If there are no issues, the server proceeds to replay the messages to the client:
            • Server responds with ← message PossDupFlag <43> = Y
            • Replay from server to client is complete
            • Server responds with ← Heartbeat <0>
        • No, value is less than expected:
          • Client disconnects or sends server → Logout <5>
          • Gemini coordinates with the client to triage so the FIX connection can be re-established
        • Otherwise:
          • Client sends server → Heartbeat <0>
3. Success! Your FIX connection is established.