After a proper SYN, SYN+ACK and ACK TCP-Handshake, the server immediately sends a FIN+ACK indicating a graceful connection closure. The client's HTTP requests results in RST packets from server. This happens once in a 100 concurrent calls. I have already investigated into SynAttackProtect parameter and Backlog Queue and these were related to connection resets, so unrelated to my problem. So in what cases will a Windows server suddenly issue a FIN, immediately after a successful TCP handshake?