W5500 can not recognize session has been disconnected in the server mode

W5500 already has timeout mechanism. timeout is occur by RTR/RCR register.
At the TCP packet retransmission timeout, W5500 transmits TCP packets(SYS, Data packets) and wait for ACK during the configured RTR and RCR time. If it is no ACK from the peer, a temporary timeout occurs and the TCP packet is retransmitted.
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Therefore, when the network is disconnected for every reason such as cable disconnecting, W5500 will close the socket by timeout.