If a client connected to ListenTCP processor, gets disconnected abruptly:
(eg.: they disconnected their VPN to NiFi's network), the client-TCP-socket on the NiFi node will be seen lingering for an extended amount of time (eg.: such socket can linger in established state even after 12 hours after disconnection)
Or till a manual workaround is done: the ListenTCP processor is restarted.
An ideal fix would be to allow the underlying OS to poll the socket if the remote side is still alive, and disconnect it if not.