The only practical way that I have ever found to determine whether a client has crashed is to oblige the protocol between the two to periodically send a “heartbeat” message if there is nothing else to be sent during some mutually agreed-upon interval. The host should record the latest timestamp as each socket sends a message, or just a boolean flag, and use a periodic timer, say about half-again as long as the heartbeat, to check for absence of response within that time. Upon which the host might presume that the client is dead. You see, you really can’t be 100% sure, AFAIK, that all of the intermediates between here-and-there are still in good working order. Maybe the host knows that the socket has been closed or that communication has failed, or maybe it doesn’t. But if a bottle washes up on shore, even if it’s empty, you know the sender is still out there.
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