Add a special message type "user are you alive"? To check status.

[MihanEntalpo]

We send this request from time to time.
If the user is alive, he sends the answer back.
Thus, we know that he is, firstly, alive, and secondly, that we still have the correct encryption keys with him.

If several such packets were sent, but there is no response, we mark the user as "possibly dead".
If the answers came, or the user himself sent the same packets, then he is alive again and we can send packets again.