How can the server know that the client has terminated TCP connections?

Question

Network administration

On one of the projects working over TCP, it was noticed that sometimes there are hanging connections from clients.
If the client closes the connection without notifying the server, what mechanisms are there at the TCP or OS level for the server to know that the client has left?

Answer 1

If the FIN packet did not arrive, then only timeouts.

Answer 2

keep-alive + timeout. Without traffic, a broken connection can hang forever.

Answer 3

Timeout only.