A TCP server application is programmed to listen on port number P on host S. A…

Explanation: what can happen to a TCP client and server when the server machine crashes and reboots.

• If the client was waiting to receive a packet, it may wait indefinitely.

• The client can remain blocked on receive because a server crash does not guarantee a FIN or RST is sent to the client; without the client sending data or using keepalive, the local TCP stack has no stimulus to detect the dead peer.

• The TCP server application on the host can listen on the same port after reboot.

• A reboot clears the kernel TCP state for previous connections, so the server process (after restarting) is normally able to bind and listen on the same port number.

• If the client sends a packet after the server reboot, it can receive a RST segment.

• When the rebooted host has no TCP state for that connection and no process is listening on the destination port, the host’s TCP implementation will normally reply with a RST to indicate the port/connection is not valid. If the server process has already restarted and is listening, the client may instead get a response or a new connection setup, so RST is possible but not guaranteed.

• If the client sends a packet after the server reboot, it will receive a FIN segment.

• A FIN indicates an orderly, graceful close initiated by the peer. A crash/reboot is abrupt and does not perform a graceful shutdown, so receiving a FIN from the rebooted host for the old connection is not the expected behavior.

Conclusion: the behaviors that are possible in this scenario are: the client may wait indefinitely when blocked on receive, the server can listen on the same port after reboot, and the client may receive a RST if it sends data after the reboot. Receiving a FIN is not generally expected.