GATE CS 2021 Set 1 Q.54 (CS Q.44): Transport Layer: UDP, TCP Flow and Congestion Control, Sockets

A TCP server application is programmed to listen on port number $P$ on host $S$ . A TCP client is connected to the TCP server over the network.

Consider that while the TCP connection was active, the server machine $S$ crashed and rebooted. Assume that the client does not use the TCP keepalive timer.
Which of the following behaviors is/are possible?

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

The TCP server application on $S$ can listen on $P$ after reboot.

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

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

Official answer and solution

Official answer: A, B and C
From: Official GATE 2021 answer key

Solution

Trace TCP state transitions, reset generation on half-open connections, and server socket rebinding after a reboot.

Option A: If the client application is blocked on a read/receive call waiting for incoming server data, and keepalive is disabled, the client's TCP layer has no awareness of the server crash. In the absence of incoming packets or local timeouts, the client will remain in ESTABLISHED state indefinitely. (Possible / Correct)

Option B: Upon reboot, all previous volatile socket bindings are cleared by the operating system kernel on $S$ . The server application can successfully launch, bind, and listen on port $P$ again. (Possible / Correct)

Option C: When the client sends a segment on the pre-existing connection, the segment arrives at host $S$ . Because $S$ lost its connection state during reboot, no socket matches the 4-tuple of the connection. In accordance with RFC 793, TCP on host $S$ responds with a RST (reset) segment to abort the half-open connection. (Possible / Correct)

Option D: A FIN segment is sent only during an orderly, graceful connection teardown by a process holding an active connection. A rebooted host with no knowledge of the connection never transmits a FIN; it transmits a RST. (Not possible / Incorrect)

Therefore, statements A, B, and C are correct.

Final answer

A, B, C

The answer is the one in the official GATE answer key. The solution is this site's own explanation.

Source

Paper: GATE 2021 Computer Science and Information Technology (CS, Set-1)
Question: 54 of 65, printed as CS Q.44
Section: Computer Science and Information Technology

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