GATE CS 2021 Set 1 — Question 54
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Computer Networks → Transport Layer → TCP Handshake
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Question
A TCP server application is programmed to listen on port number on host . A TCP client is connected to the TCP server over the network. Consider that while the TCP connection was active, the server machine crashed and rebooted. Assume that the client does not use the TCP keepalive timer. Which of the following behaviors is/are possible?
Correct answer
(A) If the client was waiting to receive a packet, it may wait indefinitely.; (B) The TCP server application on S can listen on P after reboot.; (C) If the client sends a packet after the server reboot, it will receive a RST segment.
Solution
When a server crashes and reboots, all active connection states (TCBs) are lost.
- (A) is possible: Without a keepalive timer, if the client is simply waiting for data, it has no mechanism to detect that the server has crashed and will continue to wait indefinitely.
- (B) is possible: After the machine reboots, the server application can be restarted and bind to the same port to listen for new connections.
- (C) is possible: If the client sends a segment for the old connection, the rebooted server will have no record of it. According to TCP specifications, receiving a segment for an unknown connection results in the server sending a RST (Reset) segment to the client.
- (D) is impossible: A FIN segment is used for a graceful shutdown of an established connection. Since the server has lost all state, it cannot initiate a graceful close for a connection it doesn't recognize.
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