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?

Let us analyze each scenario after the server host $S$ crashes and reboots:

1. Option (A): If the client is blocked waiting for data from the server and does not send any packets, and keepalive is disabled, the client will never know that the server has crashed. It will wait indefinitely. Hence, (A) is possible and correct.

2. Option (B): After rebooting, the operating system kernel on $S$ has cleared all previous connection states. The TCP server application can be restarted and can bind/listen on port $P$ again. Hence, (B) is possible and correct.

3. Option (C): If the client transmits a data packet to host $S$ after the reboot, host $S$ receives a segment for a connection for which it has no Transmission Control Block (TCB) entry. According to RFC 793, when a TCP endpoint receives an unexpected segment not matching any active connection, it responds with an RST (Reset) segment. Hence, (C) is possible and correct.

4. Option (D): A FIN segment is only transmitted during an orderly, graceful connection shutdown initiated by an active connection endpoint. Since host $S$ has lost all connection state across the reboot, it cannot send a FIN; it will instead send an RST. Hence, (D) is incorrect.

Therefore, the correct options are A, B, and C.