What are FIN and RST flags in TCP, and when are they used?

• A. FIN indicates no more data from the sender; RST resets a connection due to error or invalid state; FIN for graceful close, RST for abortive termination
• B. FIN is used to reset a connection; RST indicates normal termination
• C. FIN carries data; RST carries control flags only
• D. FIN and RST are used to indicate congestion control states

Answer: (A)

In TCP, these flags signal how a connection should be terminated or reset. The FIN flag means the sender has no more data to send, so the connection can be closed gracefully. When a side sends FIN, the other side acknowledges with an ACK and may continue sending data or eventually send its own FIN to close its direction of the connection. This orderly exchange lets in-flight data be acknowledged and avoids abrupt termination.

The RST flag, by contrast, is used to reset a connection immediately, typically in case of an error, protocol violation, or a segment arriving for a non-existent or unrecoverable state. It terminates the connection right away and frees resources, with no guarantee of data delivery.

So, FIN is for a graceful close, while RST is for abortive termination. The other statements are not correct: FIN does not reset a connection, RST does not indicate normal termination, FIN does not carry data, and these flags are not related to congestion control states.