What does the FIN flag mean in TCP?

FIN = Finish - it signals the sender has no more data and wants to close its half of the connection (used in the four-way termination).

FIN = Finish

The FIN flag indicates the sender has no more data to send and wants to close the connection.

Purpose:

• Signals intent to terminate the connection
• Used in the four-way termination handshake
• Each direction must be closed independently

When it appears:

• When either host wants to close the connection
• Requires acknowledgment (ACK) from the receiver

Tip: "FIN = Finished sending" - the sender is done talking but can still receive.

Go deeper:

• RFC 9293 §3.6 — Closing a Connection — how FIN closes one direction of the connection in the four-way teardown.
• TCP — Connection termination — Wikipedia's FIN/ACK teardown walkthrough.