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FIN flag

As explained in Acing the CCNA Exam: Volume 1 Fundamentals and Protocols

The FIN flag is a TCP control bit used to signal that a host has finished sending data and is closing its direction of a connection.

The FIN flag is a TCP control bit set to 1 when a host has finished sending data and is closing its direction of the connection. section 22.2.1

A typical TCP termination sequence works as follows:

  • One host sends a TCP segment with FIN after completing its data exchange. section 22.2.1
  • The other host acknowledges that FIN with an ACK segment, then sends its own FIN when it is finished. section 22.2.1
  • The first host acknowledges the second FIN, producing the typical FIN, ACK, FIN, ACK four-way handshake. section 22.2.1 section 22.3

Exam sequence

After connection establishment and data exchange, PC1 sends FIN. SRV1 replies with ACK, sends its own FIN, and receives PC1's final ACK. Remember: SYN, SYN-ACK, ACK establishes TCP; FIN, ACK, FIN, ACK typically terminates it. section 22.2.1

How Acing the CCNA Exam: Volume 1 Fundamentals and Protocols covers it

How deep it goes
Defines the control bit and typical message sequence, while deferring less-common combined ACK-FIN termination details.
Examples
Uses PC1 and SRV1 to illustrate FIN, ACK, FIN, ACK termination.
What it assumes
Assumes basic TCP connection establishment and data exchange, while distinguishing termination from the three-way handshake.
What you can do after
Evaluate TCP traces by recognizing the typical FIN, ACK, FIN, ACK termination sequence.
How it connects
Connects FIN with TCP, the ACK flag, TCP connection termination, and the four-way handshake.
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