TCP Connection Termination | FIN Segment (2024)

Three Way Handshake-

Before you go through this article, make sure that you have gone through the previous article on Three Way Handshake.

We have discussed-

• TCP uses Three Way Handshake to establish a connection between the sender and receiver.
• Connection establishment using Three Way Handshake involves the steps as shown-

Also Read- TCP Retransmission

In this article, we will discuss how a TCP connection is terminated.

TCP Connection Termination-

ATCP connection is terminated using FIN segment where FIN bit is set to 1.

Consider-

• There is a well established TCP connection between the client and server.
• Client wants to terminate the connection.

The following steps are followed in terminating the connection-

Step-01:

For terminating the connection,

• Client sends a FIN segment to the server with FIN bit set to 1.
• Client enters the FIN_WAIT_1 state.
• Client waits for an acknowledgement from the server.

Step-02:

After receiving the FIN segment,

• Server frees up its buffers.
• Server sends an acknowledgement to the client.
• Server enters the CLOSE_WAIT state.

Step-03:

After receiving the acknowledgement, client enters the FIN_WAIT_2 state.

Now,

• The connection from client to server is terminated i.e. one way connection is closed.
• Client can not send any data to the server since server has released its buffers.
• Pure acknowledgements can still be sent from the client to server.
• The connection from server to client is still open i.e. one way connection is still open.
• Server can send both data and acknowledgements to the client.

Step-04:

Now, suppose server wants to close the connection with the client.

For terminating the connection,

• Server sends a FIN segment to the client with FIN bit set to 1.
• Server waits for an acknowledgement from the client.

Step-05:

After receiving the FIN segment,

• Client frees up its buffers.
• Client sends an acknowledgement to the server (not mandatory).
• Client enters the TIME_WAIT state.

TIME_WAIT State-

• The TIME_WAIT state allows the client to resend the final acknowledgement if it gets lost.
• The time spent by the client in TIME_WAIT state depends on the implementation.
• The typical values are 30 seconds, 1 minute and 2 minutes.
• After the wait, the connection gets formally closed.

Next Article- Practice Problems On TCP

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