Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 204
Termination

Finally, TCP must be able to gracefully terminate a connection. This is accomplished using the FIN bit in the TCP header. Since TCP offers a full-duplex connection, each side of the connection must close the connection. Refer to slide 225 for two communicating devices, endstation A and host station B. The application running on endstation A indicates to host B that it wishes to close a connection by sending a packet to host station B with the FIN bit set. Host station B acknowledges that packet and no longer accepts data from endstation A. However, host station B does accept data from its application to send to endstation A. Endstation A continues to accept data from host station B. This way, station A can, at a minimum, accept a FIN packet from host station B to completely close the connection. To finalize the closing of this connection, host station B sends a packet to endstation A with the FIN bit set. Endstation A ACKs this packet and the connection is closed. If no ACK is received, FINs are retransmitted and will eventually time-out if there is no response.

Termination