TCP Connection Termination
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As the saying goes, all good things must come to an end and so it is with TCP connections. The link between a pair of devices can remain open for a considerable period of time, assuming that a problem doesn't force the connection to be aborted. Eventually, however, one or both of the processes in the connection will run out of data to send and will shut down the TCP session, or will be instructed by the user to do so.
Just as TCP follows an ordered sequence of operations to establish a connection, it includes a specific procedure for terminating a connection. As with connection establishment, each of the devices moves from one state to the next to terminate the connection. This process is more complicated than one might imagine it needs to be. In fact, an examination of the TCP finite state machine shows that there are more distinct states involved in shutting down a connection than in setting one up.
The reason that connection termination is complex is that during normal operation, both of the devices are sending and receiving data simultaneously. Usually, connection termination begins with the process on just one device indicating to TCP that it wants to close the connection. The matching process on the other device may not be aware that its peer wants to end the connection at all. Several steps are required to ensure that the connection is shut down gracefully by both devices, and that no data is lost in the process.
Ultimately, shut down of a TCP connection requires that the application processes on both ends of the connection recognize that the end is nigh for the connection and stop sending data. For this reason, connection termination is implemented so that each device terminates its end of the connection separately. The act of closing the connection by one device means that device will no longer send data, but can continue to receive it until the other device has decided to stop sending. This allows all data that is pending to be sent by both sides of the communication to be flushed before the connection is ended.
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