Internet Core Protocols: The Definitive Guide

Closing a circuit

Once the applications have exchanged all of their data, the circuit can be closed. Closing a circuit is similar to opening one, in that an application must request the action (except in those cases where the connection has collapsed, and TCP is forced to terminate it).

Either end of the connection may close the circuit at any time, using a variety of different means. The two common ways to close are " active closes” that initiate a shutdown sequence and “passive closes” that respond to an active close request.

Just as building a circuit requires a bidirectional exchange of special command segments, so does closing it. One end of the connection requests that the circuit be closed (the active close at work). The remote system then acknowledges the termination request and responds with its own termination request (the passive close). The terminating system then acknowledges the acknowledgment, and both endpoints drop the circuit. At this point, neither system is able to send any more data over the virtual circuit.

Figure 7.7 shows this process in detail. Once the HTTP client has received all of the data, it requests that the virtual circuit be closed. The HTTP server then returns an acknowledgment for the shutdown request, and also sends its own termination request. When the server’s shutdown request is received by the client, the client issues a final acknowledgment, and begins closing its end of the circuit. Once the final acknowledgment is received by the server, ...