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It takes four to terminate a connection. 

  1. One application calls close first, and we say that this end performs the active close. This end's TCP sends a FIN segment, which means it is finished sending data.

  2. The other end that receives the FIN performs the passive close. The received FIN is acknowledged by TCP. The receipt of the FIN is also passed to the application as an end-of-file (after any data that may have already been queued for the application to receive), since the receipt of the FIN means the application will not receive any additional data on the connection.

  3. Sometime later, the application that received the end-of-file will close its socket. This causes its TCP to send a FIN.

  4. The TCP on the system that receives this final FIN (the end that did the active close) acknowledges the FIN.

graphics/02fig03.gif
 A way for the application to say, "I am done sending data, so send a FIN to the other end, but I still want to receive data from the other end, until it sends me a FIN" is called as half way close in Four way TCP termination

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