tcp断开过程

      Client                            消息                                    Server

         close()
                                      ------ FIN ------->
        FIN_WAIT1                                                         CLOSE_WAIT
                                      <----- ACK -------
        FIN_WAIT2
                                                                                  close()
                                       <------ FIN ------                    
        TIME_WAIT                                                       LAST_ACK      

                                      ------ ACK ------->
                                                                                   CLOSED
           CLOSED