After all communication is completed, use the $DASSGN system
    service to free an I/O channel. A $DASSGN operation executed
    on a channel associated with a network pseudodevice does the
    following:
    o  Ends all pending operations to send or receive data at $QIO
       level ($CANCEL system service).
    o  Clears the port associated with the channel. When executing
       the $DASSGN system service for TCP sockets, the socket remains
       until the connection is closed on both the local and remote
       sides.
    o  Ends all communications with the network pseudodevice that the
       I/O channel identifies.
    o  Frees the channel associated with the network pseudodevice. An
       I/O channel can be deassigned only from an access mode equal
       to or more privileged than the access mode from which the
       original channel assignment was made.
    I/O channels assigned from user mode are automatically deassigned
    at image exit.
                                   NOTE
       Even after a $DASSGN has been issued, a TCP socket may
       remain until the TCP close timeout interval expires. The
       default and maximum timeout interval is either 10 minutes
       if the peer host is not responding or 30 seconds after
       acknowledging the socket close. Although the TCP socket
       is open, you cannot make a reference to that socket after
       issuing a $DASSGN.