def shutdown(self):
        '''
        Should be called by Switchyard user code when a network object is
        being shut down.  (This method cleans up internal threads and network
        interaction objects.)
        '''
        if not LLNetReal.running:
            return

        LLNetReal.running = False
        log_debug("Joining threads for shutdown")
        for t in self._threads:
            t.join()
        log_debug("Closing pcap devices")
        for devname,pdev in self._pcaps.items():
            pdev.close()
        for rdev in self._localsend.values():
            rdev.close()
        log_debug("Done cleaning up")
Should be called by Switchyard user code when a network object is
        being shut down.  (This method cleans up internal threads and network
        interaction objects.)