docs: 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")