code: def _stop_transport(self):
          
          try:
              self._check_is_started()
          except (BaseSSHTunnelForwarderError,
                  HandlerSSHTunnelForwarderError) as e:
              self.logger.warning(e)
          for _srv in self._server_list:
              tunnel = _srv.local_address
              if self.tunnel_is_up[tunnel]:
                  self.logger.info('Shutting down tunnel {0}'.format(tunnel))
                  _srv.shutdown()
              _srv.server_close()
              # clean up the UNIX domain socket if we're using one
              if isinstance(_srv, _UnixStreamForwardServer):
                  try:
                      os.unlink(_srv.local_address)
                  except Exception as e:
                      self.logger.error('Unable to unlink socket {0}: {1}'
                                        .format(self.local_address, repr(e)))
          self.is_alive = False
          if self.is_active:
              self._transport.close()
              self._transport.stop_thread()
          self.logger.debug('Transport is closed')
docs: Close the underlying transport when nothing more is needed