code: def receive_delivered(self, delivered: Delivered):
          
          self.raiden.on_message(delivered)
  
          message_id = delivered.delivered_message_identifier
          async_result = self.raiden.transport.messageids_to_asyncresults.get(message_id)
  
          # clear the async result, otherwise we have a memory leak
          if async_result is not None:
              del self.messageids_to_asyncresults[message_id]
              async_result.set()
          else:
              self.log.warn(
                  'Unknown delivered message received',
                  message_id=message_id,
              )
docs: Handle a Delivered message.
  
          The Delivered message is how the UDP transport guarantees persistence
          by the partner node. The message itself is not part of the raiden
          protocol, but it's required by this transport to provide the required
          properties.