def deliverResponse(self, responseMessage):
        """
        Deliver response.

        Destination is based on topmost Via header.
        """
        destVia = parseViaHeader(responseMessage.headers["via"][0])
        # XXX we don't do multicast yet
        host = destVia.received or destVia.host
        port = destVia.rport or destVia.port or self.PORT
        destAddr = URL(host=host, port=port)
        self.sendMessage(destAddr, responseMessage)

        Deliver response.

        Destination is based on topmost Via header.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/protocols/sip.py