def sendUnimplemented(self):
        """
        Send a message to the other side that the last packet was not
        understood.
        """
        seqnum = self.incomingPacketSequence
        self.sendPacket(MSG_UNIMPLEMENTED, struct.pack('!L', seqnum))

        Send a message to the other side that the last packet was not
        understood.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/conch/ssh/transport.py