def sendack(self, max_size):
    with self.llc.lock:
        for socket in self.sock_list:
            pdu = socket.sendack(max_size)
            if pdu:
                return pdu
def sendack(self, max_size):
    with self.llc.lock:
        for socket in self.sock_list:
            pdu = socket.sendack(max_size)
            if pdu:
                return pdu