protected function decodeVariableHeader(& $packet_data, & $pos)
    {
        $topic = Utility::UnpackStringWithLength($packet_data, $pos);
        $this->message->setTopic($topic);

        if ($this->getQos() > 0) {
            # Decode Packet Identifier if QoS > 0
            $this->decodePacketIdentifier($packet_data, $pos);
        }

        return true;
    }