to identify a message sent from 3rd-party servers to CCS. Therefore, it's important that this <code>message_id</code> not only be unique, but always present.</li>

  <li>For ACK/NACK messages that are special control messages, you also need to include a <code>message_type</code> field in the JSON message. For example:

<pre>message_type = ('ack' OR 'nack');</pre>
  </li>
</ul>
<p>For each message a device sends to the server, you need to send an ACK message. You never need to send a NACK message. If you don