XEP-0384: add business rule on heartbeat messages

xep-0384.xml

@@ -382,6 +382,7 @@
   <p>As there are no explicit error messages in this protocol, if a client does receive a PreKeySignalMessage using an invalid PreKey, they SHOULD respond with a KeyTransportElement, sent in a &lt;message&gt; using a PreKeySignalMessage. By building a new session with the original sender this way, the invalid session of the original sender will get overwritten with this newly created, valid session.</p>
   <p>If a PreKeySignalMessage is received as part of a &xep0313; catch-up and used to establish a new session with the sender, the client SHOULD postpone deletion of the private key corresponding to the used PreKey until after MAM catch-up is completed. If this is done, the client MUST then also send a KeyTransportMessage using a PreKeySignalMessage before sending any payloads using this session, to trigger re-keying. (as above) This practice can mitigate the previously mentioned race condition by preventing message loss.</p>
   <p>As the asynchronous nature of OMEMO allows decryption at a later time to currently offline devices client SHOULD include a &xep0334; &lt;store /&gt; hint in their OMEMO messages. Otherwise, server implementations of &xep0313; will generally not retain OMEMO messages, since they do not contain a &lt;body /&gt;</p>
+  <p>When a client receives the first message for a given ratchet key with a counter of 53 or higher, it MUST send a heartbeat message. Heartbeat messages are normal OMEMO encrypted messages where they SCE payload does not include any elements. These heartbeat messages cause the ratchet to forward, thus consequent messages will have the counter restarted from 0.</p>
 </section1>
 <section1 topic='Implementation Notes' anchor='impl'>
   <!-- TODO: I think this is still true? -->