title: How does Facebook chat tcp connection keep alive?
body: <p>I implemented a XMPP chat client with Facebook Chat API, it can log in and chat.
  But I found Facebook chat XMPP server doesn't support XEP-0199.
  So if client doesn't send or receive message for a long time(tcp connection is idle), client will be offline.</p>
  
  <p>How does Facebook chat tcp connection keep alive?</p>