title: What happens to a GCM message when the recipient's device is off?
body: <p>I'm currently working on an Android messaging app and GCM is utilized for notifications.</p>
  
  <p>When the device of the recipient, that'd already be registered on GCM, is off, what exactly happens? Does GCM wait until the device is back online? If so, does the device send a message to say it's alive, or does GCM keep pinging the device? Or does GCM continually send the message until success? Either way, how long can the message persist on the GCM server when the device is offline?</p>
  
  <p>Any direction would be helpful.</p>