title: How can a back-end server connect to a specific iphone?
body: <p>It is very usual that an iPhone send data to internet server.<br>
  But what if a server wants to connect to an iphone?<br>
  I have wondered long about it.<br>
  For example, like many web-based apps like Facebook, what is the mechanism/algorithm behind, so that when, for example, any updates on your wall can be noticed to you in a real-time manner?</p>
  
  <p>Hope some can understand my query.</p>