title: What http status code is supposed to be used to tell the client the session has timed out?
body: <p>In a webpage, it uses YUI connection manager/datasource to send AJAX requests to the server, if the session (which contains the info on whether the user has been authenticated) has already timed out, those ajax responses that can only be viewed by authenticated users should return an http status code, telling the client that the session has already timed out, then the client either simply redirects him to the login page or asks him if he wants to extend the session.</p>
  
  <p>My question is that, in this situation, what http status code is the most appropriate to tell the client the session has timed out?</p>
  
  <p><a href="http://en.wikipedia.org/wiki/List_of_HTTP_status_codes" rel="noreferrer">List of HTTP status codes from wiki</a></p>