[jQuery] Fetch (ajax) a file only if it's been modified

[Magnificent]

Hi all, I'm trying to fetch a file, but only if the Last-Modified
header has changed.  What I have is a page that fetches a data file
every X seconds (20 in my case) and displays that info to the user.
That data file is updated on the server whenever a user takes specific
action.

It works fine doing this:

//refresh data every 20 seconds
setInterval(function() {
     $('div#livedata').load('livedata_fetch.php')
}, 20000);

But that fetches the file every single time.  It's not a big file, but
I really only need to fetch it if it's changed.  I thought the
following would work, but it doesn't seem to, any ideas?

//refresh data every 20 seconds but only if data has changed
setInterval(function() {

     $.ajax({
          url: "livedata_fetch.php",
          ifModified: true,
          success: function(data){
               $("div#livedata").html(data);
          }
     });

}, 20000);