title: IF-MODIFIED-SINCE vs UNLESS-MODIFIED-SINCE
body: <p>What is the difference between if-modified-since and unless-modified-since (http headers)? What web server response should we send to this? IE9 is requesting the unless-modified-since header which we are unfamiliar with.</p>
  
  <p>Most of my research pushes is leading towards the if-modified-since header. My understanding is that the unless-modified-since header is IE specific.</p>
  
  <p>You can go to this link: <a href="http://imgur.com/am64WuG" rel="nofollow">http://imgur.com/am64WuG</a> to see the screen shot of the request header.</p>