title: If-Modified-Since HTTP header passed by IE9 includes length?
body: <h3>Please clarify this weird <code>If-Modified-Since</code> header passed by IE9</h3>
  <p>In my ASP.NET 4.0 web app, I've got a generic handler (.ashx) that serves images stored in the DB.  In the interest of efficiency, I'm handling some caching-related headers and passing cache information.</p>
  <p>I'm getting DateTime parse errors moderately frequently from trying to parse the <code>If-Modified-Since</code> header contents, usually from IE9.  Turns out it's sending something like this:</p>
  <blockquote>
  <p>Mon, 28 Nov 2011 16:34:52 GMT; length=8799</p>
  </blockquote>
  <p>I'm handling that by using a regular expression to strip out the last part.  But I'm curious: what length is it referring to, and what use is it?  Is it the size of the cached data for the requested URL?</p>