Question

Please clarify this weird If-Modified-Since header passed by IE9

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.

I’m getting DateTime parse errors moderately frequently from trying to parse the If-Modified-Since header contents, usually from IE9. Turns out it’s sending something like this:

Mon, 28 Nov 2011 16:34:52 GMT; length=8799

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?

Answer 1

According to an old post on the Squid proxy mailing list:

The length parameter to If-Modified-Since is a Netscape extension of
HTTP/1.0, meant to improve the accuracy of If-Modified-Since in case
a document is updated twice in the same second.

HTTP/1.1 solved the
same problem in a better way via the ETag header and If-None-Match.

I’m guessing that IE adapted this extension at some point and have left it in.