Microsoft published about the "improvements the way Internet Explorer 9 caches content to ensure that as many resources as possible are loaded from the cache."
- "With IE9, we now accept any value up to 2^63 for the max-age value, although internally the freshness interval will be truncated to 2^31 seconds
- IE9 enhanced support for HTTP/1.1 Vary response header scenarios. Specifically, IE9 will no longer require server revalidation for responses which contain Vary: Accept-Encoding and Vary: Host directives
- IE9 now supports caching of HTTP redirect responses, as described by RFC 2616. Responses with Permanent Redirect status (301) are cacheable unless headers with "Cache-Control: no-cache", and those with Temporary Redirect status (302 or 307) are cacheable if headers permit (Cache-Control: max-age=120)
- In IE9, unnecessary cross-host HTTPS requests are now conditional requests, so the server can simply return a HTTP/304 Not Modified response for unchanged content
- For IE9, we've made improvements so that clicking the back and forward buttons results in faster performance
- IE9 allows to configure what should happen when content is delivered without expiration information. Inside Tools > Internet Options > Browsing history > Settings, you'll see four options:
- Caching Inspector in Fiddler will show you when a response expires, based on headers provided on that response. For instance, here's what you see for a response which contains an ETAG and Last-Modified header, but no expiration information: