FortiOS web caching is not always able to cache Windows and MS-Office updates because they are downloaded using HTTP in multipart or chunked mode and typically run through multiple TCP connections. To resolve this issue in FortiOS 5.0, the first request for Windows or MS-Office updates (to download.windowsupdate.com) causes the cache process to download the new update file in the background.

Once the new update file has been downloaded to the cache, it is available to web cache users and all subsequent requests for this update will be downloaded from the cache. Because the update file will not be available in the web cache until it has completely downloaded, the first update request will not be able to get it from the web cache and neither will any updates requested while the file is downloading in the background.