Basic algorithms used by Microsoft Proxy Server to determine the cached item's Time To Live (TTL) value

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By default, the Web Proxy server is set to cache items. It determines the TTL for cached items in the following manner, listed in order of highest To lowest precedence:

1. If the page to be cached has a preset cache TTL (defined in the HTTP "Expires" header), the Proxy Server caches it for this amount of time.
2. If the administrator for the Proxy server sets an explicit TTL for a particular object (this can be done as described in Appendix D of the online documentation), then this time will be used.
3. If no explicit TTL is set for the page, Proxy Server uses the following algorithm: The TTL is 20% of the time since the object was last modified (this is specified in the "Last-Modified" HTTP header), with a minimum value of 15 minutes (MIN Interval) and a maximum value of one day (MAX Interval).

Also note that many web pages can be marked as "uncacheable" by the web administrators of sites. These sites are usually marked this way so web administrators can accurately count page hits, or update Ad banners. These pages do not get cached (www.microsoft.com is one such web page).