Automatic Proxy Result Cache

With most browsers, the PAC retrieve function is called every time a request is made.

However, Internet Explorer since 5.5 include a feature called Automatic Proxy Result Cache that caches the requested URL host name and the returned proxy address for 30 minutes.

This has the advantage of minimizing web server calls but:

1. The Automatic Proxy Result Cache use the host name for key cache, it is not possible for a PAC file to distribute traffic to distinct proxy servers based on any part of the URL in addition to the host name. It is not possible to redirect traffic to different proxy servers based on the URLs path portion for a single host.
    
2. Automatic Proxy Result Cache caches the host name/proxy result pair instead of the full proxies list. 
   The failover by PAC file will not be used.
   This feature is discussed in more detail in the Microsoft knowledge base article titled How to disable automatic proxy caching in Internet Explorer.