|Microsoft Internet Security and Acceleration Server 2000|
The Web proxy service of ISA Server maintains a cache of Web objects and attempts to fulfill Web requests from the cache. If the request cannot be fulfilled from the cache, the ISA Server computer initiates a new request on behalf of the client. Once the remote Web server responds to the ISA Server computer, the ISA Server computer caches the response to the original client request and sends a response to the client.
ISA Server supports forward caching, used for outgoing requests, and reverse caching, used for incoming requests. Clients in both forward caching and reverse caching benefit from the full gamut of ISA Server caching features.
ISA Server includes an Hypertext Transfer Protocol (HTTP) redirector filter, which allows Firewall and secure network address translation (SecureNAT) clients to also benefit from the caching features. When the HTTP redirector is enabled, Web requests from Firewall and SecureNAT clients can also be cached.
If you installed ISA Server in Firewall mode, ISA Server does not maintain a cache.
ISA Server analyzes routing rules, cache configuration, and existing cache content to determine if an object should be retrieved from the cache.
First, if the user request is allowed, ISA Server checks if the object exists in its cache. If the request is made to an array of ISA Server computers, then the Cache Array Routing Protocol (CARP) algorithm is used to determine which server's cache should be checked. If the object is not in the cache, then ISA Server checks the routing rule's action, to determine how to route the request. If the object is in the cache, then ISA Server performs the following steps:
If the object is valid, ISA Server retrieves the object from the cache and returns it to the user.