ISA Firewall Web Caching Capabilities (2)

ISA Firewall Cache Rules
ISA uses cache rules to allow you to customize what types of content will be stored in the cache and exactly how that content will be handled when a request is made for objects stored in cache.

You can create rules to control the length of time that a cache object is considered to be valid (ensuring that objects in the cache do not get hopelessly out of date), and you can specify how cached objects are to be handled after they expire.

ISA gives you the flexibility to apply cache rules to all sites or just to specific sites. A rule can further be configured to apply to all types of content or just to specified types.

Cache Rules to Specify Content Types That Can Be Cached
A cache rule lets you specify which of the following types of content are to be cached:

*Dynamic content This is content that changes frequently, and thus, is marked as not cacheable. If you select to cache dynamic content, retrieved objects will be cached even though they are marked as not cacheable.
* Content for offline browsing In order for users to be able to browse while offline (disconnected from the Internet, all content needs to be stored in the cache. Thus, when you select this option, ISA will store all content, including “non-cacheable” content, in the cache.
* Content requiring user authentication for retrieval Some sites require that users be authenticated before they can access the content. If you select this option, ISA will cache content that requires user authentication.

You can also specify a Maximum object size. By using this option, you can set limits on the size of Web objects that will be cached under a particular cache rule.

Using Cache Rules to Specify How Objects are Retrieved and Served from Cache
In addition to controlling content type and object size, a cache rule can control how ISA will handle the retrieval and service of objects from the cache. This refers to the validity of the object. An object’s validity is determined by whether its Time to Live (TTL) has expired. Expiration times are determined by the HTTP or FTP caching properties or the object’s properties. Your options include:

* Setting ISA to retrieve only valid objects from cache (those that have not expired). If the object has expired, the ISA will send the request on to the Web server where the object is stored and retrieve it from there.
* Setting ISA to retrieve requested objects from the cache even if they are not valid. In other words, if the object exists in the cache, ISA will retrieve and serve it from there even if it has expired. If there is no version of the object in the cache, the ISA will send the request to the Web server and retrieve it from there.
* Setting ISA to never route the request. In this case, the ISA relies only upon the cache to retrieve the object. Objects will be returned from cache whether or not they are valid. If there is no version of the object in the cache, the ISA will return an error. It will not send the request to the Web server.
* Setting ISA to never save the object to cache. If you configure the rule this way, the requested object will never be saved to the cache.

Note:
The default TTL for FTP objects is one day. TTL boundaries for cached HTTP objects (which are defined in the cache rule) consist of a percentage of the age of the content, based on when it was created or last changed.

You can also control whether HTTP and FTP content are to be cached for specific destinations, and you can set expiration policies for the HTTP and FTP objects. You can also control whether to enable caching of SSL content.

Because SSL content often consists of sensitive information (which is the reason it’s being protected by SSL), you might consider not enabling caching of this type of content for better security.

If you have multiple cache rules, they will be processed in order from first to last, with the default rule processed after all the custom rules. The default rule is automatically created when you install ISA. It is configured to retrieve only valid objects from cache, and to retrieve the object from the Internet if there is no valid object in the cache.