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There are two basic types of Web caching: forward and reverse. ISA Server 2004 performs both of these types of Web caching.
Forward Web caching has the advantage of making access for internal users faster because they are retrieving the Web objects (pages, graphics, sound files, and so forth) over a fast LAN connection, typically 100Mbps or more, instead of a slower Internet connection at perhaps 1.5Mbps.
The primary 'bottom line' benefit of ISA Server 2004's forward caching is cost savings realized by reduced bandwidth usage on the Internet connection.
Reverse caching reduces traffic on the internal network and speeds access for external users when the company hosts its own Web sites. Frequently requested objects on the internal Web servers are cached at the network edge on a proxy server so that the load on the Web servers is reduced.
There are two principle benefits to the reverse-caching scenario: reverse caching reduces bandwidth usage on the internal network, and reverse caching allows Web content to be available when the Web server is offline.
Multiple Web caching servers can be used together to provide for more efficient caching. There are two basic caching architectures that use multiple caching servers working together: distributed and hierarchical.
You can combine the distributed and hierarchical methods to create a hybrid caching architecture. The combination gives you the 'best of both worlds,' improving performance and efficiency.
There are a number of different protocols that can be used for communications between Web-caching servers. The most popular of these are CARP, ICP, HTCP, WCCP and Cache digests.
ISA Server 2004 uses CARP for communications between distributed Web-caching servers.
Web Proxy clients can also use CARP to locate the ISA Server that contains objects when distributed caching is enabled.
The Web Proxy filter is the mechanism that ISA Server 2004 uses to implement caching functionality.
The cache is an area on the ISA Server's hard disk that is used to store the requested Web objects. You can control the amount of disk space to be allocated to the cache (and thus, the maximum size of the cache). You can also control the maximum size of objects that can be cached to ensure that a few very large objects can't 'hog' the cache space.
Caching uses system memory. Objects are cached to RAM, as well as to disk. Objects can be retrieved from RAM more quickly than from the disk. ISA Server 2004 allows you to determine what percentage of random access memory can be used for caching (by default, ISA Server 2004 uses 10 percent of the RAM and then caches the rest of the objects to disk only).
Enabling caching on your ISA Server 2004 computer is done by configuring a cache drive. When you configure a cache drive, this enables both forward and reverse caching.
The file in which the cache objects are stored is named dir1.cdat. It is located in the urlcache folder on the drive that you have configured for caching. This file is referred to as the cache content file.
ISA Server 2004 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.
In addition to controlling content type and object size, a cache rule can control how ISA Server will handle the retrieval and service of objects from the cache. This refers to the validity of the object.
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 content download feature is used to schedule ISA Server 2004 to download new content from the Internet at pre-defined times so that when Web Proxy clients request those objects, updated versions will be in the cache.
