Caching: Concepts

Microsoft Internet Security and Acceleration (ISA) Server 2006 implements a cache of frequently requested objects to improve network performance. You can configure the cache to ensure that it contains the most frequently used data.

By default, when you install ISA Server, caching is effectively disabled. This is because no space on any disk drive on the ISA Server computer has been specified to be used for caching. When you configure a drive for caching, by specifying how much space to use on the drive, you effectively enable caching. You can further configure the cache, specifying which content can be cached.

With caching enabled, you can configure cache rules, which determine whether content from specified sites should be stored or retrieved from the ISA Server cache. A cache rule applies to requested sites, regardless of the source network.

Also, when caching is enabled, you can configure content download jobs. With content download jobs, ISA Server can be configured to download content automatically, at a specified time. Network performance can be improved, because ISA Server can be configured to access frequently requested content during off-peak hours.

For more information about ISA Server caching, see Caching Concepts in ISA Server 2006 at the Microsoft ISA Server TechCenter Web site (

Using caching

ISA Server can be deployed as a forward caching server that provides clients with faster access to requested content. ISA Server maintains a centralized cache of frequently requested objects that can be accessed by any Web browser client. Objects served from the disk cache require significantly less processing than objects served from another network. This improves client browser performance, decreases user response time, and reduces bandwidth consumption on your Internet connection.

In the following figure, clients on the Internal network request objects from servers on the External network. The figure illustrates how ISA Server responds when a user requests an object.

How caching works

The figure also illustrates how users benefit when ISA Server caches objects. Although the figure focuses on a forward caching scenario, the process is identical for reverse caching, when Internet users access a corporate Web server. The figure illustrates these steps:

  1. The first user (Client 1) requests a Web object.
  2. ISA Server checks whether the object is in the cache. Because the object is not in the ISA Server cache, ISA Server requests the object from the server on the Internet.
  3. The server on the Internet returns the object to the ISA Server computer.
  4. ISA Server retains a copy of the object in its cache, and returns the object to Client 1.
  5. Client 2 requests the same object.
  6. ISA Server returns the object from its cache, rather than obtaining it from the Internet.

ISA Server can be deployed in front of an organization's Web server that is hosting a commercial Web business or providing access to business partners. With incoming Web requests, ISA Server can impersonate a Web server to the outside world, fulfilling client requests for Web content from its cache. ISA Server forwards requests to the Web server only when the requests cannot be served from its cache.

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