The Background Intelligent Transfer Service (BITS) allows the transfer of large amounts of data without impacting network performance. It is able to dothis by transferring data in smaller chunks, using unused bandwidth when available, and then reassembling the data when it has all been recieved. One of the most popular uses for BITS is Microsoft Update.

BITS caching is useful in a scenario where there is no WSUS server, and all clients are going out to Microsoft Update to download and install hotfixes. Normally, each computer on the LAN would go out to the Microsoft Update site and each download there own set of hotfixes to be installed. If you have 20 clients and 1MB worth of hotfixes to download that equals 20MB of data. By enabling BITS caching, ISA server will cache the data downloaded from Microsoft Update and the LAN clients will retrieve the data from the cache, rather than re-downloading it. To enable BITS caching, open up the ISA Management MMC, expand the Configuration node and click on Cache. In the task bar on the right, click on "Create the Microsoft Update Cache Rule".

A wizard will start up. It is about as easy a wizard as you can get as all options are populated :)

The list of Microsoft Update URLs are also included. This can be modified by editing the Microsoft Update Domain Name Set.

Click Finish and then apply the change to the ISA server.

You now have a BITS caching rule configured and enabled!