Finally! With the release of ISA Server 2004 Service Pack 2 comes support for HTTP compression. HTTP compression is used to reduce file size by using the industry standard GZIP and Deflate algorithms. These algorithms are built into Windows 2000 Server and up, work with Internet Explorer 4.0 and up, as well as any other HTTP 1.1 compliant browser.

The algorithms compress static files, and can do on-demand compression of dynamically content before the data is sent over the network. The browser uses the same algorithms to decompress the data. The HTTP compression feature in ISA Server 2004 SP2 is a global HTTP policy that applies to all HTTP traffic passing through the ISA server. It does not compress HTTPS traffic. Two web filters are used to provide HTTP compression:

• Compression Filter - The Compression filter handles the compression and decompression of HTTP requests and responses. This filter is given a high priority, and is near the top of the list of Web filters. Because this filter handles decompression, when you enable the inspection of compressed data, the data must first be decompressed before other web filters can inspect the content. For this reason do not alter the priority of this filter.  
• Caching Compressed Content Filter - The Caching Compressed Content filter filter manages caching the compressed content and responding to requests from the compressed content in the cache. This filter has the lowest priority because data is cached after all other filters have been applied. For this reason do not alter the priority of this filter.

There are certain types of data that cannot be compressed, some examples of data that cannot be compressed include: - Audio and video - Compressed appication MIME types These cannot be compressed because they are already compressed or are streaming data types. To configure HTTP Compression, open up the ISA Management MMC, expand the Configuration node, and under the General node click on Define HTTP Compression Preferences.

Click Add and select the Network and/or Computer sets you wish to apply the compression rules to. In this example I have selected External and Internal. If you have IIS servers behind the ISA server, you can create a computer set with the IIS server(s) and apply the compression to those servers only.

The default compression level is disabled, select an element and press the Set Compression button to configure compression. Here you can specify to request or respond with compressed content.

In this example, we want to reply with compressed content to external elements and request compressed content for our internal network elements. This is a good general compression setting for everyday use.

Under the Content Types tab you can specify which types of data to apply the compression filter to. You can edit the Content Types available in the Firewall Policy. Along with the User, Computer and Domain Name sets there is a Content Types setting where you can add or remove them.

Finally, under the Content Inspection tab, you can specify if you want ISA to decompress the content to inspect the content. Ensure this box is checked.

Apply this configuration to the ISA server and ISA will begin requesting and responding to requests with compressed data.