Overview of HTTP compression

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HTTP compression in Microsoft Forefront Threat Management Gateway is a global HTTP policy setting. It applies to all HTTP traffic that passes through Forefront TMG to or from a specified network or network object, rather than to traffic handled by a specific rule. HTTP compression is provided by two Web filters:

  • Compression Filter. This filter is responsible for compression and decompression of HTTP requests and responses. This filter has a high priority and is high in the ordered list of Web filters. This is because the filter is responsible for decompression. Decompression must take place before any other Web filters inspect the content.
  • Caching Compressed Content Filter. This filter is responsible for caching of compressed content and serving a request from the compressed content in the cache. This filter has the lowest priority and is low in the ordered list of Web filters, because caching can take place after all other filters have handled the content.

Do not change the default priority and order settings of these filters.

Microsoft Forefront Threat Management Gateway Hypertext Transfer Protocol (HTTP) compression reduces file size by using algorithms to eliminate redundant data. Most common Web-related file types can safely be compressed. HTTP compression uses the industry standard GZIP algorithm, which is built into the Microsoft Windows Server 2008, Windows Vista, Windows Server 2003, and Windows 2000 Server operating systems and Microsoft Internet Explorer 6, Internet Explorer 5, and Internet Explorer 4. These algorithms compress static files and optionally perform on-demand compression of dynamically generated responses before sending them over the network. These same algorithms are again used to decompress the static files and dynamic responses on an HTTP 1.1 supported client. A client that is configured to use HTTP 1.1 will request compressed content from a Web server. Web servers indicate in their responses whether they support compression.

Notes   In Internet Explorer, configure the use of HTTP 1.1 on the Advanced tab of Internet Options, by selecting Use HTTP 1.1 through proxy connections.

HTTP Compression is available in ISA Server 2006 and later.

Compression Scenario

A company has its headquarters in New York City and branch offices throughout the world. Some of the larger branch offices may be connected directly to headquarters with high-speed connections. Others may be connected as virtual private networks (VPNs) over communication lines of varying speeds and latencies. For example, a small, remote branch office may be connected directly (over a frame-relay line) or as a VPN over a 56 or 64-kilobits per second (Kbps) line. Often, the main office proxy routes Internet requests directly to the Internet, whereas the branch offices may route their requests through headquarters.

Compression of content between the branch offices and the main office is required to preserve the limited bandwidth. Forefront TMG provides HTTP compression to reduce bandwidth usage. Note that HTTP 1.1 supports compression, whereas HTTP 1.0 does not.