SDI Seminar

Speaker: Fred Douglis, AT&T Labs - Research

Date: October 23, 1997

Where & When: WeH 8220, Noon

Potential Benefits of Delta-encoding and Data Compression for HTTP


Caching in the World Wide Web currently follows a naive model, which assumes that resources are referenced many times between changes. This model also provides no way to update a cache entry if a resource does change, except by transferring the resource's entire new value. Some previous work has proposed updating cache entries by transferring only the differences, or "delta," between the cached entry and the current value.

In this talk, I describe two studies of the potential benefits of using delta-encoded responses. The first is a prototype implementation that used two proxies to transfer deltas in a transparent fashion. Its benefits were evaluated using a static repository of versions of a relatively small collection of resources. The second study used dynamic traces of the full contents of HTTP messages at two sites, AT&T Research and Digital Equipment Corporation, to consider the benefits of delta-encoding on a live stream of accesses. It also considered the benefits of on-line compression, instead of or in addition to delta-encoding. The study showed that such encodings offer a remarkable improvement in response size and response delay for textual content types such as HTML.

Finally, I propose specific extensions to the HTTP protocol for delta-encoding and data compression. These extensions are compatible with existing implementations and specifications, yet allow efficient use of a variety of encoding techniques.

Joint work with Gaurav Banga, Anja Feldmann, Balachander Krishnamurthy, Jeffrey Mogul, and Misha Rabinovich.