Caches are used to good effect on today's web to improve response times and reduce network usage. For any given resource, such as an HTML page or an image, the client remembers the last instance it retrieved, and it may use it to satisfy future requests. However, the current-system is all-or-nothing: the resource must either be exactly the same as the cached instance, or it is downloaded from scratch. The web is moving towards dynamic content: many pages are assembled from databases or are customized for each visitor. In the existing HTTP caching system, this means that many resources cannot be cached at all. A far better approach would be for the server to download a description of the changes from the old instance to the new one: a `diff' or `delta'. Some people have proposed that the server should send the resource as an unchanging template plus variable values, or that the server should retain all old instances and so calculate the differences. These techniques have some value, but they constrain the server-side developer and seem unlikely to be widely adopted. The rproxy extensions to HTTP allow the server to generate a diff relative to the cached instance in a way that is completely general, and transparent to both the server and user agent. rproxy adds backwards-compatible extensions to HTTP that come into operation when two parties to a web request understand the `hsync' encoding. If there are no two that can handle these extensions then they are silently ignored, so that the software will interoperate smoothly with existing systems. These parties might be the user agent (browser), the origin server, or intermediate proxies. At this stage of the project special-purpose proxies are used so that neither the server nor user-agent need be changed, but we expect to integrate this extension into popular web software in the near future. Martin Pool, a lead developer of rproxy, will review the background and implementation of rproxy, present exciting experimental results, and discuss future plans. The session will be interesting to developers of Apache and of other web software, and to webmaster interested in reducing traffic volumes and improving response times. For more information, see