Optimistic Deltas for WWW Latency Reduction

When a machine is connected to the Internet via a slow network, such as a 28.8 Kbps modem, the cumulative latency to communicate over the Internet to World Wide Web servers and then transfer documents over the slow network can be significant. We have built a system that optimistically transfers data that may be out of date, then sends either a subsequent confirmation that the data is current or a delta to change the older version to the current one. In addition, if both sides of the slow link already store the same older version, just the delta need be transferred to update it. Our mechanism is optimistic because it assumes that much of the time there will be sufficient idle time to transfer most or all of the older version before the newer version is available, and because it assumes that the changes between the two versions will be small relative to the actual document. Timings of retrievals of random URLs in the Internet support the former assumption, while experiments using a version repository of Web documents bear out the latter one. Performance measurements of the optimistic delta system demonstrate that deltas significantly reduce latency when both sides cache the old version, and optimistic deltas can reduce latency, to a lesser degree, when content-provider service times are in the range of seconds or longer.