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Abstracts - 1997 ANNUAL TECHNICAL CONFERENCE
Optimistic Deltas for WWW Latency Reduction
Gaurav Banga; Rice University
Fred Douglis, Michael Rabinovich; AT&T Labs - Research
Abstract
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optimistically transfers data that may be out of date, then sends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.
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