Presence-Based Availability and P2P Systems

The availability of a P2P service is a function of the individual peers availabilities, and it is often desirable to estimate how available a particular P2P service will be given the availability of its peers. Prior work in this area has widely used the fraction of time the average peer is available as the basis for this estimate. We show here that this approach has serious drawbacks. We develop a different measure, which we call presence-based availability, which takes into account the availability of the individual peers. Using traces of live P2P systems taken from the literature, we demonstrate that presence-based availability is a more reliable indicator of potential performance than prior methods. We show that our metrics successfully estimate the availability of a P2P file-sharing system. Then, using presence-based measures to make a better estimate of a parameter in a highly-available system, we achieve a 75% decrease in resource usage relative to an existing technique relying on traditional metrics.