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3.1 Collection

We instrumented clients that we ran on PlanetLab to record the application-level latency between them and the rest of the network creating a dense latency matrix. These clients ran on 283 PlanetLab nodes for 24 days starting on July 19th 2006, collecting approximately 950,000,000 latency measurements to 156,658 Azureus nodes. To reduce these raw measurements into a dense latency matrix, we used the following process: first, we summarized each edge with the median round trip time for this edge, discarding edges with fewer than a minimum number of samples (4); second, we discarded all nodes that had fewer than half of the maximum number of edges (280). This process resulted in a 249x2902 matrix with 91 percent density, where 83 percent of the entries were the median of at least ten samples. We derived the PlanetLab data set from the Azureus matrix by simply selecting the subset of nodes that were PlanetLab hosts.