Detecting network path anomalies generally requires examining large volumes of traffic data to find misbehavior. We observe that wide-area services, such as peerto-peer systems and content distribution networks, exhibit large traffic volumes, spread over large numbers of geographically-dispersed endpoints. This makes them ideal candidates for observing wide-area network behavior. Specifically, we can combine passive monitoring of wide-area traffic to detect anomalous network behavior, with active probes from multiple nodes to quantify and characterize the scope of these anomalies. This approach provides several advantages over other techniques: (1) we obtain more complete and finergrained views of failures since the wide-area nodes already provide geographically diverse vantage points; (2) we incur limited additional measurement cost since most active probing is initiated when passive monitoring detects oddities; and (3) we detect failures at a much higher rate than other researchers have reported since the services provide large volumes of traffic to sample. This paper shows how to exploit this combination of wide-area traffic, passive monitoring, and active probing, to both understand path anomalies and to provide optimization opportunities for the host service.
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