Network failures adversely impact Internet's end-users and service providers. It is important for end-users and service providers to diagnose (i.e., detect, identify, and troubleshoot) network failures quickly and reliably to fix or bypass them. Network operators can use control data collected at network routers to detect and identify failures in the autonomous systems that they manage. Unfortunately, other than in this specific case, control data is not available and is insufficient to identify all types of failures. Hence, end-users and network operators resort to active end-to-end measurements to diagnose failures.