Network disruptions can adversely impact a users' web browsing, cause video/audio interruptions, or render web sites and services unreachable. Such problems are frustrating to Internet users, who are oblivious to the underlying problems, but completely exposed to the service degrada- tions. Ideally users' end systems would have diagnostic tools that can automatically detect, diagnose and possibly repair, performance degradations. Hopefully, this can be done without user intervention. Clearly, the first step for any such (end-host) diagnostic tool is a methodology to automatically detect performance degradations in the network that can affect a user's perception of application performance.