We have developed a method to localize and quantify interdomain congestion in the Internet. Our Time Sequence Latency Probes (TSLP) method depends on two facts: Internet traffic patterns are typically diurnal, and queues increase packet delay through a router during periods of adjacent link congestion. Repeated round trip delay measurements from a single test point to the two edges of a congested link will show sustained increased latency to the far (but not to the near) side of the link. We are designing and implementing a system for network-wide measurement of congestion using the TSLP method. We plan to support QoE measurements on this system to complement our measurement of QoS metrics such as delay and loss rate.