Abstract-Network monitoring capabilities are critical for both network operators and networked applications. In the context of an experimental test facility, network measurement is important for researchers experimenting with new network architectures and applications, as well as operators of the test facility itself. The Global Environment for Network Innovations (GENI) is a sophisticated test facility comprised of multiple "control frameworks." In this paper, we describe the design and implementation of S 3 Monitor, a scalable and extensible monitoring service for GENI. A key feature of S 3 Monitor is a flexible design that allows easy "plug in" of new network measurement tools. We discuss our deployment experiences with S 3 Monitor on GENI, and give experimental results to quantify the performance and system footprint of S 3 Monitor. We find that the S 3 Monitor service is light-weight and scales well as the number of paths to be monitored increases.