Overlay networks have recently emerged as a promising technique for deploying new network services, and more generally, for introducing disruptive technology into the Internet. However, there is a pressing need for a testbed that allows the research community to experiment with overlay services at scale, and under real-world conditions. The proposed project will build-out and enhance a global overlay network, called PlanetLab, that provides just such a capability. Enhancements to be made to the PlanetLab software include the following:  -Implement system-level mechanisms that isolate experimental services from each other, and protect the network from faulty and malicious traffic; -Build mechanisms to monitor the health of PlanetLab nodes and services, diagnose unexpected behavior, and map suspicious behavior to the responsible individuals; -Provide mechanisms to allow researchers to discover the set of available PlanetLab nodes that meet their needs and acquire resources on those nodes according to a market that respects resource availability; and provide the instrumentation hooks and measurement tools needed to collect, archive, and analyze performance data collected on PlanetLab.  Broader Impact and Intellectual Merit: PI's envision PlanetLab serving as a microcosm for the next generation Internet. For the networking research community, PlanetLab offers the opportunity to regain the now-lost ability to rapidly and seamlessly move from idea to evaluation to practice. For the educational community, it offers a rich and realistic platform for teaching and experimentation, and a body of courseware and tools to support it. For the broader community of users, commercial developers, and society at large, it offers a path to re-energizing the innovative process that has led to new Internet services, widespread consumer adoption and generation of new economic and social value. This proposal aims to enhance the software substrate and build out the number of nodes in PlanetLab - a global overlay network that will serve as both a research testbed and a deployment platform. It can play a potentially momentous role in helping define a next generation (overlay) Internet by allowing researchers to experiment at scale and under real-world conditions.