PlanetLab is a geographically distributed overlay platform designed to support the deployment and evaluation of planetary-scale network services. It currently includes over 500 machines spanning 252 sites and 29 countries, and has supported over 450 research projects. PlanetLab can support large-scale overlays running in slices (i.e., collections of distributed virtual machines). It has significantly lowered the barriers to deploying overlays. The paper describes how to lower them further by introducing a general data plane for overlay networks. Researchers can plug their own control planes into this data plane to run fully functional overlays easily on PlanetLab, and students can use the overlay data plane as a hands-on tool for learning about internetworking concepts. The overlay data plane is the first step in the "Internet-in-a-slice" project, which will let members of the PlanetLab community experiment with virtual Internets running on PlanetLab.