Several initiatives worldwide are seeking to build an open, generalpurpose, and sustainable large-scale shared experimental facility to foster the emergence of the Future Internet. This objective is ambitious as it calls for the setting up of testbeds to study solutions yet to be designed. Furthermore, any proposed new architecture must be accompanied by a transition scenario to overcome the significant obstacles that will lie in the path to its eventual adoption. The OneLab experimental facility is a leading prototype for a flexible federation of testbeds that is open to the current Internet. OneLab has pioneered the concept of testbed federation, providing a federation model that has been proven through a durable interconnection between its flagship testbed PlanetLab Europe (PLE) and the global PlanetLab infrastructure, mutualising over five hundred sites around the world. OneLab is further developing an understanding of what it means for autonomous organizations operating heterogeneous testbeds to federate their computation, storage, and network resources, including defining terminology, establishing universal design principles, and identifying candidate federation strategies