Projects of the Future Internet Research & Experimentation (FIRE) initiative are building an experimental facility that shall serve the needs of Future Internet research and development. The main design principles are virtualization of resources and federation. Federation is a means to meet requirements from Future Internet research that cannot be met by individual testbeds. In particular, to support large scale experiments utilizing heterogeneous resources, a federation of experimental facilities is needed. While several initiatives are currently establishing large scale testbeds, the mechanisms for federating such environments across the boundaries of administrative domains are unclear. This is due to the lack of established and agreed federation models, methods, and operational procedures. In this article we propose a federation model that defines high level conceptual entities for federating resources across administrative domains. A first prototype implementation of the functional components derived from the model has been realized and evaluated. This is demonstrated by the discussion of use cases that depict the flexibility of the proposed approach. The model can guide future testbed developments and harmonize the currently scattered efforts across several FIRE projects in order to establish an agreed resource federation framework. This framework shall be the basis for Future Internet research and experimentation in Europe and provide experimental facility services to academia and industry.