We are developing an Interoperable Distributed Simulation (IDSim) framework that builds upon the Open Grid Services Infrastructure (OGSI) to provide distributed simulation services to federated simulators. OGSI and web technology standards provide novel opportunities for a domain-independent, distributed simulation framework to support extensibility by users of the system. IDSim services can be extended through inheritance to add or extend operations tailored to the needs of specific simulation domains. XML-based documentation of simulation models allows the framework to operate upon the contents of shared events (e.g., content-based distribution). Additionally, exposure of the underlying framework through Grid Web Services Description Language (GWSDL) allows for tool-based extensibility; that is, tools can be developed to create and manage complex federations. This paper presents the design and architecture of IDSim with a focus on extensibility; other design concepts are presented where relevant.