The SIMNET program has been a pioneer in "multi-interconnected-simulator" training for the Army. As the benefits of this training have been demonstrated, the Department of Defense (DoD) community has recognized a need for more training of this type and therefore wants to interconnect the current inventory of simulators in a manner similar to SIMNET. In an attempt to answer this need, a standardization process was begun to allow greater interoperability of defense simulations. This project is funded by the Defense Advanced Research Projects Agency (DARPA) and administered by the Army Project Manager for Training Devices (PMTRADE). The current mission at the Institute for Simulation and Training (IST) is to develop a standard for Protocol Data Units (PDU) on the application layer of the communications software. The Simulation PDU's of the SIMNET protocol were considered as a baseline for this effort. This paper discusses the approach taken for development of the Draft Standard, lists and describes the recommended PDU's, and discusses other requirements for interoperability of defense simulations.