A Distributed Interactive Simulation Intranet Using RAMP, a Reliable Adaptive Multicast Protocol

A dynamic, heterogeneous, multicast environment has been fielded as a simulation Intranet over a wide area network (WAN) for distributed interactive simulation (DIS). By replacing the usual UDP-based unreliable broadcast and TCP-based reliable unicast mechanisms with a single multicast transport protocol (RAMP), the diverse goals of enhanced scalability, reliability (as needed), and efficient operation outside of LAN environments are achieved while still meeting the DIS stated maximum latency requirements. To further support scalability, hierarchies of reliability requirements have been established for DIS protocol data unit (PDU) types. The hierarchies are based on the reliability requirements of individual entities, and are used to dynamically select between RAMP's reliable and unreliable transport modes. By restricting use of reliability to only times when it is really needed, control channel traffic required for reliability is greatly reduced, furthering both scaleability and performance. The viability of this approach has been demonstrated through simulation exercises conducted via RAMP-enabled PDU transmissions across an Intranet comprised of multicast-capable routers and commercial WAN segments interconnecting five geographically disperse TASC simulation labs. RAMP also provides automatic fragmentation and reassembly of large packets, which facilitates transmission of DIS packets (such as environmental PDUs) that exceed the size limitations imposed by the Ethernet specification. The benefits to the DIS paradigm for a heterogeneous reliability are also shown to support the proposed next generation simulation initiatives under way with the object request broker (ORB) based high level architecture (HLA) efforts. Initiatives by the Object Management Group (OMG) to support multipoint-to-multipoint communications with the next generation of ORBs is currently underway. TASC has developed methods for embedding RAMP under IONA's ORB implementation (ORBIX) that support multipoint-to-multipoint communications, and is in the process of submitting recommendations for incorporating multicast protocols to the OMG.