Overlay Multicast Support for Distributed Simulation
Large scale distributed simulations present a variety of problems that are unique to the Modeling and
Simulation Community. That community, in turn, is mostly concentrated in Defense applications so that
there are few commercially available solutions to its specialized problems. One such problem is multicast
networking, where the network delivers each message to a group of computers (by contrast, in common unicast,
a message is delivered to one computer). Because multicasting can produce dramatic reductions in network
traffic for group applications, it is a desirable supporting technology for M&S.
A multicast version of the Internet Protocol (IP) exists, but supporting it in wide area networks is
significantly more complicated than running a regular unicast IP network, particularly where multiple
network domains interoperate (as in the Internet or the GIG). Therefore Internet Service Providers
(including DISA) have chosen not to offer multi-domain multicasting service. In the commercial domain,
what multicast does exist generally supports “one to many” applications, where one stream is sent to
many receivers (as in entertainment). In the DoD M&S Domain, the problem most often encountered is
“many to many” real-time multicast requirement among simulators. This lack of a multicast capability
has also been identified as a technology gap for M&S in the internetwork spanning DoD, from the core
Global Information Grid (GIG) (which may have multicast available, at a price) to the far-flung networks
of the Services, Defense Agencies and Joint Commands (which almost surely will not have it).
Recent academic research projects at MIT and Carnegie-Mellon have demonstrated that there is another way
to obtain the benefits of multicasting: An “overlay” can be constructed among the computers in a regular
unicast network. In this overlay, the computers attached to the network create the same capability that
the network’s routers would produce for multicasting: They cooperate to route the network traffic among
themselves such that redundant message transmissions across the underlying network are reduced or eliminated
for distributed simulation and applications that have similar requirements, such as group collaboration.
DMSO has sponsored work at the C4I Center of George Mason University (GMU) to adapt to DoD M&S the results
of academic research in overlay multicast, with particular emphasis on the many-to-many problem. The result
thus far is an architecture document and a prototype (based on previous student work at GMU and the Naval
Postgraduate School) that extends IP multicast from local networks to the wide area. GMU has demonstrated
the prototype, called Cross-Network Overlay Multicast (XOM), in small configurations. The next step in
adoption of this promising capability is trial implementation at larger scale, in a C2-Simulation environment.
DMSO is sponsoring GMU's participation in the Actionable Situation Awareness Pull (ASAP) ACTD, which in turn
will result in its evaluation for use in DISA's Network-Centric Enterprise Services (NCES) for the GIG. In
addition, the Joint Advanced Training Technology Laboratory (JATTL) of USJFCOM has committed to sponsor
application in the Joint Rapid Database Development and Deployment (JRD3) system, and the Defense Threat
Reduction Agency has funded work to provide private overlay multicast communication over the open Internet,
using e-commerce style encryption. GMU is also engeged in standardization efforts for overlay multicast
through the Internet Engineering Task Force.