RFC 2502 (rfc2502) - Page 2 of 11

Limitations of Internet Protocol Suite for Distributed Simulation the Large Multicast Environment

RFC 2502         Limitations of Internet Protocol Suite    February 1999


   Distributed Interactive Simulation is the name of a family of
   protocols used to exchange information about a virtual environment
   among hosts in a distributed system that are simulating the behavior
   of objects in that environment.  The objects are capable of physical
   interactions and can sense each other by visual and other means
   (infrared, etc.).  DIS was developed by the U.S. Department of
   Defense (DoD) to implement systems for military training, rehearsal,
   and other purposes. More information on DIS can be found in [SSM96].

   The feature of distributed simulation that drives network
   requirements is that it is intended to work with output to and input
   from humans across distributed simulators in real time. This places
   tight limits on latency between hosts.  It also means that any
   practical network will require multicasting to implement the required
   distribution of all data to all participating simulators.  Large
   distributed simulation configurations are expected to group hosts on
   multicast groups based on sharing the same sensor inputs in the
   virtual environment.  This can mean a need for thousands of multicast
   groups where objects may move between groups in large numbers at high
   rates.  Because the number of simulators is known in advance and
   their maximum output rate in packets per second and bits per second
   is specified, the overall total data rate (the sum of all multicast
   groups) is bounded. However the required data rate in any particular
   group cannot be predicted, and may change quite rapidly during the
   simulation.

   DIS real time flow consists of packets of length around 2000 bits at
   rates from .2 packets per second per simulator to 15 packets per
   second per simulator. This information is intentionally redundant and
   is normally transmitted with a best effort transport protocol (UDP).
   In some cases it also is compressed.  Required accuracy both of
   latency and of physical simulation varies with the intended purpose
   but generally must be at least sufficient to satisfy human
   perception.  For example, in tightly coupled simulations such as high
   performance aircraft maximum acceptable latency is 100 milliseconds
   between any two hosts.  At relatively rare intervals events (e.g.
   collisions) may occur which require reliable transmission of some
   data, on a unicast basis, to any other host in the system.

   The U.S. DoD has a goal to build distributed simulation systems with
   up to 100,000 simulated objects, many of them computer generated
   forces that run with minimal human intervention, acting as opposing
   force or simulating friendly forces that are not available to
   participate.  DoD would like to carry out such simulations using a
   shared WAN.  Beyond DoD many people see a likelihood that distributed
   simulation capabilities may be commercialized as entertainment.  The
   scope of such an entertainment system is hard to predict but
   conceivably could be larger than the DoD goal of 100,000.



Pullen                       Informational