RFC 2502 (rfc2502) - Page 2 of 11
Limitations of Internet Protocol Suite for Distributed Simulation the Large Multicast Environment
Alternative Format: Original Text Document
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