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Versions: 00 01 02 03 04 rfc2502                                        
INTERNET DRAFT                                       J.M.Pullen
Expiration: 25 April 1997                              George Mason U.
                                                       U.of Central Florida
                                                       SAIC, Inc.
                                                     25 November 1996

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


Status of this Memo

     This document is an Internet-Draft.  Internet-Drafts are working
     documents of the Internet Engineering Task Force (IETF), its
     areas, and its working groups.  Note that other groups may also
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1.  The Large Multicast Environment

The Large Multicast User's Group (LAMUG) was formed to create a consensus-
based requirement for Internet Protocols to support Distributed Interactive
Simulation (DIS), its successor the High Level Architecture for simulation
(HLA), and related applications.  The applications are characterized by
the need to distribute a real-time application over a shared wide-area network
in a scalable manner such that numbers of hosts from a few to tens of
thousands are able to interchange state data with sufficent reliability
and timeliness to sustain a three-dimensional virtual, visual environment
containing large numbers of moving objects.  The network supporting
such an system necessarily will be capable of multicast.

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 system for military training, rehearsal, and other purposes.
More information on DIS can be found in the references.

The feature of DIS 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 DIS 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 hundreds
of multicast groups where objects may move between groups in large
numbers at high rates.

DIS real time flow consists of packets of length around 2000 bits at
rates from .2 per second per simulator to 15  per second per simulator.
This information is intentionally redundant and is normally
transmitted with a best-effort transport protocol (UDP), and in some
cases 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.

DoD has a goal to build DIS 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 DIS-like 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.

The High Level Architecture (HLA) is a development beyond DIS that aims
at bringing DIS and other forms of distributed simulation into a unifying
system paradigm.  Thus HLA has netowrking requirements at least as
demanding as DIS.  HLA is still under development, therefore this
document will focus on the requirements of DIS.

2.  DIS network requirements.

a.  real-time packet delivery, with low packet loss (less than 2%),
predictable latency on the order of a few hundred milliseconds, and
low jitter (variation of latency), on the order of a few milliseconds, in
a shared network

b.  multicasting with thousands of multicast groups that can sustain
join/leave in less than one second at rates of hundreds of join/leaves
per second

c.  multicasting using a many-to-many paradigm in which 90% or more
of the group members act as receivers and senders to a group

d.  support for resource reservation because of the impracticality
of over-provisioning the WAN and the LAN

e.  support for secure networking, needed for classified military

3.  Internet Protocol Suite facilities needed and not yet available for large-scale DIS in shared networks.  These derive from the need for real-time multicast with established quality of service:

a.  resource reservation available in production systems (RSVP seems
to be on a path to achieving this but a mechanism is needed to group
streams such that multiple multicast groups can share the same

b.  resource-sensitive routing to be used with the resource reservation

c.  IP multicast that is capable of taking advantage of link-layer
multicast (such as ATM) for packet replication across multiple logical
IP subnets

d.  a hybrid transport protocol that can support best-effort multicast of
most data, lightweight reliable multicast of critical reference data, and
reliable unicast of occasional data

e.  network management for DIS systems (this appears to require only
a DIS MIB for use with SNMP)

f.  a session protocol to start, pause, and stop a DIS exercise over an IP
network (MMUSIC or an adaptation might work)

g.  an integrated security architecture (it is likely that the IPv6 security
architecture will meet this need)

4.  References

RFC1667, "Modeling and Simulation Requirements for IPng"
August 1994

"The DIS Vision", DIS Steering Committee, Institute for Simulation and Training, University of Central Florida, May 1994

IEEE 1278.1-1995, Standard for Distributed Interactive Simulation - Application Protocols

IEEE 1278.2-1995, Standard for Distributed Interactive Simulation - Communication services and Profiles

5.  Authors' Addresses

  J. Mark Pullen
  Computer Science/4A5
  George Mason University
  Fairfax, VA 22032

  Michael Myjak
  Institute for Simulation and Training
  Orlando, FL

  Christina Bouwens
  SAIC Inc.
  Orlando FL

Expiration: 25 April 1997