Network Working Group                                       S. Symington
Request for Comments: 1667                             MITRE Corporation
Category: Informational                                          D. Wood
                                                       MITRE Corporation
                                                               M. Pullen
                                                 George Mason University
                                                             August 1994


             Modeling and Simulation Requirements for IPng

Status of this Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

Abstract

   This document was submitted to the IETF IPng area in response to RFC
   1550.  Publication of this document does not imply acceptance by the
   IPng area of any ideas expressed within.  Comments should be
   submitted to the big-internet@munnari.oz.au mailing list.

Executive Summary

   The Defense Modeling and Simulation community is a major user of
   packet networks and as such has a stake in the definition of IPng.
   This white paper summarizes the Distributed Interactive Simulation
   environment that is under development, with regard to its real-time
   nature, scope and magnitude of networking requirements.  The
   requirements for real-time response, multicasting, and resource
   reservation are set forth, based on our best current understanding of
   the future of Defense Modeling and Simulation.

1.  Introduction

   The Internet Engineering Task Force (IETF) is now in the process of
   designing the Next Generation Internet Protocol (IPng). IPng is
   expected to be a driving force in the future of commercial off-the-
   shelf (COTS) networking technology. It will have a major impact on
   what future networking technologies are widely available, cost
   effective, and multi-vendor interoperable.  Applications that have
   all of their network-layer requirements met by the standard features
   of IPng will be at a great advantage, whereas those that don't will
   have to rely on less-widely available and more costly protocols that
   may have limited interoperability with the ubiquitous IPng-based COTS
   products.

   This paper is intended to serve as input to the IPng design effort by
   specifying the network-layer requirements of Defense Modeling and
   Simulation (M&S) applications. It is important that the M&S community
   make its unique requirements clear to IPng designers so that
   mechanisms for meeting these requirements can be considered as
   standard features for IPng. The intention is to make IPng's benefits
   of wide COTS availability, multi-vendor interoperability, and cost-
   effectiveness fully available to the M&S community.

2.  Background: Overview of Distributed Interactive Simulation

   The Defense Modeling and Simulation community requires an integrated,
   wide-area, wideband internetwork to perform Distributed Interactive
   Simulation (DIS) exercises among remote, dissimilar simulation
   devices located at worldwide sites. The network topology used in
   current M&S exercises is typically that of a high-speed cross-country
   and trans-oceanic backbone running between wideband packet switches,
   with tail circuits running from these packet switches to various
   nearby sites. At any given site involved in an exercise, there may be
   several internetworked local area networks on which numerous
   simulation entity hosts are running.  Some of these hosts may be
   executing computer-generated semi-automated forces, while others may
   be manned simulators.  The entire system must accommodate delays and
   delay variance compatible with human interaction times in order to
   preserve an accurate order of events and provide a realistic combat
   simulation. While the sites themselves may be geographically distant
   from one another, the simulation entities running at different sites
   may themselves be operating and interacting as though they are in
   close proximity to one another in the battlefield.  Our goal is that
   all of this can take place in a common network that supports all
   Defense modeling and simulation needs, and hopefully is also shared
   with other Defense applications.

   In a typical DIS exercise, distributed simulators exchange
   information over an internetwork in the form of standardized protocol
   data units (PDUs). The DIS protocols and PDU formats are currently
   under development.  The first generation has been standardized as
   IEEE 1278.1 and used for small exercises (around 100 hosts), and
   development of a second generation is underway.  The current
   Communications Architecture for DIS specifies use of Internet
   protocols.

   The amount, type, and sensitivity level of information that must be
   exchanged during a typical DIS exercise drives the communications
   requirements for that exercise, and depends on the number and type of
   participating entities and the nature and level of interaction among
   those entities.  Future DIS exercises now in planning extend to
   hundreds of sites and tens of thousands of simulation platforms

   worldwide. For example, an exercise may consist of semi-automated and
   individual manned tank, aircraft, and surface ship simulators
   interacting on pre-defined geographic terrain. The actual locations
   of these simulation entities may be distributed among sites located
   in Virginia, Kansas, Massachusetts, Germany, and Korea. The PDUs that
   are exchanged among simulation entities running at these sites must
   carry all of the information necessary to inform each site regarding
   everything relevant that occurs with regard to all other sites that
   have the potential to affect it within the simulation. Such
   information could include the location of each entity, its direction
   and speed, the orientation of its weapons systems, if any, and the
   frequency on which it is transmitting and receiving radio messages.
   If an entity launches a weapon, such as a missile, a new entity
   representing this missile will be created within the simulation and
   it will begin transmitting PDUs containing relevant information about
   its state, such as its location, and speed.

   A typical moving entity would generate between one and two PDUs per
   second, with typical PDU sizes of 220 bytes and a maximum size of
   1400 bytes, although rates of 15 PDUs/second and higher are possible.
   Stationary entities must generate some traffic to refresh receiving
   simulators; under the current standard this can be as little as 0.2
   PDUs per second.  Compression techniques reducing PDUs size by 50% or
   more are being investigated but are not included in the current DIS
   standard.

   With so much information being exchanged among simulation entities at
   numerous locations, multicasting is required to minimize network
   bandwidth used and to reduce input to individual simulation entities
   so that each entity receives only those PDUs that are of interest to
   it. For example, a given entity need only receive information
   regarding the location, speed and direction of other entities that
   are close enough to it within the geography of the simulation that it
   could be affected by those entities.  Similarly, an entity need not
   receive PDUs containing the contents of radio transmissions that are
   sent on a frequency other than that on which the entity is listening.

   Resource reservation mechanisms are also essential to guarantee
   performance requirements of DIS exercises: reliability and real-time
   transmission are necessary to accommodate the manned simulators
   participating in an exercise.

   M&S exercises that include humans in the loop and are executed in
   real-time require rapid network response times in order to provide
   realistic combat simulations.  For DIS, latency requirements between
   the output of a PDU at the application level of a simulator and input
   of that PDU at the application level of any other simulator in that
   exercise have been defined as:

      - 100 milliseconds for exercises containing simulated units
        whose interactions are tightly coupled

      - 300 milliseconds for exercises whose interactions are not
        tightly coupled [2].

   The reliability of the best-effort datagram delivery service
   supporting DIS should be such that 98% of all datagrams are delivered
   to all intended destination sites, with missing datagrams randomly
   distributed [3].

   While these numbers may be refined for some classes of simulation
   data in the future, latency requirements are expected to remain under
   a few hundred milliseconds in all cases.  It is also required that
   delay variance (jitter) be low enough that smoothing by buffering the
   data stream at the receiving simulator does not cause the stated
   latency specifications to be exceeded.

   There are currently several architectures under consideration for the
   M&S network of the future. Under fully distributed models, all
   simulation entities rely directly on the network protocols for
   multicasting and are therefore endowed with much flexibility with
   regard to their ability to join and leave multicast groups
   dynamically, in large numbers.

   In some cases, the M&S exercises will involve the transmission of
   classified data over the network. For example, messages may contain
   sensitive data regarding warfare tactics and weapons systems
   characteristics, or an exercise itself may be a rehearsal of an
   imminent military operation. This means the data communications used
   for these exercises must meet security constraints defined by the
   National Security Agency (NSA).  Some such requirements can be met in
   current systems by use of end-to-end packet encryption (E3) systems.
   E3 systems provide adequate protection from disclosure and tampering,
   while allowing multiple security partitions to use the same network
   simultaneously.

   Currently the M&S community is using the experimental Internet Stream
   protocol version 2 (ST2) to provide resource reservation and
   multicast.  There is much interest in converting to IPv4 multicast as
   it becomes available across the COTS base, but this cannot happen
   until IPv4 has a resource reservation capability.  The RSVP work
   ongoing in the IETF is being watched in expectation that it will
   provide such a capability.  Also some tests have been made of IPv4
   multicast without resource reservation; results have been positive,
   now larger tests are required to confirm the expected scalability of
   IPv4 multicast.  But issues remain: for security reasons, some M&S
   exercises will require sender-initiated joining of members to

   multicast groups. In addition, it is not clear that IPv4 multicast
   will be able to make use of link-layer multicast available in ATM
   systems, which the M&S community expects to use to achieve the
   performance necessary for large exercises.

3.  M&S Requirements for IPng

   The identified network-layer service requirements for M&S
   applications are set forth below in three major categories: real-time
   response, multicast capability, and resource reservation capability.
   All of these capabilities are considered to be absolute requirements
   for supporting DIS as currently understood by the M&S community,
   except those specifically identified as highly desirable.  By
   desirable we mean that the capabilities are not essential, but they
   will enable more direct or cost-effective networking solutions.

   It is recognized that some of the capabilities described below may be
   provided not from IPng but from companion protocols, e.g. RSVP and
   IGMP.  The M&S requirement is for a compatible suite of protocols
   that are available in commercial products.

   a.  Real-time Response

      DIS will continue to have requirements to communicate real-time
      data, therefore the extent to which IPng lends itself to
      implementing real-time networks will be a measure of its utility
      for M&S networking.  The system-level specifications for the DIS
      real-time environment are stated in Section 2 above.

   b.  Multicasting

      M&S requires a multicasting capability and a capability for
      managing multicast group membership.  These multicasting
      capabilities must meet the following requirements:

      - Scalable to hundreds of sites and, potentially, to tens of
        thousands of simulation platforms.

      - It is highly desirable that the network-layer multicasting
        protocol be able to use the multicasting capabilities of
        link-level technologies, such as broadcast LANs, Frame Relay,
        and ATM.

      - The group management mechanics must have the characteristics
        that thousands of multicast groups consisting of tens of
        thousands of members each can be supported on a given network
        and that a host should be able to belong to hundreds of multicast
        groups simultaneously.

      - Multicast group members must be able to be added to or removed
        from groups dynamically, in less than one second, at rates of
        hundreds of membership changes per second.  It is not possible
        to predict what special cases may develop, thus this requirement
        is for all members of all groups.

      - The network layer must support options for both sender- and
        receiver-initiated joining of multicast groups.

   c.  Resource Reservation

      The M&S community requires performance guarantees in supporting
      networks. This implies that IPng must be compatible with a
      capability to reserve bandwidth and other necessary allocations in
      a multicast environment, in order to guarantee network capacity
      from simulator-to-simulator across a shared network for the
      duration of the user's interaction with the network. Such a
      resource reservation capability is essential to optimizing the use
      of limited network resources, increasing reliability, and
      decreasing delay and delay variance of priority traffic,
      especially in cases in which network resources are heavily used.
      The resource reservations should be accomplished in such a way
      that traffic without performance guarantees will be re-routed,
      dropped, or blocked before reserved bandwidth traffic is affected.

      In addition, it would be highly desirable for the resource
      reservation capability to provide mechanisms for:

      - Invoking additional network resources (on-demand capacity)
        when needed.

      - The network to feed back its loading status to the applications
        to enable graceful degradation of performance.

4.  References

   [1] Cohen, D., "DSI Requirements", December 13, 1993.

   [2] Final Draft Communication Architecture for Distributed
       Interactive Simulation (CADIS), Institute for Simulation and
       Training, Orlando, Florida, June 28, 1993.

   [3] Miller, D., "Distributed Interactive Simulation Networking
       Issues", briefing presented to the ST/IP Peer Review Panel, MIT
       Lincoln Laboratory, December 15, 1993.

   [4] Pate, L., Curtis, K., and K. Shah, "Communication Service
       Requirements for the M&S Community", September 1992.

   [5] Pullen, M., "Multicast Network Architecture for DIS, briefing
       presented to the Networks Infrastructure Task Force", George
       Mason University, C3I Center/Computer Science, November 10, 1993,
       revised November 11, 1993.

5.  Authors' Addresses

       Susan Symington
       MITRE Corporation
       7525 Colshire Drive
       McLean, VA 22101-3481

       Phone: 703-883-7209
       EMail: susan@gateway.mitre.org


       David Wood
       MITRE Corporation
       7525 Colshire Drive
       McLean, VA 22101-3481

       Phone: 703-883-6394
       EMail: wood@mitre.org


       J. Mark Pullen
       Computer Science
       George Mason University
       Fairfax, VA 22030

       Phone: 703-993-1538
       EMail: mpullen@cs.gmu.edu