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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.


 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 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

Symington, Wood & Pullen [Page 1] RFC 1667 Modeling and Simulation Requirements for IPng August 1994

 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
 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

Symington, Wood & Pullen [Page 2] RFC 1667 Modeling and Simulation Requirements for IPng August 1994

 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
 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:

Symington, Wood & Pullen [Page 3] RFC 1667 Modeling and Simulation Requirements for IPng August 1994

  1. 100 milliseconds for exercises containing simulated units

whose interactions are tightly coupled

  1. 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
 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

Symington, Wood & Pullen [Page 4] RFC 1667 Modeling and Simulation Requirements for IPng August 1994

 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:
  1. Scalable to hundreds of sites and, potentially, to tens of

thousands of simulation platforms.

  1. 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.
  1. 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.

Symington, Wood & Pullen [Page 5] RFC 1667 Modeling and Simulation Requirements for IPng August 1994

  1. 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.
  1. 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:
  1. Invoking additional network resources (on-demand capacity)

when needed.

  1. 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.

Symington, Wood & Pullen [Page 6] RFC 1667 Modeling and Simulation Requirements for IPng August 1994

 [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
     David Wood
     MITRE Corporation
     7525 Colshire Drive
     McLean, VA 22101-3481
     Phone: 703-883-6394
     J. Mark Pullen
     Computer Science
     George Mason University
     Fairfax, VA 22030
     Phone: 703-993-1538

Symington, Wood & Pullen [Page 7]

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