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
