Network Working Group M. Pullen
Request for Comments: 2502 George Mason University
Category: Informational M. Myjak
The Virtual Workshop
C. Bouwens
SAIC
February 1999
Limitations of Internet Protocol Suite for Distributed Simulation
in the Large Multicast Environment
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1999). All Rights Reserved.
Abstract
The Large-Scale Multicast Applications (LSMA) working group was
chartered to produce documents aimed at a consensus based development
of the Internet protocols to support large scale multicast
applications including real-time distributed simulation. This memo
defines services that LSMA has found to be required, and aspects of
the Internet protocols that LSMA has found to need further
development in order to meet these requirements.
1. The Large Multicast Environment
The Large-Scale Multicast Applications working group (LSMA) was
formed to create a consensus based requirement for Internet Protocols
to support Distributed Interactive Simulation (DIS) [DIS94], its
successor the High Level Architecture for simulation (HLA) [DMSO96],
and related applications. The applications are characterized by the
need to distribute a real-time applications 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 sufficient
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 [IEEE95a,IEEE95b].
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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.
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The High Level Architecture (HLA) is a DoD development beyond DIS
that aims at bringing DIS and other forms of distributed simulation
into a unifying system paradigm. From a distributed systems
standpoint HLA is considerably more sophisticated than DIS. For
example attributes of distributed objects may be controlled by
different simulators. From the standpoint of the supporting network
the primary difference between HLA and DIS is that HLA does not call
for redundant transmission of object attributes; instead it specifies
a "Run Time Infrastructure" (RTI) that is responsible to transmit
data reliably, and may choose to do so by various means including
redundant transmission using best effort protocols. It is reasonable
to say that any network that can meet the needs of DIS can support
HLA by DIS-like redundant transmission, however this approach ignores
the possibility that under HLA some mixture of redundant and reliable
transmission can make significantly better use of network resources
than is possible using DIS. While HLA, like DIS, does not specify
use of a multicasting network, it has similar requirements for many-
to-many transmission of object attributes at rates in excess of one
update per object per second that cannot be met without multicasting.
Further, HLA calls for transmission of semantically organized data
(for example, groups of objects with similar capabilities such as
tanks or aircraft) in this many-to-many context.
One solution that has been employed to deal with these challenges is
to aggregate the contents of many multicast groups into a single
multicast transmission [PuWh95, CSTH95]. Termed "dual-mode" or "bi-
level" multicast, this approach takes advantage of the fact that
although the amount of traffic in any particular multicast group can
vary greatly, the aggregate of all transmissions is bounded. If the
traffic is all aggregated into one large flow, an underlying ATM
network can create multicast SVCs with acceptable QoS to support the
requirement. It also bounds the network control problem of group
joins, in that the joins take place among dedicated collections of
routers and across the dedicated SVCs, rather than contending with
other LSMAs that may be sharing the same network. But it does this at
the cost of adding to the network a new, nonstandard aggregation
element that is a hybrid of the Internet and ATM protocols. We
address below the requirement to achieve such a result using a purely
IP network with aggregated reservation via RSVP.
The defense distributed simulation community has created a number of
multicast-capable networks for various simulated exercises, ranging
from tens to hundreds of simulated objects distributed across numbers
of sites ranging from two to twenty. As the number of objects has
increased they have found that building multicasting networks
potentially supporting thousands of simultaneous multicast groups
with large group change rates is a hard problem. This defense problem
is the precursor of similar problems that can be expected in
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commercial networks. Therefore the following sections describe the
services required and the shortcomings that have been found in using
today's Internet protocols in providing these services, with the
intention of informing the IETF to enable it to produce protocols
that meet the needs in these areas.
2. Distributed Simulation (DIS and HLA) network service requirements.
a. real-time packet delivery, with low packet loss (less than 2%),
predictable latency on the order of a few hundred milliseconds, after
buffering to account for jitter (variation of latency) such that less
than 2% of packets fail to arrive within the specified latency, in a
shared network
b. multicasting with thousands of multicast groups that can support
join latencies of less than one second, at rates of hundreds of joins
per second
c. multicasting using a many-to-many paradigm in which 90% or more of
the group members act as receivers and senders within any given
multicast group
d. support for resource reservation; because of the impracticality of
over-provisioning the WAN and the LAN for large distributed
simulations, it is important to be able to reserve an overall
capacity that can be dynamically allocated among the multicast groups
e. support for a mixture of best-effort and reliable low-latency
multicast transport, where best-effort predominates in the mixture,
and the participants in the reliable multicast may be distributed
across any portion of the network
f. support for secure networking, in the form of per-packet
encryption and authentication needed for classified military
simulations
3. Internet Protocol Suite facilities needed and not yet available for
large-scale distributed simulation in shared networks: These derive
from the need for real-time multicast with established quality of
service in a shared network. (Implementation questions are not
included in this discussion. For example, it is not clear that
implementations of IP multicast exist that will support the required
scale of multicast group changes for LSMA, but that appears to be a
question of implementation, not a limitation of IP multicast.)
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3.1 Large-scale resource reservation in shared networks
The Resource reSerVation Protocol (RSVP) is aimed at providing setup
and flow-based information for managing information flows at pre-
committed performance levels. This capability is generally seen as
needed in real-time systems such as the HLA RTI. Concerns have been
raised about the scalability of RSVP, and also about its ability to
support highly dynamic flow control changes. In terms of existing
RTI capabilities, the requirement in LSMA is for rapid change of
group membership, not for rapid change of group reservations. This
is because in existing RTIs the aggregate requirement for all groups
in a large scale distributed simulation is static. However the
current RSVP draft standard for LSMA does not support aggregation of
reservation resources for groups of flows and therefore does not meet
the needs of existing RTIs. Moreover, there is at least one RTI
development underway that intends to use individual, dynamic
reservations for large numbers of groups, and therefore will require
a dynamic resource reservation capability that scales to thousands of
multicast groups.
Further, RSVP provides support only for communicating specifications
of the required information flows between simulators and the network,
and within the network. Distributing routing information among the
routers within the network is a different function altogether,
performed by routing protocols such as Multicast Open Shortest Path
First (MOSPF). In order to provide effective resource reservation in
a large shared network function, it may be necessary to have a
routing protocol that determines paths through the network within the
context of a quality of service requirement. An example is the
proposed Quality Of Service Path First (QOSPF) routing protocol
[ZSSC97]. Unfortunately the requirement for resource-sensitive
routing will be difficult to define before LSMA networks are deployed
with RSVP.
3.2 IP multicast that is capable of taking advantage of all common
link layer protocols (in particular, ATM)
Multicast takes advantage of the efficiency obtained when the
network can recognize and replicate information packets that are
destined to a group of locations. Under these circumstances, the
network can take on the job of providing duplicate copies to all
destinations, thereby greatly reducing the amount of information
flowing into and through the network.
When IP multicast operates over Ethernet, IP multicast packets are
transmitted once and received by all receivers using Ethernet-layer
multicast addressing, avoiding replication of packets. However,
with wide-area Asynchronous Transfer Mode (ATM), the ability to take
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advantage of data link layer multicast capability is not yet
available beyond a single Logical IP Subnet (LIS). This appears to
be due to the fact that (1) the switching models of IP and ATM are
sufficiently different that this capability will require a rather
complex solution, and (2) there has been no clear application
requirement for IP multicast over ATM multicast that provides for
packet replication across multiple LIS. Distributed simulation is
an application with such a requirement.
3.3 Hybrid transmission of best-effort and reliable multicast
In general the Internet protocol suite uses the Transmission Control
Protocol (TCP) for reliable end-to-end transport, and the User
Datagram Protocol (UDP) for best-effort end-to-end transport,
including all multicast transport services. The design of TCP is
only capable of unicast transmission.
Recently the IETF has seen proposals for several reliable multicast
transport protocols (see [Mont97] for a summary). A general issue
with reliable transport for multicast is the congestion problem
associated with delivery acknowledgments, which has made real-time
reliable multicast transport infeasible to date. Of the roughly 15
attempts to develop a reliable multicast transport, all have shown
to have some problem relating to positive receipt acknowledgments
(ACK) or negative acknowledgments (NAK). In any event, its seems
clear that there is not likely to be a single solution for reliable
multicast, but rather a number of solutions tailored to different
application domains. Approaches involving distributed logging seem
to hold particular promise for the distributed simulation
application.
In the DIS/HLA environment, five different transmission needs can be
identified:
(1) best-effort low-latency multicast of object attributes that often
change continuously, for example position of mobile objects;
(2) low-latency reliable multicast of object attributes that do not
change continuously but may change at arbitrary times during the
simulation, for example object appearance (An important
characteristic of this category is that only the latest value of
any attribute is needed by the receiver.);
(3) low-latency, reliable unicast of occasional data among arbitrary
members of the multicast group (This form of transmission was
specified for DIS "collisions"; it is not in the current HLA
specification but might profitably be included there. The
requirement is for occasional transaction-like exchange of data
between two arbitrary hosts in the multicast group, with a low
latency that makes TCP connection impractical.);
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(4) reliable but not necessarily real-time multicast distribution of
supporting bulk data such as terrain databases and object
enumerations; and
(5) reliable unicast of control information between individual RTI
components (this requirement is met by TCP).
All of these transmissions take place within the same large-scale
multicasting environment. The value of integrating categories (1) and
(2) into a single selectively reliable protocol was proposed by Cohen
[Cohe94]. Pullen and Laviano implemented this concept [PuLa95] and
demonstrated it within the HLA framework [PLM97] as the Selectively
Reliable Transmission Protocol (SRTP) for categories (1) through (3).
Category (4) could be supported by a reliable multicast protocol such
as the commercial multicast FTP offering from Starburst [MRTW97],
however adequate congestion control has not been demonstrated in any
such protocol. There has been some discussion of using the Real-Time
Streaming Protocol, RTSP, for this purpose, however as the databases
must be transmitted reliably and RTSP uses a best-effort model, it
does not appear to be applicable.
In summary, it is clear that a hybrid of best-effort and reliable
multicast (not necessarily all in the same protocol) is needed to
support DIS and HLA, and that the low-latency, reliable part of this
hybrid is not available in the Internet protocol suite.
3.4 Network management for distributed simulation systems
Coordinated, integrated network management is one of the more
difficult aspects of a large distributed simulation exercise. The
network management techniques that have been used successfully to
support the growth of the Internet for the past several years could
be expanded to fill this need. The technique is based on a primitive
called a Management Information Base (MIB) being polled periodically
at very low data rates. The receiver of the poll is called an Agent
and is collocated with the remote process being monitored. The agent
is simple so as to not absorb very many resources. The requesting
process is called a Manager, and is typically located elsewhere on a
separate workstation. The Manager communicates to all of the agents
in a given domain using the Simple Network Management Protocol
(SNMP). It appears that SNMP is well adapted to the purpose of
distributed simulation management, in addition to managing the
underlying simulation network resources. Creating a standard
distributed simulation MIB format would make it possible for the
simulation community to make use of the collection of powerful, off-
the-shelf network management tools that have been created around
SNMP.
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3.5 A session protocol to start, pause, and stop a distributed
simulation exercise
Coordinating start, stop, and pause of large distributed exercises is
a complex and difficult task. The Session Initiation Protocol (SIP)
recently proposed by the Multiparty Multimedia Session Control
(MMUSIC) working group serves a similar purpose for managing large
scale multimedia conferences. As proposed, SIP appears to offer
sufficient extensibility to be used for exercise session control, if
standardized by the IETF.
3.6 An integrated security architecture
It appears that this requirement will be met by IPv6 deployment. A
shortcoming of the current Internet Protocol (IPv4) implementation is
the lack of integrated security. The new IPv6 protocol requires
implementers to follow an integrated security architecture that
provides the required integrity, authenticity, and confidentiality
for use of the Internet by communities with stringent security
demands, such as the financial community. The possibility that the
IPv6 security architecture may meet military needs, when combined
either with military cryptography or government-certified commercial
cryptography, merits further study.
3.7 Low-latency multicast naming service
Name-to-address mapping in the Internet is performed by the Domain
Name Service (DNS). DNS has a distributed architecture tuned to the
needs of unicast networking with reliable transmission (TCP) that is
not considered problematic if its latency is on the order of a second
or more. The requirement of distributed simulation for agile movement
among multicast groups implies a need for name-to-multicast-address
mapping with latency of under one second for the name resolution and
group join combined. This problem has been circumvented in military
simulations by using group IP addresses rather than names. While
military simulations may be satisfied to communicate using a known
mapping from grid squares to multicast groups, growth of distributed
simulation into commercial entertainment cannot be based on such a
simple capability. The players in distributed entertainment
simulations will want to be organized symbolically by virtual world
and role. A low-latency multicast naming service will be required.
3.8 Inter-Domain Multicast Routing for LSMA
While military LSMAs typically take place within a single
administrative domain, future entertainment LSMAs can be expected to
involve heavy inter-domain multicast traffic so that players can be
supported by multiple service providers. Standardized protocols able
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to support large numbers of multicast flows across domain boundaries
will be needed for this purpose. Current work to create a Border
Gateway Multicast Protocol (BGMP) shows promise of meeting this need.
4. References
[CSTH95] Calvin, J., et. al., "STOW Realtime Information Transfer
and Networking Architecture," 12th DIS Workshop on
Standards for the Interoperability Distributed Simulations,
March 1995.
[Cohe94] Cohen, D., "Back to Basics," Proceedings of the 11th
Workshop on Standards for Distributed Interactive
Simulation, Orlando, FL, September 1994.
[DIS94] DIS Steering Committee, "The DIS Vision," Institute for
Simulation and Training, University of Central Florida, May
1994.
[DMSO96] Defense Modeling and Simulation Office, High Level
Architecture Rules Version 1.0, U.S. Department of Defense,
August 1996.
[IEEE95a] IEEE 1278.1-1995, Standard for Distributed Interactive
Simulation - Application Protocols
[IEEE95b] IEEE 1278.2-1995, Standard for Distributed Interactive
Simulation - Communication services and Profiles
[MRTW97] Miller, K., et. al. "StarBurst Multicast File Transfer
Protocol (MFTP) Specification", Work in Progress.
[Mont97] Montgomery, T., Reliable Multicast Links webpage,
http://research.ivv.nasa.gov/RMP/links.html
[PuLa95] Pullen, M. and V. Laviano, "A Selectively Reliable
Transport Protocol for Distributed Interactive Simulation",
Proceedings of the 13th Workshop on Standards for
Distributed Interactive Simulation, Orlando, FL, September
1995.
[PuWh95] Pullen, M. and E. White, "Dual-Mode Multicast: A New
Multicasting Architecture for Distributed Interactive
Simulation," 12th DIS Workshop on Standards for the
Interoperability of Distributed Simulations, Orlando, FL,
March 1995.
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[PLM97] Pullen, M., Laviano, V. and M. Moreau, "Creating A Light-
Weight RTI As An Evolution Of Dual-Mode Multicast Using
Selectively Reliable Transmission," Proceedings of the
Second Simulation Interoperability Workshop, Orlando, FL,
September 1997.
[SPW94] Symington, S., Pullen, M. and D. Wood, "Modeling and
Simulation Requirements for IPng", RFC 1667, August 1994.
[SSM96] Seidensticker, S., Smith, W. and M. Myjak, "Scenarios and
Appropriate Protocols for Distributed Interactive
Simulation", Work in Progress.
[ZSSC97] Zhang, Z., et. al., "Quality of Service Path First Routing
Protocol", Work in Progress.
4. Security Considerations
Security issues are discussed in section 3.6.
5. Authors' Addresses
J. Mark Pullen
Computer Science/C3I Center
MS 4A5
George Mason University
Fairfax, VA 22032
EMail: mpullen@gmu.edu
Michael Myjak
The Virtual Workshop
P.O. Box 98
Titusville, FL 32781
EMail: mmyjak@virtualworkshop.com
Christina Bouwens
ASSET Group, SAIC Inc.
Orlando, FL
EMail: christina.bouwens@cpmx.mail.saic.com
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