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A Taxonomy for Networked Virtual Environments
Michael R. Macedonia
Fraunhofer Center for Research in Computer Graphics
167 Angell St.
Providence, R.I. 02906
[email protected]
Michael J. Zyda
Computer Science Department
Naval Postgraduate School
Monterey, California 93943-5118 USA
[email protected]
The development of multi-user networked virtual worlds has become a major area of
interest in the computer and communications fields. However, there has been little effort to
provide a framework for understanding distributed virtual environments (VEs). In this pa-
per we discuss VEs in the context of how network communications, views, data, and pro-
cesses are distributed while emphasizing those aspects critical to scaling environments. We
find most of the systems described in this paper scale to accommodate a handful of users.
We also discuss why systems which demand strong data consistency, causality, and reliable
communications at the same time while supporting real-time interaction are not likely to
scale very well. Furthermore, if the systems are to be geographically dispersed, then high-
speed, multicast communication is required.
KEYWORDS: Virtual Reality, Distributed Interactive Simulation, Internet Protocol Multicast, Distributed
Interactive Entertainment, Large-scale Virtual Environments.
In this paper we discuss what needs to be considered when building large-scale VEs.
This frame of reference is necessary to have a common understanding of the many compo-
nents of distributed VE systems.
Currently, there are relatively few examples of academic VE systems in which to ap-
ply the framework. Research into large-scale distributed virtual worlds has been limited be-
cause of a number of practical factors. Immature network technology has relegated most
distributed VEs to Ethernet local area networks (LAN). Large-scale VEs will need to use
wide-area-networks (WANs) to expand both their physical geographic scope and number
of participating hosts. Furthermore, until recently, real-time graphics performance had been
confined to specialized and very expensive computer image generators. Software develop-
ment and graphics databases have also progressed slowly and the interfaces for immersing
the human into the environment have been primitive at best [9].
These problems are being overcome to a limited degree by the rapid growth of high-
speed internetworks, the availability of low-cost, off-the-shelf graphics workstations, and
the development of standard graphics tools and libraries. Moreover, distributed VE’s hold
the promise for new educational, training, entertainment, and medical applications. For in-
stance, over twenty companies, including Microsoft, are developing 3D computer gamin-
ing networks.
However, there are many challenges yet to be met that would appear trivial if these
applications were not distributed across wide-area networks.
Virtual environments mimic many aspects of operating systems. For example, the now
defunct system developed by the Human Interface Technology Laboratory, Virtual Envi-
ronment Operating Shell (VEOS), provided much of the functionality of a distributed op-
erating system in the way of programming language services [4]. However, we are
primarily concerned with a single application and not a general purpose computing envi-
ronment. Therefore, the most important questions about virtual environment software ar-
chitectures we address here are:
• What is distributed?
• What are the modalities of the distribution?
• Why is it distributed?
1. Network Communication
Several aspects of network communication are largely responsible for answering the
three questions above. The primary dimensions as shown for VEs are bandwidth, latency,
distribution schemes, and reliability (Figure 3).
a. Bandwidth
We pay particular attention to the effect of bandwidth in this paper because the
available network bandwidth determines the size and richness of a virtual environment. As
the number of participants increases so do the bandwidth requirements. On local area
networks (LANs), this has been not a major issue because technologies such as standard
Network Communication Issues For VEs
Broadcast, multicast or unicast
Lag, jitter
Acknowledgments, negative acknowledgments
Figure 3. Communication issues.
Ethernet (10 Mbps) are relatively inexpensive and the number of users for LAN-based VEs
has been limited. In contrast, for wide area networks (WANs) bandwidths have been
generally limited to T1 (1.5 Mbps) but the potential user base is much larger through the
However, networks are now becoming fast enough to be true extensions to the
computer’s backplane and for the development of distributed VR applications. Distributed
VR can require enormous bandwidth to support multiple users, video, audio and the
exchange of 3D graphic primitives and models in real-time. Moreover, the mix of data
requires new protocols and techniques to handle data appropriately over a network link.
The technologies providing these gains in performance blur the traditional distinction
between local area and wide area networks (LANs and WANs). There is also a convergence
between networks that traditionally carried only voice and video over point-to-point links
(circuit-switching) and those that handle packet-switched data.
The actual number of VEs to take advantage of these high speed networks have
been small and have been associated with Grand Challenge (high performance computing)
problems. The Multidimensional Applications and Gigabit Internetwork Consortium
(MAGIC) network is a gigabit-per-second Asynchronous Transfer Mode (ATM) network
that connects Minneapolis, Sioux Falls, Lawerence, Kansas, Kansas City and Ft.
Leavenworth, Kansas. MAGIC is designed to allow a commander to see three-dimensional
photo-realistic computer generated images of a very large area of interest in real time, both
from ground level and from the air, using data stored in a remote database. These images
will be generated from elevation data (Digital Elevation Maps), aerial photographs, models
of buildings, and models of vehicles whose positions will be updated in real-time via the
Global Positioning System. For example, a terrain database of Germany viewed in Kansas
on a workstation receives images from California which are texture mapped onto the terrain
in real-time [34]. The network provides trunk speeds of 2.4 Gbps and access speeds of 622
Mbps, allowing an application to use a supercomputer (CM-5) to process data from a
database at a second location, and display the results on a workstation at a third location.
The NASA Computational Aerosciences Project is planning to use high speed
networks to support visualization of large Computational Fluid Dynamics (CFD) data set
by distributing processing onto several supercomputers across the United States. Gigabit
networks will be required to move actual geometries generated by the supercomputers to
be rendered on a remote graphics workstation [14].
Similarly, the Electronic Visualization Laboratory at the University of Illinois has
used a combination of Ethernet, Fiber Distributed Data Interface (FDDI) and High-
Performance Parallel Interface (HPPI) networks to develop a distributed VE application.
The operator navigated through the VE using a CAVE (a system that projects images on
three walls or hemi-cube for simulating “walkthroughs”), which was connected to an
Silicon Graphics workstations used for rendering and control which in turn was connected
to a CM-5 used for the actual simulation [26].
Network Network
Broadcast Multicast
Figure 4. Examples of broadcast, multicast, and unicast.
b. Distribution
Some distribution schemes scale better than others. Three methods are shown in
Figure 4. Multicast services allow arbitrarily-sized groups to communicate on a network
via a single transmission by the source [19]. Multicast provides one-to-many and many-to-
many delivery services for applications such as teleconferencing and distributed simulation
in which there is a need to communicate with several other hosts simultaneously. For
example, a multicast teleconference allows a host to send voice and video simultaneously
to a set of (but not necessarily all) locations. With broadcast, data is sent to all hosts while
unicast or point-to-point establishes communication between two hosts.
Most distributed VEs have employed some form of broadcast -- hardware-based
or via the Internet Protocol (IP) -- or point-to-point communications. For example, the MR
Toolkit Peer Package, which is used for creating distributed virtual reality applications over
the Internet, uses unicast for communications among the applications though the
developers have considered using IP Multicast [29]. Unicast is also the general approach
for Grand Challenge applications like MAGIC. Another example is the NASA discussed
above in which the network is a logical part of the visualization system much in a manner
analogous to traditional image generators.
However, these schemes are bandwidth inefficient for large groups. Furthermore,
broadcast, which is used in SIMNET -- an early military SIMulator NETwork developed
by Bolt, Beranek, Newman (BBN) and Delta Graphics -- and most military distributed
interactive simulation (DIS) implementations, are not suitable for internetworks because
the network becomes flooded with unwanted traffic and it is difficult to avoid routing loops.
Moreover, IP broadcast requires that all hosts examine a packet even if the information is
not intended for that host, incurring a major performance penalty for that host because it
must interrupt operations in order to perform this task at the operating system level.
(SIMNET uses the hardware multicast capability of Ethernet but only to create a single
multicast group for the entire distributed simulation.) Point-to-point requires the
establishment of a connection or path from each node to every other node in the network
for a total of N*(N-1) virtual connections in a group (see Figure 5). For example, with a
1000 member group, each of the 1000 individual hosts would have to separately address
and send 999 identical packets.
Some researchers have proposed different ideas for using multicast to support virtual
environments. The partitioning of virtual worlds into spaces is a common metaphor for
VEs. Mitsubishi Electric Research Labs (MERL) have developed the Spline VE architec-
ture. Spline uses multicast peer-to-peer communication. It also incorporates a region-based
filtering scheme. These regions or “locales” partitioning the VE while “beacons” provide
an entity updates about other objects in the VE [44].
Multi-User Dungeons (MUDs) have used the idea of spatialization. The Jupiter from
Xerox PARC has extended the concept to associating “rooms” with multicast video and au-
dio teleconferences [42]. Also at Xerox, Schilit and Theimer developed an active map ser-
vice (AMS) that publishes the location of objects in a region using dynamic multicast
groups associated with different parts of the region. For example, the system can track per-
player player
player player
Figure 5. Distributed model.
sons in a building via the use of active badges. Using multicast for updates reduces aggre-
gate message traffic [30].
Benford has described a concept for the spatial interaction of objects in a large-scale
VE [2]. The spatial model uses different levels of awareness between objects based on their
relative distance and mediated through a negotiation mechanism. The Swedish Institute of
Computer Science’s Distributed Interactive Virtual Environment (DIVE) -- an advanced ex-
perimental VE -- implements this concept using “standard VR collision detection” to de-
termine when the transitions between awareness levels should occur [7].
Others have suggested using multicast for DIS but, very few have actually conducted
research or implemented VEs using multicast communications. Stanford Research Institute
recommended multicast in an early 1990 White Paper and it has been recommended for
IEEE 1278 standards group [33].
Van Hook at the MIT Lincoln Laboratories has also proposed using a combination of
grid-filtering to reduce the computational requirement of object filtering, an O(n2) opera-
tion [39]. Van Hook also suggested the idea of on-demand forwarding in which entities
would send a low-rate broadcast with terse state information. Each receiver would compute
a range check and send state data to the visible entities. However, object-filtering and on-
demand forwarding essentially establish a multicast group for every receiver. For example,
in Figure 6, Entity 1 and 2 join each other’s multicast groups. Entity 3 is outside the range
of 1 and 2 and therefore is a member of only its own group.
Until 1994, there was reluctance in the DIS community to use IP Multicast because of
the Defense Advanced Research Projects Agency (DARPA) support for other network
technologies, the lack of a software architecture and algorithms that could exploit it, and
limited hardware support. The status of IP Multicast in the DIS world has changed. It is now
part of the standard. Moreover, Mark Pullen at GMU and others have suggested using a
two-level architecture using IP Multicast mapping to ATM multicast facilities [23].
Multicast Group 1
Multicast Group 2
Entity 1
Entity 2
Multicast Group 3
Entity 3
Figure 6. Object-based filtering.
c. Latency
Another dimension of communication is latency which controls the interactive
and dynamic nature of the virtual environment -- how well the players mesh in behavior. If
a distributed environment is to emulate the real world, it must operate in real-time in terms
of human perception. A key challenge is that the appropriate systems involving human
operators must deliver packets with minimal latency and generate textured 3D graphics at
30-60 Hz to guarantee the illusion of reality [41]. On top of this is the need to provide real-
time audio, video, and imagery services for the simulation of player communication
Latency is a problem for network cue correlation. Sawler notes that both the delay
of an individual cue (e.g., seeing an object move) and the variation in the length of the delay
are important, particularly in closely coupled tasks which require a high degree of
correlation (e.g., flying in formation) [27]. This becomes a major challenge in systems that
use wide area networks because of delays induced by long paths, switches and routers.
Network latency can be reduced to a certain extent by using dedicated links (or virtual ones
using protocols like the Reservation Protocol [43]), improvements in router and switching
technologies, faster interfaces and computers.
However, more bandwidth is not necessarily a complete solution. Operating at
gigabit speeds presents a new set of problems. New methods of handling congestion are
required because of the high ratio of propagation time to cell transmission time [13]. By the
time that a computer in New York sends a message telling a host in San Francisco to stop
sending data, it is too late to have stopped a gigabit worth of information from being
The bottlenecks will most likely be in the network interfaces, memory
architectures and operating systems of the computers on either end. The slow progress in
increasing the interface performance of FDDI is an example of the lag in technologies we
will probably see as high speed networks are fully deployed. Nor have memory speeds kept
up with the leaps made in CPU and network performance. At the operating system level,
most VR applications are built on commercial versions of UNIX which are not designed
for real-time performance.
Other methods are available for ameliorating the effects of latency. BBN
developed dead-reckoning techniques for SIMNET which reduces communications loads
on the network and perceived delays because of predictive modeling by the local host [15].
Briefly, a local entity passes state vectors to remote simulations. Both the local and remote
simulations model the probable path of the entity. The local entity sends update state
vectors when its current location or orientation exceeds some predetermined error threshold
from the modeled path. Singhal has proposed a new dead-reckoning method that exploits
position history [32].
However, lag can never be totally eliminated and for environments where the VE
is widely distributed (e.g. Earth to Mars). Therefore, techniques such as synthetic fixtures
are used which provide force and visual clues to operators in limited domains about that
environment [28].
d. Reliability
Finally, communications reliability often forces a compromise between
bandwidth and latency. Reliability means that systems can logically assume that data sent
is always received correctly, thus obviating the need to periodically re-send the
information. Unfortunately, to guarantee delivery, the underlying network architecture
must use acknowledgment and error recovery schemes that can introduce large amounts of
delay - a common case on WANs and with large distributed systems. Additionally, some
transport protocols such as the Transport Control Protocol (TCP) use congestion control
mechanisms that are unsuitable for real-time traffic because they throttle back the packet
rate if congestion is detected.
Reliable multicast protocols are currently not practical for large groups because
in order to guarantee that a packet is properly received at every host in the group, an
acknowledgment and retransmission scheme is required [18]. With a large distributed
simulation, reliability, e.g., as provided in TCP, would penalize real-time performance
merely by having to maintain timers for each host’s acknowledgment and by holding up
flow when a packet is lost for retransmission. Flow control introduces delay to the network
to reduce congestion. Therefore, it is also not appropriate for DIS which can recover from
a lost packet more gracefully than from late arrivals -- it is impossible for real-time
simulations to go backward in time. For example, when a packet is lost the receiving host
notifies the sender, possibly invalidating a number of packets already sent because of
propagation and network processing delay. The sender must retrieve a copy of the packet
lost and retransmit it. This also affects the windowing behavior which in turn slows
However, researchers are trying to develop both a reliable and scalable multicast
service. The ISIS system, developed by Ken Birman, uses a reliable multicast service to
guarantee that the virtual environment databases are accurately and synchronously
replicated [7]. (A recent version of ISIS implements a reliable transport layer on top of IP
Multicast). However, a peer group with more than twenty or thirty members is about as
large as can be efficiently supported by ISIS.
Brian Whetten and Simon Kaplan developed the Reliable Multicast Protocol
(RMP) which is based on a token ring protocol that sits atop IP Multicast. This method uses
sequencing and negative acknowledgments (NACKs)[40]. The problem with this method
is the potential for NACK implosions, in which a group of receivers simultaneous send
NACKs, adding to congestion and consequently causing the loss of more packets which
introduce more NACKs. Again, reliable systems are not likely to operate in real-time. As
Partridge in [18] states, “the problem of reliable multicasting over internets has not been
Netrek, an Internet multiplayer game that uses X Window system graphics, took
the approach of using different degrees of reliability to gain better real time performance
[16]. (Mark Pullen of GMU has suggested a similar concept for DIS called the Selective
Reliability Transport Protocol [22].) Previous versions of the game used TCP. New
versions have a protocol that:
• guarantees the reliability of certain packets with TCP such as error conditions
and session setup and for information that is sent infrequently (server message
of the day)
• does not guarantee reliability for frequent and noncritical data such as player
state (speed, direction)
• allows switching on demand from TCP to UDP/TCP and back
• won’t hang or cause abnormal termination if a UDP packet is lost
Views are the windows into the virtual environment from the perspective of the people
or processes who use it. We define two kinds of useful views for distributed environments.
The first one is the synchronous view. An example of this is in a distributed flight simulator
where one machine controls the forward image, and two other hosts each process the left
and right cockpit window perspectives. The images are coordinated to give the illusion that
they are all part of single cockpit view. Synchronism requires both high reliability and low
latency. Therefore, virtual environments that require synchronous views are for practical
reasons restricted to local area networks. An example of such an environment is the
RAVEN simulator developed by Southwest Research Institute for NASA synchronizes
shuttle astronaut viewpoints which are rendered on different machines to improve render-
ing performance [8]. The CAVE uses a similar approach to synchronize each image frame
projected on the screen of a hemi-cube with the added component of synchronizing the sim-
ulation run separately on a CM-5. Originally, this was done using a SCRAMNET (propri-
etary fiber optic, shared memory LAN). Later, this was accomplished using multiple raster
managers on an SGI Reality Engine Onyx and shared memory.
ghost ships in wire frame (controlled by player Y has a different “live” ship
other player updates and dead reckoning) than player X
live ship (controlled by this player)
Figure 7. Two views of the simulation.
Synchronous views are also important for computer-aided design and systems used for
concurrent engineering. The Fraunhofer Institute for Computer Graphics has developed a
virtual prototyping environment that implements a shared-3D CAD viewer [45]. The VE is
designed to demonstrate the ability of engineering teams to work globally via the Fraun-
hofer Transatlantic Research and Development Environment (TRADE).
The second and most general concept is the asynchronous view. In this paradigm, mul-
tiple users have individual control over when and what they can see in the virtual environ-
ment concurrently (Figure 7). Participants can be physically separated over a local area
network or a wide area network. Their awareness of each other’s presence, if they are rep-
resented by an object, is brought about inside the virtual environment. The Naval Postgrad-
uate Schools VE, NPSNET, supports the DIS protocol and uses the asynchronous model
where each view is typically that of the simulated entity [46]. Views not associated with an
entity are often referred to as “magic carpets” or “stealth” vehicles. Stealth entities only
“listen” to the distributed world traffic because there is no need for the world to have
knowledge of the viewer. Large-scale VEs will use asynchronous views because of the cost
of synchronization over wide-area networks. Synchronous views will be important for
small VEs in which precise cooperative manipulation of objects is required and for appli-
cations or device communication distributed over LANs.
3. Data
Perhaps the most difficult decision of building a distributed environment is determin-
ing where to put the data relevant to the state of the virtual world and its objects. These de-
cisions affect the scale, communication requirements, and reliability of the VE data. For
example, a real-time system requiring strong consistency will be inherently difficult to
scale because of the need for causality and automaticity. For now, at least, large VEs only
allow weak consistency among group members.
Data Models for VEs
• Replicated homogeneous world database
• Shared, centralized
• Shared, distributed, peer-to-peer
• Shared, distributed, client-server
Figure 8. Data models for VEs.
There are many conceivable ways of distributing persistent or semi-persistent data
(see Figure 8). We present some of the most prevalent methods in current VEs:
a. Replicated homogeneous world
A common method for large VEs is to initialize the state of every system
participating in the distributed environment with a homogeneous world database
containing information about the terrain, model geometry, textures, and behavior of all that
is represented in the virtual environment. Communicated among all the users of the
environment are object state changes such as vehicle location or events such as the
detonation of a simulated missile or collisions between two objects. The advantage of this
approach is that messages are relatively small. The disadvantages are that it is relatively
inflexible and that as virtual environment content increases so must everyone’s database.
Moreover, over time, the world becomes inconsistent among the participants through the
loss of state and event messages. This is the model for SIMNET. However, once a
simulation begins, each host maintains its own database without making any effort at
guaranteeing consistency except through the use of “heartbeat” messages and event updates
b. Shared, centralized databases
On the other hand, the Virtual Space Teleconferencing System (VISTEL) uses a
shared world database. As its name implies, VISTEL is a teleconferencing system that
displays 3D models of each conference participant. Changes in a model’s shape, reflecting
changes in a person’s facial expression, are sent via messages to a central server and
redistributed. Only one user at a time can modify the database (see Figure 9)[17].
This is the model used by MUDs except that they typical employ relatively
primitive clients. Text-based MUDs use TCP connections to a central server that does
almost all the computation and maintains the state of the VW. For text communication, this
typically scales to about fifty concurrent users who “move” about among rooms, create and
delete new objects or actions, and communicate with each other. LamdaMOO from Xerox
Palo Alto Research Center is probably the most advanced MUD [42]. Using a centralized
server for 3D virtual worlds is obviously limited to a few participants because of input/
output (I/O) contention, and maintaining a dynamic object database in real-time.
Figure 9. Centralized model.
The I/O problem is demonstrated in Netrek which scales to about 18 players with
UDP and uses an asymmetric communications model [16]. The data in Figure 10 from J.
Mark Noworolski shows how the server becomes the bottleneck because it must retransmit
all other players’s state to each client. In this case communication from individual clients
are only 168 bytes per second. The server, on the other hand, must take every client
message and redistribute to it to all the other clients with an order of magnitude increase in
bandwidth required.
Server -> Client network usage:
Maximum CPS during normal play: 3588 bytes per second
Standard deviation: 918
Total bytes received 1795888, average CPS: 803.0
Client -> Server network usage:
Maximum CPS out during normal play: 168 bytes per second
Standard deviation out: 21
Total bytes sent 20580, average CPS: 18.0
Figure 10. Server vs. client communication in Netrek.
c. Shared, distributed databases with peer-to-peer updates
Many distributed systems strive to simulate shared memory architectures. For
example, DIVE has a homogeneous fully-replicated distributed database. However, unlike
SIMNET the entire database is dynamic and uses reliable multicast protocols to actively
replicate new objects. A disadvantage with this approach is that it is difficult to scale up
because of the communications costs associated with maintaining reliability and consistent
data across wide area networks. Modeling complex or dense objects, such as constructing
a large CAD model or changing a terrain database, is very expensive (though highly
desirable) in terms of the number of polygons that might be created, changed, and
communicated over a network.
Virtual environments that use Linda, the parallel programming language, also
trade performance for a relatively simple blackboard programming model. For example,
Denis Amselem of SRI developed a VE using Linda with an unusual hand-held interface -
- a portable LCD television with a space tracker for VE navigation. Performance of the
multiuser system limited it to three participants [1]. The simplicity and illusion of shared
memory presented by Linda is also the reason why this system suffered from poor
performance. Data must reside somewhere. In this case, it was on a central server.
d. Shared, distributed, client-server databases
Another technique is to use a variant of the client-server model in which the
database is partitioned among clients and communication is mediated by a central server.
For example, in BrickNet, as an entity moves through the virtual environment, its database
is updated by an object-request broker on a server that has knowledge of which client
maintains that part of the world [31]. BrickNet may be most appropriate for large CAD
environments because it attempts to tackle the walkthrough problem of a virtual
environment that has huge numbers of component models and provides multiple views
simultaneously to a group of users. However, in a dynamic large scale world, the servers
can quickly become I/O bottlenecks, increasing the inherent latency of the virtual
In a similar approach to BrickNet and DIVE, the Model, Architecture and System
for Spatial Interaction in Virtual Environments (MASSIVE) system uses a spatial model
for data partitioning among clients. In this case, an entity declares its world to a local “aura”
manager which in turn informs other aura collision managers. These managers broker
between objects by detecting proximal collisions and informing each of the peer entities of
mutual interface references[12].
Pure client-server systems that strictly use classic remote procedure calls (RPC)
do not scale well for a number of reasons. Partridge points out that the RPC is poorly suited
for high speed networks because communication is achieved by sending a message and
waiting for a reply. As relative delay of networks increases, RPCs become expensive [18].
4. Processes
Distributing processes to multiple hosts increases the aggregate computing power as-
sociated with a simulation. We can use this not only to provide the capability to distribute
views but also handle a variety of input devices. SIMNET and its descendants, such as DIS-
compliant systems, also make use of the aggregate computing power by taking advantage
of dead-reckoning to reduce the need for network communication.
With the exception of the DIS model, practically all distributed environments assume
that the same kind of processes are running on each host that has the same function (archi-
tectures may differ). The advantage of this approach is consistency. The disadvantage is
that it is very inflexible. DIS is a protocol designed so that different developers can create
different simulations on different machines that theoretically can share in the same virtual
environment because they can communicate at some common level. The problem with this
is that no protocol is complete and DIS is not an exception. For example, new objects can-
not be introduced without a change in the standard.
The AVIARY system has homogenous processes but contains Object Servers which
permits migration of lightweight objects to enable load balancing. These objects represent
the entities and the processes which control them. DIVE uses the concept of process groups
from ISIS to partition the VE into rooms or spatial regions. The MR Toolkit distributes pro-
cesses that support different components of the VE such as the input devices [29]. It pro-
vides an interpreted language, the Object Modeling Language (OML) that allows platform
independence for developing virtual environments. OML specifies the behavior and geom-
etry of VE entities. Similarly, BrickNet uses a language called Starship. BrickNet objects
can share or transfer behaviors that are specified in Starship. These behaviors are either en-
vironmentally-dependent, reactive, or capability based.
Other more general-purpose scripting languages may provide the capability to migrate
processes and objects across diverse platforms by using active messaging. Sun’s Java is the
primary example of this class. A byte-compiled, interpretive language, similar to C++, Java
is meld to the client-server architecture of the World Wide Web. Java also supports peer IP
Multicast communication and in the future a host of multimedia components.
Gavin Bell and Tony Parisi of SGI have developed the Virtual Reality Modeling Lan-
guage (VRML) which “is a language for describing multi-participant interactive simula-
tions -- virtual worlds networked via the global Internet and hyperlinked with the World
Wide Web” [20]. However, VRML merely describes a 3D scene and methods for interact-
ing with models. Though VRML 2.0 allows the use of Java to provide object behaviors,
VRML itself does not provide a mechanism for communication among distributed users.
The proprietary Telescript language is an innovative communication technology that
-- like Java -- also considers the network as a platform, on top of which you can run appli-
cations that are not bound to a specific node of the network, but, to the contrary, are intend-
ed to move around the network during their execution. Telescript itself is an interpreted
language that is specifically designed for communication. It provides primitives that allow
the script to suspend, migrate to another node of the network, and resume execution from
the same point. The key idea is procedural messaging. With Telescript, write agents are sent
around the network to accomplish the tasks you want. Instead of having a client dealing
with a server by means of a set of messages sent back and forth, you build an agent and send
it where the server is. The agent is smart enough to interact with the server, and returns to
the sender with the required information. In this way, bandwidth consumption is reduced
and we can build agents that seek information on our behalf [24].
Safe-Tcl is a language for active or agent-based mail in which the data delivered
through the mail constitute a program in a well- specified language, allowing the program
to be automatically evaluated on behalf of the recipient when the mail is “read.” The syntax
of Safe-Tcl is identical to the syntax of Tcl [3]. No syntactic constructs are changed. The
only difference, therefore, between Tcl and Safe-Tcl is the set of available primitive func-
tions and procedures. Safe-Tcl may be described as an “extended subset” of Tcl, in that the
“dangerous” primitives in Tcl have been removed, while certain new primitives have been
added. An example of a dangerous primitive is an exec call in Tcl which can start up a new
process. Eliminating this from Safe-Tcl helps avoid the possibility that a script could gen-
erate unwanted behavior by the receiving host. DIVE uses Tcl.
In a distributed VE, clients can be homogeneous as is the case for DIVE. Clients
can also be dissimilar except for the communications protocols among them, providing
interoperability (e.g., DIS and SIMNET systems, which exchange standard state and event
data). Furthermore, processes can be designed to migrate across homogeneous
architectures like AVIARY. New scripting languages like Safe-Tcl offer the opportunity
for migrating processes across heterogeneous systems, therefore permitting efficient
exchange of object behaviors as well as entity state within large, heterogenous VEs.
We have discussed VEs in t

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