Papers in PDF format

IP Multicast messages do not work over long distances, because most routers are configured to filter out
multicast messages.
2. Fault tolerance: If one IRC server is down switching to a different one is done automatically. The IRC
network itself has several mechanisms to cope with network interrupts, unreachable servers and other real
world trouble.
3. Easy startup: No special service providers or new server programs are necessary. Start up can be done by
simply providing a new VRML browser and using existing IRC servers. Even if there is a need to create
some new servers, providers will be much more willing to install an IRC server, which uses technology that
exists for quite a while and is well tested, than a brand new experimental server with unknown safety hazards.
The only drawbacks using the IRC to implement a multi user virtual reality are the limited message length
(messages are limited to 511 bytes), the delays introduces by IRC servers with enabled ‘flood control’ which allows
only one message per 2 seconds, and the difficulties in implementing semaphores (although it can be done).
4 Conclusion
We have presented a proposal to introduce multi-user interaction into VRML worlds which is based on existing
IRC server technology to handle the communication aspect and on VRML as the presentation level protocol.
The implementation of this approach together with several new features improving the way users can navigate
through threedimensional scenes is available through MRTSpace, a stand-alone VRML browser built as a platform
to implement extensions to the VRML standard and to experiment with new features of user interaction.
References
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Acknowledgment
We are grateful to all our colleagues and students contributing to the development of MRT which is the result
of a collaborative effort in the Computer Graphics Lab of the University of Bonn (see http://hyperg.cs.unibonn.de/CompGraph).
This particular work builds extensively on the contributions by H. Bendels, M. Fischer,
A. Hopp, G. Müller, and S. Schäfer.