\section{Conclusions}
\label{sec-concl}

We presented {\kf}, an infrastructure which supports building
Web-based parallel and collaborative applications.  Within the context
of current research efforts, we pointed out a set of problems and
challenges which application programmers face when using Java in
combination with Web browsers and their ability to load and execute
untrusted applets in a secure fashion.  We described the three
services provided by {\kf} addressing these issues: {\kfdirectory}
which makes it possible for users using Web browsers to find other
members of a distributed session; {\kfserver} which allows users to
start a distributed computation on any host; and {\kfapplet} to allow
direct applet-to-applet communication without the need for a
forwarding agent.  We also argued how parallel programming and
collaborative systems can benefit from using {\kf} as an underlying
layer.  We demonstrated this by utilizing {\kf} to extend Charlotte, a
parallel programming environment, as well as by building a
collaborative application.

Currently, {\kf} lacks a clean solution to inform applets when a
member of the session leaves voluntarily, for example when a user
clicks on her browser's {\tt back} button, or involuntarily because of
a failure.  Future work includes implementing a set of high-level
services to support parallel and collaborative applications
concurrently, as well as addressing the issue of fault propagation. We
are also looking at ways of supporting inter-applet communication in a
secure fashion. 


\subsection*{Acknowledgements}

We are thankful to Ulrich Gall, Li Gong, Alain Mayer, Fabian Monrose,
Toto Paxia, Avi Rubin, Peter Wyckoff, Leonid Zheleznyak, and the
anonymous referees for helpful comments and for bringing the security
concerns to our attention.

This research was sponsored by the Defense Advanced Research Projects
Agency and Rome Laboratory, Air Force Materiel Command, USAF, under
agreement number F30602-96-1-0320; by the National Science Foundation
under grant number CCR-94-11590; and by the Intel Corporation. One of
the authors was supported by Deutsche Forschungsgemeinschaft.

The U.S. Government is authorized to reproduce and distribute reprints
for Governmental purposes notwithstanding any copyright annotation
thereon.

The views and conclusions contained herein are those of the authors
and should not be interpreted as necessarily representing the official
policies or endorsements, either expressed or implied, of the Defense
Advanced Research Projects Agency, Rome Laboratory, or the
U.S. Government.

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