RMultiChat is a program that is designed to run on a LAN, where you can chat with anyone that has the program.
There is no central server, because dynamic group management is used. It uses Reliable Multicasting for broadcasting messages, which is one of the key algorithms we studied.
Either way... Nothing too interesting unless you want to learn how to use multithreaded tcp/udp sockets in C#.
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(Requires .Net framework or Mono)