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The aims of the research are to investigate techniques that support the development of highly dependable applications in a distributed system environment. Techniques we developed include redundant task allocation, load balancing, fault-tolerant computing and communication. The application we have implemented in the system is a firewall application. The firewall runs in redundant mode. Each incoming or outgoing packet is checked by two or more copies of the firewall application. Only when the majority of the firewall copies decide to accept the packet, the packet can go through the firewall. Different decisions from the different firewall copies signify a possible hardware fault or a software error in the underlying system. This paper reports a recent implementation of a simulation system, in which computing nodes, redundant copies of tasks and packet queues are implemented as independent threads, as well as experimental result we obtained based on queuing models.