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Fault-tolerant frameworks provide highly available services by means of fault detection and fault recovery mechanisms. These frameworks need to meet different constraints related to the fault model strength, performance, and resource consumption. One of the factors that led to this work is the observation that current fault-tolerant frameworks are not always adapted to existing Internet services. In fact, most of the proposed frameworks are not transport-level- or session-level-aware, although the concerned services range from regular services like HTTP and FTP to more recent Internet services such as multimodal conferencing and voice over IP. In this work we give a comprehensive overview of fault tolerance concepts, approaches, and issues. We show how the redundancy of application servers can be invested to ensure efficient failover of Internet services when the legitimate processing server goes down.