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.

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While a lot of researches focused on how to efficiently spread the offered network load on the available cluster resources, less interest has been granted to the impact of the used mechanisms on the reliable execution of the upper layer services. On the other hand, emerging NGN services as well as some of the already familiar services involve multiple flows during the lifespan of a single endto-end session, hence, raising the challenge of session awareness while processing the incoming network traffic.In this paper, we grasp the need for fine grained session awareness to efficiently allocate the cluster resources to the offered network traffic. The analysis of load balancing scenarios of some representative IP services provides us with solid reasons to use deep packet inspection to achieve fine grained network traffic load distribution, and to meet NAT and firewall traversal constraints as well.

In this paper, we investigate an economically motivated session-aware admission-control model for Internet servers subjected to future NGN session-oriented services. Our fundamental observation is that it is sometimes desirable to reject new sessions from some customers, so that others may be completed and thereby generate some revenue for the operator. Conversely, most of the already existing admission-control schemes, which operate either at the packet level or at the request level, may interrupt life-service sessions at any time during their lifespan. In this work, we evaluate novel service-oriented sessionaware admission-control strategies for controlling the acceptance of the offered network traffic to an Internet server. Conducted simulations target popular VoIP service. Simulation results demonstrate that responsive session-aware admission control improves the service provider's benefit and offers better QoS to subscribers. Particularly, we show that session-aware responsiveness provides network QoS in terms of completion of the service sessions independently of their durations.

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