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.

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.

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Internet server clustering has been widely used by operators to improve the scalability and the availability of the rendered services under heavy load condition. Load balancing is a well known mean to optimize the usage of the cluster available resources by fairly allocating them to the offered network traffic. Most state-of-the-art research advocate either session oblivious or coarse grained session aware load balancing architectures. While guaranteeing session integrity, the coarse grained session awareness leads to an unfair allocation of the cluster resources. In this work, we advocate an innovative fine grained session aware load balancing architecture of an offered IP traffic to a cluster of Internet servers. The proposed architecture aims to maximize the cluster useful throughput in terms of completed sessions per unit of time. It provides high availability capabilities in case of failure of the legitimate entry point to the cluster. Finally, it is fully client transparent and is open to adapt to any multiple-flow based NGN service such as voice over IP or video streaming.

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