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Summary form only given. One common problem with present P2P systems is the dynamic membership of the peer hosts. The arbitrary joining and leaving of the peers make it difficult to maintain the overlay structure of the system and cause large amount of nonproductive exchange of protocol overhead (e.g., updating finger entries to adjust the structure). We present a novel, decentralized and fault tolerant protocol called Alpha-Beta cluster-based protocol (ABC). With the new scheme, the function of a node is undertaken by a cluster of hosts; moreover, the size of a cluster can vary between an upper bound (Alpha) and a lower bound (Beta). This helps maintain the stability of the system even if nodes join or leave the system dynamically; it also retains the query response efficiency. With each node maintaining at most 2logn+2logn/loglogn logical links, ABC can achieve each query within at most 2logn/loglogn hops. In addition, we present a modified ABC which reduces the number of hops to at most logn/loglogn with the same number of logic links. The two protocols offer provably better performance than existing protocols while solving the stability problem.