Architectures consisting of multiple cache servers are a popular solution to deal with performance and network resource utilization issues related to the growth of the Web request. Cache cooperation is often carried out through purely hierarchical and flat schemes that suffer from scalability problems when the number of servers increases. We propose, implement and compare the performance of three novel distributed cooperation models based on a two-tier organization of the cache servers. The

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... rimental results show that the proposed architectures are effective in supporting cooperative document lookup and download. They guarantee cache hit rates comparable to those of the most performing protocols with a significant reduction of the cooperation overhead. Moreover, in case of congested network, they reduce the 90-percentile of the system response time up to nearly 30% with respect to the best pure cooperation mechanisms.