Systems consisting of multiple proxy servers are a popular solution to deal with performance and network resource utilization problems related to the growth of the Web numbers. After a first period of prevalent enthusiasm towards cooperating proxy servers, the research community is exploring in a more systematic way the real benefits and limitations of cooperative caching. Hierarchical cooperation has clearly shown its limits. We study the scalability of traditional protocols (e.g., directory-based, query-based) in flat architectures through different performance metrics and experiments using both synthetic workloads and traces. The synthetic workload is also used for sensitivity analysis with respect to various parameters while traces are used for validating our observations in a more realistic scenario. We show that ICP has a better hit rate than that of Cache Digests, but the latter has a much smaller overhead, thus making the choice between the two protocols a challenge depending ...