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Peer-to-peer file-sharing networks depend on peers uploading data to each other. Some peers, called free-riders, will not upload data unless there is an incentive to do so. Algorithms designed to prevent free-riding typically assume that connectivity is not a problem. However, on the Internet, a large fraction of the peers resides behind a firewall or NAT, making them unable to accept incoming connections. This paper proves that it is impossible to prevent free-riding when more than half of the peers are firewalled, and the paper will provide bounds on the sharing ratios (defined as the number of bytes uploaded divided by the number of bytes downloaded) of both firewalled and non-firewalled peers.
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