Winnowing: Protecting P2P systems against pollution through cooperative index filtering

Pollution (i.e., sharing of corrupted files, or contaminating index information with bogus index records) is a de facto problem in many file sharing peer-to-peer (P2P) systems in use today. Pollution squanders network resources and frustrates users with unprofitable downloads (due to corrupted files) and unproductive download trials (due to bogus index records). In this paper, we propose a novel distributed hash table (DHT)-based anti-pollution scheme called winnowing. Winnowing aims to reduce or eliminate decoy index records (pointing to nonexisting or corrupted files) held by DHT (i.e., index) nodes in the system, so that download attempts based on the remaining (clean) index records are more likely to yield satisfactory results. To achieve this goal, two techniques are used: (1) publish verification is performed by index nodes to counteract index pollution and (2) privacy-preserving object reputation is integrated into the DHT to reduce the impact of content and metadata pollution. By integrating these techniques, winnowing converges quickly to a near-optimal solution. Winnowing has the added benefit that it does not reveal a peer's download history to other downloading peers.The publish verification of winnowing has been implemented on top of the latest eMule client, and extensive data has been collected from the Kad network using this modified client. The measurement results are summarized, and the findings from the measurement study are incorporated into an analytical model. The model demonstrates the robustness of the privacy-preserving object reputation of winnowing to a variety of pollution attacks, and to attacks on winnowing itself. The results of analysis are confirmed by means of event-driven simulations.