Network coding based peer-to-peer streaming represents an effective solutionto aggregate user capacities and to increase system throughput in livemultimedia streaming. Nonetheless, such systems are vulnerable to pollutionattacks where a handful of malicious peers can disrupt the communication bytransmitting just a few bogus packets which are then recombined and relayed byunaware honest nodes, further spreading the pollution over the network. Whereasprevious research focused on malicious nodes identification schemes andpollution-resilient coding, in this paper we show pollution countermeasureswhich make a standard network coding scheme resilient to pollution attacks.Thanks to a simple yet effective analytical model of a reference nodecollecting packets by malicious and honest neighbors, we demonstrate that i)packets received earlier are less likely to be polluted and ii) shortgenerations increase the likelihood to recover a clean generation. Therefore,we propose a recombination scheme where nodes draw packets to be recombinedaccording to their age in the input queue, paired with a decoding scheme ableto detect the reception of polluted packets early in the decoding process andshort generations. The effectiveness of our approach is experimentallyevaluated in a real system we developed and deployed on hundreds to thousandspeers. Experimental evidence shows that, thanks to our simple countermeasures,the effect of a pollution attack is almost canceled and the video qualityexperienced by the peers is comparable to pre-attack levels.
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