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Can identical BitTorrent peers experience different download times ?

Understanding and characterizing the performance of BitTorrent (BT) through mathematical models has been an active topic of research mainly due to its high complexity and tremendous practical efficiency. One of the first BT models, presented in [3], predicts the download rates of homogeneous peers. More recently, BT models to capture heterogeneous peer population with respect to their upload and download capacities have also been proposed, as well as other relevant system characteristics [2, 1]. However, all models predict that identical peers (with respect to their upload capacities) will have identical performance (with respect to download times). We consider a BT swarm where all peers have identical upload capacities but unconstrained (or large) download capacities. In this context, we identify and characterize a phenomenon that has not been previously observed: homogeneous peers experience heterogeneous download rates. This is surprising as peers are identical and it has not been captured by any prior model. Moreover, this behavior has several negative impacts, including high variability of download times, unfairness with respect to peer arrival order, bursty departures and content synchronization among the peers. Although it is more prevalent in unpopular swarms (very few peers), these by far represent the most common type of swarm in BT.