  Modeling and understanding BitTorrent (BT) dynamics is a recurrent research
topic mainly due to its high complexity and tremendous practical efficiency.
Over the years, different models have uncovered various phenomena exhibited by
the system, many of which have direct impact on its performance. In this paper
we identify and characterize a phenomenon that has not been previously
observed: homogeneous peers (with respect to their upload capacities)
experience heterogeneous download times. This behavior has direct impact on
peer and system performance, such as high variability of download times,
unfairness with respect to peer arrival order, bursty departures and content
synchronization. Detailed packet-level simulations and prototype-based
experiments on the Internet were performed to characterize this phenomenon. We
also develop a mathematical model that accurately predicts the heterogeneous
download rates of the homogeneous peers as a function of their content. In
addition, we apply the model to calculate lower and upper bounds to the number
of departures that occur in a burst. The heterogeneous download rates are more
prevalent in unpopular swarms (very few peers). Although few works have
addressed this kind of swarm, these by far represent the most common type of
swarm in BT.
