Heterogeneous download times in bandwidth-homogeneous BitTorrent swarms

TL;DR

This paper reveals that peers with identical upload capacities in BitTorrent swarms can experience vastly different download times, affecting fairness and performance, supported by simulations, experiments, and a predictive mathematical model.

Contribution

It introduces and characterizes the novel phenomenon of heterogeneous download times among homogeneous peers in BitTorrent, with a predictive model and analysis of its impact.

Findings

Heterogeneous download times occur among peers with the same upload capacity.

The phenomenon is more common in small, unpopular swarms.

A mathematical model accurately predicts download rate variability.

Abstract

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.