Heterogeneous download times in a homogeneous BitTorrent swarm

TL;DR

This paper discovers and characterizes a new phenomenon in BitTorrent where peers with identical upload capacities experience diverse download rates, affecting fairness and performance, especially in small, unpopular swarms.

Contribution

It introduces the first detailed analysis and modeling of heterogeneous download times among homogeneous peers in BitTorrent, supported by simulations and real experiments.

Findings

Homogeneous peers exhibit heterogeneous download rates.

Download time variability impacts fairness and system performance.

The phenomenon is more common in small, unpopular swarms.

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 rates. The consequences of this phenomenon have 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. Although this phenomenon is more prevalent in unpopular swarms (very few peers), these by far represent the most common type of swarm in BT.