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.

中文翻译：

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带宽均匀的 BitTorrent 群中的异构下载时间

建模和理解 BitTorrent (BT) 动态是一个经常出现的研究课题，主要是由于其高复杂性和巨大的实践效率。多年来，不同的模型揭示了系统表现出的各种现象，其中许多现象对其性能有直接影响。在本文中，我们确定并描述了一种以前未曾观察到的现象：同质节点（就其上传容量而言）经历了不同的下载时间。这种行为对对等点和系统性能有直接影响，例如下载时间的高度可变性、对等点到达顺序的不公平、突发性离开和内容同步。在互联网上进行了详细的数据包级模拟和基于原型的实验来表征这种现象。我们还开发了一个数学模型，可以准确预测同质节点的异构下载率作为其内容的函数。此外，我们应用该模型来计算突发中发生的离开次数的下限和上限。异构下载率在不受欢迎的群体（很少有同行）中更为普遍。尽管很少有工作涉及这种群体，但这些代表了 BT 中最常见的群体类型。