Incentives Build Robustness in Bit-Torrent
Incentives Build Robustness in Bit-Torrent
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发表时间：
2003
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通讯作者：
B. Cohen
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作者：
B. Cohen
BitTorrent文件分发系统使用针锋相对作为寻求帕累托效率的方法。它实现了更高水平的鲁棒性和资源利用率比任何目前已知的合作技术。我们解释了BitTorrent的作用，以及如何使用经济方法来实现这一目标。1 BitTorrent做什么当一个文件使用HTTP提供时，所有上传费用都由托管机器承担。使用BitTorrent，当多个人同时下载同一个文件时，他们会将文件的片段上传给对方。这将上传的成本重新分配给下载者（通常甚至不计量），从而使托管文件的下载者数量可能不受限制。研究人员试图找到实用的技术来做到这一点之前[3]。它以前没有大规模部署，因为后勤和鲁棒性问题相当困难。简单地弄清楚哪些对等点拥有文件的哪些部分以及它们应该被发送到哪里是很难做到的，而不会产生巨大的开销。此外，真实的部署经历非常高的流失率。对等体很少连接超过几个小时，通常只有几分钟[4]。最后，还有一个普遍的公平问题[1]。从数学上来说，所有下载器的总下载速率必须等于总上传速率。分配上传的策略似乎最有可能使对等体对他们的下载速率感到满意，这是使每个对等体的下载速率与他们的上传速率成比例。在实践中，很难保持对等下载速率有时会偶然下降到零，更不用说使上传和下载速率相关了。我们将解释BitTorrent如何很好地解决所有这些问题。1.1 BitTorrent接口BitTorrent的接口几乎是最简单的。用户通过点击他们想要下载的文件的超链接来启动它，并给出一个标准的“另存为”对话框，然后是一个下载进度对话框，其中除了下载速率外，最值得注意的是有上传速率。这种极端的易用性极大地促进了BitTorrent的采用，甚至可能比本文中描述的性能和成本再分配功能更重要，尽管它肯定是补充。
The BitTorrent file distribution system uses tit-fortat as a method of seeking pareto efficiency. It achieves a higher level of robustness and resource utilization than any currently known cooperative technique. We explain what BitTorrent does, and how economic methods are used to achieve that goal. 1 What BitTorrent Does When a file is made available using HTTP, all upload cost is placed on the hosting machine. With BitTorrent, when multiple people are downloading the same file at the same time, they upload pieces of the file to each other. This redistributes the cost of upload to downloaders, (where it is often not even metered), thus making hosting a file with a potentially unlimited number of downloaders affordable. Researchers have attempted to find practical techniqes to do this before[3]. It has not been previously deployed on a large scale because the logistical and robustness problems are quite difficult. Simply figuring out which peers have what parts of the file and where they should be sent is difficult to do without incurring a huge overhead. In addition, real deployments experience very high churn rates. Peers rarely connect for more than a few hours, and frequently for only a few minutes [4]. Finally, there is a general problem of fairness [1]. The total download rate across all downloaders must, of mathematical necessity, be equal to the total upload rate. The strategy for allocating upload which seems most likely to make peers happy with their download rates is to make each peer’s download rate be proportional to their upload rate. In practice it’s very difficult to keep peer download rates from sometimes dropping to zero by chance, much less make upload and download rates be correlated. We will explain how BitTorrent solves all of these problems well. 1.1 BitTorrent Interface BitTorrent’s interface is almost the simplest possible. Users launch it by clicking on a hyperlink to the file they wish to download, and are given a standard “Save As” dialog, followed by a download progress dialog which is mostly notable for having an upload rate in addition to a download rate. This extreme ease of use has contributed greatly to BitTorrent’s adoption, and may even be more important than, although it certainly complements, the performance and cost redistribution features which are described in this paper.