David Hales and Simon Patarin, both with the Dept. of Computer Science at the Univ. of Bologna in Italy, have written an interesting paper on altruism in BitTorrent and why free riding is relatively rare.
Read the 11-page paper: How to Cheat BitTorrent and Why Nobody Does [PDF].
The abstract:
The BitTorrent peer-to-peer file-sharing system attempts to build robustness to free-riding by implementing a tit-for-tat-like strategy within its protocol. It is often believed that this strategy alone is responsible for the the high-levels of cooperation found within the BitTorrent system. However, we highlight some of the weaknesses of the approach and indicate where it would be easy to cheat and free-ride. Given that cheating of this kind currently appears rare, this motivates the question: why is the system not dominated by free-riders?
We advance a hypothesis which argues that BitTorrent may resist free-riders in a way that has not been previously fully comprehended. Ironically, this process relies on what is commonly believed to be a weakness of BitTorrent - the lack of meta-data search. One consequence of this is to partition the BitTorrent network into numerous isolated swarms - often with several independent swarms for an identical file - which is one of the necessary conditions for a kind of evolutionary group selective process, a process that has been recently identified in similar simulated systems.
A further implication of the hypothesis is that, given the choice, users may choose unconditional altruism rather than the more restrictive reciprocal tit-for-tat approach as a result of the same group selective process.
Free riding is a major issue for filesharing programs, and can greatly degrade their performance. It is a vulnerability that those who wish to make filesharing networks less popular can exploit. BitTorrent has been remarkably resiliant to this so far.
This paper demonstrates how that resiliancy may not last. The authors show how it is possible to free ride on BitTorrent; it just doesn't make a lot of sense for users.
However, and this is something the paper doesn't go into, if there is an external motivation to free ride (like, I don't know, you don't want to get sued by the MPAA for uploading), then users may find it to their advantage to free ride, thus decreasing the utility of BitTorrent. Of course, if free riding does become prevalent, there are means to thwart it. However, this would probably lead to a code arms race that would likely have unintended consequences, such as a loss of privacy.
A clever copyright industry would figure out how to take advantage of this. Luckily for users of BitTorrent, that is probably not something they need to worry about.
Ernest Miller pursues research and writing on cyberlaw, intellectual property, and First Amendment issues. Mr. Miller attended the U.S. Naval Academy before attending Yale Law School, where he was president and co-founder of the Law and Technology Society, and founded the technology law and policy news site LawMeme. He is a fellow of the Information Society Project at Yale Law School.
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