It is to our profound and lasting discredit as a field that the most important paper in multi-agent systems in the last decade came not from the academic community, but from an outsider with some of the least auspicious academic credentials imaginable (dropping out of SUNY-Buffalo).

For every bit downloaded, someone must be uploading. But the two sides are inherently asymmetric because people are selfish and want to download as fast as possible without uploading anything. BitTorrent solves this problem by incentivizing uploading by making it the key to fast downloading.

A Blend of Theory and Practice

There’s a strong real-world focus in this paper, like you’d expect from a system that has been implemented and refined based on real usage. Take the section on the “last piece” problem — what happens if the final part of the file you’re downloading is from someone with a slow connection? These are the kinds of issues you don’t know you have until you build something.

At the same time, you also see an attempt to engage with Economic theory. From Section 3.1:

Well known economic theories show that systems which are pareto efficient, meaning that no two counterparties can make an exchange and both be happier, tend to have all of the above properties [utilizing all available resources, providing consistent download rates, and being “somewhat resistant” to people downloading but not uploading] … BitTorrent’s choking algorithms attempt to achieve pareto efficiency using a more fleshed out version of tit-for-tat than that used to play prisoner’s dilemma.

Sure, okay, you can scoff at that, and the explanation isn’t quite right, fine. It’s obviously the product of an intelligent autodidact, rather than an ensconced academic. But it evinces an obvious effort to derive practical work from theoretical first principles. It’s an effort to make it real.

Impact

It would be difficult to overstate the significance of BitTorrent to the Internet. Estimates are really shaky on this, but it’s probably somewhere between 20% and 50% of all traffic.

If we were to take all the CS papers published in the 2000s and rank them in terms of real-world impact, BitTorrent is likely in the top three — I would say the other two are Google’s MapReduce and Luis von Ahn’s work on CAPTCHAs.

Finally, like any good economic system, BitTorrent has been the subject of further analysis. I plan to address the really clever client BitTyrant, an academic effort to hack the economic protocol of BitTorrent to enable massive downloads with minimal uploads. BitTyrant, of course, also did not come from the multi-agent systems community. Just what is it we do again?

Cohen — Incentives Build Robustness in BitTorrent (2003)