Traditionally, we have used a Leviathan to enforce these incentives. By propping up a central authority (usually the state, or in BitTorrent’s case, a centralized identity service), you can force each actor in a system to behave — other than the Leviathan itself, of course.
…communities with persistent reputation, but most users prefer to use BitTorrent in total anonymity. Without any incentives around the exchange of bandwidth, BitTorrent is vulnerable to this tragedy of the commons. If BitTorrent functions well, it’s only because its citizens happen to be well-behaved.