Thanks to all of the readers who responded to my query about why the Internet’s congestion control mechanisms aren’t destroyed by selfish noncompliance. Due to the volume of responses, I can’t do all of you credit here, but I’ll do my best to summarize.
Jordan Lampe, Grant Henninger, and David Spalding point out that “Internet accelerator” utilizes (like these) do exist, but they don’t seem to come from mainstream vendors. Users may be leery of some of these products or some of these vendors. Wim Lewis suggests that these utilities may work by opening multiple connections to download material in parallel, which probably qualifies as a way of gaming the system to get more than one’s “fair share” of bandwidth.
Many readers argue that the incentive to cheat is less than I had suggested.
Aaron Swartz, Russell Borogove, and Kevin Marks argue that it’s not so easy to cheat the congestion control system. You can’t just transmit at full speed, since you don’t want to oversaturate any network links with your own traffic. Still, I think that it’s possible to get some extra bandwidth by backing off more slowly than normal, and by omitting certain polite features such as the so-called “slow start” rule.
Aaron Swartz, Wim Lewis, Mark Gritter, and Seth Finkelstein argue that most congestion happens at the endpoints of the Net: either at the link connecting directly to the server, or at the “last mile” link to the user’s desktop. These links are not really shared, since they exist only for the benefit of one party (the server or the user, respectively); so the bandwidth you gained by cheating would be stolen only from yourself.
Carl Witty and Karl-Friedrich Lenz argue that most of the relevant Net activity consists of downloads from big servers, so these server sites are the most likely candidates for cheating. Big servers have a business incentive to keep the Net running smoothly, so they are less likely to cheat.
Mark Gritter argues that most Net connections are short-lived and so don’t give congestion control much of a chance to operate, one way or the other.
All of these arguments imply that the incentive to cheat is not as large as I had suggested. Still, if the incentive is still nonzero, at least for some users, we would expect to see more cheating than we do.
Russell Borogove and John Gilmore argue that if cheating became prevalent, ISPs and backbone providers could deploy countermeasures to selectively drop cheaters’ packets, thereby lowering the benefit of cheating. This is plausible, but it doesn’t explain the apparent lack of cheating we see. The greedy strategy for users is to cheat now, and then stop cheating when ISPs start fighting back. But users don’t cheat much now.
Wim Lewis and Carl Witty suggest that if we’re looking for cheaters, we might look first at users who are already breaking or stretching the rules, such as porn sites or peer-to-peer systems.
Finally, Mark Gritter observes that defections do happen now, though in indirect ways. Some denial of service attacks operate by causing congestion, and some protocols related to streaming video or peer-to-peer queries appear to bend the rules. Perhaps the main vehicle for cheating will be through new protocols and services and not by modification of existing ones.
Thanks to all of you for an amazing demonstration of the collective mind of the Net at work.
Ultimately, I think there’s still a mystery here, though it’s smaller than I originally imagined.
