May 24, 2024

Congestion Control and the Tragedy of the Commons

I have been puzzling lately over why the Internet’s congestion control mechanisms work. They are a brilliant bit of engineering, but they fail utterly to account for the incentives of the Internet’s users. By any rational analysis, they ought to fail spectacularly, causing the Net to grind to a halt. And yet, for some unfathomable reason, these mechanisms do work.

Let me explain. As a starting point, think about the cars on a busy highway. If there aren’t many cars, the road is underutilized, carrying only a fraction of its capacity. Add more cars, and the road is used more efficiently, carrying more cars per minute past any given point. Add too many cars, though, and you’ll cause a traffic jam. Traffic slows, and the road becomes much less efficient as only a few cars per minute manage to crawl past each point. The road is in congestion.

Now think of the Internet as a highway, and each packet of data on the Net as a car. Adding more traffic increases the Net’s throughput, but only up to a point. Adding too much traffic leads to congestion, with a rapid dropoff in efficiency. If too many people are sending too much data, the Net slows to a crawl.

To address this problem, the TCP protocol (upon which are built most of the popular Net services, including email and the web) includes a “congestion control” mechanism. The mechanism is subtle in its details but pretty simple in its basic concept. Whenever two computers are talking via TCP, and they detect possible congestion on the path between them, they slow down their conversation. If everybody does this, congestion is avoided, since the onset of congestion causes everybody to back off enough to stave off an Internet traffic jam.

If you back off in response to congestion, you’re making the Internet a better place. You’re accepting a slowdown in your communication, in order to make the Internet faster for everybody else.

This is a perfect Tragedy of the Commons setup. We’re all better off if everybody backs off. But backing off is voluntary, and we each have a selfish motive to skip the backoff and just grab as much bandwidth as we can.

The mystery is this: Why hasn’t the tragedy happened? Virtually everybody does back off, and the Net doesn’t collapse under congestion. This happens despite the fact that a Net inhabited by rationally self-interested people should apparently behave otherwise. What’s going on?

Nobody seems to have an adequate explanation. One theory is that the average person doesn’t know how to cheat; but others could make and sell products that offer better Net performance by not backing off. Another theory is that Microsoft supplies most of the Net’s software and is making the choice for most consumers; and Microsoft’s self-interest is in having a useful Net. But again, why don’t others show up selling add-on “booster” products that cheat? A third theory is that people really are altruistic on the Net, behaving in a more civil and community-minded fashion than they do in real life. That seems pretty unlikely.

I’m stumped. Do any of you have an explanation for this?