Tragedy of the Commons
Published in IEEE Spectrum Magazine, Jan 2006

I often have occasion to think about the tragedy of the commons – both in life and technology. It’s a powerful metaphor, first described by Garrett Hardin in an article in Science in 1968. Briefly, it says that a shared resource is inevitably ruined by uncontrolled use.

I have a cow, and there’s this nice patch of grass in the nearby park. I see my neighbors taking their cows over to the park to graze. I know somewhere in the back of my mind that all the grass in the commons is going to get eaten by all these cows, but everyone else is doing it, and I want to get the grass for my cow before it’s all gone. So off I go with my cow, doing my part to help destroy the commons.

It’s the same thing on the highways, which are another commons. Everyone takes their car out to the road, and soon the traffic is all stopped. No one gets through. I look at the other drivers and think they should have stayed home. It’s their fault that I’m stuck. Moreover, even when traffic flowing, drivers often act in their own self-interest by speeding, changing lanes, and trying to jump exit queues – all of which is at the expense of the common good.

We have many examples of commons in technology. Probably the most obvious is in communications, where a common medium must be shared among many disparate users. Will they act courteously for the public good, or hog the medium for themselves? The history isn’t good. Remember Citizen’s Band (CB) Radio? It reminds me of what Yogi Berra said about a certain restaurant: “Nobody goes there anymore; it’s too crowded.” Today we worry whether Wi-Fi will exhibit the same meltdown. There is no incentive, other than the eventual survival of the system, for users to limit their own use.

The World Wide Web is another commons. Another problem with a commons is that when everyone is allowed free use, a certain small percentage of entrants will be bad actors. It’s like someone bringing a diseased cow to graze. In the Web it’s the spammers, tearing down the public good for their own profit.

I despair of the concept of “enlightened self-interest.” I don’t see it on the highways or anywhere else. Instead, it appears that commons need to have some form of control. In the traditional telephone system it has been through the limits to access of dial tone. More recently, some freeways have regulated access with stop lights at on-ramps. In the use of frequency spectrum, the conventional approach has been to have exclusive ownership through licenses. The unlicensed bands are a more recent experiment – one that has resulted in tremendous innovation. Yet people warn that they can’t be trusted – it’s a commons, and you know what happens to commons.

Commons are not only shared by humans, but by machines, and by our design these machines exhibit their own discourtesy or courtesy to other users. For example, the original Ethernet protocol employed a collision detection and avoidance mechanism. If you tried to send a packet and it interfered with another packet, the system automatically backed off for a random amount to time before trying again. “Oops,” says the interface card, “Excuse me; I’ll be back in a little while.”

Courtesy is also built into the transmission control protocol, TCP, used to send information over the Internet. All the different instantiations of this protocol build up the speed of packet flow until unacknowledged packets begin to accumulate. Then the “window” is closed down, slowing the speed of transmission and permitting fewer unacknowledged packets. The transmission speed is only slowly increased until another problem is encountered. The vast majority of Internet users are undoubtedly unaware of their courteous behavior. They could, of course, hack the protocol stack to increase their own share of the Internet commons, but it appears that this doesn’t happen. Perhaps those that have the knowledge and skill realize that this would be bad behavior.

Imagine if our cars acted like TCP. You’d be allowed to drive as fast as you want, as long as you didn’t interfere with others. As soon as your car detected that you were interfering with others, your speed would be automatically reduced, and you could only build it back up gradually. If everyone had such a system, perhaps traffic would flow much better and the roads would be more peaceable. However, we’d all hate it. It seems that the freedom to ruin a commons is one of those inalienable rights.

Excuse me, but I’ve got this hungry cow, and everyone else is taking their cows to the park. I’ve got to run while there’s grass left.