ILLUSTRATION: HARRY CAMPBELL

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

In the classic example, I have a cow, and there's this nice patch of grass in a 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 type of commons. Everyone takes his or her car out on the road, and soon all the traffic is stalled. 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 is flowing, drivers often act in their own interest by speeding, changing lanes, and trying to jump exit queues—all at the expense of the common good.

We have many examples of technological commons. 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? And the history isn't good. Remember citizens band (CB) radio? It reminds me of what Yogi Berra once 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 ultimate survival of the system, for users to limit their use.

The World Wide Web is also a commons, which brings up another problem. When everyone is allowed free use of a commons, a small percentage of users will behave badly. It's like someone bringing a diseased cow to graze. On 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 a commons needs to have some form of control. In the traditional telephone system, this means limiting access to a dial tone.

More recently, some freeways have regulated access with stoplights at on-ramps. In the use of the radio frequency spectrum, the conventional approach has been exclusive ownership of segments 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 a commons.

These commons are shared not only by humans but by machines, and by our design, these machines exhibit discourtesy or courtesy. For example, the Ethernet Protocol used in many local area networks employs a collision-detection-and-avoidance mechanism. If you try to send a packet and it interferes with another packet, the system automatically backs off for a random amount of 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 across the Internet. Normally, this protocol increases the speed of data packets being sent by a computer until unacknowledged packets begin to accumulate, indicating the connection is getting congested. Then the computer that is sending the packets slows the speed of transmission to avoid clogging up the network.

The vast majority of Internet users are undoubtedly unaware of this courteous behavior. The sending computers could, of course, hack the protocol stack to increase their own share of the Internet commons and transmit at maximum speed at all times, but it appears that this doesn't happen. Perhaps those who 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 wanted, 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 build it back up only gradually. If everyone were subject to 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.

Now you'll have to 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.