If you're reading this story on our Web site, I don't know what you did online before you reached this page. But your Internet provider might if it engages in something called deep packet inspection. That phrase may sound like what the Transportation Security Administration does to uncooperative airline passengers, but on the Internet it means a thorough and automatic inspection of online traffic -- not just where you've been but also what you've seen. Peering inside the digital packets of data zipping across the Internet -- in real time, for tens of thousands of users at once -- was commercially impractical until recently. But the ceaseless march of processing power has made it feasible. Unsurprisingly, companies have been trying to turn this potential into profit. By tracking users' Web habits this closely, they can gain a much more detailed picture of their interests -- and then display precisely targeted, premium-priced ads. The House Committee on Energy and Commerce recently asked dozens of providers to explain whether they had done any such testing. Most companies said they had yet to try the technology and had no plans to do so. (Although AT&T allowed that if done properly," deep packet inspection "could prove quite valuable to consumers.") Taking these companies at their word, what's there to worry about? Systems such as deep packet inspection unnerve a lot of Internet users for sound reasons. One is, of course, the immensely greater surveillance they allow. Another concern is the difficulty of circumventing this constant tracking. The machinery of deep packet inspection hides out of reach in your provider's servers.

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