FOR THE PAST several months, Comcast has been covertly sending commands to your computer that tell it to stop receiving information, especially if that information is coming to you via BitTorrent, Gnutella and other file-sharing applications.

In May of this year, disgruntled Comcast users started posting on message boards about how BitTorrent and Gnutella weren't working for them anymore. So researchers at the Electronic Frontier Foundation, along with an investigative journalist at Associated Press, started running tests on the Comcast network, using software tools to examine what exactly Comcast was doing to BitTorrent.

What they found was disturbing. Without telling customers, Comcast had begun a secret program to send automatic "reset" commands to customers' computers if they were using BitTorrent, Gnutella and a few other programs. None of these programs is illegal. Moreover, Comcast had sold its services to customers without informing them that this popular Internet software wouldn't work on its network. And Comcast is still doing it.

To make matters worse, the method Comcast uses to shut down file sharing is underhanded. It stops BitTorrent by injecting "reset" data packets into information streaming between two computers on the Comcast network. Then Comcast makes the reset packets appear to be from one of the computers using BitTorrent—not from Comcast. So even if a customer knows to look for these reset packets, she'll believe the problem comes from the computer she's trying to share files with.

When EFF and angry customers confronted Comcast about its sneaky system, the company claimed that it was merely "slowing down" certain programs. But as EFF pointed out last week in a research paper on the topic, reset packets are designed only to shut down communication between two computers. If Comcast wanted to slow down BitTorrent, the company could have used a commonly used program called a traffic shaper, which can adjust data speeds.

Without further explanation from Comcast, one is left wondering why the company would engage in such outrageously anti-consumer behavior. One possibility is that it views BitTorrent as a competitor. BitTorrent has made deals with various Hollywood studios to distribute their movies online, which is also something that Comcast cable does on television. So maybe Comcast is playing dirty so that its customers will turn to cable TV for movies, instead of getting them online via BitTorrent.

For people who don't care about using BitTorrent, though, Comcast's behavior is still a gesture of bad faith. The company is demonstrating quite plainly that it won't hesitate to deny basic Internet services to its customers without warning and without even acknowledging that it's doing it. Today, those services are for file sharing. But tomorrow they could be for sending email that doesn't use Comcast's webmail system.

I also think Comcast's actions are a harbinger of what's to come as Internet service providers get sucked into larger media companies with cable or content-making divisions. No laws guarantee network neutrality online, so Comcast is free to engage in network prejudice. It can block any old service it wants, especially if there's a financial incentive. Certainly, consumers can choose to go with another Internet service provider, and I hope they do. But in the future, market competition may not be enough.

If Comcast blocks BitTorrent, then another company might welcome BitTorrent traffic but block my favorite game services. Internet service will become like cable TV, where getting the full range of channels is incredibly expensive. Except it will be worse, because the Internet is a far more rich and diverse place than cable TV. Selectively blocking the Internet is like selectively blocking expression itself.

Annalee Newitz is a surly media nerd who gets her movies on BitTorrent.

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