Comcast Corp., the second-largest U.S. internet service provider, actively interferes with attempts by some of its high-speed subscribers to share files online, a move that runs counter to the tradition of treating all types of net traffic equally.

The interference, which the Associated Press confirmed through nationwide tests in the United States, is the most drastic example yet of data discrimination by a U.S. internet service provider. It involves company computers masquerading as those of its users.

If widely applied by other ISPs, the technology Comcast is using would be a crippling blow to the BitTorrent, eDonkey and Gnutella file-sharing networks. While these are mainly known as sources of copyright music, software and movies, BitTorrent in particular is emerging as a legitimate tool for quickly disseminating legal content.

The principle of equal treatment of traffic, called "net neutrality" by proponents, is not enshrined in law but supported by some regulations. Most of the debate around the issue has centered on tentative plans, now postponed, by large internet carriers to offer preferential treatment of traffic from certain content providers for a fee.

Comcast's interference, on the other hand, appears to be an aggressive way of managing its network to keep file-sharing traffic from swallowing too much bandwidth and affecting the internet speeds of other subscribers.

Comcast, the nation's largest cable TV operator and No. 2 internet provider, would not specifically address the practice, but spokesman Charlie Douglas confirmed that it uses sophisticated methods to keep net connections running smoothly.

"Comcast does not block access to any applications, including BitTorrent," he said.

Douglas would not specify what the company means by "access." Comcast subscribers can download BitTorrent files without hindrance. Only uploads of complete files are blocked or delayed by the company, as indicated by AP tests.

But with "peer-to-peer" technology, users exchange files with each other, and one person's upload is another's download. That means Comcast's blocking of certain uploads has repercussions in the global network of file sharers.

Comcast's technology kicks in, though not consistently, when one BitTorrent user attempts to share a complete file with another user.

Each PC gets a message invisible to the user that looks like it comes from the other computer, telling it to stop communicating. But neither message originated from the other computer — it comes from Comcast. If it were a telephone conversation, it would be like the operator breaking into the conversation, telling each talker in the voice of the other: "Sorry, I have to hang up. Goodbye."

Matthew Elvey, a Comcast subscriber in the San Francisco area who has noticed BitTorrent uploads being stifled, acknowledged the company has the right to manage its network, but disapproves of the method, saying it appears to be deceptive.

"There's the wrong way of going about that and the right way," said Elvey, who is a computer consultant.

Comcast's interference affects all types of content, meaning that, for instance, an independent movie producer who wanted to distribute his work using BitTorrent and his Comcast connection could find that difficult or impossible — as would someone pirating music.

Internet service providers have long complained about the vast amounts of traffic generated by a small number of subscribers who are avid users of file-sharing programs. Peer-to-peer applications account for between 50 per cent and 90 per cent of overall internet traffic, according to a survey this year by ipoque GmbH, a German vendor of traffic-management equipment.

"We have a responsibility to manage our network to ensure all our customers have the best broadband experience possible," Douglas said. "This means we use the latest technologies to manage our network to provide a quality experience for all Comcast subscribers."

The practice of managing the flow of internet data is known as "traffic shaping," and is already widespread among internet service providers. It usually involves slowing down some forms of traffic, like file-sharing, while giving others priority. Other ISPs have attempted to block some file-sharing applications by so-called "port filtering," but that method is easily circumvented and now largely ineffective.

Comcast's approach to traffic shaping is different because of the drastic effect it has on one type of traffic — in some cases blocking it rather than slowing it down — and the method used, which is difficult to circumvent and involves the company falsifying network traffic.

The "net neutrality" debate erupted in 2005, when AT&T Inc. suggested it would like to charge some web companies more for preferential treatment of their traffic. Consumer advocates and web heavyweights such as Google Inc. and Amazon Inc. cried foul, saying it's a bedrock principle of the internet that all traffic be treated equally.

To get its acquisition of BellSouth Corp. approved by the Federal Communications Commission, AT&T agreed in late 2006 not to implement such plans or prioritize traffic based on its origin for two and a half years. However, it did not make any commitments not to prioritize traffic based on its type, which is what Comcast is doing.

The FCC's stance on traffic shaping is not clear. A 2005 policy statement says that "consumers are entitled to run applications and services of their choice," but that principle is "subject to reasonable network management." Spokeswoman Mary Diamond would not elaborate.

Free Press, a Washington-based public interest group that advocates net neutrality, opposes the kind of filtering applied by Comcast.

"We don't believe that any internet provider should be able to discriminate, block or impair their consumers' ability to send or receive legal content over the internet," Free Press spokeswoman Jen Howard said.

Paul "Tony" Watson, a network security engineer at Google Inc. who has previously studied ways hackers could disrupt internet traffic in manner similar to the method Comcast is using, said the cable company was probably acting within its legal rights.

"It's their network and they can do what they want," Watson said. "My concern is the precedent. In the past, when people got an ISP connection, they were getting a connection to the internet. The only determination was price and bandwidth. Now they're going to have to make much more complicated decisions such as price, bandwidth and what services I can get over the internet."

Several companies have sprung up that rely on peer-to-peer technology, including BitTorrent Inc., founded by the creator of the BitTorrent software (which exists in several versions freely distributed by different groups and companies).

Ashwin Navin, the company's president and co-founder, confirmed that it has noticed interference from Comcast, in addition to some Canadian internet service providers.

"They're using sophisticated technology to degrade service, which probably costs them a lot of money. It would be better to see them use that money to improve service," Navin said, noting that BitTorrent and other peer-to-peer applications are a major reason consumers sign up for broadband.

BitTorrent Inc. announced Oct. 9 that it was teaming up with online video companies to use its technology to distribute legal content.

Other companies that rely on peer-to-peer technology, and could be affected if Comcast decides to expand the range of applications it filters, include internet TV service Joost, eBay Inc.'s Skype video-conferencing program and movie download appliance Vudu. There is no sign that Comcast is hampering those services.

Comcast subscriber Robb Topolski, a former software quality engineer at Intel Corp., started noticing the interference when trying to upload with file-sharing programs Gnutella and eDonkey early this year.

In August, Topolski began to see reports on internet forum DSLreports.com from other Comcast users with the same problem. He now believes that his home town of Hillsboro, Ore., was a test market for the technology that was later widely applied in other Comcast service areas.

Topolski agrees that Comcast has a right to manage its network and slow down traffic that affects other subscribers, but disapproves of their method.

"By Comcast not acknowledging that they do this at all, there's no way to report any problems with it," Topolski said.