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NEW YORK - Comcast Corp. actively interferes with attempts by some of its high-speed Internet 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, 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. Good bye.”

Matthew Elvey, a Comcast subscriber in the San Francisco area who has noticed BitTorrent uploads being stifled, acknowledged that 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 percent and 90 percent 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 application 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.