Comcast Corp. on Tuesday acknowledged "delaying" some subscriber internet traffic, but said any roadblocks it puts up are temporary and intended to improve surfing for other users.

The statement was a response to an Associated Press report last week that detailed how the largest cable company in the United States was interfering with file sharing by some of its internet subscribers. The AP also found that Comcast's computers masqueraded as those of its users to interrupt file-sharing connections.

Internet watchdog groups denounced Comcast's actions, calling it an example of the kind of abuse that could be curbed with so-called "Net Neutrality" legislation. It would require internet providers to treat all traffic equally — as has largely been the case historically.

Comcast has repeatedly denied blocking any internet application, including "peer-to-peer" file-sharing programs like BitTorrent, which the AP used in its nationwide tests.

On Tuesday, Mitch Bowling, senior vice-president of Comcast Online Services, added a nuance to that statement, saying that while Comcast may block initial connection attempts between two computers, it eventually lets the traffic through if the computers keep trying.

"During periods of heavy peer-to-peer congestion, which can degrade the experience for all customers, we use several network management technologies that, when necessary, enable us to delay — not block — some peer-to-peer traffic. However, the peer-to-peer transaction will eventually be completed as requested," Bowling said.

Critics question explanation

The explanation is not inconsistent with the AP's tests. In one case, a BitTorrent file transfer was squelched, apparently by messages generated by Comcast, only to start 10 minutes later.

Other tests were called off after around five minutes, while the transfers were still stifled.

Comcast's statement did not mollify Markham Erickson, executive director of the Washington, D.C.-based Open Internet Coalition, a group that counts Google Inc. and eBay Inc. among its supporters.

"What applications work, what don't, and at what speeds? Only Comcast really knows," he said. "Comcast is making arbitrary bandwidth allocation decisions slowing use of basic (programs) without being clear to consumers what they really get when they buy a broadband connection."

The Electronic Frontier Foundation confirmed the AP's findings with its own tests — including spotting forged messages sent by Comcast's computers to shut down connections.

File sharing gains legitimacy

While BitTorrent connections may eventually resume after being shut down by Comcast, other file-sharing programs like Gnutella may be more severely affected by the interference, preventing any kind of upload, said Peter Eckersley, staff technologist at the online civil liberties group.

"Characterizing that as delaying traffic I think is … a stretch. What they are doing is spoofing traffic or jamming traffic," Eckersley said.

"I think they are trying to create as much confusion about this story as they can because they've done something really scurrilous and out of line for an ISP, and I'm sure they've been burned by the community's reaction to it," he added.

Applications like BitTorrent and Gnutella are often used to illegally share copyrighted files, giving the applications themselves an image of shadiness. Recently, however, several companies have started using BitTorrent to distribute legal files.

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

Large attachments blocked

However, users also reported Comcast blocking some transfers of e-mails with large attachments through an application that is fully in the legal sphere: Lotus Notes, an IBM Corp. program used in corporate settings.

Kevin Kanarski, a network engineer for a major law firm, noticed the disruption in August and eventually traced the problem to Comcast. But he got the cold shoulder from the company's customer support department.

On Tuesday, Bowling acknowledged the problem, saying it was unintentional and due to a software bug that has been fixed. Kanarski said transfers started working again last week.

"These are the kinds of software bugs you get when you have ISPs messing around with hacking techniques to get some applications running on their networks and not others," said EFF's Eckersley, who is himself a Comcast subscriber.

"The bottom line is that if ISPs start regularly engaging in conduct like this, then kids in their dorm rooms or small startup companies that are trying to develop innovative new uses of the internet are going to have to come and get permission from players like Comcast to get their protocols working properly," Eckersley said.

"That kind of veto over innovation would be very bad news."