Comcast strong-armed one of the nation's biggest internet backbone providers, Level 3, into paying extra to deliver movies to Comcast customers, Level 3 announced Monday, describing it as a violation of open-internet principles.

"Comcast is putting up a tollbooth at the borders of its broadband internet-access network, enabling it to unilaterally decide how much to charge for content which competes with its own cable TV and Xfinity-delivered content," Level 3 chief legal officer Thomas Stortz said in a press release.

Level 3 is a major backbone provider that carries large amounts of traffic from ISP to ISP and, in some cases, from large corporate clients to ISPs. One of Level 3's biggest clients is Netflix, which provides on-demand video services that have grown to an estimated 20 percent of net traffic during peak evening hours.

Comcast told Level 3 on Nov. 19 that the backbone provider would have to pay a "recurring fee" to Comcast, and the company agreed to the demand "under protest" Nov. 22, Level 3 said.

Comcast denies that the fee had anything to with the type of traffic, and was simply due to Level 3 proposing to more than double the amount of traffic it sent to Comcast's network, and that its fee was the same as it charges other Content Delivery Networks (CDN).

"Level 3 is trying to gain an unfair business advantage over its CDN competitors by claiming it's entitled to be treated differently and trying to force Comcast to give Level 3 unlimited and highly imbalanced traffic and shift all the cost onto Comcast and its customers," Comcast wrote in a blog post Monday evening. "In other words, Level 3 wants to compete with other CDNs, but pass all the costs of that business onto Comcast and Comcast's customers, instead of Level 3 and its customers."

The accusation immediately riled public interest groups who have been fighting a losing battle to have the federal government impose fairness rules, also known as net neutrality, on ISPs.

"We are informed that within a week of Level 3’s announcement on Nov. 11 that it would be primary carrier for Netflix streaming video, Comcast informed Level 3 that Comcast would, for the first time, charge Level 3 a fee to reach Comcast’s customers who had requested any content carried by Level 3," said Harold Feld, the legal director of Public Knowledge, one of the groups involved.

Feld said this added more reason for the FCC to block Comcast's proposed purchase of NBC.

It's not clear, however, that this is anything more than a typical peering dispute between internet giants. Very little is publicly known about how the internet actually interconnects, as large ISPs often jockey over who pays who to carry traffic. The contracts are almost never publicly disclosed.

ISPs that send each other roughly equal amounts of traffic often "peer" – that is, connect directly to one another's networks without charge. But when traffic levels differ, one ISP often tries to force the other to pay to carry its traffic directly.

In this case, Comcast says it feels that it is taking in more Level 3 traffic than it is sending out through the connection, and therefore wants Level 3 to pay for the disparity – regardless of the kind of content Level 3 is sending its way.

And Level 3 could be simply playing the net neutrality card in what is actually a routine business dispute. Or Comcast could be trying to increase the cost of online video to protect its cable business and its pending purchase of NBC, which owns nearly a third of the online video site Hulu.

Given how murky and complicated the net's interconnection system works it's hard to actually tell the difference between rhetoric and fact in this dispute.

UPDATE: This story was updated with comment from Comcast.

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