Comcast's recent announcement that traffic generated by Comcast's new Xbox streaming video service would not count against the cable giant's 250GB monthly data cap drew swift denunciation from the network neutrality advocates at Public Knowledge.

In an e-mailed statement, PK President Gigi Sohn said that the new policy "raises questions not only of the justification for the caps but, more importantly, of the survival of an Open Internet."

Advocates of network neutrality regulations have long worried that incumbent broadband providers would create a "fast lane" open only to content providers that paid a premium for access. Until now, those concerns have been largely theoretical. While a few controversies over non-neutral policies have cropped up over the years—most notably, the 2007 controversy over Comcast interfering with BitTorrent traffic, examples of non-neutral policies were few and far between.

On its page announcing the new policy, Comcast claims that Xbox "content is being delivered over our private IP network and not the public Internet." Presumably, the cable giant would argue that this means its new policy does not raise network neutrality issues, since all traffic delivered over the "public Internet" is still treated equally.

But the development appears to confirm a worry that Netflix raised two years ago when the FCC was considering new network neutrality rules. Those rules exempted a "managed services" category of IP-based services from the network neutrality rules. Netflix warned that "if left unchecked, the 'managed services' category could engulf the Commission’s open Internet policies altogether," making them effectively meaningless.

Sohn argues that this is exactly what Comcast is doing. "The Xbox 360 provides a number of video services to compete for customer dollars, yet only one service is not counted against the data cap—the one provided by Comcast." she said. "This is nothing less than a wake-up call to the Commission to show it is serious about protecting the Open Internet."

Not all observers were alarmed. "The ink isn't even dry on the Net neutrality rules and already pro-regulatory groups are trying to use them to ban new business models," said Adam Thierer, a scholar at the libertarian Mercatus Center and a longtime critic of network neutrality regulations. "We should give innovation and experimentation a chance instead of preemptively regulating the Internet and broadband markets."

Comcast also denies that its new app raises network neutrality concerns. "Comcast is committed to an open Internet and has pledged to abide by the FCC’s Open Internet rules," the cable giant said in an e-mailed statement. "Our policies with respect to XfinityTV and the Xbox 360 fully comply with those rules and our commitments." Comcast argues that its Xbox app effectively just behaves like an extra set-top box, and so is exempted from the data caps for the same reasons its ordinary video-on-demand services are.

Public Knowledge spokesman Art Brodsky told Ars that his group's legal team is studying Comcast's new policy, and had yet to decide whether to file a formal complaint with the FCC. But he said supporters of network neutrality needed to do something. "If nobody asks any questions, nobody raises any red flags," and other broadband providers might be tempted to adopt a similar strategy, he said.