The Federal Communications Commission has asked Comcast, AT&T, and T-Mobile USA to answer questions about their implementations of "zero-rating," a practice that exempts certain types of content from customers' data caps.

FCC officials wrote to the companies yesterday, pointing out that data cap exemptions can favor some content providers over others, whose content does count against consumers' caps. The letters asked the companies to make "relevant technical and business personnel" available for discussions with FCC staff by January 15.

FCC Chairman Tom Wheeler told reporters today that "this is not an investigation. This is not any enforcement. This is to help us stay informed as to what the practices are, as we said we would do in the Open Internet Order."

More commonly known as the FCC's net neutrality rules, the Open Internet Order did not specifically ban data caps or zero-rating. But it includes a general conduct standard in which the FCC judges on a case-by-case basis whether a practice "unreasonably interferes" with the ability of consumers to reach content or the ability of content providers to reach consumers.

"We want to ensure that we have all the facts to understand how this service relates to the Commission's goal of maintaining a free and open Internet while incentivizing innovation and investment from all sources," the letters to the companies said.

Here are links for the letters to Comcast, AT&T, and T-Mobile.

The three companies have implemented data cap exemptions in different ways. AT&T's Sponsored Data program charges third parties, such as advertisers, for the right to deliver data without counting against consumers' mobile data caps. T-Mobile exempts many music and video services from its high-speed data limits, but it does not charge for the exemptions. In the case of T-Mobile's Binge On program for video, companies must meet T-Mobile's technical requirements to get the exemption. Binge On also reduces the quality of streaming video unless customers opt out of the data cap exemption.

Comcast doesn't offer third-party services the ability to not count against its home Internet data caps, but it exempts its own "Stream TV" in-home streaming service from the cap. Comcast says this does not violate any rules because Stream TV is an IP cable service—similar to cable TV—delivered separately from Internet access.

The FCC's letters described concerns raised by consumer advocates, content providers, and journalists. The letter to AT&T said that "some have argued that sponsored data unfairly advantages incumbent content providers." Writing to T-Mobile, the FCC said that "some have argued that the technical requirements of the Binge On program may harm innovation by 'making certain video apps more attractive than others.' Others have asserted that the reduction of video quality 'has harmed some users.'"

The letter to Comcast said that "the CEO of Sling TV has suggested Comcast data usage allowances and the inapplicability of those usage allowances to the Stream TV raises 'level playing field' concerns. Others have suggested that Stream TV is a 'new way to deliver services that get around the FCC's net neutrality guidelines.'"

The letter to T-Mobile is a little surprising because Wheeler previously called Binge On "pro-competition" and "pro-innovation." But he had also said the FCC would "keep an eye on it" and measure it against the FCC's general conduct standard.

The inquiries do not have unanimous support at the FCC. Republican Commissioner Ajit Pai, an opponent of the net neutrality rules, said the letters show that "the era of permissionless innovation is over."

"Today's message from [the] FCC is clear," Pai Chief of Staff Matthew Berry tweeted. "If you come up with an innovative service, you will be hauled into [the] FCC to explain yourself."

Verizon is also planning to roll out a sponsored data program.