The Federal Communications Commission is asking major broadband carriers to explain their policies on exempting some content from data caps. The commission is looking at the thorny issue of whether such policies violate net neutrality rules.

The letters have gone to Comcast, AT&T and T-Mobile, which all offer Internet plans (cable for Comcast, mobile for the others) that include a monthly data cap.

Each has an exemption meaning that some data doesn’t count towards the cap, though they operate it in different ways:

• Comcast exempts its own TV streaming service, though this is delivered along a different connection to the user’s Internet.
• T-Mobile exempts any video service that meets its technical requirements, with no money changing hands.
• AT&T exempts content from companies who’ve paid to ‘sponsor’ their data, such as in delivering promotional trailers.

In each case, the FCC says it wants to check the policies don’t breach its Open Internet Rules. Those are the regulations — introduced after a lengthy political battle over the FCC’s rulemaking authority — which enforce the principle of net neutrality, namely to treat all legal traffic equally.

The problem is determining how strictly to enforce that principle. Many definitions suggest the concept simply means not blocking or deliberately slowing down traffic. A stricter definition might have it that exempting some content from data caps effectively makes non-exempted content more expensive to consume.

Which if any of the three companies are breaching the rules will depend on the interpretation. It’s hard to see how Comcast can be held at fault given its service doesn’t involve the customer’s main internet connection.

T-Mobile’s service also seems reasonable as it continues to allow all companies the opportunity to take part. Strictly speaking it’s favoring video content over non-video content, but that’s not really a practical issue given that video is the main content type for which data caps are a potential barrier to use.

AT&T’s service is the one that seems most open to question in terms of net neutrality. Whether or not you think what it’s doing is fair or reasonable, it does seem to be a case of a carrier treating traffic in a different way because the content provider is paying a fee. The question here is whether net neutrality applies only to technical measures (such as blocks or slowdowns) or if financial considerations come into play.

It should be noted at this stage that the FCC isn’t proposing a formal investigation or potential regulatory action. It says instead that it would like to meet the companies involved to discuss the issue by mid-January.