Recently, a few bloggers conducted tests that seem to show Comcast’s traffic to its Xbox 360 running Xfinity TV On Demand, which does not count against a user’s usage cap (as announced in March), uses a different type of traffic routing. The effect is that the Xfinity TV service has its own dedicated channel on a given Internet connection, through what Comcast calls a “separate service flow.” At the time, Comcast claimed it was serving its Xfinity TV service through a “private IP network” rather than the public Internet. But it's a claim that appears to not actually be true.

Many argued this means Comcast is prioritizing traffic, a charge the company denies. So what exactly is Comcast doing? Who’s right, and why does it matter?

The short answer: Comcast is doing some type of traffic management. It comes down to how “prioritization” is defined—if the company would be found to be favoring one type of traffic over another, that would be a violation of federal regulations.

“This is definitely in a gray zone,” said Peter Eckersley, the technology projects director at the Electronic Frontier Foundation, in an interview with Ars.

Last week, Bryan Berg, the CTO of San Francisco startup Mixed Media Labs, wrote a detailed post that showed how Comcast appears to be favoring the Xfinity traffic. Berg concluded that this constitutes “reasonable evidence of the prioritization of traffic in violation of the terms of the Comcast/NBCU Consent Decree.”

Comcast acknowledges it is applying a different quality of service value (known in the industry as Differentiated Services Code Point, DiffServ, or DSCP value) that normally denotes priority levels for traffic. But the company states, “that is not their only application—and that is not what they are being used for here.”

Within that DSCP value, there are a number of ranges of possible sub-values defined as a way to manage traffic. These have names like Class Selector, Assured Forwarding, Expedited Forwarding, and others. In this case, Berg and Dan Rayburn both showed that Comcast appears to be primarily using the Class Selector value to distinguish its traffic. This has a range of values from CS1 (lowest value, also known as “bulk”) to CS5, which takes higher priority. The company is taking advantage of this 6-bit field in the IP header of each packet as a way to distinguish the Xfinity traffic as it is sent down to the customer.

When a Comcast customer starts the Xfinity app on the Xbox, that request for video data is sent out from the Xbox, through the router and cable modem, and then to the cable modem termination system (CMTS). That is the company’s central hub for a group of cable modem subscribers. Once there, the request is sent on to the video-on-demand host, which responds by serving the video to the customer.

Defining DSCP values is typically done by the end host (in this case, Comcast’s video host), or may be included later downstream by an ISP that offers “integrated services.” Those might include TV, phone, and Internet, as a way to preserve a certain quality of service for various applications. Here, Comcast is arguing that it’s not giving CS5 traffic a higher priority when compared to its normal Internet service. In a sense, the company is merely providing a separate-but-equal channel to serve bits to customers.

“Is it OK to have one lane for all the Internet traffic and another lane for Xfinity?” Eckersley added. “The Xfinity isn’t inherently faster, but it’s a separate lane. That’s the policy question you want to ask.”

Comcast did not respond to questions from Ars before this article was published.

Netflix, Vint Cerf remain unconvinced of Comcast's arguments

Under the regulatory conditions of Comcast’s merger with NBC Universal, it is not allowed to prioritize its own services above those of others. As the conditions state (PDF):

“If Comcast offers consumers Internet Access Service under a package that includes caps, tiers, metering, or other usage-based pricing, it shall not measure, count, or otherwise treat Defendants’ affiliated network traffic differently from unaffiliated network traffic. Comcast shall not prioritize Defendants’ Video Programming or other content over other Persons’ Video Programming or other content.”

Netflix CEO Reed Hastings isn’t convinced, as he described in a Facebook post last month.

"For example, if I watch last night's SNL episode on my Xbox through the Hulu app, it eats up about one gigabyte of my cap. But if I watch that same episode through the Xfinity Xbox app, it doesn't use up my cap at all," he wrote. "The same device, the same IP address, the same WiFi, the same Internet connection, but totally different cap treatment. In what way is this neutral?"

The Federal Communications Commission did not respond as of press time to comment on whether or not this practice may be a potential violation.

However, Vint Cerf—considered by many to be the “father of the Internet,” and the co-author of the TCP/IP protocol—wrote to Ars saying that Comcast’s argument didn’t make much sense to him, either.

“The analysis sounds as if they are prioritizing to say nothing of not counting against the caps,” he wrote in an e-mail. “I am disturbed by what I have heard so far and hope to get a much more detailed summary from Comcast about this practice.”