Comcast Prioritizing Their Video Content Over Competitors Traffic, Here’s The Proof

[Updated 5/22: In a blog post, Comcast says they are not prioritizing traffic, but Level 3 says they are.]

Last year, when Comcast acquired NBC Universal they had to agree to terms as set forth by the Department Of Justice and the FCC regarding how they would treat competitive content delivered over their network. One of the points in that document says that, "Comcast shall not prioritize Defendants’ Video Programming or other content over other Persons’ Video Programming or other content." While Comcast agreed to these terms and said they would not prioritize their video traffic over someone like Netflix, that's exactly what they are doing.

Based on details I have gotten from those who have looked at how packets are marked on their home broadband connections provided by Comcast, packets are in fact being marked with Quality of Service tags. Services from MLB, Hulu and Netflix are marked with CS1 tags and Xfinity is marked as CS5. In tests, the Xfinity traffic originates from servers inside Comcast's network and the other traffic originates on Level 3 and Akamai servers outside Comcast's network. All of these QoS tags are put on by Comcast.

According to the IETF the QoS tags have the following definitions;

  • CS1: This service is the low class of service and effectively gets any bandwidth left after every other service has transmitted over the link. "The fundamental service offered to the Low-Priority Data service class is best-effort service with zero bandwidth assurance". This class gets any of the scraps left over after all other classes have had their packets forwarded.
  • CS5: This service is considered a high class of service that gets processed such that it is ensured to get forwarded: "The Signaling service class SHOULD use the Class Selector (CS) PHB, defined in [RFC2474]. This service class SHOULD be configured to provide a minimum bandwidth assurance for CS5 marked packets to ensure they get forwarded."

All Internet traffic from at least the cable modem termination system to the home cable modem shares exactly the same path. There is no "private IP network" between those points. While I don't know how the Comcast servers that deliver the Xfinity traffic connect to the cable modem termination system, and while that part could be part of a private network, I think that's unlikely.

The part of the network from the cable modem termination system to the homes is usually the most congested and that equipment in the cable modem termination system is capable of acting on those QoS tags and so could treat the traffic differently. What I don't know is if they are treating the two tags differently, but the tags are there. If they aren't making use of them why are they adding them?

And if they are using them I think you could then make a reasonable assertion that the spirit, if not the letter, of the DoJ statement is being broken by Comcast. I've put in a request to Comcast to see if they want to address this and will update the post once I hear back.

Updated: If you'd like to do this testing for yourself, here is a PDF that shows you how to do it.