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Comcast loses P2P religion, goes agnostic on throttling

Deep packet inspection is out at Comcast as the cable operator explains its …

Nate Anderson, Eric Bangeman – Sep 19, 2008 9:27 PM | 0

Comcast has finally provided the FCC with details of its new congestion management system, and there's a lot to like. Deep packet inspection it out. P2P protocol targeting is out. Management systems that affect entire regions are out.

Instead, Comcast will deploy a highly-targeted, protocol-agnostic management system that has nothing to do with the applications a customer uses and everything to do with the total bandwidth being used in the last few minutes. It's a huge step forward from the previous system that relied on falsified TCP reset packets to interrupt P2P uploads. The company plans on rolling it out later this year despite its lawsuit against the FCC over the ruling that forced this new approach.

When the FCC issued its Order against the company's P2P throttling scheme last month, Comcast was given 30 days to describe its current congestion management and the changes that the company would make to comply with the FCC ruling. Those 30 days are up today, and Comcast has just filed its document with the agency. Ars also spoke to the company to flesh out our understanding of the new system. Here's how it works.

Ars Video

A better congestion management system

Comcast's new technique is based on a simple premise: during periods of congestion, heavy users of bandwidth on a local node ought to see speed reductions before light users. To make that happen, the system tracks each customer's uploads and downloads separately using software from Sandvine that runs on Linux servers (Comcast stresses to us that this is not deep packet inspection software, but basic "shallow inspection" code that simply counts packets.)

"Our management system is designed so that the effect on all customers is as minimal as possible, even those whose traffic is being managed," Comcast spokesperson Sena Fitzmaurice told Ars. "We want to make sure that all of our users are getting a fair share of resources during times of congestion."

When any port (think neighborhood node) on the Cable Modem Termination System (CMTS) in the local cable company office enters a "near congestion" state, the system looks up the heaviest users of bandwidth during the preceding few minutes. Those users then have their traffic tagged as "Best Effort" rather than the default "Priority Best Effort." At this point, nothing happens to anyone's traffic.

When congestion actually occurs, the Priority Best Effort users should see no slowdown in their connections; all traffic will go through ahead of the Best Effort traffic. Best Effort folks may not notice any slowdown, either. They are not speed-limited, but they do go to the back of the quality of service (QoS) line. At this point, if traffic does in fact fill the pipe, users in the Best Effort category will experience delays in their connections, though their traffic will still be sent on whenever possible.

A CMTS has separate upload and download ports, and the new management system is port-specific. If congestion only appears on the upload link, no one's download links will be affected. In addition, when the system begins to tag users as Best Effort, it only considers their traffic on the specific port being managed; heavy downloaders won't be tagged if it is only the upload link that starts to have problems.

Fitzmaurice told Ars that the traffic reprioritzation would typically affect only a handful of customers when it goes into effect. "On a typical day, less than one percent of people had their traffic put in the Best Efforts category," she said. "It's measured in 15-minute increments and is only in effect for as long as your traffic level was over a specific threshold."

The new system also applies management only to the nodes actually experiencing congestion, which means that neighborhoods, rather than entire cities, would be affected. If your friend across town is in a neighborhood filled with crazed P2P uploaders, her network segment may experience management even while yours remains completely unaffected.

The moment that congestion vanishes, all traffic flows without delay even though some users are still tagged with "Best Effort" (the system monitors congestion and responds nearly in realtime). The tag will only be applied for a few minutes at a time and has nothing do with the company's new 250GB monthly data limit, which remains in effect.

Comcast has trialled the new approach in five cities: Chambersburg, PA, ?Colorado Springs, CO, East Orange, FL, Lake City, FL, and Warrenton, VA. In the course of its testing, Comcast has paid special attention to the way that the system will affect realtime protocols like VoIP and gaming. Somewhat incredibly, it claims to have received zero complaints about the new approach from any household in the five markets. "We have had no consumer complaints that can be traced to the new practices in the pilot areas," Fitzmaurice told Ars. "It's compatible with Internet standards and supportive of Internet innovation."

What the FCC will think of the new system remains to be seen.

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