As we've reported, in addition to a new 250GB cap starting October 1, Comcast has been testing a system that throttles users back to "above DSL speeds." These trials have been conducted for several months in Comcast's Chambersburg, Pennsylvania, Warrenton, Virginia, Colorado Springs, Colorado and East Orange and Lake City Florida markets. While the company has promised transparency in this new management technique, they'd yet to offer specifics as to what customers had to do in order to be throttled.
That's changed somewhat with Comcast's latest filing
(pdf) with the FCC, which goes in to significant detail concerning network architecture, and how exactly Comcast plans to target high-consumption users. Previously, Comcast was using Sandvine hardware to forge customer TCP packets, resulting in degraded upstream BitTorrent connectivity for all
users, regardless of consumption. You'll note their filing on existing practices
(pdf) with the FCC avoids using the phrase "packet forgery."
The new plan is to -- for a change of pace -- actually target the users causing the congestion. According to Comcast, they'll be deploying new hardware and software close to the company's Regional Network Routers (RNRs). This hardware will flip a user from the standard "Priority Best-Effort" traffic (PBE) to lower quality of service (QoS) "Best-Effort" traffic (BE) if
a particular CMTS port is congested, and
if that user has been identified as a primary reason why. What's Comcast's definition for one of these users? From the filing:
Following lab tests, simulations, technical trials, customer feedback, vendor evaluations, and a third-party consulting analysis, we have determined that the appropriate starting point for the User Consumption Threshold is 70 percent of a subscribers provisioned upstream or downstream bandwidth, and that the appropriate starting point for the User Consumption Duration is 15 minutes.
In other words, if a heavy user on a congested CMTS is using seventy percent or more of their allotted up or downstream bandwidth for more than fifteen minutes, they're tagged as a glutton and treated as a second class customer until his or her consumption eases up. That user has their QoS lowered until their usage drops to 50 percent of their provisioned upstream or downstream bandwidth for "a period of approximately 15 minutes". Note that if you're just a heavy user on an uncongested portion of the network, no action is taken. Also worth noting: upstream and downstream bandwidth is both managed separately.
Depending upon the level of congestion in the CMTS port, this designation (BE) may or may not result in the users traffic being delayed or, in extreme cases, dropped before PBE traffic is dropped.
While Comcast has said users will be throttled to "above DSL speeds", they're still not particularly clear on exactly what speeds a throttled user will see. Being tagged a lower class "BE" user "may or may not result in the users traffic being delayed or, in extreme cases, dropped before PBE traffic is dropped," according to the filing. Comcast pulls out a bus metaphor to explain the difference between being a throttled "BE" and an unthrottled "PBE" user. Apparently, kids placed on the "special bus" should still be treated fairly:
If there is no congestion, packets from a user in a BE state should have little trouble getting on the bus when they arrive at the bus stop. If, on the other hand, there is congestion in a particular instance, the bus may become filled by packets in a PBE state before any BE packets can get on. In that situation, the BE packets would have to wait for the next bus that is not filled by PBE packets. In reality, this all takes place in twomillisecond increments, so even if the packets miss 50 busses, the delay only will be about one-tenth of a second.
Some of this is still a little murky, and you get the idea Comcast won't have greater specifics on real world speed impact until they've wrapped up throttling trials and get this running on more congested portions of the network. For their part, Comcast is insisting that in trial markets (which you can guess probably weren't Comcast's most congested), less than 1 percent of customers had their traffic managed during a regular day.
What's more, Comcast claims they saw no complaints in trial markets. "To date, Comcast has yet to receive a single customer complaint in any of the trial markets that can be traced to the new congestion management practices, despite having broadly publicized its trials," says the company. Comcast says that should they change the criteria used to determine congestion, they'll notify both the public and the FCC two weeks ahead of time.