Hogging the Trough: The EFF Strikes Back

The FCC is to investigate Comcast's network management practices. Last month here I gave an expert view on how the EFF, and other campaigners who called for an inquiry, don't understand the problems. Now Peter Eckserley, a copyright academic at the EFF, has responded to my article. Let's recap the story first.

To avoid congestion on its cable network, Comcast uses a technique to throttle uploads for BitTorrent users; BitTorrent downloads continue to proceed smoothly. Net Neutrality campaigners leapt onto the issue, insisting that Comcast's methods are illegitimate.

Comcast has little choice but to do what they're doing, given the three problems BitTorrent causes for their network - and their customers who don't use BitTorrent.

The first is the nature of BitTorrent itself. BitTorrent's behavior on the Comcast network is like a glutton at an all-you-can-eat buffet who insists on planting himself at the trough, preventing others from getting to the food. This causes multiple problems for DOCSIS cable networks, which caused Comcast's network managers to throttle uploads under high-load conditions (but not prohibit them outright) using a technique called Reset Spoofing.

The EFF has a preferred way of dealing with this, random packet drop. For EFF this is the One True Method of traffic management. But as I've explained to Eckersley both in print and over the phone, the problems BitTorrent causes can't be healed by random packet drop.

Packet drop would work with the regular diner who takes a plateful and moves on, but not with this super-hungry dude.

(I don't attribute malicious intent to BitTorrent's designer Bram Cohen; software often has bugs, even when it wasn't born in Redmond.)

But the EFF soldiers on, its honor now at stake.

Hogging the pipe

The EFF simply dismisses the Denial-of-Service-like effects of BitTorrent handshakes, in which a given Comcast customer's PC can become quite attractive to BitTorrent downloaders because of its favored position in the Tracker's list.

Eckersley correctly noticed that I gave an incorrect reason for this positioning: it's not because of response time, as I said, it's because of the Comcast-resident system's possession of rare file parts, which is even worse. Watch BitTorrent in operation and you'll see cycles of high popularity come and go. It's immune to packet dropping; a connect request comes into the Comcast network, and the user's system responds immediately, regardless of congestion, TCP window sizes, or load.

This highlights a design flaw in the Internet's reliance on TCP packet drop to control congestion generally; packet drop only slows traffic on established streams, not on sessions in the process of becoming established or on non-TCP streams.

Another, more persistent problem that the EFF dodges has been explained by Professor Jim Martin of Clemson University, the world's leading expert on TCP/BitTorrent interaction, and it's simple enough that a copyright expert can certainly grasp it if he wants to.

Residential networks, Comcast's being no exception, are designed on the assumption that users do more downloading than uploading. BitTorrent strives for a symmetric interchange of data, offering as much (or slightly more) in the upload direction as in the download direction. Hence, a small number of BitTorrent sessions will exhaust the network's upload capacity long before it's stressed in the download path. Professor Martin's paper, Assessing the Impact of BitTorrent on DOCSIS Networks [PDF, 450kb] predicts that fifteen BitTorrent sessions significantly slow down web browsing for the neighbors.

The web response time statistic increased from a value of 0.25 seconds when no BitTorrent users were active to 0.65 seconds when 15 BitTorrent users were active. This suggests that 15 BitTorrent users can cause a drop in performance by a factor of 2.5. When the number of BitTorrent users exceeds 30 performance degrades beyond the 1 second metric threshold.

Eckersley is certainly familiar with Martin's work, as he cites him in his original "research" on BitTorrent and Comcast.

An additional problem arises from BitTorrent's tendency to punish users on fast connections with greater traffic loads. Even though Comcast limits upstream traffic to 384kbit/s, a small fraction of its basic 4Mbit/s download rate, upstream traffic moves considerably faster on its network than it does on a standard DSL connection.

Consequently, BitTorrent downloaders will gravitate to peers on Comcast over those on DSL through their own performance assessment.