Feeds

Hogging the Trough: The EFF Strikes Back

The art of voodoo network management

Build a business case: developing custom apps

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.

The essential guide to IT transformation

Next page: Conclusion

More from The Register

next story
6 Obvious Reasons Why Facebook Will Ban This Article (Thank God)
Clampdown on clickbait ... and El Reg is OK with this
So, Apple won't sell cheap kit? Prepare the iOS garden wall WRECKING BALL
It can throw the low cost race if it looks to the cloud
Time Warner Cable customers SQUEAL as US network goes offline
A rude awakening: North Americans greeted with outage drama
Shoot-em-up: Sony Online Entertainment hit by 'large scale DDoS attack'
Games disrupted as firm struggles to control network
BT customers face broadband and landline price hikes
Poor punters won't be affected, telecoms giant claims
Netflix swallows yet another bitter pill, inks peering deal with TWC
Net neutrality crusader once again pays up for priority access
EE plonks 4G in UK Prime Minister's backyard
OK, his constituency. Brace yourself for EXTRA #selfies
prev story

Whitepapers

Top 10 endpoint backup mistakes
Avoid the ten endpoint backup mistakes to ensure that your critical corporate data is protected and end user productivity is improved.
Implementing global e-invoicing with guaranteed legal certainty
Explaining the role local tax compliance plays in successful supply chain management and e-business and how leading global brands are addressing this.
Backing up distributed data
Eliminating the redundant use of bandwidth and storage capacity and application consolidation in the modern data center.
The essential guide to IT transformation
ServiceNow discusses three IT transformations that can help CIOs automate IT services to transform IT and the enterprise
Next gen security for virtualised datacentres
Legacy security solutions are inefficient due to the architectural differences between physical and virtual environments.