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P2P BitTorrent Filtering by Comcast & Verizon
by Iljitsch van Beijnum

Overview


What would happen if every American decided to take a shower at the same time? The water pipes weren't created for such an usual peak capacity, so very likely, the water flow would reduce to a mere trickle until a large part of the shower-takers decided to do something else. It's the same thing with the internet: even though ISPs sell "unlimited" service, if everyone would start to use their available bandwidth to the limit—actually, not even close to the limit—ISP networks would overload and only be able to get a small part of the requested data through.
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This is not an academic situation. We saw two news stories related to this issue recently: Comcast decided to to "delay" peer-to-peer traffic, and Verizon decided to settle with the state New York over its use of the term "unlimited" even though it disconnected users that would generate too much traffic over its wireless broadband service.

Comcast has been extremely tight-lipped about what it is they're doing, but it seems that they use devices that recognize peer-to-peer protocols such as BitTorrent and Gnutella (and mis-recognize Lotus Notes as collateral damage).
 
IPv6

When these devices see a certain usage pattern, they will send "TCP RST" packets to disrupt the communication between a Comcast user and a peer elsewhere. They only seem to do this when the Comcast user is uploading, not when they're downloading. Even though ADSL, cable and wireless broadband all have much lower upload speeds than download speeds, the uploading can still be the problematic part. Being a good citizen in the BitTorrent world means uploading as much as you download. So BitTorrent and other peer-to-peer file sharing applications use the upload capacity to the max. However, cable networks have very limited return channel capacities, because they were originally designed for one-way distribution of TV signals. So even though Comcast's connections to the rest of the internet can easily carry the peer-to-peer uploads, it's the local cable infrastructure that gets into trouble with too many peer-to-peer uploads: this will delay the traffic of other users.

IPv6   So Comcast decided to stop these uploads. Verizon on the other hand, disconnected "excessive" users. New York's Attorney General found that this had happened in no less than 13,000 cases. Rather than taking its chances in court, Verizon chose to reimburse disconnected users the costs of their laptop cards or cell phones, and pay $150,000 in penalties in costs to the state.

Also, they now make their data limit explicit: it's 5 gigabytes per month, and certain types of applications (such as peer-to-peer file sharing) aren't allowed. Unlimited is no longer unlimited. The filing of a class action lawsuit agains Comcast seems to be only a matter of time.

That case will probably be more complex, because Comcast doesn't disconnect users, but rather, gets in the way of them using the service to a certain degree. It will be especially interesting to see whether Comcast really only delays the uploads by letting the file sharing application retry them successfully later, and whether that makes a difference in court. In the meantime, Comcast users who enable IPv6 tunneling and then use an IPv6-aware peer-to-peer application such as Azureus, can probably fly under the radar for a while.