First thoughts on Comcast's BitTorrent (and others) blocking

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It's now been pretty widely reported that Comcast is blocking BitTorrent (as well as other apps such as Gnutella and allegedly Lotus Notes) traffic. (Good summary by Ars Technica here and here).

The technical issue here is pretty straightforward; Comcast seems to be forging TCP RST (Reset) segments from one side of the connection to the other, causing the receiving TCP implementation to terminate its side of the connection. The evidence here is that people have taken packet dumps on both sides of the connection and neither peer is generating the RSTs, so it's clearly someone in the middle, and the pattern of which subscribers are affected looks like it implicates Comcast. Note: I'm going purely by others reports. I have Comcast myself, but I haven't tested this.

More interesting is the pattern of what is being blocked. According to TorrentFreak, Comcast is only blocking people seeding files:

Unfortunately, these more aggressive throttling methods can't be circumvented by simply enabling encryption in your BitTorrent client. It is reported that Comcast is using an application from Sandvine to throttle BitTorrent traffic. Sandvine breaks every (seed) connection with new peers after a few seconds if it's not a Comcast user. This makes it virtually impossible to seed a file, especially in small swarms without any Comcast users. Some users report that they can still connect to a few peers, but most of the Comcast customers see a significant drop in their upload speed.

The throttling works like this: A few seconds after you connect to someone in the swarm the Sandvine application sends a peer reset message (RST flag) and the upload immediately stops. Most vulnerable are users in a relatively small swarm where you only have a couple of peers you can upload the file to. Only seeding seems to be prevented, most users are able to upload to others while the download is still going, but once the download is finished, the upload speed drops to 0. Some users also report a significant drop in their download speeds, but this seems to be less widespread. Worse on private trackers, likely that this is because of the smaller swarm size

Assuming this is correct, Comcast is targetting files which Comcast users are serving to non-Comcast users. This mostly doesn't degrade your perceived performance if you're a Comcast user downloading content, but if you're (1) a non-Comcast customer trying to download traffic from a Comcast customer or (2) actually trying to push something into the P2P network, then this is going to seriously impact your experience. Since most customers are probably in the downloader category, this is actually a pretty attractive way to reduce network traffic without overly annoying too many of your users. By contrast, if Comcast just blocked all BitTorrent, then everyone trying to download the next episode of Lost would be pretty unhappy and would most likely be pretty intolerable to a sizable enough percentage of customers that you couldn't just stonewall.


Fanatics of the P2P super power gave birth to the devil.
It is the strongest P2P file sharing system Share NT.
And, Because UDP is used, even the band limiting that the internet service provider does is exceeded.

Share (P2P) - Wikipedia
Share NT -

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