Moxie, a Comcast customer who replied to a post on Silicon Alley, points out that when you log your network activity with an application like Wireshark, you’ll notice that Comcast servers start sending reset messages as soon as a download is finished, exactly as we described it.

With Wireshark in the background run your BitTorrent application. Wait until completed and watch Wireshark, notice when it finishes seeding Comcast servers send out a reset command every second to your computer noted by the highlighted red line in Wireshark. It is 8:30 pm Monday pst and Comcast is still resting my BitTorrent connections. Maybe the PR guy didn’t get the email from the VP of Networking.

It might be that not every Comcast customer is equally affected, but a significant percentage is. Not only the 10+ users we talked to before we first reported this issue, but also hundreds of additional commenters here on TorrentFreak, and elsewhere. Some users even captured the throttling in progress on video (download), and anyone has to agree that this does look very suspicious.

More evidence comes from Robb Topolski, a networking and protocol expert with more than 25 years of experience, who first wrote about this issue on DSLReports. He told TorrentFreak: “We have had two Comcast techs confirm Sandvine in use, but neither confirmed or denied its connection with the RST interference. For me, seeding is possible. I can reach my upload speed limit, but there sure is a lot of interference. Since your article came out, I too have received many reports of seeding being impossible. I’m not sure if it’s regional, or what!”

For the networking savvy people among us, here’s an example of real RST interference on an unencrypted BitTorrent connection. In this case, it happens right after the bitfields are exchanged

Nevertheless Comcast spokesman Charlie Douglas said in a response to Light Reading: “We’re not blocking access to any application, and we don’t throttle any traffic”. He might be right here semantically speaking, they are not throttling anything, they just kill all outgoing connections when a clients starts to seed a file. But the fact is that Comcast is making it impossible for (at least some) customers to share files with non-Comcast users over BitTorrent.

Luckily there is a fix for this problem, and we know that at least two BitTorrent client developers are including this fix in their next update.

Source: TorrentFreak, for the latest info on copyright, file-sharing and anonymous VPN services.

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

Although BitTorrent protocol encryption seems to work against most forms of traffic shaping, it doesn’t help in this specific case. Setting up a secure connection through VPN or over SSH seems to be the only solution. More info about how to setup BitTorrent over SSH can be found here.

Last year we had a discussion whether traffic shaping is good or bad, and ISPs made it pretty clear that they do not like P2P applications like BitTorrent. One of the ISPs that joined our discussions said: “The fact is, P2P is (from my point of view) a plague – a cancer, that will consume all the bandwidth that I can provide. It’s an insatiable appetite.”, and another one stated: “P2P applications can cripple a network, they’re like leaches. Just because you pay 49.99 for a 1.5-3.0mbps connection doesn’t mean your entitled to use whatever protocols you wish on your ISP’s network without them provisioning it to make the network experience good for all users involved.”

Customers on the other hand like to fully use their connection, and don’t agree that traffic shaping is the correct solution. One reader commented: “If you pay for an internet connection, that’s what you should get from your ISP — an internet connection. Not a connection that will let you browse the web and check email, but little else. If an ISP has issues with the amount of data a customer is transferring, then the ISP needs to address that issue with that customer, and not restrict every user in one class of traffic.”

I guess this battle will go on for a while and I would advise Comcast users to try setting up a VPN connection to get around the traffic shaping, other users who find out that they are throttles might try BitTorrent encryption first, that seems to work quite well in most cases.

More details about the Sandvine application can be found here.

Source: TorrentFreak, for the latest info on copyright, file-sharing and anonymous VPN services.