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How to Thwart Comcast's BitTorrent Shenanigans
Dissecting the cable giant's traffic shaping efforts...
by Karl Bode 01:36PM Wednesday Aug 29 2007
A few months ago, users in our forums started noticing that Comcast (in addition to their invisible caps) was using Sandvine traffic-shaping hardware installed at the CMTSs to limit the effectiveness of BitTorrent seeding. Here's how it works, according to resident user funchords, who has been dissecting the practice for weeks:
"The Sandvine application reads packets that are

traversing the network boundary. If the application senses that outbound P2P traffic is higher than a threshold determined by Comcast, Sandvine begins to interrupt P2P protocol sequences that would initiate a new transfer from within the Comcast network to a peer outside of the Comcast network. The interruption is accomplished by sending a perfectly forged TCP packet (correct peer, port, and sequence numbering) with the RST (reset) flag set. This packet is obeyed by the network stack or operating system which drops the connection."
Comcast's goal is to manage peak p2p traffic, not destroy it. While Comcast issued some very carefully worded denials, using semantics to diffuse attention, insiders have privately been confirming that this information is accurate. Funchords continues to dissect Comcast's efforts, and has since come up with some solutions:

•Encrypted tunnels to a point outside the Comcast network (VPN, SSH tunnel, etc.)•Forcing encryption.•When downloading, make sure you've met your uploading goal by the time that the download completes. The easiest way to do this is to set a download rate slower than the uploading rate.•Tolerating the up to 40% rate RSTs when using BitTorrent to upload a file and not forcing encryption. He says he still hits his 16 KB/s (256 Kbps) preferred upload limit.

The question now becomes whether this evolves into a game of cat and mouse now familiar to subscribers of Canadian cable operator Rogers, who took traffic shaping to an entirely new level by using deep packet inspection to strangle all VPN and encrypted traffic in order to thwart such workarounds.

142 comments .. click to read

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Mountain View, CA

2 recommendations

reply to Rick

Re: I think it's safe to say that we can now officially....

Rick, I respect what you have to say, and I acknowledge your point fully. It's a legitimate statement, and I sincerely do understand it.

Now I ask you to understand my point, which is (I believe) the point most of technically-savvy Comcast customers, and probably the majority of DSLR/BBR users are making:

Comcast is completely violating (that is, destroying) the legitimacy of the TCP protocol.

In English: Comcast is screwing up TCP packets with their equipment.

I completely understand Comcast's need to try and "tune" their network based on bandwidth concerns or heavy users. I respect that. But I CANNOT respect a company who fucks with stateful TCP sessions by injecting TCP RST into the stream prematurely using ANY piece of equipment (Sandvine or otherwise).

What they are doing violates core pieces of how IP (e.g. TCP/IP) works in general. They're playing a risky game by doing this. It's getting a lot of attention in the networking community, and the attention is not positive.

An ISP's job is to provide me transit -- as far as throttling what's important vs. what isn't, that's for me to decide, not for them.

And for those considering the "So pick another ISP" rebuttal: we have no other choices in the Bay Area. It's AT&T/SBC who offers DSL only (max. 6mbit, and CO distance plays a HUGE ROLE -- in my case, I cannot even get 1.5mbit due to my CO distance being over 12000 electrical feet). Wireless IS NOT considered broadband around here.

So please understand where I'm coming from. I want an ISP that provides me transit, and does not (pardon my language) fuck with packets. It's not their job to fuck with packets. It's their job to provide me unaltered service.

Making life hard for others since 1977.
I speak for myself and not my employer/affiliates of my employer.

Waterbury, CT

5 recommendations

take this issue to the same level as the "capping" issue.

And, by that ..I mean ...Let's have BBR just continually rip this company to shreds for taking steps to preserve the integrity of their network for those of us who do NOT abuse it.

Heck, why not just post the "solutions" for how people can even get around this and continue on with their ever loving ways of soaking up so much bandwidth that there's simply none left for the rest of us to use and enjoy.

Oh.Wait. That's exactly what this story now does.
Silly me for thinking that is what would come next.

Well, allow me this post to post my opposing view.
As a comcast customer.

Dear Comcast..
Keep right on doing whatever it is you're doing.
We love you for it.

Some of us are of the belief that you give us a tremendous amount of value for our money. And even believe that 300 gigs per month is still a WHOLE lot of data..and those who seek to use their connection more than that really should
a)cough up the money for a business class connection or
b) get a secondary line to split your use between the two services.

Ladies and Gents..Comcast does not owe us the world for 42.95 per month. Nor do they owe the abusers an unlimited license to take whatever it is they want..at whatever cost it is to comcast..in order to satisfy their own demands.

You see...the problem with that..is that SOMEONE is going to pay anyway..and who it will be will be YOU..and I..
those who do use our connections for very reasonable and then some...downloading and uploading.

Comcast is VERY fair when it comes to their limits..
and those limits are in place for the benefit of us all..not the detriment to us all.

Comcast should NOT be engaged in the business of allowing the kinds of copyright stealing abusers who exist out there to seed an entire p2p network. And, for this website..BBR..to post "solutions" and "workarounds" to that is really..just flat out wrong.

And, you elevate yourself to a much LOWER level..IMHO.

Listen. I'm not against people connecting to work..nor even using their connections..a LOT.
And Comcast does allow for that.
I'm always using my connection. It's my right arm...and it's always there to serve me..no problems..no questions asked.

What that tells me is that the people who ARE having problems are by far..exceeding EVERY reasonable boundary there is.

And quite frankly..for BBR to be aiding that kind of behavior..and worse yet..to be presenting Comcast in an unfavorable and negative light because of it..almost non stop these days..

Is really...

Just plain..

Flat out...


Thank you for allowing me my op Ed piece.

Satisfied Comcast Customer
The Coyote captured the RR! Roadrunner Rick is now Comcastic!