Comcast Stifling BitTorrent

This was expected (cablecos, like telcos, want "freedom to degrade"), but is now confirmed:

Comcast Corp. actively interferes with attempts by some of its high-speed Internet subscribers to share files online, a move that runs counter to the tradition of treating all types of Net traffic equally.

The interference, which The Associated Press confirmed through nationwide tests, is the most drastic example yet of data discrimination by a U.S. Internet service provider. It involves company computers masquerading as those of its users.

If widely applied by other ISPs, the technology Comcast is using would be a crippling blow to the BitTorrent, eDonkey and Gnutella file-sharing networks. While these are mainly known as sources of copyright music, software and movies, BitTorrent in particular is emerging as a legitimate tool for quickly disseminating legal content.

— Comcast blocks some Internet traffic, Tests confirm data discrimination by number 2 U.S. service provider, by Jeff Chiu, AP, 19 Oct 2007

Comcast was denying blocking or throttling as recently as August (and as near as I can tell they still do deny it). Numerous users reported it, and the AP has now confirmed it. However, what Comcast is doing isn't precisely throttling.

The AP story indicates that Comcast is faking packets to appear as if they came from computers participating in BitTorrent, telling the recipient computer that its peer can't continue. It's not clear from the story exactly what packets they're faking. Other sources indicate it's a TCP connection RST (reset) message. They're using Sandvine traffic shaping hardware to do it. Sandvine fakes the correct IP addresses, port, and sequence number for the RST packet, so the recipient has no way to determine that it's fake. Well, not directly at the TCP level; they can notice by observing repeatedly reset TCP connections, however. Workarounds are already being attempted, such as encrypting the BitTorrent TCP connections, with limited success. There are also rumors that some BitTorrent clients are unaffected by Comcast's RSTs, which will be very interesting if whatever the difference is can be replicated in other clients.

Anyway, Comcast is not throttling or blocking; they're faking a packet that cuts off a TCP connection. The connection starts, so they're not blocking. And while it's running, it goes at whatever speed it goes, so they're not throttling. But they are cutting off the connection.

Comcast apparently also disconnects some users, for

...abusive activity that adversely impacts on everybody else’s experience.

Such activity apparently includes moving large files, which is ironic for users who have paid for unlimited access. Perhaps more than ironic; I wonder if anybody has tried suing yet.

So we appear to have a cable company acting as unilateral Internet gatekeeper, acting on some undefined and unannounced bandwidth limit, applied according to the evidence we've seen to one particular application, without admitting what they are doing, and with no oversight.

That's the thing about letting a very few private companies control a public good: everyone is at the mercy of whatever those companies want to do.

If the market for Internet access is going as peachy-keen as FTC and FCC think it is, why is Comcast so short of bandwidth that it is resorting to such measures? If the market isn't that good, why are we pretending there's no market failure?

-jsq