Comcast: How We’ve Been Slowing Down BitTorrent

Comcast submitted a detailed description its ongoing network management practices in a filing with the FCC last night (PDF download), revealing that it has been testing controversial network management technologies since May of 2005. The company also laid out a plan for its transition to protocol-agnostic network management, which Stacey Higginbotham covered over at GigaOM. The filings are response to the FCC’s order against the company issued a month ago.

Comcast has previously been less than forthcoming about its network management. In fact, the company denied for months that it was interfering with P2P traffic at all. Today’s filing not only confirms many suspicions about the practice in place, it also shows a different reason why Comcast is moving away from it: The current BitTorrent blocks just aren’t that effective.

Comcast acknowledges in today’s filing that it tested Sandvine’s Policy Traffic Switch 8210 as early as May 2005. The company started to roll out these devices on a massive basis in 2006 and “achieved wide-scale deployment in 2007″, the filing states.

So how does the Policy Traffic Switch work? The device has been used to block P2P based on the total number of connections in a certain area, according to the filing. Sandvine’s switch only looks for unidirectional traffic, meaning it just targets people who are uploading or seeding downloads that have already completed, according to the filing. Comcast has been using the Sandvine devices to block five different P2P protocols: Ares, BitTorrent, eDonkey, FastTrack, and Gnutella.

Each protocol has been assigned a different threshold value of seeding connections per device, with Bitorrent having the lowest value. The logic behind this is that BitTorrent has a large number of bidirectional file transfers, meaning that users upload data chunks to each other while they’re downloading files. That’s not necessarily the case with other protocols, which is why Comcast has been allowing 80 Gnutella uploads, but only eight BitTorrent uploads per node before the network management kicks in.

Comcast explained in a filing submitted to the FCC earlier this year that each of its residential nodes serves about 450 households. In other words: Your BitTorrent uploads are blocked if even just one of your 400-and then some neighbors is seeding eight files.

It’s also important to notice that this practice is solely based on the number of connections, and not on the number of bytes or the types of files transferred . Your uploads are blocked even if those eight other uploads are very, very slow. Meanwhile, someone else might cause a ton of traffic by uploading one file after another via Gnutella, FTP or any other unmanaged protocol.

That doesn’t sound to effective, does it? Apparently, it isn’t. The filing also reveals that Comcast is still seeing around 50 percent of its upstream bandwidth consumed by P2P applications. In some areas, file swapping even causes up to two-thirds of all upstream bandwidth. Comcast stated in an earlier filing with the FCC that it was necessary to use these technologies because otherwise 80 percent of the bandwidth would be consumed by 20 percent of its users. Maybe getting that number down to 60-some percent wasn’t really worth all that uproar.

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