The PTS inspects the traffic as it goes by, looking in each packet header to determine what protocol is being used. It doesn’t bother with the content of the packets but it looks specifically for headers identifying the traffic as belonging to BitTorrent, Gnutella, Ares, eDonkey, or FastTrack. As it does this, it counts the number of packets using these protocols passing on to the upstream router.
Comcast determined that “the number of simultaneous unidirectional upload sessions of any particular P2P protocol at any given time serves as a useful proxy for determining the level of overall network congestion." In other words, when a certain percentage of all traffic is on the upload side, congestion is soon to follow.
Their task was complicated by the fact that the different file sharing networks have different ratios of bidirectional and unidirectional traffic. BitTorrent, for example runs at about 20:1, which is consistent with its being a very distributed network used for sharing large files. (Torrents can consist of a file on a single user’s hard drive or the same file distributed across thousands of computers. This is very efficient, and it is the advantage of P2P networking.)
The PTS sits and watches, and when a certain number of unidirectional uploads for a protocol, say BitTorrent, rises above the preset thresholds, the PTS kicks in. In each package it identifies as part of that protocol, it inserts a “reset" flag. This is the equivalent of an error message in the network. Named “RST," it tells the computers involved to reset the connection.
Sending out the reset message not only terminates the connection, it prevents the BitTorrent client from “seeding." Seeders are PCs that have complete copies of the files involved. Thus Comcast’s network practice was not only interfering with the user’s traffic flow, but that of other users of BitTorrent.
Comcast claims that that P2P traffic now comprises about half of its network’s upload capacity. Even at this level, they say, less than 10% of the flow is affected by Comcast’s traffic management.
And they are going to stop doing it. In their consent to the FCC’s “Memorandum Opinion & Order," Comcast will stop using reset packets to manage its high speed Internet Networks by the end of 2008.
So now Comcast has agreed to manage their network in a “protocol agnostic" manner. We’ll see how they plan to do that in the next article.