Felten on Comcast and BitTorrent

by on October 23, 2007 · 0 comments

Ed Felten isn’t impressed with Comcast’s traffic shaping techniques:

Comcast is using an unusual and nonstandard form of blocking. There are well-established mechanisms for dealing with traffic congestion on the Internet. Networks are supposed to respond to congestion by dropping packets; endpoint computers notice that their packets are being dropped and respond by slowing their transmissions, thus relieving the congestion. The idea sounds simple, but getting the details right, so that the endpoints slow down just enough but not too much, and the network responds quickly to changes in traffic level but doesn’t overreact, required some very clever, subtle engineering.

What Comcast is doing instead is to cut off connections by sending forged TCP Reset packets to the endpoints. Reset packets are supposed to be used by one endpoint to tell the other endpoint that an unexplained, unrecoverable error has occurred and therefore communication cannot continue. Comcast’s equipment (apparently made by a company called Sandvine) seems to send both endpoints a Reset packet, purporting to come from the other endpoint, which causes both endpoints to break the connection. Doing this is a violation of the TCP protocol, which has at least two ill effects: it bypasses TCP’s well-engineered mechanisms for handling congestion, and it erodes the usefulness of Reset packets as true indicators of error.

This brings to mind a question: as I understand it, TCP relies to some extent on clients being well-behaved and voluntarily backing off when faced with congestion problems. Is it possible that part of the reason that Comcast chose to target P2P applications specifically is that these aren’t “well-behaved” applications in this sense? Richard seems to be implying that this is the case. Is he right?

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