µTorrent's switch to UDP

In action movies, when the hero has only minutes before the readout on the bomb counts down to zero, with many miles to drive through a congested city in order to get there in time, he'll just ignore traffic lights and floor it. Wouldn't it be great if you could do the same thing on the Internet if your torrents don't download fast enough? Now you can. Maybe.

In a new alpha version of the popular BitTorrent client µTorrent, uTP has been made the default instead of TCP. uTP sends the actual downloads over the UDP protocol rather than TCP. The existing version 1.8.1 also supports uTP, but it's not enabled by default. This simple swapping of the transport protocol that sits between the IP layer and the application has gotten our friends over at The Register all worked up: "By most estimates, P2P accounts for close to half of internet traffic today. When this traffic is immune to congestion control, the remaining half will stumble along at roughly a quarter of the bandwidth it has available today: half the raw bandwidth, used with half efficiency, by 95% of internet users."

The half of the internet traffic figure is already highly debatable, as services like Hulu and iTunes deliver non-P2P video to ever more users. But the quarter of the bandwidth number is entirely speculation. If µTorrent is indeed eschewing congestion control, however, and this catches on, it could be pretty bad.

The TCP protocol that nearly all applications use—especially the ones sending a lot of data—has a traffic management system built in that is known as congestion control. The congestion control algorithms make TCP slow down when packets get lost, eventually reaching an equilibrium between packet loss and sending rate. Other TCP sessions flowing through the same part of the network see the same packet loss rate, so all these TCPs converge on a similar sending rate, making them share the limited bandwidth equally. UDP, on the other hand, doesn't have any congestion control built in. It's basically an extremely lightweight wrapper around the IP protocol that allows applications to send packets any way they feel like, without providing any feedback about congestion in the network.

So a naive UDP application will just send packets at the highest rate the network interface can support—or maybe even higher—almost certainly overloading some part of the network. However, this is not what µTorrent does. uTP avoids overloading the uplink of the user running the application—removing the need for BitTorrenters to impose limits on the BitTorrent upload and download speeds to avoid slowing down other applications.