LEDBAT: The New BitTorrent Congestion Control Protocol

A few months ago, BitTorrent developers announced that the transfer of torrent data in the official client was about to switch to a new application-layer congestion-control protocol using UDP at the transport-layer. This announcement immediately raised an unmotivated buzz about a new, imminent congestion collapse of the whole Internet. As the new congestion control aims at offering a lower than best effort transport service, this reaction was not built on solid technical foundation. Nevertheless, a legitimate question remains: whether this new protocol is a necessary building block for future Internet applications, or whether it may result in an umpteenth addition to the already well populated world of Internet congestion control algorithms. To tackle this issue, we implement the novel congestion control algorithm and investigate its performance by means of packet-level simulations. Considering a simple bottleneck scenario, where the new BitTorrent competes against either TCP or other BitTorrent flows, we evaluate the fairness of resource share as well as the protocol efficiency. Our results show that the new protocol successfully meets some of its design goals, as for instance the efficiency one. At the same time, we also identify some potential fairness issues, that need to be dealt with. Finally, we point out that end-users will be the final judges of the new protocol: therefore, further research should evaluate the effects of its adoption on the performance of the applications ultimately relying on it.