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Micro Transport Protocol or µTP (sometimes also uTP) is an open UDP-based variant of the BitTorrent peer-to-peer file sharing protocol intended to mitigate poor latency and other congestion control issues found in conventional BitTorrent over TCP, while providing reliable, ordered delivery.
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It was devised to automatically slow down the rate at which packets of data are transmitted between users of peer-to-peer file sharing torrents when it interferes with other applications. For example, the protocol should automatically allow the sharing of an ADSL line between a BitTorrent application and a web browser.
Development
µTP emerged from research at Internet2 on QoS and high-performance bulk transport, was adapted for use as a background transport protocol by Plicto, that was founded by Stanislav Shalunov and later it was acquired by BitTorrent, Inc. in 2006, and further developed within its new owner. It was first introduced in the µTorrent 1.8.x beta branches, and publicized in the alpha builds of µTorrent 1.9.
The implementation of µTP used in µTorrent was later separated into the "libutp" library and published under the MIT license.
The first free software client to implement µTP was KTorrent 4.0. libtorrent implements µTP since version 0.16.0 and it is used in qBittorrent since 2.8.0. Tixati implements µTP since version 1.72. Vuze (formerly Azureus) implements µTP since version 4.6.0.0. Transmission implements µTP since version 2.30.
µTP congestion control
The congestion control algorithm used by µTP, known as Low Extra Delay Background Transport (LEDBAT), aims to decrease the latency caused by applications using the protocol while maximizing bandwidth when latency is not excessive. Additionally, information from the µTP congestion controller can be used to choose the transfer rate of TCP connections.
LEDBAT is described in RFC 6817. As of 2009, the details of the µTP implementation were different from those of the then-current Internet Draft.
µTP also adds support for NAT traversal using UDP hole punching between two port-restricted peers where a third unrestricted peer acts as a STUN server.