Micro Transport Protocol

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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.

It was devised to automatically slow down^[1] 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, which was acquired by BitTorrent, Inc. in 2006, and further developed within its new owner.^[2] It was first introduced in the µTorrent 1.8.x beta branches, and publicized in the alpha builds of µTorrent 1.9.^[3][4]

The implementation of µTP used in µTorrent was later separated into the "libutp" library and published under the MIT license.^[5][6]

The first free software client to implement µTP was KTorrent 4.0.^[7][8] libtorrent currently has an incomplete implementation of µTP in a development branch.^[9][10][11] Azureus (now called Vuze) Bittorrent Client also implements µTP (since Azureus 4.5.0.5 on Microsoft Windows and Mac OS X 10.5 or higher only).^[12] Transmission implements µTP since version 2.30^[13].

Operation

µTP consists of two parts: a framing scheme and a congestion control algorithm that is less aggressive than TCP's.^{[citation needed]}

µTP framing

µTP stores data within UDP datagrams using its own framing scheme. µTP's framing scheme has functionality roughly equivalent to TCP's (with timestamps and SACK), but it is realised in a completely incompatible manner.

The µTP framing scheme is believed by some to be far from optimal.^[14]

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.^[15][16] Additionally, information from the µTP congestion controller can be used to choose the transfer rate of TCP connections.^[17]

LEDBAT has been described in an Internet-Draft,^[18] but the details of the µTP implementation are different from those of the draft.^[19]

See also

• µTorrent
• BitTorrent
• TCP congestion avoidance algorithm

References

1. ^ http://forum.bittorrent.org/viewtopic.php?pid=762#p762
2. ^ http://www.slideshare.net/eCommConf/eric-klinker-presentation-at-emerging-communication-conference-awards-2010-america/11 uTP timeline, slide 11, "This Green Revolution-improving the yield of your network investment", eComm America Conference, San Francisco, CA, April 2010.
3. ^ µTorrent's switch to UDP and why the sky isn't falling
4. ^ uTorrent shifts towards UDP to make it work better
5. ^ http://forum.utorrent.com/viewtopic.php?id=76640
6. ^ https://github.com/bittorrent/libutp
7. ^ http://torrentfreak.com/ktorrent-first-bittorrent-client-to-adopt-open-source-utp-100528/
8. ^ http://ktorrent.org/?q=node/42
9. ^ https://libtorrent.svn.sourceforge.net/svnroot/libtorrent/branches/libtorrent_utp/
10. ^ http://libtorrent.svn.sourceforge.net/viewvc/libtorrent/branches/libtorrent_utp/
11. ^ http://code.google.com/p/libtorrent/issues/detail?id=6
12. ^ http://azureus.sourceforge.net/plugin_details.php?plugin=azutp
13. ^ https://trac.transmissionbt.com/ticket/2338
14. ^ http://forum.bittorrent.org/viewtopic.php?id=131
15. ^ Technical information about UDP
16. ^ http://forum.bittorrent.org/viewtopic.php?pid=762#p762
17. ^ "Post by developer "Greg Hazel" in thread "µTorrent 1.9 alpha"". 2008-11-26. http://forum.utorrent.com/viewtopic.php?pid=377209#p377209. Retrieved 2009-03-08. 
18. ^ "Low Extra Delay Background Transport (LEDBAT)". 2010-03-22. http://tools.ietf.org/html/draft-ietf-ledbat-congestion-01. Retrieved 2010-05-30. 
19. ^ http://forum.bittorrent.org/viewtopic.php?pid=753#p753

External links

• Official uTP protocol page
• BitTorrent Enhancement Proposal 29 - µTorrent transport protocol
• TCP-LP