Micro Transport Protocol

Micro Transport Protocol (μTP, sometimes uTP) is an open User Datagram Protocol-based (UDP-based) variant of the BitTorrent peer-to-peer file-sharing protocol intended to mitigate poor latency and other congestion control problems found in conventional BitTorrent over Transmission Control Protocol (TCP), while providing reliable, ordered delivery.

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 a DSL 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, founded by Stanislav Shalunov and Ben Teitelbaum and later 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.

Protocol

Traditional BitTorrent runs over TCP, which only slows down once packets are dropped. This leads to the user's modem buffers being full, and web browsing, calls or games on the same line get stuck behind BitTorrent traffic. μTP addresses this by running over UDP instead and handling reliability, ordering and congestion control by itself. It is specified in BitTorrent Enhancement Proposal 29.

Each packet starts with a 20-byte header:

FieldBitsDescription
type4Packet type
ver4Protocol version (currently 1)
extension8First extension header, if any
connection_id16Connection identifier
timestamp_microseconds32Send time
timestamp_difference_microseconds32Most recent measured delay
wnd_size32Receive window in bytes
seq_nr16Sequence number
ack_nr16Last packet received in order

There are five packet types: ST_DATA, ST_FIN, ST_STATE (an acknowledgment with no data), ST_RESET and ST_SYN. A connection opens with a two-way handshake: the initiator sends an ST_SYN and the other side answers with an ST_STATE. Unlike TCP, sequence numbers count packets, not bytes.

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.

See also

References

  1. ↑ "Forum.bittorrent.org / A few comments about µTP (BEP-29)". Archived from the original on March 1, 2010. Retrieved November 15, 2009.
  2. ↑ This Is How Your BitTorrent Downloads Move So Fast, Fastcolabs, 2013-07-29, Retrieved November 6, 2013
  3. ↑ 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.
  4. ↑ Beijnum, Iljitsch van (2008-12-02). "?Torrent's switch to UDP and why the sky isn't falling". Ars Technica. Retrieved 2021-05-11.
  5. ↑ "uTorrent shifts towards UDP to make it work better". www.thinkbroadband.com. 3 December 2008. Retrieved 2021-05-11.
  6. ↑ "Libutp, an open source implementation of µTP, has now been released (Page 1) / Announcements / µTorrent Community Forums". Archived from the original on 2011-01-23. Retrieved 2011-01-15.
  7. ↑ bittorrent/libutp, BitTorrent Inc., 2021-05-08, retrieved 2021-05-11
  8. ↑ "KTorrent First BitTorrent Client To Adopt Open Source uTP * TorrentFreak". Retrieved 2021-05-11.
  9. ↑ "KTorrent 4.0 is out". 24 May 2010. Archived from the original on 2015-04-02.
  10. ↑ "Google Code Archive - Long-term storage for Google Code Project Hosting". code.google.com. Retrieved 2021-05-11.
  11. ↑ "qBittorrent Official Website". www.qbittorrent.org. Archived from the original on 2019-12-13. Retrieved 2021-05-11.
  12. ↑ "Tixati.com - News". www.tixati.com. Retrieved 2021-05-11.
  13. ↑ "Version 4400 4702 Changelog - VuzeWiki". wiki.vuze.com. Archived from the original on 2015-04-02. Retrieved 2021-05-11.
  14. ↑ "Changes – Transmission". Archived from the original on 2015-05-24. Retrieved 2012-07-12.
  15. 1 2 3 4 Norberg, Arvid (2009-06-22). "BEP 29: uTorrent transport protocol". BitTorrent.org. Retrieved 2026-11-01.
  16. ↑ Testa, Claudio; Rossi, Dario; Rao, Ashwin; Legout, Arnaud (2012). Experimental Assessment of BitTorrent Completion Time in Heterogeneous TCP/uTP Swarms. Traffic Monitoring and Analysis (TMA 2012). Springer. pp. 52–65. arXiv:1110.5068.
  17. ↑ "Technical information about UDP". Archived from the original on 2009-05-12. Retrieved 2008-12-23.
  18. ↑ "Forum.bittorrent.org / A few comments about µTP (BEP-29)". Archived from the original on March 1, 2010. Retrieved November 15, 2009.
  19. ↑ "Post by developer "Greg Hazel" in thread "μTorrent 1.9 alpha"". 2008-11-26. Archived from the original on February 8, 2009. Retrieved 2009-03-08.
  20. ↑ "Forum.bittorrent.org / A few comments about µTP (BEP-29)". Archived from the original on July 25, 2011. Retrieved November 15, 2009.[full citation needed]
  21. ↑ "ΜTorrent 1.9 alpha 15380 (Page 1) / Announcements / µTorrent Community Forums". Archived from the original on March 25, 2013. Retrieved December 23, 2012.
  22. ↑ "Distributed UDP hole punching for the BitTorrent protocol (Page 1) / Protocol Design Discussion / µTorrent Community Forums". Archived from the original on June 19, 2013. Retrieved December 23, 2012.