LEDBAT
Low Extra Delay Background Transport (LEDBAT) is a way to transfer data on the Internet quickly without clogging the network.[1] LEDBAT was invented by Stanislav Shalunov[2][3] and is used by Apple for software updates, by BitTorrent for most of its transfers[4] and by Microsoft SCCM software distribution points.[5] At one point in time, LEDBAT was estimated to carry 13–20% of Internet traffic.[4][6][3]
LEDBAT is a delay-based congestion control algorithm that uses all the available bandwidth while limiting the increase in delay;[2][7] it does so by measuring one-way delay and using changes in the measurements to limit congestion that the LEDBAT flow itself induces in the network. LEDBAT is described in RFC 6817.
Design goals
[edit]LEDBAT congestion control has the following goals:[2]
- Use all available bandwidth, and to maintain a low queueing delay when no other traffic is present,
- Limit the queuing delay it adds to that induced by other traffic, and
- To yield quickly to standard TCP that share the same bottleneck link.
Implementations and deployment
[edit]The two main implementations are uTP by BitTorrent and as part of TCP by Apple. BitTorrent uses uTP for most traffic and makes the code available under an open-source license.[8] Apple uses LEDBAT for Software Updates so that large software downloads to macOS computers and iOS devices do not interfere with normal user activities; Apple also makes the source code available.[9]
Both of the above implementations aim to limit the network queuing delay to 100ms. This is the maximum allowed for by the standardized protocol. If one used a lower value, then it would be starved when the other was in use.[2][9]
Windows 10 Anniversary Update introduced support for LEDBAT via undocumented socket option as an experimental Windows TCP Congestion Control Module and Windows Server 2019.[10][11][12]
Example
[edit]Assumptions:
- The clocks of the sender and the receiver are not synchronized.
- The sender sends at a fixed rate.
The sender sends 5 packets of data every 10 clock counts: 10, 20, 30, 40, 50. The units are unimportant. The receiver is receiving data not only from this particular sender but also from other sources. For the 5 packets that were sent, the receiver receives them at the following clock counts: 112, 135, 176, 250, 326. The first differences (one way delay) between the received and sent clock counts are: 102, 115, 146, 210, 276. The second differences (change in one way delay) are: 13 (115 - 102), 31, 64 and 66. The receiver will infer from the positive increase in one way delays that congestion is increasing and adjust the transfer rate accordingly.
See also
[edit]References
[edit]- ^ McMillan, Robert. "How the Large Hadron Collider Will Bring the Internet to Everything". WIRED. Retrieved 2018-10-09.
- ^ a b c d Mirja, Kuehlewind; Greg, Hazel; Stanislav, Shalunov; Janardhan, Iyengar (December 2012). "Low Extra Delay Background Transport (LEDBAT)". tools.ietf.org. doi:10.17487/RFC6817.
- ^ a b Tammy Parker (December 10, 2012). "Open Garden to enable channel bonding over Wi-Fi, 3G and 4G - FierceWirelessTech". Archived from the original on 2016-03-30.
- ^ a b This Is How Your BitTorrent Downloads Move So Fast, July 29, 2013, Retrieved November 24, 2013
- ^ aczechowski. "Content management fundamentals - Configuration Manager". docs.microsoft.com. Retrieved 2019-04-02.
- ^ By Gabe Stein, Former BitTorrent Engineer Thinks He Can Fix Your Wi-Fi--For Good, July 24, 2013, Retrieved November 24, 2013
- ^ BY ROBERT MCMILLAN, THE INTERNET OF THINGS, July 21, 2013, Retrieved November 24, 2013
- ^ Libutp - The uTorrent Transport Protocol library, Retrieved November 24, 2013
- ^ a b "Tcp_ledbat.c".
- ^ "Announcing: New Transport Advancements in the Anniversary Update for Windows 10 and Windows Server 2016". Archived from the original on 2018-10-09. Retrieved 2017-04-17.
- ^ Microsoft (2017). "LEDBAT++: Low priority TCP Congestion Control in Windows" (PDF).
- ^ "Top 10 Networking Features in Windows Server 2019: #9 LEDBAT – Latency Optimized Background Transport". Archived from the original on 2018-08-17. Retrieved 2018-08-17.