[IETF] [BitTorrent] Low Extra Delay Background Transport (LEDBAT)

Reference

[1] Low Extra Delay Background Transport (LEDBAT)

http://tools.ietf.org/html/rfc6817

What is LEDBAT?

LEDBAT, Developed by BitTorrent and approved by IETF in Dec.2012, is a new application layer congestion control algorithm, LEDBAT aims to enable the background bulk-transfer applications (e.g. large file transfers, software updating, etc) to be assigned a priority lower than standard TCP and interactive traffic (e.g. VoIP, online gaming, etc). It can be used with any application.

Basic idea

Standard TCP congestion control is activated when loss is detected or ECN mark is received, which occurs only when bottleneck link overflows or active queue management (ARQ) is applied. In summary, the increase in queueing delay precedes the phenomenon of loss and ECN marking. LEDBAT utilizes the increase in queueing delay as an early indication of congestion, enabling itselft to report congestion earlier than standard TCP.

LEDBAT employs one-way delay measurements to estimate queueing delay. When the estimated queueing delay is less than a predetermined target, LEDBAT infers that the network is not yet congested and increases its sending rate to utilize any spare capacity in the network.  When the estimated queueing delay becomes greater than the predetermined target, LEDBAT decreases its sending rate as a response to potential congestion in the network.

Queueing Delay Estimation

LEDBAT uses data packets to estimate queueing delay, without additional measurement packets. 

End-to-end delay consists of 1) transmission delay 2) propagation delay 3) processing delay 4) queueing delay. Except the queueing delay, the sum of the first three delays (base delay) are constant (in the absence of any route changes), thus, queueing delay is additive to the end-to-end delay. LEDBAT estimates the queuesing delay simply equals the difference of the end-to-end delay measurement and the current estimate of base delay. The queueing delay needs to be filtered to eliminated noise, such as spike in processing delay.

Features of LEDBAT

• avoid bulk-transfer flows from persistently delaying other flows sharing the same queue.

Applicability [1]

works with transport protocol and applications which are capable of carrying timestamps (for queueing delay estimation) and acknowledging data frequently (for reporting congestion), such as TCP, SCTP, DCCP, P2P over UDP, etc.