LEDBAT (Low Extra Delay Background Transport) [RFC6817] is a congestion-control algorithm that implements a less-than-best-effort (LBE) traffic class.
When LEDBAT traffic shares a bottleneck with one or more TCP connections using standard congestion control algorithms such as Cubic [RFC8312] (hereafter standard-TCP for short), it reduces its sending rate earlier and more aggressively than standard-TCP congestion control, allowing standard-TCP traffic to use more of the available capacity. In the absence of competing standard-TCP traffic, LEDBAT aims to make an efficient use of the available capacity, while keeping the queuing delay within predefined bounds.
LEDBAT reacts both to packet loss and to variations in delay. Regarding to packet loss, LEDBAT reacts with a multiplicative decrease, similar to most TCP congestion controllers. Regarding
delay, LEDBAT aims for a target queueing delay. When the measured current queueing delay is below the target, LEDBAT increases the sending rate and when the delay is above the target, it reduces the sending rate. LEDBAT estimates the queuing delay by subtracting the measured current one-way delay from the estimated base one-way delay (i.e. the one way delay in the absence of queues).