Manual Reference Pages - CC_HD (4)
- HD Congestion Control Algorithm
The HD congestion control algorithm is an implementation of the Hamilton
Institutes delay-based congestion control which aims to keep network queuing
delays below a particular threshold (queue_threshold).
HD probabilistically reduces the congestion window (cwnd) based on its estimate
of the network queuing delay.
The probability of reducing cwnd is zero at hd_qmin or less, rising to a maximum
at queue_threshold, and then back to zero at the maximum queuing delay.
Loss-based congestion control algorithms such as NewReno probe for network
capacity by filling queues until there is a packet loss.
HD competes with loss-based congestion control algorithms by allowing its
probability of reducing cwnd to drop from a maximum at queue_threshold to be
zero at the maximum queuing delay.
This has been shown to work well when the bottleneck link is highly multiplexed.
The algorithm exposes the following tunable variables in the
branch of the
Queueing congestion threshold (qth) in ticks.
Default is 20.
Per packet maximum backoff probability as a percentage.
Default is 5.
Minimum queuing delay threshold (qmin) in ticks.
Default is 5.
A strategy for fair coexistence of loss and delay-based congestion control algorithms
Development and testing of this software were made possible in part by grants
from the FreeBSD Foundation and Cisco University Research Program Fund at
Community Foundation Silicon Valley.
The Hamilton Institute have recently made some improvements to the algorithm
implemented by this module and have called it Coexistent-TCP (C-TCP).
The improvements should be evaluated and potentially incorporated into this
congestion control module first appeared in
.Fx 9.0 .
The module was first released in 2010 by David Hayes whilst working on the
NewTCP research project at Swinburne University of Technologys Centre for
Advanced Internet Architectures, Melbourne, Australia.
More details are available at:
congestion control module and this manual page were written by
.An David Hayes Aq firstname.lastname@example.org .
Visit the GSP FreeBSD Man Page Interface.
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