Manual Reference Pages - CC_CHD (4)
- CHD Congestion Control Algorithm
CHD enhances the HD algorithm implemented in
It provides tolerance to non-congestion related packet loss and improvements to
coexistence with traditional loss-based TCP flows, especially when the
bottleneck link is lightly multiplexed.
Like HD, the algorithm aims to keep network queuing delays below a particular
threshold (queue_threshold) and decides to reduce the congestion window (cwnd)
probabilistically based on its estimate of the network queuing delay.
It differs from HD in three key aspects:
- The probability of cwnd reduction due to congestion is calculated once per round
trip time instead of each time an acknowledgement is received as done by
- Packet losses that occur while the queuing delay is less than queue_threshold
do not cause cwnd to be reduced.
- CHD uses a shadow window to help regain lost transmission opportunities when
competing with loss-based TCP flows.
The algorithm exposes the following tunable variables in the
branch of the
Queueing congestion threshold (qth) in ticks.
Default is 20.
Per RTT maximum backoff probability as a percentage.
Default is 50.
Minimum queuing delay threshold (qmin) in ticks.
Default is 5.
If 1, cwnd is adjusted using the shadow window when a congestion
related loss is detected.
Default is 1.
If 1, the maximum RTT seen within the measurement period is used as the basic
delay measurement for the algorithm, otherwise a sampled RTT measurement
Default is 1.
Improved coexistence and loss tolerance for delay based TCP congestion control
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.
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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