cc_chd —
CHD
Congestion Control Algorithm
CHD enhances the HD algorithm implemented in
cc_hd(4). 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
cc_hd(4).
- 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
net.inet.tcp.cc.chd branch of the
sysctl(3) MIB:
- queue_threshold
- Queueing congestion threshold (qth) in ticks. Default is
20.
- pmax
- Per RTT maximum backoff probability as a percentage.
Default is 50.
- qmin
- Minimum queuing delay threshold (qmin) in ticks. Default is
5.
- loss_fair
- If 1, cwnd is adjusted using the shadow window when a
congestion related loss is detected. Default is 1.
- use_max
- 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 is used. Default is 1.
cc_cubic(4),
cc_hd(4),
cc_htcp(4),
cc_newreno(4),
cc_vegas(4),
h_ertt(4),
mod_cc(4),
tcp(4),
khelp(9),
mod_cc(9)
D. A. Hayes and
G. Armitage, Improved coexistence
and loss tolerance for delay based TCP congestion control,
in 35th Annual IEEE Conference on Local Computer Networks,
24-31, October 2010.
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
cc_chd congestion control module first appeared
in
FreeBSD 9.0.
The module was first released in 2010 by David Hayes whilst working on the
NewTCP research project at Swinburne University of Technology's Centre for
Advanced Internet Architectures, Melbourne, Australia. More details are
available at:
http://caia.swin.edu.au/urp/newtcp/
The
cc_chd congestion control module and this
manual page were written by
David Hayes
<
[email protected]>.