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cc_chd(4)
CC_CHD(4) BSD Kernel Interfaces Manual CC_CHD(4)
NAME
cc_chd — CHD Congestion Control Algorithm
DESCRIPTION
CHD enhances the HD algorithm implemented in cc_hd(4). It provides tol‐
erance to non-congestion related packet loss and improvements to coexis‐
tence with traditional loss-based TCP flows, especially when the bottle‐
neck link is lightly multiplexed.
Like HD, the algorithm aims to keep network queuing delays below a par‐
ticular 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 opportuni‐
ties when competing with loss-based TCP flows.
MIB Variables
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 algo‐
rithm, otherwise a sampled RTT measurement is used.
Default is 1.
SEE ALSO
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.
ACKNOWLEDGEMENTS
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.
HISTORY
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 Cen‐
tre for Advanced Internet Architectures, Melbourne, Australia. More
details are available at:
http://caia.swin.edu.au/urp/newtcp/
AUTHORS
The cc_chd congestion control module and this manual page were written by
David Hayes <david.hayes@ieee.org>.
BSD September 15, 2011 BSD