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Manual Pages  — CC_CHD


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:

MIB Variables

The algorithm exposes the following tunable variables in the net.inet.tcp.cc.chd branch of the sysctl(3) MIB:
  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 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, G. Armitage, in 35th Annual IEEE Conference on Local Computer Networks, 24-31, Improved coexistence and loss tolerance for delay based TCP congestion control, 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:



The cc_chd congestion control module and this manual page were written by David Hayes <Mt david.hayes@ieee.org>.

CC_CHD (4) September 15, 2011

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