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CC_CHD(4)              FreeBSD Kernel Interfaces Manual              CC_CHD(4)

     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:

     +o   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).

     +o   Packet losses that occur while the queuing delay is less than
         queue_threshold do not cause cwnd to be reduced.

     +o   CHD uses a shadow window to help regain lost transmission
         opportunities when competing with loss-based TCP flows.

MIB Variables
     The algorithm exposes the following tunable variables in the 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

     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:

     The cc_chd congestion control module and this manual page were written by
     David Hayes <>.

FreeBSD 11.0-PRERELEASE       September 15, 2011       FreeBSD 11.0-PRERELEASE


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