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CC_HD(4)               FreeBSD Kernel Interfaces Manual               CC_HD(4)

     cc_hd - HD Congestion Control Algorithm

     The HD congestion control algorithm is an implementation of the Hamilton
     Institute's delay-based congestion control which aims to keep network
     queuing delays below a particular threshold (queue_threshold).

     HD probabilistically reduces the congestion window (cwnd) based on its
     estimate of the network queuing delay.  The probability of reducing cwnd
     is zero at hd_qmin or less, rising to a maximum at queue_threshold, and
     then back to zero at the maximum queuing delay.

     Loss-based congestion control algorithms such as NewReno probe for
     network capacity by filling queues until there is a packet loss.  HD
     competes with loss-based congestion control algorithms by allowing its
     probability of reducing cwnd to drop from a maximum at queue_threshold to
     be zero at the maximum queuing delay.  This has been shown to work well
     when the bottleneck link is highly multiplexed.

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 packet maximum backoff probability as a
                          percentage.  Default is 5.

     qmin                 Minimum queuing delay threshold (qmin) in ticks.
                          Default is 5.

     cc_chd(4), cc_cubic(4), cc_htcp(4), cc_newreno(4), cc_vegas(4),
     h_ertt(4), mod_cc(4), tcp(4), khelp(9), mod_cc(9)

     L. Budzisz, R. Stanojevic, R. Shorten, and F. Baker, "A strategy for fair
     coexistence of loss and delay-based congestion control algorithms", IEEE
     Commun. Lett., 7, 13, 555-557, Jul 2009.

     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 Hamilton Institute have recently made some improvements to the
     algorithm implemented by this module and have called it Coexistent-TCP
     (C-TCP).  The improvements should be evaluated and potentially
     incorporated into this module.

     The cc_hd 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_hd 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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