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     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


     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