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 net-
     work capacity by filling queues until there is a packet loss.  HD com-
     petes with loss-based congestion control algorithms by allowing its prob-
     ability 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(4), cc_chd(4), cc_cubic(4), cc_htcp(4), cc_newreno(4), cc_vegas(4),
     h_ertt(4), tcp(4), cc(9), khelp(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 algo-
     rithm implemented by this module and have called it Coexistent-TCP (C-
     TCP).  The improvments 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 Cen-
     tre 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 9.0		       February 15, 2011		   FreeBSD 9.0

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