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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	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.	De-
		      fault 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	algo-
     rithm 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 Cen-
     tre for Advanced Internet Architectures, Melbourne, Australia.  More de-
     tails are available at:

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

FreeBSD	13.0		      September	15, 2011		  FreeBSD 13.0


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