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CC_CHD(4)		 BSD Kernel Interfaces Manual		     CC_CHD(4)

     cc_chd -- CHD Congestion Control Algorithm

     CHD enhances the HD algorithm implemented in cc_hd(4).  It	provides tol-
     erance to non-congestion related packet loss and improvements to coexis-
     tence with	traditional loss-based TCP flows, especially when the bottle-
     neck link is lightly multiplexed.

     Like HD, the algorithm aims to keep network queuing delays	below a	par-
     ticular 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 opportuni-
	 ties 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.	De-
		      fault is 5.

     loss_fair	      If 1, cwnd is adjusted using the shadow window when a
		      congestion related loss is detected.  Default is 1.

     use_max	      If 1, the	maximum	RTT seen within	the measurement	period
		      is used as the basic delay measurement for the algo-
		      rithm, otherwise a sampled RTT measurement is used.  De-
		      fault 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 Cen-
     tre for Advanced Internet Architectures, Melbourne, Australia.  More de-
     tails are available at:

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

BSD			      September	15, 2011			   BSD


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