CC_CHD(4)	       FreeBSD 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.
		      Default 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.
		      Default is 1.


     cc(4), cc_cubic(4), cc_hd(4), cc_htcp(4), cc_newreno(4), cc_vegas(4),
     h_ertt(4), tcp(4), cc(9), khelp(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
     details are available at:


     The cc_chd 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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