1.\" 2.\" Copyright (c) 2010-2011 The FreeBSD Foundation 3.\" 4.\" This documentation was written at the Centre for Advanced Internet 5.\" Architectures, Swinburne University of Technology, Melbourne, Australia by 6.\" David Hayes under sponsorship from the FreeBSD Foundation. 7.\" 8.\" Redistribution and use in source and binary forms, with or without 9.\" modification, are permitted provided that the following conditions 10.\" are met: 11.\" 1. Redistributions of source code must retain the above copyright 12.\" notice, this list of conditions and the following disclaimer. 13.\" 2. Redistributions in binary form must reproduce the above copyright 14.\" notice, this list of conditions and the following disclaimer in the 15.\" documentation and/or other materials provided with the distribution. 16.\" 17.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 18.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 19.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 20.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR 21.\" ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 22.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 23.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 24.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 25.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 26.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 27.\" SUCH DAMAGE. 28.\" 29.\" $FreeBSD$ 30.\" 31.Dd May 13, 2021 32.Dt CC_CHD 4 33.Os 34.Sh NAME 35.Nm cc_chd 36.Nd CHD Congestion Control Algorithm 37.Sh DESCRIPTION 38CHD enhances the HD algorithm implemented in 39.Xr cc_hd 4 . 40It provides tolerance to non-congestion related packet loss and improvements to 41coexistence with traditional loss-based TCP flows, especially when the 42bottleneck link is lightly multiplexed. 43.Pp 44Like HD, the algorithm aims to keep network queuing delays below a particular 45threshold (queue_threshold) and decides to reduce the congestion window (cwnd) 46probabilistically based on its estimate of the network queuing delay. 47.Pp 48It differs from HD in three key aspects: 49.Bl -bullet 50.It 51The probability of cwnd reduction due to congestion is calculated once per round 52trip time instead of each time an acknowledgement is received as done by 53.Xr cc_hd 4 . 54.It 55Packet losses that occur while the queuing delay is less than queue_threshold 56do not cause cwnd to be reduced. 57.It 58CHD uses a shadow window to help regain lost transmission opportunities when 59competing with loss-based TCP flows. 60.El 61.Sh MIB Variables 62The algorithm exposes the following tunable variables in the 63.Va net.inet.tcp.cc.chd 64branch of the 65.Xr sysctl 3 66MIB: 67.Bl -tag -width ".Va queue_threshold" 68.It Va queue_threshold 69Queueing congestion threshold (qth) in ticks. 70Default is 20. 71.It Va pmax 72Per RTT maximum backoff probability as a percentage. 73Default is 50. 74.It Va qmin 75Minimum queuing delay threshold (qmin) in ticks. 76Default is 5. 77.It Va loss_fair 78If 1, cwnd is adjusted using the shadow window when a congestion 79related loss is detected. 80Default is 1. 81.It Va use_max 82If 1, the maximum RTT seen within the measurement period is used as the basic 83delay measurement for the algorithm, otherwise a sampled RTT measurement 84is used. 85Default is 1. 86.El 87.Sh SEE ALSO 88.Xr cc_cdg 4 , 89.Xr cc_cubic 4 , 90.Xr cc_dctcp 4 , 91.Xr cc_hd 4 , 92.Xr cc_htcp 4 , 93.Xr cc_newreno 4 , 94.Xr cc_vegas 4 , 95.Xr h_ertt 4 , 96.Xr mod_cc 4 , 97.Xr tcp 4 , 98.Xr khelp 9 , 99.Xr mod_cc 9 100.Rs 101.%A "D. A. Hayes" 102.%A "G. Armitage" 103.%T "Improved coexistence and loss tolerance for delay based TCP congestion control" 104.%J "in 35th Annual IEEE Conference on Local Computer Networks" 105.%D "October 2010" 106.%P "24-31" 107.Re 108.Sh ACKNOWLEDGEMENTS 109Development and testing of this software were made possible in part by grants 110from the FreeBSD Foundation and Cisco University Research Program Fund at 111Community Foundation Silicon Valley. 112.Sh HISTORY 113The 114.Nm 115congestion control module first appeared in 116.Fx 9.0 . 117.Pp 118The module was first released in 2010 by David Hayes whilst working on the 119NewTCP research project at Swinburne University of Technology's Centre for 120Advanced Internet Architectures, Melbourne, Australia. 121More details are available at: 122.Pp 123http://caia.swin.edu.au/urp/newtcp/ 124.Sh AUTHORS 125.An -nosplit 126The 127.Nm 128congestion control module and this manual page were written by 129.An David Hayes Aq Mt david.hayes@ieee.org . 130