1.\" 2.\" Copyright (c) 2010-2011 The FreeBSD Foundation 3.\" All rights reserved. 4.\" 5.\" This documentation was written at the Centre for Advanced Internet 6.\" Architectures, Swinburne University of Technology, Melbourne, Australia by 7.\" David Hayes under sponsorship from the FreeBSD Foundation. 8.\" 9.\" Redistribution and use in source and binary forms, with or without 10.\" modification, are permitted provided that the following conditions 11.\" are met: 12.\" 1. Redistributions of source code must retain the above copyright 13.\" notice, this list of conditions and the following disclaimer. 14.\" 2. Redistributions in binary form must reproduce the above copyright 15.\" notice, this list of conditions and the following disclaimer in the 16.\" documentation and/or other materials provided with the distribution. 17.\" 18.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 19.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 20.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 21.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR 22.\" ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 23.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 24.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 25.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 26.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 27.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 28.\" SUCH DAMAGE. 29.\" 30.\" $FreeBSD$ 31.\" 32.Dd May 13, 2021 33.Dt CC_CHD 4 34.Os 35.Sh NAME 36.Nm cc_chd 37.Nd CHD Congestion Control Algorithm 38.Sh DESCRIPTION 39CHD enhances the HD algorithm implemented in 40.Xr cc_hd 4 . 41It provides tolerance to non-congestion related packet loss and improvements to 42coexistence with traditional loss-based TCP flows, especially when the 43bottleneck link is lightly multiplexed. 44.Pp 45Like HD, the algorithm aims to keep network queuing delays below a particular 46threshold (queue_threshold) and decides to reduce the congestion window (cwnd) 47probabilistically based on its estimate of the network queuing delay. 48.Pp 49It differs from HD in three key aspects: 50.Bl -bullet 51.It 52The probability of cwnd reduction due to congestion is calculated once per round 53trip time instead of each time an acknowledgement is received as done by 54.Xr cc_hd 4 . 55.It 56Packet losses that occur while the queuing delay is less than queue_threshold 57do not cause cwnd to be reduced. 58.It 59CHD uses a shadow window to help regain lost transmission opportunities when 60competing with loss-based TCP flows. 61.El 62.Sh MIB Variables 63The algorithm exposes the following tunable variables in the 64.Va net.inet.tcp.cc.chd 65branch of the 66.Xr sysctl 3 67MIB: 68.Bl -tag -width ".Va queue_threshold" 69.It Va queue_threshold 70Queueing congestion threshold (qth) in ticks. 71Default is 20. 72.It Va pmax 73Per RTT maximum backoff probability as a percentage. 74Default is 50. 75.It Va qmin 76Minimum queuing delay threshold (qmin) in ticks. 77Default is 5. 78.It Va loss_fair 79If 1, cwnd is adjusted using the shadow window when a congestion 80related loss is detected. 81Default is 1. 82.It Va use_max 83If 1, the maximum RTT seen within the measurement period is used as the basic 84delay measurement for the algorithm, otherwise a sampled RTT measurement 85is used. 86Default is 1. 87.El 88.Sh SEE ALSO 89.Xr cc_cdg 4 , 90.Xr cc_cubic 4 , 91.Xr cc_dctcp 4 , 92.Xr cc_hd 4 , 93.Xr cc_htcp 4 , 94.Xr cc_newreno 4 , 95.Xr cc_vegas 4 , 96.Xr h_ertt 4 , 97.Xr mod_cc 4 , 98.Xr tcp 4 , 99.Xr khelp 9 , 100.Xr mod_cc 9 101.Rs 102.%A "D. A. Hayes" 103.%A "G. Armitage" 104.%T "Improved coexistence and loss tolerance for delay based TCP congestion control" 105.%J "in 35th Annual IEEE Conference on Local Computer Networks" 106.%D "October 2010" 107.%P "24-31" 108.Re 109.Sh ACKNOWLEDGEMENTS 110Development and testing of this software were made possible in part by grants 111from the FreeBSD Foundation and Cisco University Research Program Fund at 112Community Foundation Silicon Valley. 113.Sh HISTORY 114The 115.Nm 116congestion control module first appeared in 117.Fx 9.0 . 118.Pp 119The module was first released in 2010 by David Hayes whilst working on the 120NewTCP research project at Swinburne University of Technology's Centre for 121Advanced Internet Architectures, Melbourne, Australia. 122More details are available at: 123.Pp 124http://caia.swin.edu.au/urp/newtcp/ 125.Sh AUTHORS 126.An -nosplit 127The 128.Nm 129congestion control module and this manual page were written by 130.An David Hayes Aq Mt david.hayes@ieee.org . 131