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.Dd May 13, 2021 30.Dt CC_CHD 4 31.Os 32.Sh NAME 33.Nm cc_chd 34.Nd CHD Congestion Control Algorithm 35.Sh DESCRIPTION 36CHD enhances the HD algorithm implemented in 37.Xr cc_hd 4 . 38It provides tolerance to non-congestion related packet loss and improvements to 39coexistence with traditional loss-based TCP flows, especially when the 40bottleneck link is lightly multiplexed. 41.Pp 42Like HD, the algorithm aims to keep network queuing delays below a particular 43threshold (queue_threshold) and decides to reduce the congestion window (cwnd) 44probabilistically based on its estimate of the network queuing delay. 45.Pp 46It differs from HD in three key aspects: 47.Bl -bullet 48.It 49The probability of cwnd reduction due to congestion is calculated once per round 50trip time instead of each time an acknowledgement is received as done by 51.Xr cc_hd 4 . 52.It 53Packet losses that occur while the queuing delay is less than queue_threshold 54do not cause cwnd to be reduced. 55.It 56CHD uses a shadow window to help regain lost transmission opportunities when 57competing with loss-based TCP flows. 58.El 59.Sh MIB Variables 60The algorithm exposes the following tunable variables in the 61.Va net.inet.tcp.cc.chd 62branch of the 63.Xr sysctl 3 64MIB: 65.Bl -tag -width ".Va queue_threshold" 66.It Va queue_threshold 67Queueing congestion threshold (qth) in ticks. 68Default is 20. 69.It Va pmax 70Per RTT maximum backoff probability as a percentage. 71Default is 50. 72.It Va qmin 73Minimum queuing delay threshold (qmin) in ticks. 74Default is 5. 75.It Va loss_fair 76If 1, cwnd is adjusted using the shadow window when a congestion 77related loss is detected. 78Default is 1. 79.It Va use_max 80If 1, the maximum RTT seen within the measurement period is used as the basic 81delay measurement for the algorithm, otherwise a sampled RTT measurement 82is used. 83Default is 1. 84.El 85.Sh SEE ALSO 86.Xr cc_cdg 4 , 87.Xr cc_cubic 4 , 88.Xr cc_dctcp 4 , 89.Xr cc_hd 4 , 90.Xr cc_htcp 4 , 91.Xr cc_newreno 4 , 92.Xr cc_vegas 4 , 93.Xr h_ertt 4 , 94.Xr mod_cc 4 , 95.Xr tcp 4 , 96.Xr khelp 9 , 97.Xr mod_cc 9 98.Rs 99.%A "D. A. Hayes" 100.%A "G. Armitage" 101.%T "Improved coexistence and loss tolerance for delay based TCP congestion control" 102.%J "in 35th Annual IEEE Conference on Local Computer Networks" 103.%D "October 2010" 104.%P "24-31" 105.Re 106.Sh ACKNOWLEDGEMENTS 107Development and testing of this software were made possible in part by grants 108from the FreeBSD Foundation and Cisco University Research Program Fund at 109Community Foundation Silicon Valley. 110.Sh HISTORY 111The 112.Nm 113congestion control module first appeared in 114.Fx 9.0 . 115.Pp 116The module was first released in 2010 by David Hayes whilst working on the 117NewTCP research project at Swinburne University of Technology's Centre for 118Advanced Internet Architectures, Melbourne, Australia. 119More details are available at: 120.Pp 121http://caia.swin.edu.au/urp/newtcp/ 122.Sh AUTHORS 123.An -nosplit 124The 125.Nm 126congestion control module and this manual page were written by 127.An David Hayes Aq Mt david.hayes@ieee.org . 128