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_HD 4 33.Os 34.Sh NAME 35.Nm cc_hd 36.Nd HD Congestion Control Algorithm 37.Sh DESCRIPTION 38The HD congestion control algorithm is an implementation of the Hamilton 39Institute's delay-based congestion control which aims to keep network queuing 40delays below a particular threshold (queue_threshold). 41.Pp 42HD probabilistically reduces the congestion window (cwnd) based on its estimate 43of the network queuing delay. 44The probability of reducing cwnd is zero at hd_qmin or less, rising to a maximum 45at queue_threshold, and then back to zero at the maximum queuing delay. 46.Pp 47Loss-based congestion control algorithms such as NewReno probe for network 48capacity by filling queues until there is a packet loss. 49HD competes with loss-based congestion control algorithms by allowing its 50probability of reducing cwnd to drop from a maximum at queue_threshold to be 51zero at the maximum queuing delay. 52This has been shown to work well when the bottleneck link is highly multiplexed. 53.Sh MIB Variables 54The algorithm exposes the following tunable variables in the 55.Va net.inet.tcp.cc.hd 56branch of the 57.Xr sysctl 3 58MIB: 59.Bl -tag -width ".Va queue_threshold" 60.It Va queue_threshold 61Queueing congestion threshold (qth) in ticks. 62Default is 20. 63.It Va pmax 64Per packet maximum backoff probability as a percentage. 65Default is 5. 66.It Va qmin 67Minimum queuing delay threshold (qmin) in ticks. 68Default is 5. 69.El 70.Sh SEE ALSO 71.Xr cc_cdg 4 , 72.Xr cc_chd 4 , 73.Xr cc_cubic 4 , 74.Xr cc_dctcp 4 , 75.Xr cc_htcp 4 , 76.Xr cc_newreno 4 , 77.Xr cc_vegas 4 , 78.Xr h_ertt 4 , 79.Xr mod_cc 4 , 80.Xr tcp 4 , 81.Xr khelp 9 , 82.Xr mod_cc 9 83.Rs 84.%A "L. Budzisz" 85.%A "R. Stanojevic" 86.%A "R. Shorten" 87.%A "F. Baker" 88.%T "A strategy for fair coexistence of loss and delay-based congestion control algorithms" 89.%J "IEEE Commun. Lett." 90.%D "Jul 2009" 91.%V "13" 92.%N "7" 93.%P "555-557" 94.Re 95.Sh ACKNOWLEDGEMENTS 96Development and testing of this software were made possible in part by grants 97from the FreeBSD Foundation and Cisco University Research Program Fund at 98Community Foundation Silicon Valley. 99.Sh FUTURE WORK 100The Hamilton Institute have recently made some improvements to the algorithm 101implemented by this module and have called it Coexistent-TCP (C-TCP). 102The improvements should be evaluated and potentially incorporated into this 103module. 104.Sh HISTORY 105The 106.Nm 107congestion control module first appeared in 108.Fx 9.0 . 109.Pp 110The module was first released in 2010 by David Hayes whilst working on the 111NewTCP research project at Swinburne University of Technology's Centre for 112Advanced Internet Architectures, Melbourne, Australia. 113More details are available at: 114.Pp 115http://caia.swin.edu.au/urp/newtcp/ 116.Sh AUTHORS 117.An -nosplit 118The 119.Nm 120congestion control module and this manual page were written by 121.An David Hayes Aq Mt david.hayes@ieee.org . 122