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_HD 4 31.Os 32.Sh NAME 33.Nm cc_hd 34.Nd HD Congestion Control Algorithm 35.Sh DESCRIPTION 36The HD congestion control algorithm is an implementation of the Hamilton 37Institute's delay-based congestion control which aims to keep network queuing 38delays below a particular threshold (queue_threshold). 39.Pp 40HD probabilistically reduces the congestion window (cwnd) based on its estimate 41of the network queuing delay. 42The probability of reducing cwnd is zero at hd_qmin or less, rising to a maximum 43at queue_threshold, and then back to zero at the maximum queuing delay. 44.Pp 45Loss-based congestion control algorithms such as NewReno probe for network 46capacity by filling queues until there is a packet loss. 47HD competes with loss-based congestion control algorithms by allowing its 48probability of reducing cwnd to drop from a maximum at queue_threshold to be 49zero at the maximum queuing delay. 50This has been shown to work well when the bottleneck link is highly multiplexed. 51.Sh MIB Variables 52The algorithm exposes the following tunable variables in the 53.Va net.inet.tcp.cc.hd 54branch of the 55.Xr sysctl 3 56MIB: 57.Bl -tag -width ".Va queue_threshold" 58.It Va queue_threshold 59Queueing congestion threshold (qth) in ticks. 60Default is 20. 61.It Va pmax 62Per packet maximum backoff probability as a percentage. 63Default is 5. 64.It Va qmin 65Minimum queuing delay threshold (qmin) in ticks. 66Default is 5. 67.El 68.Sh SEE ALSO 69.Xr cc_cdg 4 , 70.Xr cc_chd 4 , 71.Xr cc_cubic 4 , 72.Xr cc_dctcp 4 , 73.Xr cc_htcp 4 , 74.Xr cc_newreno 4 , 75.Xr cc_vegas 4 , 76.Xr h_ertt 4 , 77.Xr mod_cc 4 , 78.Xr tcp 4 , 79.Xr khelp 9 , 80.Xr mod_cc 9 81.Rs 82.%A "L. Budzisz" 83.%A "R. Stanojevic" 84.%A "R. Shorten" 85.%A "F. Baker" 86.%T "A strategy for fair coexistence of loss and delay-based congestion control algorithms" 87.%J "IEEE Commun. Lett." 88.%D "Jul 2009" 89.%V "13" 90.%N "7" 91.%P "555-557" 92.Re 93.Sh ACKNOWLEDGEMENTS 94Development and testing of this software were made possible in part by grants 95from the FreeBSD Foundation and Cisco University Research Program Fund at 96Community Foundation Silicon Valley. 97.Sh FUTURE WORK 98The Hamilton Institute have recently made some improvements to the algorithm 99implemented by this module and have called it Coexistent-TCP (C-TCP). 100The improvements should be evaluated and potentially incorporated into this 101module. 102.Sh HISTORY 103The 104.Nm 105congestion control module first appeared in 106.Fx 9.0 . 107.Pp 108The module was first released in 2010 by David Hayes whilst working on the 109NewTCP research project at Swinburne University of Technology's Centre for 110Advanced Internet Architectures, Melbourne, Australia. 111More details are available at: 112.Pp 113http://caia.swin.edu.au/urp/newtcp/ 114.Sh AUTHORS 115.An -nosplit 116The 117.Nm 118congestion control module and this manual page were written by 119.An David Hayes Aq Mt david.hayes@ieee.org . 120