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, Melbourne, Australia by David Hayes 7.\" 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 February 15, 2011 33.Dt CC_HD 4 34.Os 35.Sh NAME 36.Nm cc_hd 37.Nd HD Congestion Control Algorithm 38.Sh DESCRIPTION 39The HD congestion control algorithm is an implementation of the Hamilton 40Institute's delay-based congestion control which aims to keep network queuing 41delays below a particular threshold (queue_threshold). 42.Pp 43HD probabilistically reduces the congestion window (cwnd) based on its estimate 44of the network queuing delay. 45The probability of reducing cwnd is zero at hd_qmin or less, rising to a maximum 46at queue_threshold, and then back to zero at the maximum queuing delay. 47.Pp 48Loss-based congestion control algorithms such as NewReno probe for network 49capacity by filling queues until there is a packet loss. 50HD competes with loss-based congestion control algorithms by allowing its 51probability of reducing cwnd to drop from a maximum at queue_threshold to be 52zero at the maximum queuing delay. 53This has been shown to work well when the bottleneck link is highly multiplexed. 54.Sh MIB Variables 55The algorithm exposes the following tunable variables in the 56.Va net.inet.tcp.cc.hd 57branch of the 58.Xr sysctl 3 59MIB: 60.Bl -tag -width ".Va queue_threshold" 61.It Va queue_threshold 62Queueing congestion threshold (qth) in ticks. 63Default is 20. 64.It Va pmax 65Per packet maximum backoff probability as a percentage. 66Default is 5. 67.It Va qmin 68Minimum queuing delay threshold (qmin) in ticks. 69Default is 5. 70.El 71.Sh SEE ALSO 72.Xr cc 4 , 73.Xr cc_chd 4 , 74.Xr cc_cubic 4 , 75.Xr cc_htcp 4 , 76.Xr cc_newreno 4 , 77.Xr cc_vegas 4 , 78.Xr h_ertt 4 , 79.Xr tcp 4 , 80.Xr cc 9 , 81.Xr khelp 9 82.Rs 83.%A "L. Budzisz" 84.%A "R. Stanojevic" 85.%A "R. Shorten" 86.%A "F. Baker" 87.%T "A strategy for fair coexistence of loss and delay-based congestion control algorithms" 88.%J "IEEE Commun. Lett." 89.%D "Jul 2009" 90.%V "13" 91.%N "7" 92.%P "555-557" 93.Re 94.Sh ACKNOWLEDGEMENTS 95Development and testing of this software were made possible in part by grants 96from the FreeBSD Foundation and Cisco University Research Program Fund at 97Community Foundation Silicon Valley. 98.Sh FUTURE WORK 99The Hamilton Institute have recently made some improvements to the algorithm 100implemented by this module and have called it Coexistent-TCP (C-TCP). 101The improvments should be evaluated and potentially incorporated into this 102module. 103.Sh HISTORY 104The 105.Nm 106congestion control module first appeared in 107.Fx 9.0 . 108.Pp 109The module was first released in 2010 by David Hayes whilst working on the 110NewTCP research project at Swinburne University's Centre for Advanced Internet 111Architectures, Melbourne, Australia. 112More details are available at: 113.Pp 114http://caia.swin.edu.au/urp/newtcp/ 115.Sh AUTHORS 116.An -nosplit 117The 118.Nm 119congestion control module and this manual page were written by 120.An David Hayes Aq david.hayes@ieee.org . 121