129f269dcSLawrence Stewart.\" 229f269dcSLawrence Stewart.\" Copyright (c) 2010-2011 The FreeBSD Foundation 329f269dcSLawrence Stewart.\" 429f269dcSLawrence Stewart.\" This documentation was written at the Centre for Advanced Internet 5891b8ed4SLawrence Stewart.\" Architectures, Swinburne University of Technology, Melbourne, Australia by 6891b8ed4SLawrence Stewart.\" David Hayes under sponsorship from the FreeBSD Foundation. 729f269dcSLawrence Stewart.\" 829f269dcSLawrence Stewart.\" Redistribution and use in source and binary forms, with or without 929f269dcSLawrence Stewart.\" modification, are permitted provided that the following conditions 1029f269dcSLawrence Stewart.\" are met: 1129f269dcSLawrence Stewart.\" 1. Redistributions of source code must retain the above copyright 1229f269dcSLawrence Stewart.\" notice, this list of conditions and the following disclaimer. 1329f269dcSLawrence Stewart.\" 2. Redistributions in binary form must reproduce the above copyright 1429f269dcSLawrence Stewart.\" notice, this list of conditions and the following disclaimer in the 1529f269dcSLawrence Stewart.\" documentation and/or other materials provided with the distribution. 1629f269dcSLawrence Stewart.\" 1729f269dcSLawrence Stewart.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 1829f269dcSLawrence Stewart.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 1929f269dcSLawrence Stewart.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 2029f269dcSLawrence Stewart.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR 2129f269dcSLawrence Stewart.\" ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 2229f269dcSLawrence Stewart.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 2329f269dcSLawrence Stewart.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 2429f269dcSLawrence Stewart.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 2529f269dcSLawrence Stewart.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 2629f269dcSLawrence Stewart.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 2729f269dcSLawrence Stewart.\" SUCH DAMAGE. 2829f269dcSLawrence Stewart.\" 29*8ea5eeb9SMichael Tuexen.Dd May 13, 2021 3029f269dcSLawrence Stewart.Dt CC_HD 4 3129f269dcSLawrence Stewart.Os 3229f269dcSLawrence Stewart.Sh NAME 3329f269dcSLawrence Stewart.Nm cc_hd 3429f269dcSLawrence Stewart.Nd HD Congestion Control Algorithm 3529f269dcSLawrence Stewart.Sh DESCRIPTION 3629f269dcSLawrence StewartThe HD congestion control algorithm is an implementation of the Hamilton 3729f269dcSLawrence StewartInstitute's delay-based congestion control which aims to keep network queuing 3829f269dcSLawrence Stewartdelays below a particular threshold (queue_threshold). 3929f269dcSLawrence Stewart.Pp 4029f269dcSLawrence StewartHD probabilistically reduces the congestion window (cwnd) based on its estimate 4129f269dcSLawrence Stewartof the network queuing delay. 4229f269dcSLawrence StewartThe probability of reducing cwnd is zero at hd_qmin or less, rising to a maximum 4329f269dcSLawrence Stewartat queue_threshold, and then back to zero at the maximum queuing delay. 4429f269dcSLawrence Stewart.Pp 4529f269dcSLawrence StewartLoss-based congestion control algorithms such as NewReno probe for network 4629f269dcSLawrence Stewartcapacity by filling queues until there is a packet loss. 4729f269dcSLawrence StewartHD competes with loss-based congestion control algorithms by allowing its 4829f269dcSLawrence Stewartprobability of reducing cwnd to drop from a maximum at queue_threshold to be 4929f269dcSLawrence Stewartzero at the maximum queuing delay. 5029f269dcSLawrence StewartThis has been shown to work well when the bottleneck link is highly multiplexed. 5129f269dcSLawrence Stewart.Sh MIB Variables 5229f269dcSLawrence StewartThe algorithm exposes the following tunable variables in the 5329f269dcSLawrence Stewart.Va net.inet.tcp.cc.hd 5429f269dcSLawrence Stewartbranch of the 5529f269dcSLawrence Stewart.Xr sysctl 3 5629f269dcSLawrence StewartMIB: 5729f269dcSLawrence Stewart.Bl -tag -width ".Va queue_threshold" 5829f269dcSLawrence Stewart.It Va queue_threshold 5929f269dcSLawrence StewartQueueing congestion threshold (qth) in ticks. 6029f269dcSLawrence StewartDefault is 20. 6129f269dcSLawrence Stewart.It Va pmax 6229f269dcSLawrence StewartPer packet maximum backoff probability as a percentage. 6329f269dcSLawrence StewartDefault is 5. 6429f269dcSLawrence Stewart.It Va qmin 6529f269dcSLawrence StewartMinimum queuing delay threshold (qmin) in ticks. 6629f269dcSLawrence StewartDefault is 5. 6729f269dcSLawrence Stewart.El 6829f269dcSLawrence Stewart.Sh SEE ALSO 69*8ea5eeb9SMichael Tuexen.Xr cc_cdg 4 , 7029f269dcSLawrence Stewart.Xr cc_chd 4 , 7129f269dcSLawrence Stewart.Xr cc_cubic 4 , 72*8ea5eeb9SMichael Tuexen.Xr cc_dctcp 4 , 7329f269dcSLawrence Stewart.Xr cc_htcp 4 , 7429f269dcSLawrence Stewart.Xr cc_newreno 4 , 7529f269dcSLawrence Stewart.Xr cc_vegas 4 , 7629f269dcSLawrence Stewart.Xr h_ertt 4 , 77f772f9feSLawrence Stewart.Xr mod_cc 4 , 7829f269dcSLawrence Stewart.Xr tcp 4 , 79f772f9feSLawrence Stewart.Xr khelp 9 , 80f772f9feSLawrence Stewart.Xr mod_cc 9 8129f269dcSLawrence Stewart.Rs 8229f269dcSLawrence Stewart.%A "L. Budzisz" 8329f269dcSLawrence Stewart.%A "R. Stanojevic" 8429f269dcSLawrence Stewart.%A "R. Shorten" 8529f269dcSLawrence Stewart.%A "F. Baker" 8629f269dcSLawrence Stewart.%T "A strategy for fair coexistence of loss and delay-based congestion control algorithms" 8729f269dcSLawrence Stewart.%J "IEEE Commun. Lett." 8829f269dcSLawrence Stewart.%D "Jul 2009" 8929f269dcSLawrence Stewart.%V "13" 9029f269dcSLawrence Stewart.%N "7" 9129f269dcSLawrence Stewart.%P "555-557" 9229f269dcSLawrence Stewart.Re 9329f269dcSLawrence Stewart.Sh ACKNOWLEDGEMENTS 9429f269dcSLawrence StewartDevelopment and testing of this software were made possible in part by grants 9529f269dcSLawrence Stewartfrom the FreeBSD Foundation and Cisco University Research Program Fund at 9629f269dcSLawrence StewartCommunity Foundation Silicon Valley. 9729f269dcSLawrence Stewart.Sh FUTURE WORK 9829f269dcSLawrence StewartThe Hamilton Institute have recently made some improvements to the algorithm 9929f269dcSLawrence Stewartimplemented by this module and have called it Coexistent-TCP (C-TCP). 100f93fa1cbSUlrich SpörleinThe improvements should be evaluated and potentially incorporated into this 10129f269dcSLawrence Stewartmodule. 10229f269dcSLawrence Stewart.Sh HISTORY 10329f269dcSLawrence StewartThe 10429f269dcSLawrence Stewart.Nm 10529f269dcSLawrence Stewartcongestion control module first appeared in 10629f269dcSLawrence Stewart.Fx 9.0 . 10729f269dcSLawrence Stewart.Pp 10829f269dcSLawrence StewartThe module was first released in 2010 by David Hayes whilst working on the 109891b8ed4SLawrence StewartNewTCP research project at Swinburne University of Technology's Centre for 110891b8ed4SLawrence StewartAdvanced Internet Architectures, Melbourne, Australia. 11129f269dcSLawrence StewartMore details are available at: 11229f269dcSLawrence Stewart.Pp 11329f269dcSLawrence Stewarthttp://caia.swin.edu.au/urp/newtcp/ 11429f269dcSLawrence Stewart.Sh AUTHORS 11529f269dcSLawrence Stewart.An -nosplit 11629f269dcSLawrence StewartThe 11729f269dcSLawrence Stewart.Nm 11829f269dcSLawrence Stewartcongestion control module and this manual page were written by 1196c899950SBaptiste Daroussin.An David Hayes Aq Mt david.hayes@ieee.org . 120