Copyright (c) 2010-2011 The FreeBSD Foundation
All rights reserved.
This documentation was written at the Centre for Advanced Internet
Architectures, Swinburne University of Technology, Melbourne, Australia by
David Hayes under sponsorship from the FreeBSD Foundation.
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
1. Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
2. Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the distribution.
THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR
ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
SUCH DAMAGE.
$FreeBSD: head/share/man/man4/cc_hd.4 225583 2011-09-15 12:15:36Z lstewart $
.Dd September 15, 2011 .Dt CC_HD 4 .Os .Sh NAME .Nm cc_hd .Nd HD Congestion Control Algorithm .Sh DESCRIPTION The HD congestion control algorithm is an implementation of the Hamilton Institute's delay-based congestion control which aims to keep network queuing delays below a particular threshold (queue_threshold).
p HD probabilistically reduces the congestion window (cwnd) based on its estimate of the network queuing delay. The probability of reducing cwnd is zero at hd_qmin or less, rising to a maximum at queue_threshold, and then back to zero at the maximum queuing delay.
p Loss-based congestion control algorithms such as NewReno probe for network capacity by filling queues until there is a packet loss. HD competes with loss-based congestion control algorithms by allowing its probability of reducing cwnd to drop from a maximum at queue_threshold to be zero at the maximum queuing delay. This has been shown to work well when the bottleneck link is highly multiplexed. .Sh MIB Variables The algorithm exposes the following tunable variables in the .Va net.inet.tcp.cc.hd branch of the .Xr sysctl 3 MIB: l -tag -width ".Va queue_threshold" t Va queue_threshold Queueing congestion threshold (qth) in ticks. Default is 20. t Va pmax Per packet maximum backoff probability as a percentage. Default is 5. t Va qmin Minimum queuing delay threshold (qmin) in ticks. Default is 5. .El .Sh SEE ALSO .Xr cc_chd 4 , .Xr cc_cubic 4 , .Xr cc_htcp 4 , .Xr cc_newreno 4 , .Xr cc_vegas 4 , .Xr h_ertt 4 , .Xr mod_cc 4 , .Xr tcp 4 , .Xr khelp 9 , .Xr mod_cc 9 .Rs .%A "L. Budzisz" .%A "R. Stanojevic" .%A "R. Shorten" .%A "F. Baker" .%T "A strategy for fair coexistence of loss and delay-based congestion control algorithms" .%J "IEEE Commun. Lett." .%D "Jul 2009" .%V "13" .%N "7" .%P "555-557" .Re .Sh ACKNOWLEDGEMENTS Development and testing of this software were made possible in part by grants from the FreeBSD Foundation and Cisco University Research Program Fund at Community Foundation Silicon Valley. .Sh FUTURE WORK The Hamilton Institute have recently made some improvements to the algorithm implemented by this module and have called it Coexistent-TCP (C-TCP). The improvements should be evaluated and potentially incorporated into this module. .Sh HISTORY The .Nm congestion control module first appeared in .Fx 9.0 .
p The module was first released in 2010 by David Hayes whilst working on the NewTCP research project at Swinburne University of Technology's Centre for Advanced Internet Architectures, Melbourne, Australia. More details are available at:
p http://caia.swin.edu.au/urp/newtcp/ .Sh AUTHORS .An -nosplit The .Nm congestion control module and this manual page were written by .An David Hayes Aq david.hayes@ieee.org .