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.\"
.\" 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.
.\"
.\" $FreeBSD$
.\"
.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).
.Pp
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.
.Pp
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:
.Bl -tag -width ".Va queue_threshold"
.It Va queue_threshold
Queueing congestion threshold (qth) in ticks.
Default is 20.
.It Va pmax
Per packet maximum backoff probability as a percentage.
Default is 5.
.It 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 .
.Pp
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:
.Pp
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 .
|
