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