129 lines
4.4 KiB
Groff
129 lines
4.4 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_CHD 4
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.Os
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.Sh NAME
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.Nm cc_chd
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.Nd CHD Congestion Control Algorithm
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.Sh DESCRIPTION
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CHD enhances the HD algorithm implemented in
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.Xr cc_hd 4 .
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It provides tolerance to non-congestion related packet loss and improvements to
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coexistence with traditional loss-based TCP flows, especially when the
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bottleneck link is lightly multiplexed.
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.Pp
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Like HD, the algorithm aims to keep network queuing delays below a particular
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threshold (queue_threshold) and decides to reduce the congestion window (cwnd)
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probabilistically based on its estimate of the network queuing delay.
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.Pp
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It differs from HD in three key aspects:
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.Bl -bullet
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.It
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The probability of cwnd reduction due to congestion is calculated once per round
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trip time instead of each time an acknowledgement is received as done by
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.Xr cc_hd 4 .
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.It
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Packet losses that occur while the queuing delay is less than queue_threshold
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do not cause cwnd to be reduced.
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.It
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CHD uses a shadow window to help regain lost transmission opportunities when
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competing with loss-based TCP flows.
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.El
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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.chd
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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 RTT maximum backoff probability as a percentage.
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Default is 50.
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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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.It Va loss_fair
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If 1, cwnd is adjusted using the shadow window when a congestion
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related loss is detected.
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Default is 1.
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.It Va use_max
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If 1, the maximum RTT seen within the measurement period is used as the basic
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delay measurement for the algorithm, otherwise a sampled RTT measurement
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is used.
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Default is 1.
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.El
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.Sh SEE ALSO
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.Xr cc_cubic 4 ,
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.Xr cc_hd 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 "D. A. Hayes"
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.%A "G. Armitage"
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.%T "Improved coexistence and loss tolerance for delay based TCP congestion control"
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.%J "in 35th Annual IEEE Conference on Local Computer Networks"
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.%D "October 2010"
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.%P "24-31"
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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 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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