8ea5eeb913
Reviewed by: bcr, gbe MFC after: 3 days Sponsored by: Netflix, Inc. Differential Revision: https://reviews.freebsd.org/D30240
157 lines
5.4 KiB
Groff
157 lines
5.4 KiB
Groff
.\"
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.\" Copyright (c) 2013 Swinburne University of Technology, Melbourne, Australia
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.\" All rights reserved.
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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 May 13, 2021
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.Dt CC_CDG 4
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.Os
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.Sh NAME
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.Nm cc_cdg
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.Nd CDG Congestion Control Algorithm
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.Sh DESCRIPTION
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CAIA-Delay Gradient (CDG) is a hybrid congestion control algorithm which reacts
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to both packet loss and inferred queuing delay.
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It attempts to operate as a delay-based algorithm where possible, but utilises
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heuristics to detect loss-based TCP cross traffic and will compete effectively
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as required.
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CDG is therefore incrementally deployable and suitable for use on shared
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networks.
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.Pp
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During delay-based operation, CDG uses a delay-gradient based probabilistic
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backoff mechanism, and will also try to infer non congestion related
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packet losses and avoid backing off when they occur.
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During loss-based operation, CDG essentially reverts to
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.Xr cc_newreno 4 Ns - Ns like
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behaviour.
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.Pp
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CDG switches to loss-based operation when it detects that a configurable number
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of consecutive delay-based backoffs have had no measurable effect.
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It periodically attempts to return to delay-based operation, but will keep
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switching back to loss-based operation as required.
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.Sh MIB Variables
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The algorithm exposes the following variables in the
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.Va net.inet.tcp.cc.cdg
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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 exp_backoff_scale"
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.It Va version
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Current algorithm/implementation version number.
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.It Va beta_delay
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Delay-based window decrease factor as a percentage (on delay-based backoff, w =
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w * beta_delay / 100).
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Default is 70.
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.It Va beta_loss
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Loss-based window decrease factor as a percentage (on loss-based backoff, w =
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w * beta_loss / 100).
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Default is 50.
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.It Va exp_backoff_scale
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Scaling parameter for the probabilistic exponential backoff.
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Default is 2.
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.It Va smoothing_factor
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Number of samples used for moving average smoothing (0 means no smoothing).
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Default is 8.
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.It Va loss_compete_consec_cong
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Number of consecutive delay-gradient based congestion episodes which will
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trigger loss-based CC compatibility.
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Default is 5.
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.It Va loss_compete_hold_backoff
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Number of consecutive delay-gradient based congestion episodes to hold the
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window backoff for loss-based CC compatibility.
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Default is 5.
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.It Va alpha_inc
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If non-zero, this enables an experimental mode where CDG's window increase
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factor (alpha) is increased by 1 MSS every
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.Va alpha_inc
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RTTs during congestion avoidance mode.
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(Setting
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.Va alpha_inc
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to 1 results in the most aggressive growth of the window increase factor over
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time.
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Use higher
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.Va alpha_inc
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values for slower growth.)
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Default is 0.
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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_dctcp 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 "Revisiting TCP Congestion Control using Delay Gradients"
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.%J "Networking 2011 Proceedings, Part II"
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.%D "May 2011"
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.%P "328-341"
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.Re
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.Rs
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.%A "N. Khademi"
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.%A "G. Armitage"
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.%T "Minimising RTT across homogeneous 802.11 WLANs with CAIA Delay-Gradient TCP (v0.1)"
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.%R "CAIA Technical Report 121113A"
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.%D "November 2012"
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.%U "http://caia.swin.edu.au/reports/121113A/CAIA-TR-121113A.pdf"
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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 The Cisco University Research Program Fund, a
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corporate advised fund of Silicon Valley Community Foundation.
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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 9.2 .
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.Pp
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The module was first released in 2011 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 was written by
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.An David Hayes Aq Mt david.hayes@ieee.org .
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This manual page was written by
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.An Lawrence Stewart Aq Mt lstewart@FreeBSD.org
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and
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.An Grenville Armitage Aq Mt garmitage@swin.edu.au .
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.Sh BUGS
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The underlying algorithm and parameter values are still a work in progress and
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may not be optimal for some network scenarios.
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