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.\"
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.\" Copyright (c) 2014 Midori Kato <katoon@sfc.wide.ad.jp>
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.\" Copyright (c) 2014 The FreeBSD Foundation
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.\" All rights reserved.
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.\"
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.\" Portions of this documentation were written at Keio University, Japan.
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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 January 12, 2015
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.Dt CC_DCTCP 4
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.Os
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.Sh NAME
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.Nm cc_dctcp
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.Nd DCTCP Congestion Control Algorithm
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.Sh DESCRIPTION
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The DCTCP (data center TCP) congestion control algorithm aims to maximise
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throughput and minimise latency in data center networks by utilising the
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proportion of Explicit Congestion Notification (ECN) marks received from capable
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hardware as a congestion signal.
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.Pp
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DCTCP uses fraction of ECN marked packets to update congestion window. The
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window reduction ratio is always <= 1/2. Only when all of the packets are
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marked, congestion window is halved.
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.Pp
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In order to keep the accuracy of the ECN marked fraction, a DCTCP receiver
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mirrors back incoming (or missing) CE marks by setting (or clearing) ECE marks.
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This feedback methodology is also adopted when the receiver uses delayed ACK.
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.Pp
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FreeBSD DCTCP implementation includes two minor modifications for the one-sided
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deployment. Considering the situation that DCTCP is used as sender and classic
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ECN is used as receiver, DCTCP sets the CWR flag as the reaction to the ECE
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flag. In addition, when classic ECN is used as sender and DCTCP is used as
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receiver, DCTCP avoids to mirror back ACKs only when the CWR flag is
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set in the incoming packet.
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.Pp
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The other specifications are based on the paper and Internet Draft referenced
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in the
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.Sx SEE ALSO
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section below.
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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.dctcp
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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 alpha"
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.It Va alpha
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An initial estimator of the congestion on the link.
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Default is 0.
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.It Va dctcp_shift_g
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An estimation gain in the alpha calculation.
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Default is 16.
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.It Va slowstart
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A trigger to halve congestion window after slow start.
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Default does nothing to halve window.
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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_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 mod_cc 4 ,
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.Xr tcp 4 ,
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.Xr mod_cc 9
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.Rs
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.%A "Mohammad Alizadeh"
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.%A "Albert Greenberg"
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.%A "David A. Maltz"
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.%A "Jitendra Padhye"
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.%A "Parveen Patel"
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.%A "Balaji Prabhakar"
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.%A "Sudipta Sengupta"
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.%A "Murari Sridharan"
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.%T "Data Center TCP (DCTCP)"
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.%U "http://research.microsoft.com/pubs/121386/dctcp-public.pdf"
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.%J "ACM SIGCOMM 2010"
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.%D "July 2010"
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.%P "63-74"
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.Re
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.Rs
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.%A "Stephen Bensley"
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.%A "Lars Eggert"
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.%A "Dave Thaler"
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.%T "Microsoft's Datacenter TCP (DCTCP): TCP Congestion Control for Datacenters"
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.%U "http://tools.ietf.org/html/draft-bensley-tcpm-dctcp-01"
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.Re
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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 11.0 .
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.Pp
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The module was first released in 2014 by Midori Kato studying at Keio
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University, Japan.
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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 Midori Kato katoon@sfc.wide.ad.jp
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and
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.An Lars Eggert lars@netapp.com
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with help and modifications from
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.An Hiren Panchasara hiren@FreeBSD.org
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