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.\" Copyright (c) 2014 Midori Kato <katoon@sfc.wide.ad.jp>
.\" Copyright (c) 2014 The FreeBSD Foundation
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.\" Portions of this documentation were written at Keio University, Japan.
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.Dd May 13, 2021
.Dt CC_DCTCP 4
.Os
.Sh NAME
.Nm cc_dctcp
.Nd DCTCP Congestion Control Algorithm
.Sh DESCRIPTION
The DCTCP (data center TCP) congestion control algorithm aims to maximise
throughput and minimise latency in data center networks by utilising the
proportion of Explicit Congestion Notification (ECN) marks received from capable
hardware as a congestion signal.
.Pp
DCTCP uses fraction of ECN marked packets to update congestion window.
The window reduction ratio is always <= 1/2.
Only when all of the packets are
marked, congestion window is halved.
.Pp
In order to keep the accuracy of the ECN marked fraction, a DCTCP receiver
mirrors back incoming (or missing) CE marks by setting (or clearing) ECE marks.
This feedback methodology is also adopted when the receiver uses delayed ACK.
.Pp
The
.Fx
DCTCP implementation includes two minor modifications for the one-sided
deployment.
Considering the situation that DCTCP is used as sender and classic
ECN is used as receiver, DCTCP sets the CWR flag as the reaction to the ECE
flag.
In addition, when classic ECN is used as sender and DCTCP is used as
receiver, DCTCP avoids to mirror back ACKs only when the CWR flag is
set in the incoming packet.
.Pp
The other specifications are based on the paper and the RFC referenced
in the
.Sx SEE ALSO
section below.
.Sh MIB Variables
The algorithm exposes the following tunable variables in the
.Va net.inet.tcp.cc.dctcp
branch of the
.Xr sysctl 3
MIB:
.Bl -tag -width ".Va slowstart"
.It Va alpha
The initial value to estimate the congestion on the link.
The valid range is from 0 to 1024, where 1024 reduces the congestion
window to half, if a CE is observed in the first window and
.Va alpha
could not yet adjust to the congestion level on that path.
Default is 1024.
.It Va shift_g
An estimation gain in the
.Va alpha
calculation.
This influences the responsiveness when adjusting alpha
to the most recent observed window.
Valid range from 0 to 10, the default is 4, resulting in an effective
gain of 1 / ( 2 ^
.Va shift_g
), or 1/16th.
.It Va slowstart
A flag if the congestion window should be reduced by one half after slow start.
Valid settings 0 and 1, default 0.
.El
.Sh SEE ALSO
.Xr cc_cdg 4 ,
.Xr cc_chd 4 ,
.Xr cc_cubic 4 ,
.Xr cc_hd 4 ,
.Xr cc_htcp 4 ,
.Xr cc_newreno 4 ,
.Xr cc_vegas 4 ,
.Xr mod_cc 4 ,
.Xr tcp 4 ,
.Xr mod_cc 9
.Rs
.%A "Mohammad Alizadeh"
.%A "Albert Greenberg"
.%A "David A. Maltz"
.%A "Jitendra Padhye"
.%A "Parveen Patel"
.%A "Balaji Prabhakar"
.%A "Sudipta Sengupta"
.%A "Murari Sridharan"
.%T "Data Center TCP (DCTCP)"
.%U "http://research.microsoft.com/pubs/121386/dctcp-public.pdf"
.%J "ACM SIGCOMM 2010"
.%D "July 2010"
.%P "63-74"
.Re
.Rs
.%A "Stephen Bensley"
.%A "Dave Thaler"
.%A "Praveen Balasubramanian"
.%A "Lars Eggert"
.%A "Glenn Judd"
.%T "Data Center TCP (DCTCP): TCP Congestion Control for Data Centers"
.%U "https://tools.ietf.org/html/rfc8257"
.Re
.Sh HISTORY
The
.Nm
congestion control module first appeared in
.Fx 11.0 .
.Pp
The module was first released in 2014 by Midori Kato studying at Keio
University, Japan.
.Sh AUTHORS
.An -nosplit
The
.Nm
congestion control module and this manual page were written by
.An Midori Kato Mt katoon@sfc.wide.ad.jp
and
.An Lars Eggert Mt lars@netapp.com
with help and modifications from
.An Hiren Panchasara Mt hiren@FreeBSD.org