.\"
.\" 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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.\" $FreeBSD: head/share/man/man4/cc_dctcp.4 277063 2015-01-12 10:40:28Z brueffer $
.\"
.Dd January 12, 2015
.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 Internet Draft 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 alpha"
.It Va alpha
An initial estimator of the congestion on the link.
Default is 0.
.It Va dctcp_shift_g
An estimation gain in the alpha calculation.
Default is 16.
.It Va slowstart
A trigger to halve congestion window after slow start.
Default does nothing to halve window.
.El
.Sh SEE ALSO
.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 "Lars Eggert"
.%A "Dave Thaler"
.%T "Microsoft's Datacenter TCP (DCTCP): TCP Congestion Control for Datacenters"
.%U "http://tools.ietf.org/html/draft-bensley-tcpm-dctcp-01"
.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