1.\" 2.\" Copyright (c) 2014 Midori Kato <katoon@sfc.wide.ad.jp> 3.\" Copyright (c) 2014 The FreeBSD Foundation 4.\" All rights reserved. 5.\" 6.\" Portions of this documentation were written at Keio University, Japan. 7.\" 8.\" Redistribution and use in source and binary forms, with or without 9.\" modification, are permitted provided that the following conditions 10.\" are met: 11.\" 1. Redistributions of source code must retain the above copyright 12.\" notice, this list of conditions and the following disclaimer. 13.\" 2. Redistributions in binary form must reproduce the above copyright 14.\" notice, this list of conditions and the following disclaimer in the 15.\" documentation and/or other materials provided with the distribution. 16.\" 17.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 18.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 19.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 20.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR 21.\" ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 22.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 23.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 24.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 25.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 26.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 27.\" SUCH DAMAGE. 28.\" 29.Dd November 8, 2022 30.Dt CC_DCTCP 4 31.Os 32.Sh NAME 33.Nm cc_dctcp 34.Nd DCTCP Congestion Control Algorithm 35.Sh DESCRIPTION 36The DCTCP (data center TCP) congestion control algorithm aims to maximise 37throughput and minimise latency in data center networks by utilising the 38proportion of Explicit Congestion Notification (ECN) marks received from capable 39hardware as a congestion signal. 40.Pp 41DCTCP uses fraction of ECN marked packets to update congestion window. 42The window reduction ratio is always <= 1/2. 43Only when all of the packets are 44marked, congestion window is halved. 45.Pp 46In order to keep the accuracy of the ECN marked fraction, a DCTCP receiver 47mirrors back incoming (or missing) CE marks by setting (or clearing) ECE marks. 48This feedback methodology is also adopted when the receiver uses delayed ACK. 49.Pp 50The 51.Fx 52DCTCP implementation includes two minor modifications for the one-sided 53deployment. 54Considering the situation that DCTCP is used as sender and classic 55ECN is used as receiver, DCTCP sets the CWR flag as the reaction to the ECE 56flag. 57In addition, when classic ECN is used as sender and DCTCP is used as 58receiver, DCTCP avoids to mirror back ACKs only when the CWR flag is 59set in the incoming packet. 60.Pp 61The other specifications are based on the paper and the RFC referenced 62in the 63.Sx SEE ALSO 64section below. 65.Sh MIB Variables 66The algorithm exposes the following tunable variables in the 67.Va net.inet.tcp.cc.dctcp 68branch of the 69.Xr sysctl 3 70MIB: 71.Bl -tag -width ".Va slowstart" 72.It Va alpha 73The initial value to estimate the congestion on the link. 74The valid range is from 0 to 1024, where 1024 reduces the congestion 75window to half, if a CE is observed in the first window and 76.Va alpha 77could not yet adjust to the congestion level on that path. 78Default is 1024. 79.It Va shift_g 80An estimation gain in the 81.Va alpha 82calculation. 83This influences the responsiveness when adjusting alpha 84to the most recent observed window. 85Valid range from 0 to 10, the default is 4, resulting in an effective 86gain of 1 / ( 2 ^ 87.Va shift_g 88), or 1/16th. 89.It Va slowstart 90A flag if the congestion window should be reduced by one half after slow start. 91Valid settings 0 and 1, default 0. 92.It Va ect1 93Controls if a DCTCP session should use IP ECT(0) marking when sending out 94segments (default), or ECT(1) marking making use of L4S infrastructure. 95Changes to this setting will only affect new sessions, existing sessions will 96retain their previous marking value. 97.El 98.Sh SEE ALSO 99.Xr cc_cdg 4 , 100.Xr cc_chd 4 , 101.Xr cc_cubic 4 , 102.Xr cc_hd 4 , 103.Xr cc_htcp 4 , 104.Xr cc_newreno 4 , 105.Xr cc_vegas 4 , 106.Xr mod_cc 4 , 107.Xr tcp 4 , 108.Xr mod_cc 9 109.Rs 110.%A "Mohammad Alizadeh" 111.%A "Albert Greenberg" 112.%A "David A. Maltz" 113.%A "Jitendra Padhye" 114.%A "Parveen Patel" 115.%A "Balaji Prabhakar" 116.%A "Sudipta Sengupta" 117.%A "Murari Sridharan" 118.%T "Data Center TCP (DCTCP)" 119.%U "http://research.microsoft.com/pubs/121386/dctcp-public.pdf" 120.%J "ACM SIGCOMM 2010" 121.%D "July 2010" 122.%P "63-74" 123.Re 124.Rs 125.%A "Stephen Bensley" 126.%A "Dave Thaler" 127.%A "Praveen Balasubramanian" 128.%A "Lars Eggert" 129.%A "Glenn Judd" 130.%T "Data Center TCP (DCTCP): TCP Congestion Control for Data Centers" 131.%U "https://tools.ietf.org/html/rfc8257" 132.Re 133.Sh HISTORY 134The 135.Nm 136congestion control module first appeared in 137.Fx 11.0 . 138.Pp 139The module was first released in 2014 by Midori Kato studying at Keio 140University, Japan. 141.Sh AUTHORS 142.An -nosplit 143The 144.Nm 145congestion control module and this manual page were written by 146.An Midori Kato Mt katoon@sfc.wide.ad.jp 147and 148.An Lars Eggert Mt lars@netapp.com 149with help and modifications from 150.An Hiren Panchasara Mt hiren@FreeBSD.org 151