2 .\" Copyright (c) 2014 Midori Kato <katoon@sfc.wide.ad.jp>
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6 .\" Portions of this documentation were written at Keio University, Japan.
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36 .Nd DCTCP Congestion Control Algorithm
38 The DCTCP (data center TCP) congestion control algorithm aims to maximise
39 throughput and minimise latency in data center networks by utilising the
40 proportion of Explicit Congestion Notification (ECN) marks received from capable
41 hardware as a congestion signal.
43 DCTCP uses fraction of ECN marked packets to update congestion window.
44 The window reduction ratio is always <= 1/2.
45 Only when all of the packets are
46 marked, congestion window is halved.
48 In order to keep the accuracy of the ECN marked fraction, a DCTCP receiver
49 mirrors back incoming (or missing) CE marks by setting (or clearing) ECE marks.
50 This feedback methodology is also adopted when the receiver uses delayed ACK.
54 DCTCP implementation includes two minor modifications for the one-sided
56 Considering the situation that DCTCP is used as sender and classic
57 ECN is used as receiver, DCTCP sets the CWR flag as the reaction to the ECE
59 In addition, when classic ECN is used as sender and DCTCP is used as
60 receiver, DCTCP avoids to mirror back ACKs only when the CWR flag is
61 set in the incoming packet.
63 The other specifications are based on the paper and the RFC referenced
68 The algorithm exposes the following tunable variables in the
69 .Va net.inet.tcp.cc.dctcp
73 .Bl -tag -width ".Va slowstart"
75 The initial value to estimate the congestion on the link.
76 The valid range is from 0 to 1024, where 1024 reduces the congestion
77 window to half, if a CE is observed in the first window and
79 could not yet adjust to the congestion level on that path.
82 An estimation gain in the
85 This influences the responsiveness when adjusting alpha
86 to the most recent observed window.
87 Valid range from 0 to 10, the default is 4, resulting in an effective
92 A flag if the congestion window should be reduced by one half after slow start.
93 Valid settings 0 and 1, default 0.
107 .%A "Mohammad Alizadeh"
108 .%A "Albert Greenberg"
110 .%A "Jitendra Padhye"
112 .%A "Balaji Prabhakar"
113 .%A "Sudipta Sengupta"
114 .%A "Murari Sridharan"
115 .%T "Data Center TCP (DCTCP)"
116 .%U "http://research.microsoft.com/pubs/121386/dctcp-public.pdf"
117 .%J "ACM SIGCOMM 2010"
122 .%A "Stephen Bensley"
124 .%A "Praveen Balasubramanian"
127 .%T "Data Center TCP (DCTCP): TCP Congestion Control for Data Centers"
128 .%U "https://tools.ietf.org/html/rfc8257"
133 congestion control module first appeared in
136 The module was first released in 2014 by Midori Kato studying at Keio
142 congestion control module and this manual page were written by
143 .An Midori Kato Mt katoon@sfc.wide.ad.jp
145 .An Lars Eggert Mt lars@netapp.com
146 with help and modifications from
147 .An Hiren Panchasara Mt hiren@FreeBSD.org