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     cc_dctcp — DCTCP Congestion Control Algorithm


     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.

     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

     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.

     The FreeBSD 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.

     The other specifications are based on the paper and the RFC referenced in the SEE ALSO
     section below.

MIB Variables

     The algorithm exposes the following tunable variables in the branch of
     the sysctl(3) MIB:

     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 alpha could not yet adjust to the congestion
                level on that path.  Default is 1024.

     shift_g    An estimation gain in the 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 ^ shift_g ), or

     slowstart  A flag if the congestion window should be reduced by one half after slow start.
                Valid settings 0 and 1, default 0.


     cc_chd(4), cc_cubic(4), cc_hd(4), cc_htcp(4), cc_newreno(4), cc_vegas(4), mod_cc(4), tcp(4),

     Mohammad Alizadeh, Albert Greenberg, David A. Maltz, Jitendra Padhye, Parveen Patel, Balaji
     Prabhakar, Sudipta Sengupta, and Murari Sridharan, “Data Center TCP (DCTCP)”, ACM SIGCOMM
     2010,, 63-74, July 2010.

     Stephen Bensley, Dave Thaler, Praveen Balasubramanian, Lars Eggert, and Glenn Judd, Data
     Center TCP (DCTCP): TCP Congestion Control for Data Centers,


     The cc_dctcp congestion control module first appeared in FreeBSD 11.0.

     The module was first released in 2014 by Midori Kato studying at Keio University, Japan.


     The cc_dctcp congestion control module and this manual page were written by Midori Kato and Lars Eggert with help and modifications from Hiren