CC_DCTCP(4)            FreeBSD Kernel Interfaces Manual            CC_DCTCP(4)

NAME
     cc_dctcp - DCTCP Congestion Control Algorithm

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.

     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.

     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
     net.inet.tcp.cc.dctcp 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 1/16th.

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

SEE ALSO
     cc_cdg(4), cc_chd(4), cc_cubic(4), cc_hd(4), cc_htcp(4), cc_newreno(4),
     cc_vegas(4), mod_cc(4), tcp(4), mod_cc(9)

     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,
     http://research.microsoft.com/pubs/121386/dctcp-public.pdf, 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, https://tools.ietf.org/html/rfc8257.

HISTORY
     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.

AUTHORS
     The cc_dctcp congestion control module and this manual page were written
     by Midori Kato katoon@sfc.wide.ad.jp and Lars Eggert lars@netapp.com with
     help and modifications from Hiren Panchasara hiren@FreeBSD.org

FreeBSD 13.1-RELEASE-p6          May 13, 2021          FreeBSD 13.1-RELEASE-p6