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.

     ect1       Controls if a DCTCP session should use IP ECT(0) marking when
                sending out segments (default), or ECT(1) marking making use
                of L4S infrastructure.  Changes to this setting will only
                affect new sessions, existing sessions will retain their
                previous marking value.

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 15.1-STABLE-HBSD       November 8, 2022                    CC_DCTCP(4)