cc_dctcp — DCTCP Congestion Control Algorithm

       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

Debian                                            July 29, 2019                                      CC_DCTCP(4)

       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.

       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 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.

       cc_dctcp — DCTCP Congestion Control Algorithm

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)”,   ACMSIGCOMM2010,
       http://research.microsoft.com/pubs/121386/dctcp-public.pdf, 63-74, July 2010.

       Stephen Bensley, Dave Thaler, Praveen Balasubramanian, Lars Eggert,  and  Glenn  Judd,  DataCenterTCP(DCTCP):TCPCongestionControlforDataCenters, https://tools.ietf.org/html/rfc8257.