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 Internet Draft 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  An initial estimator of the congestion on the link.  Default is 0.

       dctcp_shift_g
              An estimation gain in the alpha calculation.  Default is 16.

       slowstart
              A trigger to halve congestion window after slow start.  Default does nothing to halve window.

SEE ALSO

       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, Lars Eggert, and Dave Thaler, Microsoft's Datacenter TCP (DCTCP): TCP Congestion Control
       for Datacenters, http://tools.ietf.org/html/draft-bensley-tcpm-dctcp-01.

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

Debian                                          January 12, 2015                                     CC_DCTCP(4)