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

     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 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 branch of the sysctl(3) MIB:

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

                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.

     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,, 63-74, July

     Stephen Bensley, Lars Eggert, and Dave Thaler, Microsoft's Datacenter TCP
     (DCTCP): TCP Congestion Control for Datacenters,

     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 [email protected] and Lars Eggert [email protected] with
     help and modifications from Hiren Panchasara [email protected]

FreeBSD 11.1-RELEASE-p4        January 12, 2015        FreeBSD 11.1-RELEASE-p4
Command Section