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