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CC_DCTCP(4)		 BSD 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	pack-
     ets are marked, congestion	window is halved.

     In	order to keep the accuracy of the ECN marked fraction, a DCTCP re-
     ceiver mirrors back incoming (or missing) CE marks	by setting (or clear-
     ing) ECE marks.  This feedback methodology	is also	adopted	when the re-
     ceiver 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	refer-
     enced 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.

	    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 and Lars Eggert with
     help and modifications from Hiren Panchasara

BSD			       January 12, 2015				   BSD


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