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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	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 the RFC 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	The initial value to estimate the congestion on	the link.  The
		valid range is from 0 to 1024, where 1024 reduces the conges-
		tion 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 ob-
		served window.	Valid range from 0 to 10, the default is 4,
		resulting in an	effective gain of 1 / (	2 ^ shift_g ), or

     slowstart	A flag if the congestion window	should be reduced by one half
		after slow start.  Valid settings 0 and	1, default 0.

     cc_cdg(4),	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, Dave Thaler, Praveen Balasubramanian, Lars Eggert, and
     Glenn Judd, Data Center TCP (DCTCP): TCP Congestion Control for Data

     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

FreeBSD	13.0			 May 13, 2021			  FreeBSD 13.0


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