DCTCP (Data Center TCP) implementation.

DCTCP congestion control algorithm aims to maximise throughput and minimise
latency in data center networks by utilising the proportion of Explicit
Congestion Notification (ECN) marked packets received from capable hardware as a
congestion signal.

Highlights:
Implemented as a mod_cc(4) module.
ECN (Explicit congestion notification) processing is done differently from
RFC3168.
Takes one-sided DCTCP into consideration where only one of the sides is using
DCTCP and other is using standard ECN.

IETF draft: http://tools.ietf.org/html/draft-bensley-tcpm-dctcp-00
Thesis report by Midori Kato: https://eggert.org/students/kato-thesis.pdf

Submitted by:	Midori Kato <katoon@sfc.wide.ad.jp> and
		Lars Eggert <lars@netapp.com>
		with help and modifications from
		hiren
Differential Revision:	https://reviews.freebsd.org/D604
Reviewed by:	gnn

3 files changed, 130 insertions, 1 deletions

diff --git a/share/man/man4/Makefile b/share/man/man4/Makefile
index b646906..f93cdb2 100644
--- a/share/man/man4/Makefile
+++ b/share/man/man4/Makefile
@@ -86,6 +86,7 @@ MAN=	aac.4 \
 	cc_cdg.4 \
 	cc_chd.4 \
 	cc_cubic.4 \
+	cc_dctcp.4 \
 	cc_hd.4 \
 	cc_htcp.4 \
 	cc_newreno.4 \
diff --git a/share/man/man4/cc_dctcp.4 b/share/man/man4/cc_dctcp.4
new file mode 100644
index 0000000..07587b7
--- /dev/null
+++ b/share/man/man4/cc_dctcp.4
@@ -0,0 +1,127 @@
+.\"
+.\" Copyright (c) 2014 Midori Kato <katoon@sfc.wide.ad.jp>
+.\" Copyright (c) 2014 The FreeBSD Foundation
+.\" All rights reserved.
+.\"
+.\" Portions of this documentation were written at Keio University, Japan.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR
+.\" ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+.\" SUCH DAMAGE.
+.\"
+.\" $FreeBSD$
+.\"
+.Dd January 12, 2015
+.Dt CC_DCTCP 4
+.Os
+.Sh NAME
+.Nm cc_dctcp
+.Nd DCTCP Congestion Control Algorithm
+.Sh 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.
+.Pp
+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.
+.Pp
+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.
+.Pp
+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.
+.Pp
+The other specifications are based on the paper and Internet Draft referenced
+in the
+.Sx SEE ALSO
+section below.
+.Sh MIB Variables
+The algorithm exposes the following tunable variables in the
+.Va net.inet.tcp.cc.dctcp
+branch of the
+.Xr sysctl 3
+MIB:
+.Bl -tag -width ".Va alpha"
+.It Va alpha
+An initial estimator of the congestion on the link.
+Default is 0.
+.It Va dctcp_shift_g
+An estimation gain in the alpha calculation.
+Default is 16.
+.It Va slowstart
+A trigger to halve congestion window after slow start.
+Default does nothing to halve window.
+.Sh SEE ALSO
+.Xr cc_chd 4 ,
+.Xr cc_cubic 4 ,
+.Xr cc_hd 4 ,
+.Xr cc_htcp 4 ,
+.Xr cc_newreno 4 ,
+.Xr cc_vegas 4 ,
+.Xr mod_cc 4 ,
+.Xr tcp 4 ,
+.Xr mod_cc 9
+.Rs
+.%A "Mohammad Alizadeh"
+.%A "Albert Greenberg"
+.%A "David A. Maltz"
+.%A "Jitendra Padhye"
+.%A "Parveen Patel"
+.%A "Balaji Prabhakar"
+.%A "Sudipta Sengupta"
+.%A "Murari Sridharan"
+.%T "Data Center TCP (DCTCP)"
+.%U "http://research.microsoft.com/pubs/121386/dctcp-public.pdf"
+.%J "ACM SIGCOMM 2010"
+.%D "July 2010"
+.%P "63-74"
+.Re
+.Rs
+.%A "Stephen Bensley"
+.%A "Lars Eggert"
+.%A "Dave Thaler"
+.%T "Microsoft's Datacenter TCP (DCTCP): TCP Congestion Control for Datacenters"
+.%U "http://tools.ietf.org/html/draft-bensley-tcpm-dctcp-01"
+.Re
+.Sh HISTORY
+The
+.Nm
+congestion control module first appeared in
+.Fx 11.0 .
+.Pp
+The module was first released in 2014 by Midori Kato studying at Keio
+University, Japan.
+.Sh AUTHORS
+.An -nosplit
+The
+.Nm
+congestion control module and this manual page were written by
+.An Midori Kato katoon@sfc.wide.ad.jp
+and
+.An Lars Eggert lars@netapp.com
+with help and modifications from
+.An Hiren Panchasara hiren@FreeBSD.org
diff --git a/share/man/man4/mod_cc.4 b/share/man/man4/mod_cc.4
index b280d36..f5f4493 100644
--- a/share/man/man4/mod_cc.4
+++ b/share/man/man4/mod_cc.4
@@ -30,7 +30,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd December 26, 2014
+.Dd January 12, 2015
 .Dt MOD_CC 4
 .Os
 .Sh NAME
@@ -78,6 +78,7 @@ MIB variable.
 .Xr cc_cdg 4 ,
 .Xr cc_chd 4 ,
 .Xr cc_cubic 4 ,
+.Xr cc_dctcp 4 ,
 .Xr cc_hd 4 ,
 .Xr cc_htcp 4 ,
 .Xr cc_newreno 4 ,