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.\" This documentation was written at the Centre for Advanced Internet
.\" Architectures, Swinburne University of Technology, Melbourne, Australia by
.\" David Hayes and Lawrence Stewart under sponsorship from the FreeBSD
.\" Foundation.
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.\" $FreeBSD: releng/12.2/share/man/man4/mod_cc.4 352002 2019-09-07 12:00:17Z tuexen $
.\"
.Dd August 6, 2019
.Dt MOD_CC 4
.Os
.Sh NAME
.Nm mod_cc
.Nd Modular congestion control
.Sh DESCRIPTION
The modular congestion control framework allows the TCP implementation to
dynamically change the congestion control algorithm used by new and existing
connections.
Algorithms are identified by a unique
.Xr ascii 7
name.
Algorithm modules can be compiled into the kernel or loaded as kernel modules
using the
.Xr kld 4
facility.
.Pp
The default algorithm is NewReno, and all connections use the default unless
explicitly overridden using the
.Dv TCP_CONGESTION
socket option (see
.Xr tcp 4
for details).
The default can be changed using a
.Xr sysctl 3
MIB variable detailed in the
.Sx MIB Variables
section below.
.Pp
Algorithm specific parameters can be set or queried using the
.Dv TCP_CCALGOOPT
socket option (see
.Xr tcp 4
for details).
Callers must pass a pointer to an algorithm specific data, and specify
its size.
.Sh MIB Variables
The framework exposes the following variables in the
.Va net.inet.tcp.cc
branch of the
.Xr sysctl 3
MIB:
.Bl -tag -width ".Va abe_frlossreduce"
.It Va available
Read-only list of currently available congestion control algorithms by name.
.It Va algorithm
Returns the current default congestion control algorithm when read, and changes
the default when set.
When attempting to change the default algorithm, this variable should be set to
one of the names listed by the
.Va net.inet.tcp.cc.available
MIB variable.
.It Va abe
Enable support for RFC 8511,
which alters the window decrease factor applied to the congestion window in
response to an ECN congestion signal.
Refer to individual congestion control man pages to determine if they implement
support for ABE and for configuration details.
.It Va abe_frlossreduce
If non-zero, apply standard beta instead of ABE-beta during ECN-signalled
congestion recovery episodes if loss also needs to be repaired.
.El
.Sh SEE ALSO
.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 ,
.Xr cc_vegas 4 ,
.Xr tcp 4 ,
.Xr mod_cc 9
.Sh ACKNOWLEDGEMENTS
Development and testing of this software were made possible in part by grants
from the FreeBSD Foundation and Cisco University Research Program Fund at
Community Foundation Silicon Valley.
.Sh HISTORY
The
.Nm
modular congestion control framework first appeared in
.Fx 9.0 .
.Pp
The framework was first released in 2007 by James Healy and Lawrence Stewart
whilst working on the NewTCP research project at Swinburne University of
Technology's Centre for Advanced Internet Architectures, Melbourne, Australia,
which was made possible in part by a grant from the Cisco University Research
Program Fund at Community Foundation Silicon Valley.
More details are available at:
.Pp
http://caia.swin.edu.au/urp/newtcp/
.Sh AUTHORS
.An -nosplit
The
.Nm
facility was written by
.An Lawrence Stewart Aq Mt lstewart@FreeBSD.org ,
.An James Healy Aq Mt jimmy@deefa.com
and
.An David Hayes Aq Mt david.hayes@ieee.org .
.Pp
This manual page was written by
.An David Hayes Aq Mt david.hayes@ieee.org
and
.An Lawrence Stewart Aq Mt lstewart@FreeBSD.org .