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.Dd September 13, 2022
.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 CUBIC, 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.
.Pp
Unloading a congestion control module will fail if it is used as a
default by any Vnet.
When unloading a module, the Vnet default is
used to switch a connection to an alternate congestion control.
Note that the new congestion control module may fail to initialize its
internal memory, if so it will fail the module unload.
If this occurs often times retrying the unload will succeed since the temporary
memory shortage as the new CC module malloc's memory, that prevented the
switch is often transient.
.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 hystartplusplus.css_growth_div"
.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.
.It Va hystartplusplus.bblogs
This boolean controls if black box logging will be done for hystart++ events.
If set to zero (the default) no logging is performed.
If set to one then black box logs will be generated on all hystart++ events.
.It Va hystartplusplus.css_rounds
This value controls the number of rounds that CSS runs for.
The default value matches the current internet-draft of 5.
.It Va hystartplusplus.css_growth_div
This value controls the divisor applied to slowstart during CSS.
The default value matches the current internet-draft of 4.
.It Va hystartplusplus.n_rttsamples
This value controls how many rtt samples must be collected in each round for
hystart++ to be active.
The default value matches the current internet-draft of 8.
.It Va hystartplusplus.maxrtt_thresh
This value controls the maximum rtt variance clamp when considering if CSS is needed.
The default value matches the current internet-draft of 16000 (in microseconds).
For further explanation please see the internet-draft.
.It Va hystartplusplus.minrtt_thresh
This value controls the minimum rtt variance clamp when considering if CSS is needed.
The default value matches the current internet-draft of 4000 (in microseconds).
For further explanation please see the internet-draft.
.El
.Pp
Each congestion control module may also expose other MIB variables
to control their behaviour.
Note that both NewReno and CUBIC now support Hystart++ based on the version 3 of the internet-draft.
.Sh Kernel Configuration
All of the available congestion control modules may also be loaded
via kernel configuration options.
A kernel configuration is required to have at least one congestion control
algorithm built into it via kernel option and a system default specified.
Compilation of the kernel will fail if these two conditions are not met.
.Sh Kernel Configuration Options
The framework exposes the following kernel configuration options.
.Bl -tag -width ".Va CC_NEWRENO"
.It Va CC_NEWRENO
This directive loads the NewReno congestion control algorithm.
.It Va CC_CUBIC
This directive loads the CUBIC congestion control algorithm and is included
in GENERIC by default.
.It Va CC_VEGAS
This directive loads the vegas congestion control algorithm, note that
this algorithm also requires the TCP_HHOOK option as well.
.It Va CC_CDG
This directive loads the cdg congestion control algorithm, note that
this algorithm also requires the TCP_HHOOK option as well.
.It Va CC_DCTCP
This directive loads the dctcp congestion control algorithm.
.It Va CC_HD
This directive loads the hd congestion control algorithm, note that
this algorithm also requires the TCP_HHOOK option as well.
.It Va CC_CHD
This directive loads the chd congestion control algorithm, note that
this algorithm also requires the TCP_HHOOK option as well.
.It Va CC_HTCP
This directive loads the htcp congestion control algorithm.
.It Va CC_DEFAULT
This directive specifies the string that represents the name of the system default algorithm, the GENERIC kernel
defaults this to CUBIC.
.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 config 5 ,
.Xr config 8 ,
.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 .