MOD_CC(4) FreeBSD Kernel Interfaces Manual MOD_CC(4)
NAME
mod_cc - Modular congestion control
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 ascii(7)
name. Algorithm modules can be compiled into the kernel or loaded as
kernel modules using the kld(4) facility.
The default algorithm is CUBIC, and all connections use the default
unless explicitly overridden using the TCP_CONGESTION socket option (see
tcp(4) for details). The default can be changed using a sysctl(3) MIB
variable detailed in the MIB Variables section below.
Algorithm specific parameters can be set or queried using the
TCP_CCALGOOPT socket option (see tcp(4) for details). Callers must pass
a pointer to an algorithm specific data, and specify its size.
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.
MIB Variables
The framework exposes the following variables in the net.inet.tcp.cc
branch of the sysctl(3) MIB:
available Read-only list of currently available
congestion control algorithms by name.
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
net.inet.tcp.cc.available MIB variable.
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.
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.
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.
hystartplusplus.css_rounds This value controls the number of rounds
that CSS runs for. The default value
matches the current internet-draft of 5.
hystartplusplus.css_growth_div This value controls the divisor applied
to slowstart during CSS. The default
value matches the current internet-draft
of 4.
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.
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.
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.
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.
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.
Kernel Configuration Options
The framework exposes the following kernel configuration options.
CC_NEWRENO This directive loads the NewReno congestion control
algorithm.
CC_CUBIC This directive loads the CUBIC congestion control algorithm
and is included in GENERIC by default.
CC_VEGAS This directive loads the vegas congestion control algorithm,
note that this algorithm also requires the TCP_HHOOK option
as well.
CC_CDG This directive loads the cdg congestion control algorithm,
note that this algorithm also requires the TCP_HHOOK option
as well.
CC_DCTCP This directive loads the dctcp congestion control algorithm.
CC_HD This directive loads the hd congestion control algorithm,
note that this algorithm also requires the TCP_HHOOK option
as well.
CC_CHD This directive loads the chd congestion control algorithm,
note that this algorithm also requires the TCP_HHOOK option
as well.
CC_HTCP This directive loads the htcp congestion control algorithm.
CC_DEFAULT This directive specifies the string that represents the name
of the system default algorithm, the GENERIC kernel defaults
this to CUBIC.
SEE ALSO
cc_cdg(4), cc_chd(4), cc_cubic(4), cc_dctcp(4), cc_hd(4), cc_htcp(4),
cc_newreno(4), cc_vegas(4), tcp(4), config(5), config(8), mod_cc(9)
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.
HISTORY
The mod_cc modular congestion control framework first appeared in
FreeBSD 9.0.
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:
http://caia.swin.edu.au/urp/newtcp/
AUTHORS
The mod_cc facility was written by Lawrence Stewart
<lstewart@FreeBSD.org>, James Healy <jimmy@deefa.com> and David Hayes
<david.hayes@ieee.org>.
This manual page was written by David Hayes <david.hayes@ieee.org> and
Lawrence Stewart <lstewart@FreeBSD.org>.
FreeBSD 15.1-STABLE-HBSD September 13, 2022 MOD_CC(4)