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MOD_CC(4)		    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 un-
       less  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 de-
       fault 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 at-
				       tempting	to change  the	default	 algo-
				       rithm,  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 al-
				       ters the	window decrease	factor applied
				       to the congestion window	in response to
				       an ECN congestion signal.  Refer	to in-
				       dividual	congestion control  man	 pages
				       to  determine if	they implement support
				       for ABE and for configuration details.

       abe_frlossreduce		       If non-zero, apply  standard  beta  in-
				       stead  of ABE-beta during ECN-signalled
				       congestion recovery  episodes  if  loss
				       also needs to be	repaired.

       hystartplusplus.bblogs	       This boolean controls if	black box log-
				       ging 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 hys-
				       tart++ 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 sam-
				       ples must be collected  in  each	 round
				       for  hystart++  to  be active.  The de-
				       fault value matches the current	inter-
				       net-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  configutation  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 algo-
		   rithm.

       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	 algo-
		   rithm, 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	 algo-
		   rithm.

       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 Pro-
       gram 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 Uni-
       versity 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 Sil-
       icon 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	14.3		      September	13, 2022		     MOD_CC(4)

Want to link to this manual page? Use this URL:
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