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 NewReno, 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.

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.

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), 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 13.1-RELEASE-p6         August 6, 2019         FreeBSD 13.1-RELEASE-p6