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MOD_CC(4) Device Drivers Manual MOD_CC(4)


Modular congestion control


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 branch of the sysctl(3) MIB:
Read-only list of currently available congestion control algorithms by name.
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 MIB variable.
Enable support for draft-ietf-tcpm-alternativebackoff-ecn, 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.
If non-zero, apply standard beta instead of ABE-beta during ECN-signalled congestion recovery episodes if loss also needs to be repaired.


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)


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.


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:


The mod_cc facility was written by Lawrence Stewart <>, James Healy <> and David Hayes <>.

This manual page was written by David Hayes <> and Lawrence Stewart <>.

March 19, 2018 Linux 4.19.0-6-amd64