mod_cc4freebsd

Overview

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.

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.