freebsd-dev/share/man/man4/mod_cc.4
Lawrence Stewart 370efe5ac8 Add support for the experimental Internet-Draft "TCP Alternative Backoff with
ECN (ABE)" proposal to the New Reno congestion control algorithm module.
ABE reduces the amount of congestion window reduction in response to
ECN-signalled congestion relative to the loss-inferred congestion response.

More details about ABE can be found in the Internet-Draft:
https://tools.ietf.org/html/draft-ietf-tcpm-alternativebackoff-ecn

The implementation introduces four new sysctls:

- net.inet.tcp.cc.abe defaults to 0 (disabled) and can be set to non-zero to
  enable ABE for ECN-enabled TCP connections.

- net.inet.tcp.cc.newreno.beta and net.inet.tcp.cc.newreno.beta_ecn set the
  multiplicative window decrease factor, specified as a percentage, applied to
  the congestion window in response to a loss-based or ECN-based congestion
  signal respectively. They default to the values specified in the draft i.e.
  beta=50 and beta_ecn=80.

- net.inet.tcp.cc.abe_frlossreduce defaults to 0 (disabled) and can be set to
  non-zero to enable the use of standard beta (50% by default) when repairing
  loss during an ECN-signalled congestion recovery episode. It enables a more
  conservative congestion response and is provided for the purposes of
  experimentation as a result of some discussion at IETF 100 in Singapore.

The values of beta and beta_ecn can also be set per-connection by way of the
TCP_CCALGOOPT TCP-level socket option and the new CC_NEWRENO_BETA or
CC_NEWRENO_BETA_ECN CC algo sub-options.

Submitted by:	Tom Jones <tj@enoti.me>
Tested by:	Tom Jones <tj@enoti.me>, Grenville Armitage <garmitage@swin.edu.au>
Relnotes:	Yes
Differential Revision:	https://reviews.freebsd.org/D11616
2018-03-19 16:37:47 +00:00

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4.7 KiB
Groff

.\"
.\" Copyright (c) 2010-2011 The FreeBSD Foundation
.\" All rights reserved.
.\"
.\" This documentation was written at the Centre for Advanced Internet
.\" Architectures, Swinburne University of Technology, Melbourne, Australia by
.\" David Hayes and Lawrence Stewart under sponsorship from the FreeBSD
.\" Foundation.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in the
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.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" $FreeBSD$
.\"
.Dd March 19, 2018
.Dt MOD_CC 4
.Os
.Sh NAME
.Nm mod_cc
.Nd Modular congestion control
.Sh 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
.Xr ascii 7
name.
Algorithm modules can be compiled into the kernel or loaded as kernel modules
using the
.Xr kld 4
facility.
.Pp
The default algorithm is NewReno, and all connections use the default unless
explicitly overridden using the
.Dv TCP_CONGESTION
socket option (see
.Xr tcp 4
for details).
The default can be changed using a
.Xr sysctl 3
MIB variable detailed in the
.Sx MIB Variables
section below.
.Pp
Algorithm specific parameters can be set or queried using the
.Dv TCP_CCALGOOPT
socket option (see
.Xr tcp 4
for details).
Callers must pass a pointer to an algorithm specific data, and specify
its size.
.Sh MIB Variables
The framework exposes the following variables in the
.Va net.inet.tcp.cc
branch of the
.Xr sysctl 3
MIB:
.Bl -tag -width ".Va abe_frlossreduce"
.It Va available
Read-only list of currently available congestion control algorithms by name.
.It Va 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
.Va net.inet.tcp.cc.available
MIB variable.
.It Va abe
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.
.It Va 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.
.El
.Sh SEE ALSO
.Xr cc_cdg 4 ,
.Xr cc_chd 4 ,
.Xr cc_cubic 4 ,
.Xr cc_dctcp 4 ,
.Xr cc_hd 4 ,
.Xr cc_htcp 4 ,
.Xr cc_newreno 4 ,
.Xr cc_vegas 4 ,
.Xr tcp 4 ,
.Xr mod_cc 9
.Sh 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.
.Sh HISTORY
The
.Nm
modular congestion control framework first appeared in
.Fx 9.0 .
.Pp
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:
.Pp
http://caia.swin.edu.au/urp/newtcp/
.Sh AUTHORS
.An -nosplit
The
.Nm
facility was written by
.An Lawrence Stewart Aq Mt lstewart@FreeBSD.org ,
.An James Healy Aq Mt jimmy@deefa.com
and
.An David Hayes Aq Mt david.hayes@ieee.org .
.Pp
This manual page was written by
.An David Hayes Aq Mt david.hayes@ieee.org
and
.An Lawrence Stewart Aq Mt lstewart@FreeBSD.org .