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freebsd-nq/share/man/man4/cc_newreno.4
Randall Stewart b8d60729de tcp: Congestion control cleanup. 
NOTE: HEADS UP read the note below if your kernel config is not including GENERIC!!

This patch does a bit of cleanup on TCP congestion control modules. There were some rather
interesting surprises that one could get i.e. where you use a socket option to change
from one CC (say cc_cubic) to another CC (say cc_vegas) and you could in theory get
a memory failure and end up on cc_newreno. This is not what one would expect. The
new code fixes this by requiring a cc_data_sz() function so we can malloc with M_WAITOK
and pass in to the init function preallocated memory. The CC init is expected in this
case *not* to fail but if it does and a module does break the
"no fail with memory given" contract we do fall back to the CC that was in place at the time.

This also fixes up a set of common newreno utilities that can be shared amongst other
CC modules instead of the other CC modules reaching into newreno and executing
what they think is a "common and understood" function. Lets put these functions in
cc.c and that way we have a common place that is easily findable by future developers or
bug fixers. This also allows newreno to evolve and grow support for its features i.e. ABE
and HYSTART++ without having to dance through hoops for other CC modules, instead
both newreno and the other modules just call into the common functions if they desire
that behavior or roll there own if that makes more sense.

Note: This commit changes the kernel configuration!! If you are not using GENERIC in
some form you must add a CC module option (one of CC_NEWRENO, CC_VEGAS, CC_CUBIC,
CC_CDG, CC_CHD, CC_DCTCP, CC_HTCP, CC_HD). You can have more than one defined
as well if you desire. Note that if you create a kernel configuration that does not
define a congestion control module and includes INET or INET6 the kernel compile will
break. Also you need to define a default, generic adds 'options CC_DEFAULT=\"newreno\"
but you can specify any string that represents the name of the CC module (same names
that show up in the CC module list under net.inet.tcp.cc). If you fail to add the
options CC_DEFAULT in your kernel configuration the kernel build will also break.

Reviewed by: Michael Tuexen
Sponsored by: Netflix Inc.
RELNOTES:YES
Differential Revision: https://reviews.freebsd.org/D32693
2021-11-11 06:28:18 -05:00

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.\"
.\" Copyright (c) 2009 Lawrence Stewart <lstewart@FreeBSD.org>
.\" Copyright (c) 2011 The FreeBSD Foundation
.\" All rights reserved.
.\"
.\" Portions of this documentation were written at the Centre for Advanced
.\" Internet Architectures, Swinburne University of Technology, Melbourne,
.\" Australia by 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
.\" documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR
.\" ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\" $FreeBSD$
.\"
.Dd May 13, 2021
.Dt CC_NEWRENO 4
.Os
.Sh NAME
.Nm cc_newreno
.Nd NewReno Congestion Control Algorithm
.Sh SYNOPSIS
.In netinet/cc/cc_newreno.h
.Sh DESCRIPTION
The NewReno congestion control algorithm is the default for TCP.
Details about the algorithm can be found in RFC5681.
.Sh Socket Options
The
.Nm
module supports a number of socket options under TCP_CCALGOOPT (refer to
.Xr tcp 4
and
.Xr mod_cc 9 for details)
which can
be set with
.Xr setsockopt 2
and tested with
.Xr getsockopt 2 .
The
.Nm
socket options use this structure defined in
<sys/netinet/cc/cc_newreno.h>:
.Bd -literal
struct cc_newreno_opts {
int name;
uint32_t val;
}
.Ed
.Bl -tag -width ".Va CC_NEWRENO_BETA_ECN"
.It Va CC_NEWRENO_BETA
Multiplicative window decrease factor, specified as a percentage, applied to
the congestion window in response to a congestion signal per: cwnd = (cwnd *
CC_NEWRENO_BETA) / 100.
Default is 50.
.It Va CC_NEWRENO_BETA_ECN
Multiplicative window decrease factor, specified as a percentage, applied to
the congestion window in response to an ECN congestion signal when
.Va net.inet.tcp.cc.abe=1
per: cwnd = (cwnd * CC_NEWRENO_BETA_ECN) / 100.
Default is 80.
.It Va CC_NEWRENO_ENABLE_HYSTART
will enable or disable the application of Hystart++.
The current implementation allows the values 0, 1, 2 and 3.
A value of 0 (the default) disables the use of Hystart++.
Setting the value to 1 enables Hystart++.
Setting the value to 2 enables Hystart++ but also will cause, on exit from Hystart++'s CSS, to
set the cwnd to the value of where the increase in RTT first began as
well as setting ssthresh to the flight at send when we exit CSS.
Setting a value of 3 will keep the setting of the cwnd the same as 2, but will cause ssthresh
to be set to the average value between the lowest fas rtt (the value cwnd is
set to) and the fas value at exit of CSS.
.PP
Note that currently the only way to enable
hystart++ is to enable it via socket option.
When enabling it a value of 1 will enable precise internet-draft behavior
(subject to any MIB variable settings), other setting (2 and 3) are experimental.
.El
.PP
Note that hystart++ requires the TCP stack be able to call to the congestion
controller with both the
.Va newround
function as well as the
.Va rttsample
function.
Currently the only TCP stacks that provide this feedback to the
congestion controller is rack.
.Pp
.Sh MIB Variables
The algorithm exposes these variables in the
.Va net.inet.tcp.cc.newreno
branch of the
.Xr sysctl 3
MIB:
.Bl -tag -width ".Va beta_ecn"
.It Va beta
Multiplicative window decrease factor, specified as a percentage, applied to
the congestion window in response to a congestion signal per: cwnd = (cwnd *
beta) / 100.
Default is 50.
.It Va beta_ecn
Multiplicative window decrease factor, specified as a percentage, applied to
the congestion window in response to an ECN congestion signal when
.Va net.inet.tcp.cc.abe=1
per: cwnd = (cwnd * beta_ecn) / 100.
Default is 80.
.It Va hystartplusplus.bblogs
This boolean controls if black box logging 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 hystart++ events.
.It Va hystartplusplus.css_rounds
This value controls the number of rounds that CSS runs for.
The default value matches the current internet-draft of 5.
.It Va hystartplusplus.css_growth_div
This value controls the divisor applied to slowstart during CSS.
The default value matches the current internet-draft of 4.
.It Va hystartplusplus.n_rttsamples
This value controls how many rtt samples must be collected in each round for
hystart++ to be active.
The default value matches the current internet-draft of 8.
.It Va 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.
.It Va 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.
.It Va hystartplusplus.lowcwnd
This value controls what is the lowest congestion window that the tcp
stack must be at before hystart++ engages.
The default value matches the current internet-draft of 16.
.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_vegas 4 ,
.Xr mod_cc 4 ,
.Xr tcp 4 ,
.Xr mod_cc 9
.Rs
.%A "Mark Allman"
.%A "Vern Paxson"
.%A "Ethan Blanton"
.%T "TCP Congestion Control"
.%O "RFC 5681"
.Re
.Rs
.%A "Naeem Khademi"
.%A "Michael Welzl"
.%A "Grenville Armitage"
.%A "Gorry Fairhurst"
.%T "TCP Alternative Backoff with ECN (ABE)"
.%O "RFC 8511"
.Re
.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
congestion control algorithm first appeared in its modular form in
.Fx 9.0 .
.Pp
The module 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
congestion control module was written by
.An James Healy Aq Mt jimmy@deefa.com ,
.An Lawrence Stewart Aq Mt lstewart@FreeBSD.org
and
.An David Hayes Aq Mt david.hayes@ieee.org .
.Pp
Support for TCP ABE was added by
.An Tom Jones Aq Mt tj@enoti.me .
.Pp
This manual page was written by
.An Lawrence Stewart Aq Mt lstewart@FreeBSD.org .
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