1.\" 2.\" Copyright (c) 2009 Lawrence Stewart <lstewart@FreeBSD.org> 3.\" Copyright (c) 2011 The FreeBSD Foundation 4.\" All rights reserved. 5.\" 6.\" Portions of this documentation were written at the Centre for Advanced 7.\" Internet Architectures, Swinburne University of Technology, Melbourne, 8.\" Australia by Lawrence Stewart under sponsorship from the FreeBSD Foundation. 9.\" 10.\" Redistribution and use in source and binary forms, with or without 11.\" modification, are permitted provided that the following conditions 12.\" are met: 13.\" 1. Redistributions of source code must retain the above copyright 14.\" notice, this list of conditions and the following disclaimer. 15.\" 2. Redistributions in binary form must reproduce the above copyright 16.\" notice, this list of conditions and the following disclaimer in the 17.\" documentation and/or other materials provided with the distribution. 18.\" 19.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 20.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 21.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 22.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR 23.\" ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 24.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 25.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 26.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 27.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 28.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 29.\" SUCH DAMAGE. 30.\" 31.\" $FreeBSD$ 32.\" 33.Dd May 13, 2021 34.Dt CC_NEWRENO 4 35.Os 36.Sh NAME 37.Nm cc_newreno 38.Nd NewReno Congestion Control Algorithm 39.Sh SYNOPSIS 40.In netinet/cc/cc_newreno.h 41.Sh DESCRIPTION 42The NewReno congestion control algorithm is the default for TCP. 43Details about the algorithm can be found in RFC5681. 44.Sh Socket Options 45The 46.Nm 47module supports a number of socket options under TCP_CCALGOOPT (refer to 48.Xr tcp 4 49and 50.Xr mod_cc 9 for details) 51which can 52be set with 53.Xr setsockopt 2 54and tested with 55.Xr getsockopt 2 . 56The 57.Nm 58socket options use this structure defined in 59<sys/netinet/cc/cc_newreno.h>: 60.Bd -literal 61struct cc_newreno_opts { 62 int name; 63 uint32_t val; 64} 65.Ed 66.Bl -tag -width ".Va CC_NEWRENO_BETA_ECN" 67.It Va CC_NEWRENO_BETA 68Multiplicative window decrease factor, specified as a percentage, applied to 69the congestion window in response to a congestion signal per: cwnd = (cwnd * 70CC_NEWRENO_BETA) / 100. 71Default is 50. 72.It Va CC_NEWRENO_BETA_ECN 73Multiplicative window decrease factor, specified as a percentage, applied to 74the congestion window in response to an ECN congestion signal when 75.Va net.inet.tcp.cc.abe=1 76per: cwnd = (cwnd * CC_NEWRENO_BETA_ECN) / 100. 77Default is 80. 78.It Va CC_NEWRENO_ENABLE_HYSTART 79will enable or disable the application of Hystart++. 80The current implementation allows the values 0, 1, 2 and 3. 81A value of 0 (the default) disables the use of Hystart++. 82Setting the value to 1 enables Hystart++. 83Setting the value to 2 enables Hystart++ but also will cause, on exit from Hystart++'s CSS, to 84set the cwnd to the value of where the increase in RTT first began as 85well as setting ssthresh to the flight at send when we exit CSS. 86Setting a value of 3 will keep the setting of the cwnd the same as 2, but will cause ssthresh 87to be set to the average value between the lowest fas rtt (the value cwnd is 88set to) and the fas value at exit of CSS. 89.PP 90Note that currently the only way to enable 91hystart++ is to enable it via socket option. 92When enabling it a value of 1 will enable precise internet-draft behavior 93(subject to any MIB variable settings), other setting (2 and 3) are experimental. 94.El 95.PP 96Note that hystart++ requires the TCP stack be able to call to the congestion 97controller with both the 98.Va newround 99function as well as the 100.Va rttsample 101function. 102Currently the only TCP stacks that provide this feedback to the 103congestion controller is rack. 104.Pp 105.Sh MIB Variables 106The algorithm exposes these variables in the 107.Va net.inet.tcp.cc.newreno 108branch of the 109.Xr sysctl 3 110MIB: 111.Bl -tag -width ".Va beta_ecn" 112.It Va beta 113Multiplicative window decrease factor, specified as a percentage, applied to 114the congestion window in response to a congestion signal per: cwnd = (cwnd * 115beta) / 100. 116Default is 50. 117.It Va beta_ecn 118Multiplicative window decrease factor, specified as a percentage, applied to 119the congestion window in response to an ECN congestion signal when 120.Va net.inet.tcp.cc.abe=1 121per: cwnd = (cwnd * beta_ecn) / 100. 122Default is 80. 123.It Va hystartplusplus.bblogs 124This boolean controls if black box logging will be done for hystart++ events. If set 125to zero (the default) no logging is performed. 126If set to one then black box logs will be generated on all hystart++ events. 127.It Va hystartplusplus.css_rounds 128This value controls the number of rounds that CSS runs for. 129The default value matches the current internet-draft of 5. 130.It Va hystartplusplus.css_growth_div 131This value controls the divisor applied to slowstart during CSS. 132The default value matches the current internet-draft of 4. 133.It Va hystartplusplus.n_rttsamples 134This value controls how many rtt samples must be collected in each round for 135hystart++ to be active. 136The default value matches the current internet-draft of 8. 137.It Va hystartplusplus.maxrtt_thresh 138This value controls the maximum rtt variance clamp when considering if CSS is needed. 139The default value matches the current internet-draft of 16000 (in microseconds). 140For further explanation please see the internet-draft. 141.It Va hystartplusplus.minrtt_thresh 142This value controls the minimum rtt variance clamp when considering if CSS is needed. 143The default value matches the current internet-draft of 4000 (in microseconds). 144For further explanation please see the internet-draft. 145.It Va hystartplusplus.lowcwnd 146This value controls what is the lowest congestion window that the tcp 147stack must be at before hystart++ engages. 148The default value matches the current internet-draft of 16. 149.El 150.Sh SEE ALSO 151.Xr cc_cdg 4 , 152.Xr cc_chd 4 , 153.Xr cc_cubic 4 , 154.Xr cc_dctcp 4 , 155.Xr cc_hd 4 , 156.Xr cc_htcp 4 , 157.Xr cc_vegas 4 , 158.Xr mod_cc 4 , 159.Xr tcp 4 , 160.Xr mod_cc 9 161.Rs 162.%A "Mark Allman" 163.%A "Vern Paxson" 164.%A "Ethan Blanton" 165.%T "TCP Congestion Control" 166.%O "RFC 5681" 167.Re 168.Rs 169.%A "Naeem Khademi" 170.%A "Michael Welzl" 171.%A "Grenville Armitage" 172.%A "Gorry Fairhurst" 173.%T "TCP Alternative Backoff with ECN (ABE)" 174.%O "RFC 8511" 175.Re 176.Sh ACKNOWLEDGEMENTS 177Development and testing of this software were made possible in part by grants 178from the FreeBSD Foundation and Cisco University Research Program Fund at 179Community Foundation Silicon Valley. 180.Sh HISTORY 181The 182.Nm 183congestion control algorithm first appeared in its modular form in 184.Fx 9.0 . 185.Pp 186The module was first released in 2007 by James Healy and Lawrence Stewart whilst 187working on the NewTCP research project at Swinburne University of Technology's 188Centre for Advanced Internet Architectures, Melbourne, Australia, which was made 189possible in part by a grant from the Cisco University Research Program Fund at 190Community Foundation Silicon Valley. 191More details are available at: 192.Pp 193http://caia.swin.edu.au/urp/newtcp/ 194.Sh AUTHORS 195.An -nosplit 196The 197.Nm 198congestion control module was written by 199.An James Healy Aq Mt jimmy@deefa.com , 200.An Lawrence Stewart Aq Mt lstewart@FreeBSD.org 201and 202.An David Hayes Aq Mt david.hayes@ieee.org . 203.Pp 204Support for TCP ABE was added by 205.An Tom Jones Aq Mt tj@enoti.me . 206.Pp 207This manual page was written by 208.An Lawrence Stewart Aq Mt lstewart@FreeBSD.org . 209