.\" .\" Copyright (c) 2009 Lawrence Stewart .\" 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 : .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 .