1.\" 2.\" Copyright (c) 2020, Gordon Bergling <gbe@FreeBSD.org> 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 13.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 14.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 15.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 16.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR 17.\" ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 18.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 19.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 20.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 21.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 22.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 23.\" SUCH DAMAGE. 24.\" 25.Dd December 17, 2023 26.Dt TCP_BBR 4 27.Os 28.Sh NAME 29.Nm tcp_bbr 30.Nd TCP Bottleneck Bandwidth and Round-Trip Time Algorithm 31.Sh SYNOPSIS 32To load the driver as a module at boot time, place the following line in 33.Xr loader.conf 5 : 34.Bd -literal -offset indent 35tcp_bbr_load="YES" 36.Ed 37.Pp 38To enable the TCP stack you must place the following line in the 39.Xr sysctl.conf 5 : 40.Bd -literal -offset indent 41net.inet.tcp.functions_default=bbr 42.Ed 43.Sh DESCRIPTION 44Bottleneck bandwidth and round-trip time (BBR) is a congestion control algorithm which 45seeks high throughput with a small queue by probing BW and RTT. 46It is a round-up redesign of congestion control, which is not loss-based, delay-based, 47ECN-based or AIMD-based. 48.Pp 49The core design of BBR is about creating a model graph of the network path by 50estimating the maximum BW and minimum RTT on each ACK. 51.Sh MIB Variables 52The algorithm exposes the following scopes in the 53.Va net.inet.tcp.bbr 54branch of the 55.Xr sysctl 3 56MIB: 57.Bl -tag -width ".Va exp_backoff_scale" 58.It Va cwnd 59Cwnd controls, for example "target cwnd rtt measurement" and "BBR initial window". 60.It Va measure 61Measurement controls. 62.It Va pacing 63Connection pacing controls. 64.It Va policer 65Policer controls, for example "false detection threshold" and "loss threshold". 66.It Va probertt 67Probe RTT controls. 68.It Va startup 69Startup controls. 70.It Va states 71State controls. 72.It Va timeout 73Time out controls. 74.El 75.Pp 76Besides the variables within the above scopes the following 77variables are also exposed in the 78.Va net.inet.tcp.bbr 79branch: 80.Bl -tag -width ".Va exp_backoff_scale" 81.It Va clrlost 82Clear lost counters. 83.It Va software_pacing 84Total number of software paced flows. 85.It Va hdwr_pacing 86Total number of hardware paced flows. 87.It Va enob_no_hdwr_pacing 88Total number of enobufs for non-hardware paced flows. 89.It Va enob_hdwr_pacing 90Total number of enobufs for hardware paced flows. 91.It Va rtt_tlp_thresh 92What divisor for TLP rtt/retran will be added (1=rtt, 2=1/2 rtt etc). 93.It Va reorder_fade 94Does reorder detection fade, if so how many ms (0 means never). 95.It Va reorder_thresh 96What factor for rack will be added when seeing reordering (shift right). 97.It Va bb_verbose 98Should BBR black box logging be verbose. 99.It Va sblklimit 100When do we start ignoring small sack blocks. 101.It Va resend_use_tso 102Can resends use TSO? 103.It Va data_after_close 104Do we hold off sending a RST until all pending data is ack'd. 105.It Va kill_paceout 106When we hit this many errors in a row, kill the session? 107.It Va error_paceout 108When we hit an error what is the min to pace out in usec's? 109.It Va cheat_rxt 110Do we burst 1ms between sends on retransmissions (like rack)? 111.It Va minrto 112Minimum RTO in ms. 113.El 114.Sh SEE ALSO 115.Xr cc_chd 4 , 116.Xr cc_cubic 4 , 117.Xr cc_hd 4 , 118.Xr cc_htcp 4 , 119.Xr cc_newreno 4 , 120.Xr cc_vegas 4 , 121.Xr h_ertt 4 , 122.Xr mod_cc 4 , 123.Xr tcp 4 , 124.Xr tcp_rack 4 , 125.Xr mod_cc 9 126.Rs 127.%A "Neal Cardwell" 128.%A "Yuchung Cheng" 129.%A "Stephen Gunn" 130.%A "Soheil Hassas Yeganeh" 131.%A "Van Jacobson" 132.%T "BBR: Congestion-Based Congestion Control" 133.%J "ACM Queue, Vol. 14" 134.%D "September / October 2016" 135.Re 136.Rs 137.%A "Dominik Scholz" 138.%A "Benedikt Jaeger" 139.%A "Lukas Schwaighofer" 140.%A "Daniel Raumer" 141.%A "Fabien Geyer" 142.%A "Georg Carle" 143.%T "Towards a Deeper Understanding of TCP BBR Congestion Control" 144.%J "IFIP Networking 2018" 145.%D "May 2018" 146.%U "http://www.net.in.tum.de/fileadmin/bibtex/publications/papers/IFIP-Networking-2018-TCP-BBR.pdf" 147.Re 148.Sh HISTORY 149The 150.Nm 151congestion control module first appeared in 152.Fx 13.0 . 153.Sh AUTHORS 154.An -nosplit 155The 156.Nm 157congestion control module was written by 158.An Randall Stewart Aq Mt rrs@FreeBSD.org 159and sponsored by Netflix, Inc. 160This manual page was written by 161.An Gordon Bergling Aq Mt gbe@FreeBSD.org . 162