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download PDF document: tcp_bbr.4.pdf

TCP_BBR(4) FreeBSD Kernel Interfaces Manual TCP_BBR(4)
NAME tcp_bbr - TCP Bottleneck Bandwidth and Round-Trip Time Algorithm
SYNOPSIS To use this TCP stack you have to place the following line in your kernel configuration file:
options TCPHPTS
To load the driver as a module at boot time, place the following line in loader.conf(5):
tcp_bbr_load="YES"
To enable the TCP stack you must place the following line in the sysctl.conf(5):
net.inet.tcp.functions_default=bbr
DESCRIPTION Bottleneck bandwidth and round-trip time (BBR) is a congestion control algorithm which seeks high throughput with a small queue by probing BW and RTT. It is a round-up redesign of congestion control, which is not loss-based, delay-based, ECN-based or AIMD-based.
The core design of BBR is about creating a model graph of the network path by estimating the maximum BW and minimum RTT on each ACK.
MIB Variables The algorithm exposes the following scopes in the net.inet.tcp.bbr branch of the sysctl(3) MIB:
cwnd Cwnd controls, for example "target cwnd rtt measurement" and "BBR initial window".
measure Measurement controls.
pacing Connection pacing controls.
policer Policer controls, for example "false detection threshold" and "loss threshold".
probertt Probe RTT controls.
startup Startup controls.
states State controls.
timeout Time out controls.
Besides the variables within the above scopes the following variables are also exposed in the net.inet.tcp.bbr branch:
clrlost Clear lost counters.
software_pacing Total number of software paced flows.
rtt_tlp_thresh What divisor for TLP rtt/retran will be added (1=rtt, 2=1/2 rtt etc).
reorder_fade Does reorder detection fade, if so how many ms (0 means never).
reorder_thresh What factor for rack will be added when seeing reordering (shift right).
bb_verbose Should BBR black box logging be verbose.
sblklimit When do we start ignoring small sack blocks.
resend_use_tso Can resends use TSO?
data_after_close Do we hold off sending a RST until all pending data is ack'd.
kill_paceout When we hit this many errors in a row, kill the session?
error_paceout When we hit an error what is the min to pace out in usec's?
cheat_rxt Do we burst 1ms between sends on retransmissions (like rack)?
minrto Minimum RTO in ms.
SEE ALSO cc_chd(4), cc_cubic(4), cc_hd(4), cc_htcp(4), cc_newreno(4), cc_vegas(4), h_ertt(4), mod_cc(4), tcp(4), tcp_rack(4), mod_cc(9)
Neal Cardwell, Yuchung Cheng, Stephen Gunn, Soheil Hassas Yeganeh, and Van Jacobson, "BBR: Congestion-Based Congestion Control", ACM Queue, Vol. 14, September / October 2016.
Dominik Scholz, Benedikt Jaeger, Lukas Schwaighofer, Daniel Raumer, Fabien Geyer, and Georg Carle, "Towards a Deeper Understanding of TCP BBR Congestion Control", IFIP Networking 2018, http://www.net.in.tum.de/fileadmin/bibtex/publications/papers/IFIP-Networking-2018-TCP-BBR.pdf, May 2018.
HISTORY The tcp_bbr congestion control module first appeared in FreeBSD 13.0.
AUTHORS The tcp_bbr congestion control module was written by Randall Stewart <rrs@FreeBSD.org> and sponsored by Netflix, Inc. This manual page was written by Gordon Bergling <gbe@FreeBSD.org>.
FreeBSD 14.0-RELEASE-p11 November 7, 2022 FreeBSD 14.0-RELEASE-p11