TCP_BBR(4) FreeBSD Kernel Interfaces Manual TCP_BBR(4)
NAME
tcp_bbr - TCP Bottleneck Bandwidth and Round-Trip Time Algorithm
SYNOPSIS
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.
hdwr_pacing Total number of hardware paced flows.
enob_no_hdwr_pacing
Total number of enobufs for non-hardware paced flows.
enob_hdwr_pacing Total number of enobufs for hardware 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
<
[email protected]> and sponsored by Netflix, Inc. This manual page was
written by Gordon Bergling <
[email protected]>.
FreeBSD 14.1-RELEASE-p8 December 17, 2023 FreeBSD 14.1-RELEASE-p8