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TCP_BBR(4) Device Drivers Manual TCP_BBR(4)

tcp_bbr — TCP Bottleneck Bandwidth and Round-Trip Time Algorithm

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

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.

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.

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.

The tcp_bbr congestion control module first appeared in FreeBSD 13.0.

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>.

December 17, 2023 dev