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
     <rrs@FreeBSD.org> and sponsored by Netflix, Inc.  This manual page was
     written by Gordon Bergling <gbe@FreeBSD.org>.

FreeBSD 15.1-STABLE-HBSD       December 17, 2023                    TCP_BBR(4)