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.

     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), 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 13.1-RELEASE-p6       September 24, 2020       FreeBSD 13.1-RELEASE-p6