---
name: "tcp_bbr"
section: "4"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["freebsd"]
url: "https://new.carta.tech/tcp_bbr"
---

# tcp\_bbr(4)

TCP Bottleneck Bandwidth and Round-Trip Time Algorithm

## 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)](/loader.conf/5.md):

```
tcp_bbr_load="YES"
```

To enable the TCP stack you must place the following line in the [sysctl.conf(5)](/sysctl.conf/5.md):

```
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)](/sysctl/3.md) 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_chd/4.md), [cc\_cubic(4)](/cc_cubic/4.md), [cc\_hd(4)](/cc_hd/4.md), [cc\_htcp(4)](/cc_htcp/4.md), [cc\_newreno(4)](/cc_newreno/4.md), [cc\_vegas(4)](/cc_vegas/4.md), [h\_ertt(4)](/h_ertt/4.md), [mod\_cc(4)](/mod_cc/4.md), [tcp(4)](/tcp/4.md), [tcp\_rack(4)](/tcp_rack/4.md), [mod\_cc(9)](/mod_cc/9.md) 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*, May 2018, https://www.net.in.tum.de/fileadmin/bibtex/publications/papers/IFIP-Networking-2018-TCP-BBR.pdf.

## 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*⟩.