---
name: "tcp_rack"
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_rack"
---

# tcp\_rack(4)

TCP RACK-TLP Loss Detection Algorithm for TCP

## NAME

**tcp\_rack** – TCP RACK-TLP Loss Detection Algorithm for TCP

## SYNOPSIS

To load the TCP stack as a module at boot time, place the following line in [loader.conf(5)](/loader.conf/5.md):

```
tcp_rack_load="YES"
```

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

```
net.inet.tcp.functions_default=rack
```

## DESCRIPTION

RACK-TLP uses per-segment transmit timestamps and selective acknowledgments (SACKs) and has two parts. Recent Acknowledgment (RACK) starts fast recovery quickly using time-based inferences derived from acknowledgment (ACK) feedback, and Tail Loss Probe (TLP) leverages RACK and sends a probe packet to trigger ACK feedback to avoid retransmission timeout (RTO) events.

Compared to the widely used duplicate acknowledgment (DupAck) threshold approach, RACK-TLP detects losses more efficiently when there are application-limited flights of data, lost retransmissions, or data packet reordering events.

It is intended to be an alternative to the DupAck threshold approach.

## MIB Variables

The algorithm exposes the following scopes in the *net.inet.tcp.rack* branch of the [sysctl(3)](/sysctl/3.md) MIB:

- *net.inet.tcp.rack.misc*

  Misc related controls

- *net.inet.tcp.rack.features*

  Feature controls

- *net.inet.tcp.rack.measure*

  Measure related controls

- *net.inet.tcp.rack.timers*

  Timer related controls

- *net.inet.tcp.rack.tlp*

  TLP and Rack related Controls

- *net.inet.tcp.rack.timely*

  Rack Timely RTT Controls

- *net.inet.tcp.rack.hdwr\_pacing*

  Pacing related Controls

- *net.inet.tcp.rack.pacing*

  Pacing related Controls

- *net.inet.tcp.rack.tp*

  Rack tracepoint facility

- *net.inet.tcp.rack.probertt*

  ProbeRTT related Controls

- *net.inet.tcp.rack.stats*

  Rack Counters

- *net.inet.tcp.rack.sack\_attack*

  Rack Sack Attack Counters and Controls

Besides the variables within the above scopes the following variables are also exposed in the *net.inet.tcp.rack* branch:

- *net.inet.tcp.rack.clear*

  Clear counters

- *net.inet.tcp.rack.opts*

  RACK Option Stats

- *net.inet.tcp.rack.outsize*

  MSS send sizes

- *net.inet.tcp.rack.req\_measure\_cnt*

  If doing dynamic pacing, how many measurements must be in before we start pacing?

- *net.inet.tcp.rack.use\_pacing*

  If set we use pacing, if clear we use only the original burst mitigation

- *net.inet.tcp.rack.rate\_sample\_method*

  What method should we use for rate sampling 0=high, 1=low

## 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\_bbr(4)](/tcp_bbr/4.md), [mod\_cc(9)](/mod_cc/9.md) Neal Cardwell, Yuchung Cheng, Nandita Dukkipati, and Priyaranjan Jha, *The RACK-TLP Loss Detection Algorithm for TCP*, February 2021, RFC 8985. M. Allman, V. Paxson, and E. Blanton, *TCP Congestion Control*, September 2009, RFC 5681. M. Mathis, Nandita Dukkipati, and Yuchung Cheng, *Proportional Rate Reduction for TCP*, May 2013, RFC 6937.

## HISTORY

The **tcp\_rack** congestion control module first appeared in FreeBSD 13.0.

## AUTHORS

The **tcp\_rack** 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*⟩.