> For the complete documentation index, see [llms.txt](https://book.bsdcn.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://book.bsdcn.org/man/man4/tcp_rack.4.md).

# tcp\_rack.4

`tcp_rack` — TCP RACK-TLP 丢失检测算法

## 名称

`tcp_rack`

## 概要

`要在引导时以模块形式加载 TCP 栈，请在 loader.conf(5) 中加入以下行：`

```sh
tcp_rack_load="YES"
```

`要启用 TCP 栈，请在 sysctl.conf(5) 中加入以下行：`

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

## 描述

RACK-TLP 使用每段传输时间戳和选择性确认（SACK），包含两部分。最近确认（RACK）利用从确认（ACK）反馈得出的基于时间的推断快速启动快速恢复，尾部丢失探测（TLP）利用 RACK 发送探测数据包以触发 ACK 反馈，从而避免重传超时（RTO）事件。

与广泛使用的重复确认（DupAck）阈值方法相比，RACK-TLP 在应用受限数据传输、重传丢失或数据包重排事件中能更高效地检测丢失。

它旨在作为 DupAck 阈值方法的替代。

## MIB 变量

该算法在 sysctl(3) MIB 的 `net.inet.tcp.rack` 分支下暴露以下作用域：

**`net.inet.tcp.rack.misc`** 杂项相关控制

**`net.inet.tcp.rack.features`** 特性控制

**`net.inet.tcp.rack.measure`** 测量相关控制

**`net.inet.tcp.rack.timers`** 定时器相关控制

**`net.inet.tcp.rack.tlp`** TLP 和 Rack 相关控制

**`net.inet.tcp.rack.timely`** Rack Timely RTT 控制

**`net.inet.tcp.rack.hdwr_pacing`** Pacing 相关控制

**`net.inet.tcp.rack.pacing`** Pacing 相关控制

**`net.inet.tcp.rack.tp`** Rack tracepoint 设施

**`net.inet.tcp.rack.probertt`** ProbeRTT 相关控制

**`net.inet.tcp.rack.stats`** Rack 计数器

**`net.inet.tcp.rack.sack_attack`** Rack Sack Attack 计数器和控制

除上述作用域中的变量外，`net.inet.tcp.rack` 分支下还暴露以下变量：

**`net.inet.tcp.rack.clear`** 清除计数器

**`net.inet.tcp.rack.opts`** RACK 选项统计

**`net.inet.tcp.rack.outsize`** MSS 发送大小

**`net.inet.tcp.rack.req_measure_cnt`** 如果进行动态 pacing，开始 pacing 之前必须有多次测量？

**`net.inet.tcp.rack.use_pacing`** 如果设置则使用 pacing，如果清除则仅使用原始突发缓解

**`net.inet.tcp.rack.rate_sample_method`** 速率采样应使用什么方法 0=高，1=低

## 参见

[cc\_chd(4)](/man/man4/cc_chd.4.md), [cc\_cubic(4)](/man/man4/cc_cubic.4.md), [cc\_hd(4)](/man/man4/cc_hd.4.md), [cc\_htcp(4)](/man/man4/cc_htcp.4.md), [cc\_newreno(4)](/man/man4/cc_newreno.4.md), [cc\_vegas(4)](/man/man4/cc_vegas.4.md), [h\_ertt(4)](/man/man4/h_ertt.4.md), [mod\_cc(4)](/man/man4/mod_cc.4.md), [tcp(4)](/man/man4/tcp.4.md), [tcp\_bbr(4)](/man/man4/tcp_bbr.4.md), [mod\_cc(9)](https://github.com/FreeBSD-Ask/freebsd-man-sc/blob/main/man9/mod_cc.9.md)

> Neal Cardwell, Yuchung Cheng, Nandita Dukkipati, Priyaranjan Jha, "The RACK-TLP Loss Detection Algorithm for TCP", February 2021, RFC 8985.

> M. Allman, V. Paxson, E. Blanton, "TCP Congestion Control", September 2009, RFC 5681.

> M. Mathis, Nandita Dukkipati, Yuchung Cheng, "Proportional Rate Reduction for TCP", May 2013, RFC 6937.

## 历史

`tcp_rack` 拥塞控制模块首次出现于 FreeBSD 13.0。

## 作者

`tcp_rack` 拥塞控制模块由 Randall Stewart <rrs@FreeBSD.org> 编写，由 Netflix, Inc. 赞助。本手册页由 Gordon Bergling <gbe@FreeBSD.org> 编写。


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter:

```
GET https://book.bsdcn.org/man/man4/tcp_rack.4.md?ask=<question>&goal=<endgoal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal.

The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.