Linux TC(HOWTO)：详解流量控制原理与组件指南

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本篇文章是关于Linux Traffic Control (TC) 的一份技术指南，由Martin A. Brown撰写，发布于2006年10月28日，版本号为1.0.2。这份HOWTO旨在介绍流量控制的概念，概述传统的流量管理元素，解析Linux内核中的流量控制实现，并提供一些实用的指导原则。首先，文章从基础概念开始，解释了什么是流量控制，它在网络通信中起着关键作用，确保数据传输的稳定性和效率。流量控制涉及如何管理发送方和接收方之间的数据流，防止拥塞和数据包丢失，从而优化网络性能。接着，作者详细介绍了Linux TC的组成部分，这包括但不限于： 1. **传统元素**：涵盖了诸如重传、窗口机制（如TCP滑动窗口）、拥塞避免（如随机早期检测，RED）和拥塞感知（如快速恢复算法，FQ-Codel）等基本流量控制策略。 2. **Linux实现**：讲解了Linux内核中对流量控制的支持，涉及到的接口如`iptables`、`tc`命令行工具以及更底层的netfilter和socket编程接口。这部分会深入探讨高级特性，如QoS（Quality of Service）控制、分类（Classful）和无类别（Classless）流量处理。 3. **高级组件**：包括流量整形（shaping）、流量监管（ policing）、流量整形与监管的组合（queuing disciplines，如WFQ，黄蓝队列），以及高级队列调度算法（如Hierarchical Fair Queuing，HFSC）。 4. **软件规则**：这部分展示了如何编写和配置TC规则来定制流量控制策略，以适应不同的网络应用场景，比如Web服务器、游戏服务器或VoIP通信。 5. **元素与组件**：这部分将流量控制的各个元素和组件组织成一个系统化的框架，便于理解和管理复杂的网络流量环境。随着版本的更新，如1.0.1和1.0.2，作者持续添加了新的参考资料，如Leonardo Balliache的文档，并修正了错误，以保持内容的准确性和时效性。同时，修订历史记录表明了作者与社区的紧密合作，以确保这份指南能够满足Linux网络管理员的需求并符合TLDP（The Linux Documentation Project）的标准。 Linux Traffic Control HOWTO是一份详尽的教程，对于希望深入了解Linux网络流量管理的用户来说，无论是网络管理员、系统工程师还是开发者，都具有很高的参考价值。通过阅读和实践这份指南，读者可以掌握如何在Linux环境中有效地管理和优化网络流量，提升网络服务质量。

2. Overview of Concepts

This section will introduce traffic control and examine reasons for it, identify a few advantages and

disadvantages and introduce key concepts used in traffic control.

2.1. What is it?

Traffic control is the name given to the sets of queuing systems and mechanisms by which packets are

received and transmitted on a router. This includes deciding which (and whether) packets to accept at what

rate on the input of an interface and determining which packets to transmit in what order at what rate on the

output of an interface.

In the overwhelming majority of situations, traffic control consists of a single queue which collects entering

packets and dequeues them as quickly as the hardware (or underlying device) can accept them. This sort of

queue is a FIFO.

The default qdisc under Linux is the pfifo_fast, which is slightly more complex than the FIFO.

There are examples of queues in all sorts of software. The queue is a way of organizing the pending tasks or

data (see also Section 2.5). Because network links typically carry data in a serialized fashion, a queue is

required to manage the outbound data packets.

In the case of a desktop machine and an efficient webserver sharing the same uplink to the Internet, the

following contention for bandwidth may occur. The web server may be able to fill up the output queue on the

router faster than the data can be transmitted across the link, at which point the router starts to drop packets

(its buffer is full!). Now, the desktop machine (with an interactive application user) may be faced with packet

loss and high latency. Note that high latency sometimes leads to screaming users! By separating the internal

queues used to service these two different classes of application, there can be better sharing of the network

resource between the two applications.

Traffic control is the set of tools which allows the user to have granular control over these queues and the

queuing mechanisms of a networked device. The power to rearrange traffic flows and packets with these tools

is tremendous and can be complicated, but is no substitute for adequate bandwidth.

The term Quality of Service (QoS) is often used as a synonym for traffic control.

2.2. Why use it?

Packet−switched networks differ from circuit based networks in one very important regard. A

packet−switched network itself is stateless. A circuit−based network (such as a telephone network) must hold

state within the network. IP networks are stateless and packet−switched networks by design; in fact, this

statelessness is one of the fundamental strengths of IP.

The weakness of this statelessness is the lack of differentiation between types of flows. In simplest terms,

traffic control allows an administrator to queue packets differently based on attributes of the packet. It can

even be used to simulate the behaviour of a circuit−based network. This introduces statefulness into the

stateless network.

2. Overview of Concepts 3

There are many practical reasons to consider traffic control, and many scenarios in which using traffic control

makes sense. Below are some examples of common problems which can be solved or at least ameliorated with

these tools.

The list below is not an exhaustive list of the sorts of solutions available to users of traffic control, but

introduces the types of problems that can be solved by using traffic control to maximize the usability of a

network connection.

Common traffic control solutions

Limit total bandwidth to a known rate; TBF, HTB with child class(es).•

Limit the bandwidth of a particular user, service or client; HTB classes and classifying with a

filter. traffic.

•

Maximize TCP throughput on an asymmetric link; prioritize transmission of ACK packets,

wondershaper.

•

Reserve bandwidth for a particular application or user; HTB with children classes and classifying.•

Prefer latency sensitive traffic; PRIO inside an HTB class.•

Managed oversubscribed bandwidth; HTB with borrowing.•

Allow equitable distribution of unreserved bandwidth; HTB with borrowing.•

Ensure that a particular type of traffic is dropped; policer attached to a filter with a drop

action.

•

Remember, too that sometimes, it is simply better to purchase more bandwidth. Traffic control does not solve

all problems!

2.3. Advantages

When properly employed, traffic control should lead to more predictable usage of network resources and less

volatile contention for these resources. The network then meets the goals of the traffic control configuration.

Bulk download traffic can be allocated a reasonable amount of bandwidth even as higher priority interactive

traffic is simultaneously serviced. Even low priority data transfer such as mail can be allocated bandwidth

without tremendously affecting the other classes of traffic.

In a larger picture, if the traffic control configuration represents policy which has been communicated to the

users, then users (and, by extension, applications) know what to expect from the network.

2.4. Disdvantages

Complexity is easily one of the most significant disadvantages of using traffic control. There are ways to

become familiar with traffic control tools which ease the learning curve about traffic control and its

mechanisms, but identifying a traffic control misconfiguration can be quite a challenge.

Traffic control when used appropriately can lead to more equitable distribution of network resources. It can

just as easily be installed in an inappropriate manner leading to further and more divisive contention for

resources.

The computing resources required on a router to support a traffic control scenario need to be capable of

handling the increased cost of maintaining the traffic control structures. Fortunately, this is a small

incremental cost, but can become more significant as the configuration grows in size and complexity.

Traffic Control HOWTO

2. Overview of Concepts 4

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