史蒂文斯经典：TCP/IP详解卷一 - The Protocols

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"TCP/IP详解卷一：The Protocols，由W. Richard Stevens撰写，是深入理解TCP/IP协议栈的经典著作。" 在本书中，作者深入浅出地讲解了TCP/IP协议族的基本概念和工作原理，包括各层协议、网络地址、域名系统、封装与解复用、客户端-服务器模型、端口号、标准化过程、RFC文档、标准服务以及互联网的实现等核心主题。第一章"Introduction"介绍了TCP/IP的基础知识，包括分层模型的概念，TCP/IP协议的四层结构（链路层、网络层、传输层和应用层），互联网地址的组成，以及如何通过域名系统（DNS）将人类可读的域名转换为IP地址。此外，还讨论了数据包在网络中的封装和解复用过程，客户端-服务器模型的工作原理，以及端口号在标识不同服务中的作用。标准化过程和RFC文档是互联网发展的重要组成部分，本章也对此进行了简要介绍。第二章"Link Layer"专注于链路层，这是TCP/IP模型的最底层。这里详细讲述了以太网和IEEE 802封装协议，如Trailer Encapsulation，以及用于串行线路的SLIP（Serial Line IP）和压缩SLIP。PPP（点对点协议）是连接远程网络时常用的一种协议，本章也对其进行了讨论。此外，还提到了Loopback接口，MTU（最大传输单元）和Path MTU发现，以及如何计算串行线路的吞吐量。第三章"IP: Internet Protocol"主要探讨网络层的IP协议。IP头的结构被详细解析，包括IP路由的工作原理，如何通过子网寻址来划分网络，以及如何使用子网掩码来确定主机部分和网络部分。特殊IP地址，如环回地址，也有提及。通过一个子网示例，读者可以更好地理解如何实际配置和使用子网。最后，介绍了`ifconfig`命令的使用，这是在许多操作系统中配置网络接口的工具。此书对TCP/IP协议的讲解不仅限于理论，还包含了大量的实例和实践指导，对于学习网络编程和理解互联网运作机制的读者来说，是一份极其宝贵的资源。通过阅读此书，读者将能够深入理解网络通信的各个环节，从而更好地设计和实现网络应用程序。

Chapter 1. Introduction

Introduction

1.1 Introduction

The TCP/IP protocol suite allows computers of all sizes, from many different computer vendors,

running totally different operating systems, to communicate with each other. It is quite amazing

because its use has far exceeded its original estimates. What started in the late 1960s as a

government-financed research project into packet switching networks has, in the 1990s, turned

into the most widely used form of networking between computerrs. It is truly an open system in

that the definition of the protocol suite and many of its implementations are publicly available at

little or no charge. It forms the basis for what is called the worldwide Internet, or the Internet, a

wide area network (WAN) of more than one million computers that literally spans the globe.

This chapter provides an overview of the TCP/IP protocol suite, to establish an adequate

background for the remaining chapters. For a historical perspective on the early development of

TCP/IP see [Lynch 1993].

1.2 Layering

Networking protocols are normally developed in layers, with each layer responsible for a

different facet of the communications. A protocol suite, such as TCP/IP, is the combination of

different protocols at various layers. TCP/IP is normally considered to be a 4-layer system, as

shown in Figure 1.1.

Application Telnet, FTP, e-mail, etc.

Transport TCP, UDP

Network IP, ICMP, IGMP

Link device driver and interface card

Figure 1.1 The four layers of the TCP/IP protocol suite.

Each layer has a different responsibility.

1. The link layer, sometimes called the data-link layer or network interface layer, normally

includes the device driver in the operating system and the corresponding network

interface card in the computer. Together they handle all the hardware details of physically

interfacing with the cable (or whatever type of media is being used).

2. The network layer (sometimes called the internet layer) handles the movement of packets

around the network. Routing of packets, for example, takes place here. IP (Internet

Protocol), ICMP (Internet Control Message Protocol), and IGMP (Internet Group

Management Protocol) provide the network layer in the TCP/IP protocol suite.

3. The transport layer provides a flow of data between two hosts, for the application layer

above. In the TCP/IP protocol suite there are two vastly different transport protocols:

TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).

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Chapter 1. Introduction

applications are designed so that one end is the client and the other side the server. The server

provides some type of service to clients, in this case access to files on the server host. In the

remote login application, Telnet, the service provided to the client is the ability to login to the

server's host.

Each layer has one or more protocols for communicating with its peer at the same layer. One

protocol, for example, allows the two TCP layers to communicate, and another protocol lets the

two IP layers communicate.

On the right side of Figure 1.2 we have noted that normally the application layer is a user

process while the lower three layers are usually implemented in the kernel (the operating

system). Although this isn't a requirement, it's typical and this is the way it's done under Unix.

There is another critical difference between the top layer in Figure 1.2 and the lower three

layers. The application layer is concerned with the details of the application and not with the

movement of data across the network. The lower three layers know nothing about the application

but handle all the communication details.

We show four protocols in Figure 1.2, each at a different layer. FTP is an application layer

protocol, TCP is a transport layer protocol, IP is a network layer protocol, and the Ethernet

protocols operate at the link layer. The TCP/IP protocol suite is a combination of many

protocols. Although the commonly used name for the entire protocol suite is TCP/IP, TCP and

IP are only two of the protocols. (An alternative name is the Internet Protocol Suite.)

The purpose of the network interface layer and the application layer are obvious-the former

handles the details of the communication media (Ethernet, token ring, etc.) while the latter

handles one specific user application (FTP, Telnet, etc.). But on first glance the difference

between the network layer and the transport layer is somewhat hazy. Why is there a distinction

between the two? To understand the reason, we have to expand our perspective from a single

network to a collection of networks.

One of the reasons for the phenomenal growth in networking during the 1980s was the

realization that an island consisting of a stand-alone computer made little sense. A few stand-

alone systems were collected together into a network. While this was progress, during the 1990s

we have come to realize that this new, bigger island consisting of a single network doesn't make

sense either. People are combining multiple networks together into an internetwork, or an

internet. An internet is a collection of networks that all use the same protocol suite.

The easiest way to build an internet is to connect two or more networks with a router. This is

often a special-purpose hardware box for connecting networks. The nice thing about routers is

that they provide connections to many different types of physical networks: Ethernet, token ring,

point-to-point links, FDDI (Fiber Distributed Data Interface), and so on.

These boxes are also called IP routers, but we'll use the term router.

Historically these boxes were called gateways, and this term is used throughout much of the TCP/IP literature.

Today the term gateway is used for an application gateway: a process that connects two different protocol suites

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Chapter 1. Introduction

(say, TCP/IP and IBM's SNA) for one particular application (often electronic mail or file transfer).

Figure 1.3 shows an internet consisting of two networks: an Ethernet and a token ring, connected

with a router. Although we show only two hosts communicating, with the router connecting the

two networks, any host on the Ethernet can communicate with any host on the token ring.

In Figure 1.3 we can differentiate between an end system (the two hosts on either side) and an

intermediate system (the router in the middle). The application layer and the transport layer use

end-to-end protocols. In our picture these two layers are needed only on the end systems. The

network layer, however, provides a hop-by-hop protocol and is used on the two end systems and

every intermediate system.

Figure 1.3 Two networks connected with a router.

In the TCP/IP protocol suite the network layer, IP, provides an unreliable service. That is, it does

its best job of moving a packet from its source to its final destination, but there are no

guarantees. TCP, on the other hand, provides a reliable transport layer using the unreliable

service of IP To provide this service, TCP performs timeout and retransmission, sends and

receives end-to-end acknowledgments, and so on. The transport layer and the network layer have

distinct responsibilities.

A router, by definition, has two or more network interface layers (since it connects two or more

networks). Any system with multiple interfaces is called multihomed. A host can also be

multihomed but unless it specifically forwards packets from one interface to another, it is not

called a router. Also, routers need not be special hardware boxes that only move packets around

an internet. Most TCP/IP implementations allow a multihomed host to act as a router also, but

the host needs to be specifically configured for this to happen. In this case we can call the system

either a host (when an application such as FTP or Telnet is being used) or a router (when it's

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