TCP/IP详解卷1：协议

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"TCP/IP Illustrated, Volume 1: The Protocols by W. Richard Stevens" 本书是TCP/IP协议详解的第一卷，作者W. Richard Stevens是一位知名的网络协议专家。这本书深入浅出地介绍了TCP/IP协议族的基本概念和工作原理，是IT专业人士和网络工程师的重要参考资料。 1. **介绍** - 书中的第一章介绍了TCP/IP的基础知识，包括协议的分层结构，以及TCP/IP如何在这些层次上实现功能。 2. **分层模型** - TCP/IP协议栈通常分为四层：链路层、网络层、传输层和应用层。这一部分会解释每一层的主要职责和它们之间的交互。 3. **互联网地址** - 书中详细讲解了IPv4地址的格式，以及地址的分类（如A类、B类、C类地址）和子网掩码的概念。 4. **域名系统(DNS)** - DNS是将人类可读的域名转换为IP地址的系统。这部分会介绍DNS的工作机制和查询过程。 5. **封装** - 数据在网络中传输时，会被封装在不同协议的头部，形成数据包。书中会展示如何在不同层次进行封装。 6. **复用与去复用** - 复用是指在发送端将多个应用的数据合并到一个网络包中，而去复用则是在接收端将数据包拆分成对应的应用数据。 7. **客户端-服务器模型** - 描述了互联网服务如何通过客户端发起请求，服务器响应的模型，并且提到了端口号在识别不同服务中的作用。 8. **端口号** - 端口号用于区分在同一台机器上的不同服务，标准服务有预定义的端口号。 9. **标准化过程** - 书中提到了互联网协议的标准化过程，主要通过RFC（Request for Comments）文档来定义和更新。 10. **RFCs** - RFC是互联网标准的正式文档，书中会解释其重要性以及如何查找和理解RFC。 11. **标准和简单服务** - 讨论了像HTTP、FTP等常见服务的标准及其工作方式。 12. **互联网的演进** - 书中涵盖了互联网的历史和发展，以及各种实现技术。 13. **应用程序接口(API)** - API允许程序员访问操作系统或网络服务的功能，书中可能会讨论如何在不同操作系统下使用TCP/IP API。 14. **测试网络** - 可能会介绍如何设置和使用测试环境来学习和实验TCP/IP协议。 15. **链路层** - 部分内容涉及链路层协议，如以太网、IEEE 802封装、Trailer Encapsulation、SLIP、压缩SLIP、PPP、Loopback接口，以及MTU和Path MTU的概念。 16. **IP协议** - IP协议部分讲解了IP头的结构，IP路由原理，子网寻址，以及特殊IP地址的处理。每章末尾通常会有总结，帮助读者回顾和巩固所学知识。通过阅读这本书，读者可以全面了解TCP/IP协议族的各个方面，从而更好地理解和解决网络相关问题。

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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(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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