TCP/IP详解卷1：协议基础与网络层解析

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"TCP/IP卷1-第三版-英文版，由W.Richard Stevens撰写，是网络技术的基础经典，详细阐述了TCP/IP协议的各个层面，包括介绍、链路层、互联网协议和地址解析协议等内容。" 《TCP/IP Illustrated, Volume 1》是网络技术领域的权威著作，尽管自初次出版以来已有30多年，但其内容仍然极具价值，不受时间的限制。书中的知识涵盖了网络通信的基础，对于理解TCP/IP协议族的工作原理至关重要。在第一章"Introduction"中，作者引入了网络通信的基本概念，如分层模型，TCP/IP的四层结构（应用层、传输层、网络层和链路层），以及互联网地址的重要性。同时，书中讲解了域名系统（DNS）如何解析人类可读的域名，以及封装和复用在通信过程中的作用。客户端-服务器模型、端口号的分配、标准化过程和请求评论文档（RFCs）的角色也在此部分得到阐述。此外，标准服务、互联网实现、应用编程接口（APIs）和测试网络的构建方法也有涉及。第二章"Link Layer"关注链路层，这是物理网络连接的基础。书中讨论了以太网和IEEE 802封装、尾部封装、串行线路IP（SLIP）、压缩SLIP、点对点协议（PPP）以及环回接口。此外，最大传输单元（MTU）的概念、路径MTU发现、串行线路的吞吐量计算都在这一章中详细解释。第三章"IP: Internet Protocol"深入到网络层，介绍了IP协议头的结构、IP路由工作原理，以及子网寻址。子网掩码的概念用于划分网络地址，特殊IP地址的情况如广播和回环地址也有详述。书中通过实例解释了如何使用`ifconfig`和`netstat`命令来管理网络接口，并展望了IP协议的未来发展趋势。第四章"ARP: Address Resolution Protocol"聚焦于地址解析协议，这是将IP地址映射到物理地址的关键机制。ARP的工作流程、缓存和 ARP欺骗等安全问题在此章节中得到了详细介绍。《TCP/IP Illustrated, Volume 1》不仅是一本全面了解TCP/IP协议的宝典，也是深入学习网络技术的重要参考资料。无论是初学者还是经验丰富的专业人士，都能从中受益匪浅，深入理解网络通信的底层机制。

application layer can ignore all these details.

UDP, on the other hand, provides a much simpler service to the application layer. It just sends packets

of data called datagrams from one host to the other, but there is no guarantee that the datagrams reach

the other end. Any desired reliability must be added by the application layer.

There is a use for each type of transport protocol, which we'll see when we look at the different

applications that use TCP and UDP.

The application layer handles the details of the particular application. There are many common

TCP/IP applications that almost every implementation provides:

Telnet for remote login,

❍

FTP, the File Transfer Protocol,❍

SMTP, the Simple Mail Transfer protocol, for electronic mail,❍

SNMP, the Simple Network Management Protocol,❍

and many more, some of which we cover in later chapters.

If we have two hosts on a local area network (LAN) such as an Ethernet, both running FTP, Figure 1.2

shows the protocols involved.

Figure 1.2 Two hosts on a LAN running FTP.

We have labeled one application box the FTP client and the other the FTP server. Most network 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

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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 (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.

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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 forwarding packets from one network to another). We'll use whichever term makes sense

given the context.

One of the goals of an internet is to hide all the details of the physical layout of the internet from the

applications. Although this isn't obvious from our two-network internet in Figure 1.3, the application layers

can't care (and don't care) that one host is on an Ethernet, the other on a token ring, with a router between.

There could be 20 routers between, with additional types of physical interconnections, and the applications

would run the same. This hiding of the details is what makes the concept of an internet so powerful and

useful.

Another way to connect networks is with a bridge. These connect networks at the link layer, while routers

connect networks at the network layer. Bridges makes multiple LANs appear to the upper layers as a single

LAN.

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Trivial File Transfer Protocol), and Chapter 16 (the Bootstrap Protocol) look at some applications that use

UDP. SNMP (the Simple Network Management Protocol) also uses UDP, but since it deals with many of the

other protocols, we save a discussion of it until Chapter 25.

IP is the main protocol at the network layer. It is used by both TCP and UDP. Every piece of TCP and UDP

data that gets transferred around an internet goes through the IP layer at both end systems and at every

intermediate router. In Figure 1.4 we also show an application accessing IP directly. This is rare, but

possible. (Some older routing protocols were implemented this way. Also, it is possible to experiment with

new transport layer protocols using this feature.) Chapter 3 looks at IP, but we save some of the details for

later chapters where their discussion makes more sense. Chapters 9 and 10 look at how IP performs routing.

ICMP is an adjunct to IP. It is used by the IP layer to exchange error messages and other vital information

with the IP layer in another host or router. Chapter 6 looks at ICMP in more detail. Although ICMP is used

primarily by IP, it is possible for an application to also access it. Indeed we'll see that two popular diagnostic

tools, Ping and Traceroute (Chapters 7 and 8), both use ICMP.

IGMP is the Internet Group Management Protocol. It is used with multicasting: sending a UDP datagram to

multiple hosts. We describe the general properties of broadcasting (sending a UDP datagram to every host

on a specified network) and multicasting in Chapter 12, and then describe IGMP itself in Chapter 13.

ARP (Address Resolution Protocol) and RARP (Reverse Address Resolution Protocol) are specialized

protocols used only with certain types of network interfaces (such as Ethernet and token ring) to convert

between the addresses used by the IP layer and the addresses used by the network interface. We examine

these protocols in Chapters 4 and 5, respectively.

1.4 Internet Addresses

Every interface on an internet must have a unique Internet address (also called an IP address). These

addresses are 32-bit numbers. Instead of using a flat address space such as 1, 2, 3, and so on, there is a

structure to Internet addresses. Figure 1.5 shows the five different classes of Internet addresses.

These 32-bit addresses are normally written as four decimal numbers, one for each byte of the address. This

is called dotted-decimal notation. For example, the class B address of the author's primary system is

140.252.13.33.

The easiest way to differentiate between the different classes of addresses is to look at the first number of a

dotted-decimal address. Figure 1.6 shows the different classes, with the first number in boldface.

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