Java TCP/IP Sockets实战指南：第二版——连接理论与实践

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《TCP/IP sockets在Java中的实用指南（第二版）》是由肯尼斯·卡尔韦特（University of Kentucky）和迈克尔·唐纳霍（Baylor University）合著的一本专为程序员设计的实践性教材。本书针对网络编程中至关重要的TCP/IP协议和套接字技术，提供了深入浅出的讲解和丰富的实例，旨在弥补传统计算机网络课程中缺乏实际操作经验的问题。在过去的教学中，计算机网络课程往往侧重于理论概念，如方程式、分析和抽象化的协议栈描述，很少有让学生亲手实践的机会。然而，随着互联网的普及，这种状况正在改变。学生可以轻易地接入互联网并利用其服务，同时也能接触到大量的软件代码，包括优秀的和有问题的，这些都成为学习者学习和理解TCP/IP和套接字原理的实际案例来源。 TCP/IP（Transmission Control Protocol/Internet Protocol）是互联网的基础协议，负责数据包的可靠传输，而套接字（sockets）则是程序间通信的重要接口，使得应用程序能够连接到网络并与远程主机进行通信。在Java中，Java Socket API（JSR 166）为开发者提供了方便的工具，使得他们能够轻松构建TCP/IP客户端和服务器应用。本书第二版不仅涵盖了TCP/IP协议的底层原理，还结合Java编程语言，提供了详尽的步骤和示例，帮助读者理解如何创建TCP连接、数据封装与解封装、错误处理以及性能优化等关键概念。通过实例项目，读者将学会如何实现简单的聊天应用、文件传输、多线程处理以及网络安全措施，如SSL/TLS加密。此外，书中还会探讨一些现代网络编程的最佳实践，如异步I/O模型、网络编程并发控制，以及在分布式系统中的套接字应用。对于想要深入理解和运用TCP/IP套接字的Java开发者来说，这是一本不可或缺的参考书。《TCP/IP sockets in Java 2nd》提供了一个从基础到进阶的学习路径，使读者能够将理论知识与实际操作相结合，从而更好地掌握网络编程的核心技能。无论是初学者还是经验丰富的开发人员，都能在本书中找到适合自己的学习材料和实战指导。

1.2 About Addresses 5

(Ethernet) and wireless (WiFi) networks. Because each such network connection belongs to a

single host, an Internet address identiﬁes a host as well as its connection to the network.

However, the converse is not true, because a single host can have multiple interfaces, and each

interface can have multiple addresses. (In fact, the same interface can have both IPv4 and IPv6

addresses.)

The port number in TCP or UDP is always interpreted relative to an Internet address.

Returning to our earlier analogies, a port number corresponds to a room number at a given

street address, say, that of a large building. The postal service uses the street address to get the

letter to a mailbox; whoever empties the mailbox is then responsible for getting the letter to the

proper room within the building. Or consider a company with an internal telephone system:

to speak to an individual in the company, you ﬁrst dial the company’s main phone number to

connect to the internal telephone system and then dial the extension of the particular telephone

of the individual you wish to speak with. In these analogies, the Internet address is the street

address or the company’s main number, whereas the port corresponds to the room number

or telephone extension. Port numbers are 16-bit unsigned binary numbers, so each one is in

the range 1 to 65,535. (0 is reserved.)

In each version of IP, certain special-purpose addresses are deﬁned. One of these that

is worth knowing is the loopback address, which is always assigned to a special loopback

interface, a virtual device that simply echoes transmitted packets right back to the sender.

The loopback interface is very useful for testing because packets sent to that address are

immediately returned back to the destination. Moreover, it is present on every host, and can

be used even when a computer has no other interfaces (i.e., is not connected to the network).

The loopback address for IPv4 is 127.0.0.1;

for IPv6 it is 0:0:0:0:0:0:0:1.

Another group of IPv4 addresses reserved for a special purpose includes those reserved

for “private use.” This group includes all IPv4 addresses that start with 10 or 192.168, as well

as those whose ﬁrst number is 172 and whose second number is between 16 and 31. (There is

no corresponding class for IPv6.) These addresses were originally designated for use in private

networks that are not part of the global Internet. Today they are often used in homes and small

oﬃces that are connected to the Internet through a network address translation (NAT) device.

Such a device acts like a router that translates (rewrites) the addresses and ports in packets as

it forwards them. More precisely, it maps (private address, port) pairs in packets on one of its

interfaces to (public address, port) pairs on the other interface. This enables a small group of

hosts (e.g., those on a home network) to eﬀectively “share” a single IP address. The importance

of these addresses is that they cannot be reached from the global Internet. If you are trying out

the code in this book on a machine that has an address in the private-use class, and you are

trying to communicate with another host that does not have one of these addresses, typically

you will only succeed if the host with the private address initiates communication.

A related class contains the link-local, or “autoconﬁguration” addresses. For IPv4, such

addresses begin with 169.254. For IPv6, any address whose ﬁrst 16-bit chunk starts with FE8

Technically any IPv4 address beginning with 127 should loop back.

6 Chapter 1: Introduction

is a link-local address. Such addresses can only be used for communication between hosts

connected to the same network; routers will not forward them.

Finally, another class consists of multicast addresses. Whereas regular IP (sometimes

called “unicast”) addresses refer to a single destination, multicast addresses potentially refer

to an arbitrary number of destinations. Multicasting is an advanced subject that we cover

brieﬂy in Chapter 4. In IPv4, multicast addresses in dotted-quad format have a ﬁrst number in

the range 224 to 239. In IPv6, multicast addresses start with FF.

1.3 About Names

Most likely you are accustomed to referring to hosts by name (e.g., host.example.com). How-

ever, the Internet protocols deal with addresses (binary numbers), not names. You should

understand that the use of names instead of addresses is a convenience feature that is inde-

pendent of the basic service provided by TCP/IP—you can write and use TCP/IP applications

without ever using a name. When you use a name to identify a communication endpoint, the

system does some extra work to resolve the name into an address. This extra step is often

worth it for a couple of reasons. First, names are obviously easier for humans to remember

than dotted-quads (or, in the case of IPv6, strings of hexadecimal digits). Second, names pro-

vide a level of indirection, which insulates users from IP address changes. During the writing

of the ﬁrst edition of this book, the address of the Web server www.mkp.com changed. Because

we always refer to that Web server by name, and because the change was quickly reﬂected in

the service that maps names to addresses (about which we’ll say more shortly)—www.mkp.com

resolves to the current Internet address instead of 208.164.121.48—the change is transparent

to programs that use the name to access the Web server.

The name-resolution service can access information from a wide variety of sources. Two

of the primary sources are the Domain Name System (DNS) and local conﬁguration databases.

The DNS [10] is a distributed database that maps domain names such as www.mkp.com to

Internet addresses and other information; the DNS protocol [11] allows hosts connected to

the Internet to retrieve information from that database using TCP or UDP. Local conﬁguration

databases are generally OS-speciﬁc mechanisms for local name-to-Internet address mappings.

1.4 Clients and Servers

In our postal and telephone analogies, each communication is initiated by one party, who sends

a letter or makes the telephone call, while the other party responds to the initiator’s contact by

sending a return letter or picking up the phone and talking. Internet communication is similar.

The terms client and server refer to these roles: the client program initiates communication,

while the server program waits passively for and then responds to clients that contact it.

1.5 What Is a Socket? 7

Together, the client and server compose the application. The terms client and server are

descriptive of the typical situation in which the server makes a particular capability—for

example, a database service—available to any client that is able to communicate with it.

Whether a program is acting as a client or server determines the general form of its

use of the sockets API to establish communication with its peer. (The client is the peer of

the server and vice versa.) Beyond that, the client-server distinction is important because

the client needs to know the server’s address and port initially, but not vice versa. With the

sockets API, the server can, if necessary, learn the client’s address information when it receives

the initial communication from the client. This is analogous to a telephone call—in order to

be called, a person does not need to know the telephone number of the caller. As with a

telephone call, once the connection is established, the distinction between server and client

disappears.

How does a client ﬁnd out a server’s IP address and port number? Usually, the client

knows the name of the server it wants—for example, from a Universal Resource Locator (URL)

such as http://www.mkp.com—and uses the name-resolution service to learn the correspond-

ing Internet address.

Finding a server’s port number is a diﬀerent story. In principle, servers can use any port,

but the client must be able to learn what it is. In the Internet, there is a convention of assigning

well-known port numbers to certain applications. The Internet Assigned Number Authority

(IANA) oversees this assignment. For example, port number 21 has been assigned to the File

Transfer Protocol (FTP). When you run an FTP client application, it tries to contact the FTP

server on that port by default. A list of all the assigned port numbers is maintained by the

numbering authority of the Internet (see http://www.iana.org/assignments/port-numbers).

1.5 What Is a Socket?

A socket is an abstraction through which an application may send and receive data, in much

the same way as an open ﬁle handle allows an application to read and write data to stable

storage. A socket allows an application to plug in to the network and communicate with other

applications that are plugged in to the same network. Information written to the socket by

an application on one machine can be read by an application on a diﬀerent machine and vice

versa.

Diﬀerent types of sockets correspond to diﬀerent underlying protocol suites and

diﬀerent stacks of protocols within a suite. This book deals only with the TCP/IP protocol

suite. The main types of sockets in TCP/IP today are stream sockets and datagram sockets.

Stream sockets use TCP as the end-to-end protocol (with IP underneath) and thus provide

a reliable byte-stream service. A TCP/IP stream socket represents one end of a TCP connec-

tion. Datagram sockets use UDP (again, with IP underneath) and thus provide a best-eﬀort

datagram service that applications can use to send individual messages up to about 65,500

bytes in length. Stream and datagram sockets are also supported by other protocol suites, but

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