C# TCP/IP Sockets实战指南

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"TCP/IP Sockets in C# - Practical Guide for Programmers" 本书《TCP/IP Sockets in C#》是程序员的实用指南，由David Makofske、Michael J. Donahoo和Kenneth L. Calvert共同编写，属于Morgan Kaufmann的“Practical Guides Series”系列。这本书专注于使用C#语言进行Socket编程，旨在帮助读者理解并应用TCP/IP套接字进行网络通信。 Socket编程是网络编程的基础，它允许程序通过网络进行数据传输。在C#中，Socket类库提供了一组接口和方法，使得开发人员能够创建跨平台的网络应用程序，支持TCP（传输控制协议）和UDP（用户数据报协议）等传输层协议。TCP是一种面向连接的、可靠的协议，而UDP则是无连接的、不可靠的协议，适用于不同的应用场景。本书首先会介绍TCP/IP协议栈的基本概念，包括IP地址、端口号、网络层次结构以及TCP/IP协议族的工作原理。然后，深入讲解C#中的Socket类和相关的.NET Framework类库，如NetworkStream、TcpClient、TcpListener、UdpClient等。读者将学习如何创建Socket对象，建立客户端和服务器之间的连接，发送和接收数据，以及处理并发连接。书中还涵盖了错误处理、异常处理、异步编程和多线程技术，这些都是在网络编程中不可或缺的部分。此外，作者可能会讨论到网络安全问题，如加密和身份验证，以及如何利用SSL/TLS协议来保护数据的安全传输。对于更复杂的应用场景，例如服务器的负载均衡、高可用性和性能优化，本书可能也会有所涉及。此外，书中的一些实例代码将帮助读者更好地理解和应用所学知识，从而开发出功能完备、高效的网络应用程序。《TCP/IP Sockets in C#》是一本全面且实践性强的指南，适合有C#基础并希望学习网络编程的开发者，无论是初学者还是有一定经验的程序员，都能从中受益。通过阅读本书，读者将能够掌握使用C#进行TCP/IP套接字编程的技术，进而构建自己的网络服务和客户端应用程序。

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1.1 Networks, Packets, and Protocols 3

Application

Socket

IP Channel

(e.g., Ethernet)

RouterHost Host

UDP TCP

Channel

Application

Socket

UDPTCP

Figure 1.1: A TCP/IP network.

The Internet Protocol provides a datagram service: Every packet is handled and deliv-

ered by the network independently, like telegrams or parcels sent via the postal system.

To make this work, each IP packet has to contain the address of its destination, just as every

package you mail is addressed to somebody. (We’ll say more about addresses shortly.)

Although most parcel delivery companies guarantee delivery of a package, IP is only a

best-effort protocol: It attempts to deliver each packet, but it can (and occasionally does)

lose, reorder, or duplicate packets in transit through the network.

The layer above IP is called the transport layer. It offers a choice between two

protocols: TCP and UDP. Each builds on the service provided by IP, but they do so in dif-

ferent ways to provide different kinds of channels, which are used by application protocols

with different needs. TCP and UDP have one function in common: addressing. Recall that

IP delivers packets to hosts; clearly, a ﬁner granularity of addressing is needed to get a

packet to a particular application, perhaps one of many using the network in the same

host. Both TCP and UDP use addresses called port numbers so that applications within

hosts can be identiﬁed. They are called end-to-end transport protocols because they carry

data all the way from one program to another (whereas IP carries data from one host to

another).

TCP is designed to detect and recover from the losses, duplications, and other errors

that may occur in the host-to-host channel provided by IP. TCP provides a reliable

byte-stream channel, so that applications don’t have to deal with these problems. It

is a connection-oriented protocol: Before using it to communicate, two programs must

ﬁrst establish a TCP connection, which involves completing an exchange of handshake

messages between the TCP implementations on the two communicating computers. Using

TCP is similar to ﬁle input/output (I/O). In fact, a ﬁle that is written by one program and

read by another is a reasonable mode of communication over a TCP connection. UDP,

on the other hand, does not attempt to recover from errors experienced by IP; it simply

extends the IP best-effort datagram service so that it works between applications programs

4 Chapter 1: Introduction

■

instead of between hosts. Thus, applications that use UDP must be prepared to deal with

losses, reordering, and so on.

1.2 About Addresses

When you mail a letter, you provide the address of the recipient in a form that the postal

service can understand. Before you can talk to somebody on the phone, you must supply

their number to the telephone system. In a similar way, before a program can communicate

with another program, it must tell the network where to ﬁnd the other program. In TCP/IP,

it takes two pieces of information to identify a particular program: an Internet address,

used by IP, and a port number, the additional address interpreted by the transport protocol

(TCP or UDP).

Internet addresses are 32-bit binary numbers.

In writing down Internet addresses

for human consumption (as opposed to using them inside applications), we typically show

them as a string of four decimal numbers separated by periods (e.g., 10.1.2.3); this is called

the dotted-quad notation. The four numbers in a dotted-quad string represent the contents

of the four bytes of the Internet address, thus each is a number between 0 and 255.

Technically, each Internet address refers to the connection between a host and an

underlying communication channel, such as a dial-up modem or Ethernet card. 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, because a host can have multi-

ple physical connections (interfaces) to the network, one host can have multiple Internet

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 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 of 1 to 65,535 (0 is reserved).

Throughout this book the term Internet address refers to the addresses used with the current version

of IP, which is version 4 [11]. Because it is expected that a 32-bit address space will be inadequate for

future needs, a new version of IP has been deﬁned [5]; it provides the same service but has much bigger

Internet addresses (128 bits). IPv6, as the new version is known, has not been widely deployed; the

sockets API will require some changes to deal with its much larger addresses [6]. The .NET framework

does support IPv6 addresses, but they are not covered in this book.

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1.3 About Names 5

1.3 About Names

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

However, the Internet protocols deal with numerical addresses, not names. You should

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

independent of the basic service provided by TCP/IP—you can write and use TCP/IP appli-

cations without ever using a name. When you use a name to identify a communication

endpoint, the system has to do 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 generally

easier for humans to remember than dotted-quads. Second, names provide a level of indi-

rection, which insulates users from IP address changes. During the writing of this book, the

Web server for the publisher of this text, Morgan Kaufmann, changed Internet addresses

from 213.38.165.180 to 129.35.78.178. However, because we refer to that Web server

as www.mkp.com (clearly much easier to remember than 213.38.165.180), and because

the change is reﬂected in the system that maps names to addresses (www.mkp.com now

resolves to the new Internet address instead of 213.38.165.180), 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 conﬁgura-

tion databases. The DNS [8] is a distributed database that maps domain names such as

www.mkp.com to Internet addresses and other information; the DNS protocol [9] 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. Microsoft Windows provides a hosts text ﬁle where

IP-to-domain-name mappings can be hard-coded or overridden. UNIX-based systems

typically have a ﬁle called /etc/hosts that does the same thing.

1.4 Clients and Servers

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

sends a letter or dials a 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 commu-

nication 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. 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 communicate 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

6 Chapter 1: Introduction

■

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

corresponding 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. 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 www.iana.org/assignments/portnumbers).

There are also numerous standards, protocols, and proposals for directory services,

by which a client can query the services and locations available from servers from a direc-

tory. Of course, the client must know the address and port to contact the directory services

server on in order to ﬁnd this information! Again, this is typically deﬁned and published

as being at a “well-known” location for the intended clients.

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 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 also 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 proto-

col 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. Datagram sockets use UDP (again, end-to-end 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 sock-

ets are also supported by other protocol suites, but this book deals only with TCP stream

sockets and UDP datagram sockets. A TCP/IP socket is uniquely identiﬁed by an Internet

address, an end-to-end protocol (TCP or UDP), and a port number. As you proceed, you

will encounter several ways for a socket to become bound to an address.

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