专业P2P应用开发揭秘：《Cracking the Code》

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"《对等网络应用开发：破解代码》（Peer-to-Peer Application Development: Cracking the Code）是由Dreamtech Software Team编著的一本专业级书籍，着重介绍了在P2P（Peer-to-Peer）模式下进行即时通讯（IM）应用程序的开发。这本书特别关注于C#和JAVA这两种编程语言的应用，提供了超过9,800行的源代码示例，详细展示了实际项目中的专业级P2P文件共享技术。作者团队以清晰的线性方式剖析了一个专业级别的P2P应用，深入到每一行代码，让读者能够理解并掌握这一领域的核心技术。书中不仅涵盖了理论概念，还包括了实践操作，适合那些希望在P2P架构中构建功能丰富的IM系统或者对底层技术感兴趣的开发者。本书由HungryMinds, Inc.出版，享有2002年的版权保护，所有内容，包括内部设计、封面设计和图标，未经许可不得任何形式复制或传输。该书已获得美国国会图书馆的版权注册号，并配以ISBN-10:0-7645-4904-9。此书在美国印刷，由HungryMinds, Inc.在全美范围内分销。通过阅读这本书，读者将收获关于P2P架构的设计原则、协议实现、数据交换机制、错误处理策略以及如何在C#和JAVA环境中构建高效、可靠的P2P应用的宝贵知识。对于想要在这个高度活跃且不断发展的领域提升技能的IT专业人士来说，这是一本不可或缺的参考资料。"

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2 Chapter 1: An Introduction to the Peer-To-Peer Programming Model

As a result of the increased demand for Web-based services, users felt the need to control, exchange, and

share resources directly. In the meantime, in the late 1990s, PCs became increasingly powerful in terms

of speed and processing power. Hence, software developers realized that they could put server software

on individual PCs and initiate a direct two-way flow of information between peers. This led to the revival

of P2P.

Interestingly, the IP routing infrastructure is still P2P. Internet routers act as peers in finding the best

route from one point on the net to another. Yet overlaid on this structure are several layers of hierarchy.

Users get their Internet connectivity from ISPs who, in turn, connect to each other in hierarchies hidden

from the end user. These ISPs depend on the same P2P architecture. Similarly, a network of peered-mail

servers routes e-mail. Between 1999 and 2000, when Napster was revolutionizing the way users shared

music on the Internet, people began to realize the true potential of P2P. Napster is the first large-scale

commercial success of P2P in resource sharing.

From the Client/Server Model to P2P

As against the client/server architecture, the greatest strengths of P2P-based models are their decreased

dependency on the server and their decentralization of control from servers, which used to be

workstations, to peers. Some P2P models do not require servers. End users can directly establish

connections with other users without involving servers. Users have more command in P2P-based models

than in the typical client /server architecture, in which conventional rules must be followed. Unlike in the

C/S system, there is no single point of failure in P2P; in some models, in which P2P puts the server in

place, the role of the server is restricted to a bare minimum. To share files, users do not have to seek the

help of the server, as they can do this directly among themselves.

In view of these advantages, many corporate houses and computing firms consider the P2P model to be

as important as the C/S model. Both models have advantages as well as disadvantages. For example, in

the C/S model, the server becomes a bottleneck when too many users log in to download information. In

contrast to the P2P model, too many requests sent across the network among users keeps network

administrators busy and puts a load on the network itself. Besides, in terms of financial management and

administration, the P2P model definitely has an edge over the C/S model. However, practical realization

of the pure P2P model still remains to be achieved. Companies such as Intel and IBM are spending

millions of dollars and expending significant labor on P2P applications.

In the 1990s, client/server computing architecture was at the peak of its popularity. It attained popularity

because it broke down the regime of monopoly of a few data providers across the world; also, it

encouraged resource sharing and provided various firewalls to its users. However, in 1999 Napster

challenged the C/S architecture. Napster, a P2P-based application, stretched the meaning of sharing

beyond the imagination of the C/S creators. With its file-sharing system of MP3 files, Napster gave a new

boost and dimension to the network and optimized its role toward greater scalability.

With regard to P2P's popularity and utility, many business organizations are seeking to incorporate it in

their purview. Another advantage of P2P is that companies can build collective computing powers and

thereby forget servers and expensive storage devices. P2P has shaken the boundaries of networking in

terms of sharing resources and costs incurred on servers. Compared with the C/S model, P2P is the better

alternative, being more flexible and versatile.

Various P2P Models

P2P models can be divided into the following categories:

♦ Pure peer-to-peer

♦ Peer-to-peer with a simple discovery server

Chapter 1: An Introduction to the Peer-To-Peer Programming Model 3

♦ Peer-to-peer with discovery and lookup servers

♦ Peer-to-peer with discovery, lookup, and content servers

Pure P2P Model

The pure P2P model entirely depends on computers (clients in the C/S model). This may seem

contradictory because each networking model involves both the computer and the server, like the typical

client/server architecture. But the pure P2P model works without relying on any central server. Once the

P2P application is downloaded in the memory of the machine, peers find other connected peers on the

network dynamically. The entire communication occurs among connected peers without any assistance

from a server (see Figure 1-1). By communication, we mean transferring data in the form of uploading

and downloading files, carrying out on-line activities, sending requests, receiving responses, and so on.

Figure 1-1: The pure peer-to-peer model

This feature of the pure P2P-based model breaks the conventional method of communication in

client/server-based models in which the entire communication process between the client and server takes

place based on rules the server sets. The pure P2P-based model allows users to set their own rules and to

set up their own networking environments. The P2P model completely eliminates the headache of

becoming part of any server or ISP to utilize the Internet.

Pure P2P models provide almost plug-and-play features for working with the Internet, in the sense that

you just connect to the Internet and you can use the P2P feature. Another advantage of the pure P2P

model is that it not only works efficiently for the Internet but also is quite beneficial for LAN or an

intranet.

The only problem with the pure P2P model is finding peers on the network. Because no central

administration registers the entry of peers that log in to the network, the users themselves have to locate

other peers.

P2P with Simple Discovery Server

The very name of this model suggests its constitution. Such P2P models do not actually involve a server.

To affect some administration, server boundaries have been laid down in this model. But the role of the

server in this model is restricted to providing the names of already connected peers to the incoming peer,

which notifies the server about its presence by logging in. It must be noted that the server only assists

peers by providing a list of connected peers and that establishing connection and communication still

6 Chapter 1: An Introduction to the Peer-To-Peer Programming Model

When you send a request or search for a particular song, the central Napster server looks for the song in

the indexes of the users currently online. The Napster server then displays the list of the currently

connected users whose collections contain the song you request.

You can then click any name from the list and open a direct link with that user. The central server

connects you to the user and gets out of the way. The file is downloaded from one user to the other. The

actual file is never stored on the Napster server.

Napster does not sell, store, or transfer music files. It is only a platform to enable you to find a file that is

requested by the peer. The Napster server enables you to search music available on other PCs and allows

direct traffic among users. The Napster central server records its registered users and the music files

available on their hard disks. This record is updated each time you log in. When you download and install

the Napster program on your PC, it asks you which folders you want to share with other users.

Gnutella

In 1999, Napster forced serious thinkers to reconsider what network is and how Napster had redefined it

by stretching the rules of the information technology industry. In spite of criticism from various copyright

lawyers and music industries, Napster’s popularity never diminished but kept increasing. Inspired by the

way Napster changed the rules, another P2P-based mode, Gnutella, entered the market. It went one step

beyond Napster. Napster was the catch phrase of the late ’90s, but this century belongs to Gnutella.

Gnutella works more or less like a pure P2P model. Gnutella is downloaded and installed on the

computer. Once the computer on which you have installed Gnutella, is connected with the network, a

message is sent to a computer, which passes it to the other computers, which are also connected to the

Gnutella network via the Internet, to let them know about your presence. Message forwarding works this

way: You get connected on the network and inform a computer, which, in turn, informs 10 others. These

10 computers inform 9 more computers, and so on.

Gnutella is a client-based software that has a file-sharing service and a mini search engine. Once Gnutella

is installed on your local machine, you can serve the requests of other users, acting somewhat like a

server and, on the other hand, can find the desired content by sending requests on the Gnutella network

for locating the user who has the requested content. You can directly download the content from its

machine. Searching on Gnutella is almost like working with any other search engine. In the same way,

Gnutella works on the network processes and shows the client what has been requested. Searching on

Gnutella is more convenient and promising, as the search occurs directly on Gnutella users’ computers

without involving any centralized server search. One limitation is that if the load on the server increases

or the server slows down due to some technical problem, all services slow down or come to a halt.

Though the searching service in Gnutella is free, this also has some limitations. From the data seeker’s

point of view, there is no certainty of the direction in which the request is proceeding, as many data

providers are on the network, making it impossible for the client to discover who fulfills the request. This

information may be needed for future references. From the data provider’s point of view, there is no

guarantee that you hear all the queries.

But in terms of its file-sharing system, Gnutella certainly maintains an edge over other P2P models.

Napster encourages its users to share music files via the centralized server solely. Gnutella facilitates not

only your ability to share music files but also to share all kinds of files, from MP3 to executable files,

without a server.

Some key terms often used in Gnutella networking are the following:

♦ Servent: Because in Gnutella architecture users can be either clients or service providers,

depending on the situation, they are collectively referred to as servents.

♦ Horizon: A horizon refers to a group of servents. A horizon does not represent the entire Gnutella

network but is itself divided into various horizons.

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