内容分发网络架构与实践详解

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《内容分发网络：架构、协议与实践》（Content Networking: Architecture, Protocols, and Practice）由Markus Hofmann和Leland R. Beaumont合著，是一本在内容分发网络领域具有高度影响力的教材和技术指南。本书详尽阐述了内容分发网络的基础理论和实践，为读者揭示了这一技术的核心概念和实际操作。内容涵盖了一系列关键知识点，包括但不限于： 1. 架构设计：深入剖析了CDN（Content Delivery Network，内容分发网络）的构建原理，涉及如何通过分布式节点布局来优化内容的访问速度和可靠性，以及边缘计算在其中的作用。 2. 网络协议：书中详细介绍了与CDN相关的各种协议，如HTTP/HTTPS、DNS、TCP/IP等，这些协议在数据传输和路由决策中扮演着核心角色。此外，还可能讨论到如何利用CDN协议实现缓存机制，以减少服务器压力和提高用户体验。 3. 技术基础：涵盖了网络算法、流量管理和带宽优化等内容，帮助读者理解如何设计高效的CDN系统，确保内容在网络中的高效分发。 4. 恢复与保护：如果章节涉及，可能还会介绍针对光网络、SONET/SDH、IP和MPLS等网络基础设施的保护和恢复策略，这对于保障CDN服务的连续性和稳定性至关重要。 5. 路由与容量设计：探讨了通信和计算机网络中的路由策略以及如何根据流量需求进行容量规划，确保CDN网络能够适应不断变化的负载。 6. 无线传感器网络：虽然这不是CDN的主要组成部分，但可能包含对无线网络环境下的内容分发策略的讨论，展示了无线技术如何融入到整体网络架构中。 7. 通信与分析方法：通过对比和分析不同方法，强调了内容分发网络中的数据分析和决策制定对于提升服务质量的重要性。 8. 互联网及其协议：介绍了互联网基础协议，如TCP/IP，以及它们在内容分发网络中的应用，帮助读者理解协议如何支撑起庞大的CDN网络。 9. 现代电缆电视技术：尽管看似与主题不直接相关，但现代通信技术的演进也影响了CDN的发展，特别是在视频内容的分发方面。 10. 蓝牙编程：虽然不是主要内容，但可能涉及如何通过蓝牙API实现智能设备之间的协同工作，这在某些CDN应用场景下是不可或缺的。 11. 政策驱动的网络管理：探讨了如何运用策略来有效管理CDN网络，以适应未来网络环境的变化和业务需求。《内容分发网络：架构、协议与实践》不仅为读者提供了全面的技术背景，还深入剖析了CDN的实际应用，是任何想要深入了解这一领域的专业人士不可多得的参考书。

CHAPTER 1

Introduction

Over the last few decades, the Internet has revolutionized our society and our

economy. It has changed the way people communicate with each other and the

way business is conducted. The Internet has created a global environment that is

drawing people from all over the world closer together. Collaboration and inter-

action of individuals through their networked computers have been main appli-

cations on the Internet since the beginning. Electronic mail and Internet chat

rooms are just two examples of popular applications. Over the last decade, the

Internet has been used ever more as a mechanism for information dissemination

and broadcasting, mainly driven by the emergence of the World Wide Web—

also referred to as WWW or the Web. The Web forms a universe of information

accessible via networked computers, offering content in the form of Web pages,

images, text, animations, or audio and video streams. This book examines the

technical concepts and the challenges of distributing, delivering, and servicing

content over the Internet. Business-related aspects are considered when they

have impact on the underlying technology. The focus is on fundamental princi-

ples and concepts rather than providing a reference for specific communication

protocols or implementation details.

The first chapter serves as an introduction, explaining the notion of content

networking and establishing the underlying key concepts. A brief look at the

early days of information access over the Internet segues to the roots of modern

content networking—the World Wide Web. The chapter continues with a flash-

back to the first half of the 1990s, with a history of the Web setting the stage for

a discussion of underlying concepts and principles. These include the represen-

tation, identification, and transport of Web objects, which are most often referred

to as Hypertext Markup Language (HTML), Universal Resource Identifier (URI),

and Hypertext Transport Protocol (HTTP), respectively. The power of URIs

and hyperlinks allows a variety of protocols to link new content types together

and add richness to the original WWW. For example, other protocols such as

RTSP and RTP allow other object types, such as multimedia streams, to be

Ch01.qxd 01/19/2005 12:49 PM Page 1

linked into the WWW. The chapter continues looking at Web applications as a

driving force for the evolution of the Web and for adopting new technology. It

identifies the shortcomings of today’s Web architecture and outlines an evolu-

tionary path toward advanced communication architectures of the future. The

technology-focused part is complemented with a description of the various Web

beneficiaries and their diversity of interests. The chapter concludes with a tour

through the book that outlines the remaining ten chapters.

1.1 The Early Days of Content Delivery over the Internet

Until about a decade ago, most of the world knew little or nothing about the

Internet. It was used largely by the scientific community for sharing resources on

computers and for interacting with colleagues in their respective research fields.

When work on the ARPANET—the origin of today’s Internet—started in the

late 1960s and the 1970s, the prevailing applications were as follows: access to

remote machines, exchange of e-mails, and copying files between computers.

Electronic distribution of documents soon gained importance, as it became

apparent that the traditional academic publication process was too slow for the

fast-paced information exchange essential for creating the Internet. When the

File Transfer Protocol (FTP) [Bhu71, RFC 959] came into use in the early 1970s,

documents were prepared as online files and made accessible on servers via FTP.

Interested parties used an FTP client program to establish a connection to the

server for downloading the document. Over the years, FTP evolved into the pri-

mary means for document retrieval and software distribution over the Internet.

In the early 1990s, FTP accounted for almost half of the Internet traffic [Mer1].

However, FTP did not solve all the problems related to information retrieval

over the Internet—it enabled downloading files from remote machines, but it did

not support users facing the daunting task of navigating through the Internet and

in locating relevant resources. Retrieving documents via FTP required users to

know in advance the exact server to contact and the name of the file to download.

Knowing just the title and the authors of a research paper, for example, was not

sufficient for retrieving an electronic copy of the paper. Moreover, the user was

required to figure out which FTP server was storing the paper and which file name

had been used. The Internet worked very much like a library without a catalog or

index cards—users had to know where to look to find the content they needed.

Locating relevant files on the Internet was simplified to some extent with the

introduction of archie in 1991 [ED92]. The archie system made use of a special

“anonymous” account on FTP servers, which gave arbitrary users limited access

without having to enter a password. Using these “anonymous” accounts, archie

servers periodically searched FTP servers throughout the Internet and recorded

the names of files they found. This information was used to create and maintain

a global catalog of files available for download. Users could use this catalog to

search for file names matching certain patterns. When matches were found,

archie also indicated the FTP servers on which the files were available.

2 CHAPTER 1 Introduction

Ch01.qxd 01/19/2005 12:49 PM Page 2

A major restriction of archie was its limitation to pattern matching on file

names rather than the actual content of the files. The Wide Area Information

Server (WAIS) project [KM91] implemented a more powerful concept by

searching through the text of documents in addition to their file names or titles.

Suppose you are interested in finding articles on Michael Jordan’s second come-

back to professional basketball, and you perform an archie search using

“Jordan” as your keyword. Even if the file named “NBA-News-September-

2001.txt” includes a story covering Jordan’s comeback, it would not turn up

under an archie search. As WAIS digs through the entire text of the article, that

file would appear with a WAIS search. Moreover, the WAIS mechanism pro-

vided a scored response, ranking retrieved information based on the quantity of

keyword appearances in the text and on how close to the document’s beginning

they turned up. WAIS was originally developed at the beginning of the 1990s by

a consortium of companies that included Thinking Machines Inc., Dow Jones,

Apple Computer, and KPMG Peat Marwick. The first version of WAIS was

available in the public domain in 1991. By summer 1992, the project had evolved

into a separate company called—not surprisingly—WAIS Inc. This company

can be considered the first to commercialize technology related to content

retrieval over the Internet.

However, the WAIS system was not perfect—the user interface was relatively

difficult to use and the search capabilities were initially limited to text docu-

ments. Besides, it scored documents based on the absolute number of keyword

appearances rather than the density of their appearance. As a result, long docu-

ments were more likely than short documents to end up at the top of the list.

WAIS further lacked the capability for hierarchical organization of content

resources—a feature introduced by the Gopher system [RFC 1436].

Gopher was developed at the University of Minnesota in 1991 and named

after the school’s furry mascot. It let users retrieve data over the Internet with-

out using complicated commands and addresses. Gopher servers searched the

Internet using WAIS and arranged the results in hierarchical menus, using plain

language. As users selected menu items, they were lead to other menus, files,

or images, which might not even have resided on the local Gopher server.

References could move users to remote servers or fetch files from distant loca-

tions. Gopher significantly simplified information retrieval on the Internet. It

handled the details of actually getting requested information, without requiring

users to know how and from where to retrieve those resources. Initially deployed

only on the University of Minnesota campus, other institutions quickly discov-

ered Gopher’s versatility and set up their own Gopher servers. At one time, there

were a few thousand Gopher servers registered with the top-level server

“Gopher Central” at the University of Minnesota or its counterparts in other

countries.

Archie, WAIS, and Gopher emerged in the same era and coexisted for some

time. They all had their advantages and disadvantages, and occasionally, they

are still used today. Nevertheless, in the course of the 1990s, they all were

subsumed into yet another system—the World Wide Web (WWW).

1.1 The Early Days of Content Delivery over the Internet 3

Ch01.qxd 01/19/2005 12:49 PM Page 3

1.2 The World Wide Web—Where It Came From and What It Is

The World Wide Web is an Internet facility that links information accessible via

networked computers. This information is typically represented in the form of

Web pages, which can contain text, graphics, animations, audio/video, and

hyperlinks. Embedding hyperlinks in documents is an important feature of the

Web and differentiates it clearly from Gopher and other approaches. Embedded

hyperlinks connect a Web page to other resources either locally or on remote

computers. Users can follow the links and access referenced resources simply by

pointing to the hyperlink and clicking a mouse button. This intuitive mechanism

allows browsing through a collection of information resources without having to

worry about their actual location or their format.

This section will briefly describe the origin of the Web, where it came from

and why it has been so successful. A description of the architectural components

will help in the understanding of the fundamental design of the Web and, at the

same time, motivate the evolution of the Web. A detailed introduction to the

Web can be found in [KR01].

1.2.1 The Origin of the World Wide Web

The World Wide Web has its origin at the European Organization for Nuclear

Research (CERN) near Geneva, Switzerland. It was initially proposed by Tim

Berners-Lee in 1989 to improve information access and help communication

within the particle physics community [Ber89]. The community included several

hundred members all scattered among various research institutes and universi-

ties. Although the groups were formally organized into a hierarchical manage-

ment structure, the actual working and communication structure looked more

like a loosely coupled mesh whose linkages evolved over time. A researcher look-

ing for specific information was typically given a few references to experts who

may prove helpful. In order to get the desired information, the researcher used

the provided information to contact the respective colleagues. While this com-

munication scheme was principally working fine, a high turnover of people

made project record keeping and locating expertise increasingly difficult. A solu-

tion was required that would support dynamic, non-centralized interaction and

quick access to documents stored at secluded locations.

In this situation, Tim Berners-Lee proposed to his management the idea of

using hypertext for linking information available on individual computers

[Ber89]. The hypertext concept had been envisioned earlier as a method for mak-

ing computers respond to the way humans think and require information

[Bus45, Nel67, EE68]. Hypertext documents embed so-called hyperlinks, which

can be represented as underlined text or as icons in any size and shape. By select-

ing and clicking on a hyperlink, associated information is loaded and displayed.

Tim’s proposal extended the hypertext concept to allow linking of information

not only on a single local machine, but also of information that can be stored on

4 CHAPTER 1 Introduction

Ch01.qxd 01/19/2005 12:49 PM Page 4

remote computers connected via a network. Retrieving the associated informa-

tion over the network is transparent to the user, without burdening the user with

having to know the resource location and the network protocol to be used for

retrieval. This scheme proved to be very powerful as it allows users transparent

accesses to documents on remote computers with a click of the mouse.

The CERN management approved the proposal and launched the project in

the second half of 1990. Tim started implementing a hypertext browser/editor

and finished the first version at the end of 1990. The program was running on

a NeXT computer and offered a graphical user interface. It was called

WorldWideWeb but later renamed Nexus to avoid confusion with the abstract

concept of the World Wide Web itself. At the same time, the implementation was

complemented with a separate line-mode browser written by CERN student

Nicola Pellow. Other people soon started implementing browsers on different

platforms. By 1992, first versions of Erwise, ViolaWWW, and MidasWWW were

introduced for the X/Motif system, followed by a CERN implementation for the

Apple Macintosh in 1993.

At that time, there were around 50 known Web servers deployed, and the

WWW was accounting for about 0.1% of the Internet traffic. It was a promising

approach, but the real breakthrough came with the creation of Mosaic, the first

widespread graphical Web browser. Mosaic development was started at the

National Center for Supercomputing Applications (NCSA) by Marc Andreesen

and Eric Bina. They realized that broad acceptance of Web technology would

require a more user-friendly interface. Their browser software added clickable

buttons for easy navigation and controls that let users scroll through text. More

important, Marc and Eric were the first ones to get embedded images working.

Earlier browsers allowed viewing of pictures only in separate windows, while

Mosaic made it possible for images and text to appear in the same window. The

application was trivial to install and the team followed up coding with very fast

customer support. Overall, Mosaic drastically simplified the first step onto the

Web and even allowed beginners to take advantage of the new, exciting Web

technology. The Unix version of Mosaic was available for download from

NCSA in early 1993. The software was provided free of charge and within weeks

tens of thousands of people had downloaded it. Software versions for the PC

and Macintosh followed later the same year, boosting its popularity even

further. The Web started eclipsing competing systems, as it subsumed their main

features and functionality. Users could conveniently access FTP servers as well

as Archie, WAIS, and Gopher from their Web browsers, thus eliminating the

need for these specialized applications.

By 1994, Marc and Eric had graduated and headed for Silicon Valley to com-

mercialize their software. Initially called Mosaic Communications Corporation,

their company was soon renamed Netscape Communications Corporation—the

birthplace of the famous Netscape browser family, also known as Netscape

Navigator and Netscape Communicator. The Web’s popularity increased, and the

number of Web sites grew from approximately 500 in 1994 to nearly 10,000 by the

beginning of 1995. Netscape quickly became the dominant browser and by 1996,

1.2 The World Wide Web—Where It Came From and What It Is 5

Ch01.qxd 01/19/2005 12:49 PM Page 5

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