内容分发网络：架构、协议与实践探索

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"Co Content Networking - Architecture Protocols and Practice" 内容分发网络（CDN，Content Delivery Network）是一种网络架构，其主要目标是通过将内容分布式存储在多个地理位置的服务器上，来提高网络服务的性能、可靠性和可扩展性。随着互联网用户对高质量、低延迟的内容访问需求增长，CDN已经成为现代互联网基础设施的关键组成部分。 "Content Networking: Architecture, Protocols, and Practice" 这本书可能详细阐述了CDN的构建原理、协议设计以及实际应用。在CDN的架构方面，它可能涵盖以下几个关键点： 1. 边缘服务器：这些服务器位于用户附近，负责缓存并分发内容，减少了数据传输的延迟，提高了用户体验。 2. 负载均衡：通过智能DNS解析或者全局负载均衡技术，确保用户请求被导向最近或最合适的边缘服务器。 3. 内容管理和更新：讨论如何有效地更新和管理分布在各地的缓存内容，以保持信息的实时性。 4. 安全性：包括防止DDoS攻击、保护内容安全和用户隐私的策略和机制。 5. 自适应流媒体：对于视频等动态内容，CDN可能会涉及自适应比特率技术，以适应不同网络条件下的流媒体传输。 6. 监控与分析：监控CDN性能，收集数据进行分析，以便优化网络配置和资源分配。在协议方面，可能涉及以下几点： 1. HTTP/HTTPS：基础的Web内容分发协议，通常用于静态内容传输。 2. RTMP/RTSP：实时流媒体协议，用于视频直播和点播。 3. P2P（对等网络）技术：如BitTorrent，允许用户之间共享内容，减轻服务器压力。 4. CDNs还可能利用TCP优化协议，如QUIC，来减少延迟和提高连接稳定性。实践部分可能包含CDN在实际应用场景中的案例，如大型网站、视频流服务、云服务提供商等如何利用CDN提升服务质量。此外，还会探讨CDN如何与新兴技术如5G、边缘计算和物联网(IoT)相结合，以适应未来网络环境的需求。 "Co Content Networking - Architecture Protocols and Practice" 是一本深入探讨CDN技术的书籍，对理解CDN的工作原理、设计和应用具有重要价值。对于从事网络工程、系统架构和网络管理的人来说，这本书可能是提升专业技能的重要参考资料。

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

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