Instant Messaging
`in Java
`The Jabber Protocols
`
`IAIN SHIGEOKA
`
`M A N N I N G
`
`Greenwich
`(74° w. long.)
`
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`Manning Publications Co.
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`
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`
`1 2 3 4 5 6 7 8 9 10 - VHG 05 04 03 02
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`contents
`
`xiii
`preface
`xv
`about this book
`xxi
`author online
`xxii
`acknowledgments
`about the cover illustration xxiii
`
`PART I
`
`INSTANT MESSAGING PRIMER . . . . . . . . . . . . . . . . 1
`
`1 Introduction to IM concepts
`
`3
`1.1 Background on messaging 4
`A brief history of Jabber 11 ■ Goals of the Jabber project 12
`1.2 What is Jabber? 13
`Jabber’s XML-based data formats 14 ■ Jabber’s simple
`architecture 18 ■ Jabber’s four core concepts 21
`1.3 Benefits of the Jabber protocols 27
`1.4 Drawbacks of the Jabber protocols 28
`1.5 Conclusion 29
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`contents
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`2 Jabber technical fundamentals
`
`31
`2.1 The Jabber messaging model 32
`Benefits 35 ■ Drawbacks 36 ■ Relying on distributed
`servers 38 ■ How Jabber packet routing works 39
`Step-by-step: a message’s path through Jabber 44
`2.2 The core Jabber protocols 45
`Message: Delivering data 45 ■ Presence: updating user
`online status 45 ■ Info/Query: handling everything else 45
`Jabber session example 47
`2.3
`2.4 Conclusions 56
`
`PART II PROTOCOLS, CODE, AND ADVANCED IM . . . . . . 57
`
`3 IM concepts and Jabber protocols
`
`59
`
`3.1 A basic Java Jabber server 60
`Goals for our server 60 ■ Our server software 61
`The basic server design 62
`3.2 The session pool maintains client connections 63
`The Session class represents a connection 64
`The SessionIndex class provides session lookup 66
`3.3 XML parsing subsystem 69
`Representing Jabber packets 69 ■ The PacketQueue class as
`server focal point 77 ■ SAX parsing in Java 80
`3.4 Packet handling and server threads 87
`Packet handling in QueueThread 90 ■ Parsing XML in the
`ProcessThread 95 ■ The main application Server class 97
`3.5 Testing the server 98
`3.6 Conclusion 100
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`ix
`
`4 The Jabber message protocols
`
`101
`4.1 Messaging is the heart of IM 102
`4.2 The message protocol 103
`Normal messages 104 ■ Chat messages 105
`Groupchat messages 106 ■ Headline messages 108
`Error messages 109 ■ Out-of-band messages 110 ■ Reality check:
`one message, many user interfaces 112
`Java Jabber client 113
`Goals 114 ■ The client design 115 ■ The client
`model 116 ■ Using the client model 123 ■ Results 131
`4.4 Conclusions 132
`
`4.3
`
`5 The presence protocols
`
`133
`5.1 The need for presence 134
`5.2 The presence protocol 134
`5.3 Adding groupchat support 138
`Groupchat protocols 138 ■ Server modifications 144
`Client modifications 157
`Shortcomings of our server and basic groupchat 161
`5.4
`5.5 Conclusions 162
`
`6.1
`
`6 Info/Query and client registration
`
`163
`Info/Query: the catch-all protocol 164
`The IQ protocol 165 ■ IQ extensions 167
`6.2 Registration creates and manages accounts 169
`User accounts 170 ■ The register protocol 171
`6.3 The Jabber server modifications 175
`Adding user accounts 175 ■ Adding registration support 183
`6.4 Conclusions 186
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`7 Client authentication
`
`189
`7.1 Authentication controls account access 190
`The authentication protocol 191
`7.2 The Jabber server modifications 199
`7.3 The Jabber client modifications 205
`Modifying the JabberModel 206 ■ The client IQ packet
`handlers 211
`7.4 Conclusions 215
`
`8 Roster and user presence
`
`217
`8.1 Roster protocol: presence’s missing piece 218
`The roster protocol 221
`8.2 The Jabber server modifications 224
`Representing user presence 224 ■ Adding a roster
`subsystem 226 ■ The roster packet handlers 233
`8.3 The Jabber client modifications 236
`Adding minimal roster support 236 ■ Testing the server 238
`8.4 Conclusions 241
`
`9 Creating a complete Jabber system 243
`
`9.2
`
`9.1 Creating Jabber-compliant implementations 244
`Setting standards: the Jabber Software Foundation 245
`Enforcing standards: Jabber Powered applications 245
`Organizing standards: Jabber environments 246
`Today’s options for achieving server compliance 247
`Server missing pieces 248
`Server-to-server communications: federating Jabber domains 248
`Dialback authentication: S2S security 250 ■ Transports:
`integrating with other IM systems 257 ■ Deployment of Jabber servers
`and components 260 ■ Server security: creating protected Jabber
`services 262 ■ Jabber server management 264 ■ Adding reliability
`and availability 265
`9.3 Client missing pieces 266
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`xi
`
`9.4 User agent clients 266
`Enhancing existing applications 268 ■ Chatbots:
`creating IM services 268
`9.5 Conclusions 270
`
`10 Enterprise Jabber
`
`271
`10.1 What is needed to support enterprise messaging 273
`Enhancing Jabber security 273 ■ Guaranteed quality
`of service 278 ■ Creating system administration tools
`an techniques 279
`10.2 The promise of MOM 280
`Jabber as middleware 281 ■ Jabber and the J2EE Java
`Messaging Service 282 ■ Jabber, .NET, and SOAP 290
`10.3 Examples of Jabber applications 292
`Jabber groupware 292 ■ Jabber network
`services 293 ■ Applications enhanced by Jabber 293
`10.4 Distributed application alternatives to Jabbers 296
`RPCs: oldies but goodies 296 ■ P2P systems:
`the new challenger 298 ■ Hybrid systems:
`a better compromise 299
`10.5 Conclusions 300
`
`Jabber reference 301
`references 369
`index 373
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`4
`
`CHAPTER 1
`Introduction to IM concepts
`
`Instant messaging (IM) systems using the Java programming language are poised
`to become a major part of both consumer and enterprise networking, and will
`play a core communication role similar to email.
` Messaging of course has always been a core feature of the Internet. For exam-
`ple, one of the first and still most pervasive Internet technologies is email. It
`remains an Internet killer app. However, we all know that Internet communica-
`tion can be even more interesting and powerful than “plain old email.” We should
`be able to better exploit it as an inexpensive medium for transferring data almost
`instantaneously. And that is the aim of this book.
` Most of the examples and IM issues discussed here are from the point of view
`of an enterprise developer interested in creating systems for medium or large
`businesses. This is primarily a bias on my part, as my projects tend to fall into this
`category. However, the same issues that face enterprises also will be important to
`any other developer who wants to create reliable, secure systems.
` To make the discussion concrete, we’ll implement an IM system in Java that
`complies with the Jabber protocols (which you can find at www.jabber.org). By
`examining the Jabber protocols in particular, we’ll get a real sense for what makes
`a working IM system tick and why I think they may very well be the best ones to
`implement in the fragmented field of IM. Finally, the Jabber protocols have suf-
`fered from a lack of documentation that I hope this book begins to address.
` In this chapter, we’ll examine IM systems in general and Jabber in particular.
`The discussion will be largely nontechnical so we can concentrate on why and
`where we need IM. Discussion of the technical aspects of the Jabber protocols will
`occupy the remainder of the book and explain the how of Jabber. We will develop
`a basic Jabber server and client in Java throughout chapters 3–8 to help make the
`concepts concrete. The chapter closes with a look at the benefits and drawbacks
`associated with Jabber IM.
`
`1.1
`
`Background on messaging
`
`The idea of instant messaging has been around for a long time. All of the visible
`IM features like one-on-one chat and group chats existed in other Internet appli-
`cations long before IM entered the scene. For example, the classic Unix talk
`application allowed users to chat over the network years before IM ever appeared,
`and group chats have been carried out on Internet Relay Chat (IRC) systems
`almost as long as talk has been around.
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`Background on messaging
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`5
`
` The innovation in today’s IM systems is the packaging of these separate systems
`into a managed messaging platform. Jabber takes this further and establishes a
`universal messaging address and the concept of presence that can be applied to that
`address for an even simpler communication experience. A common address and
`presence are relatively simple technologies that have existed in other forms. How-
`ever, it’s the packaging of the concepts into a single, easy to use, cohesive messag-
`ing system that has really caught on. The need for ease of use has increased as
`more people (many of whom have less technical knowledge or inclination than
`earlier Internet users) join the network community.
` IM addresses are an extension of the well-established email addresses almost
`everyone has today. However, your email address can be used only to receive
`email. In contrast, IM ties many message types together using a single IM address
`(figure 1.1). In an IM system, you can receive a message, chat, or groupchat1 with
`a person using one IM address.
`
`email
`
`IM application
`
`IM Address
`
`chat
`
`group chat
`
`IM application
`
`Figure 1.1 IM addresses serve as universal communication destinations. They aggregate
`many different messaging sources into a single user message endpoint.
`
`Chat and group chat are conversational, and require you to be online at the same
`time as the person you are chatting with. To make the online rendezvous simple,
`your IM presence will constantly inform other users when you are online and
`available for chatting (figure 1.2). IM presence makes communication through IM
`
`1 IM messages are like memos or emails and tend to be complete like a letter. Chat and groupchat allows
`you to quickly exchange short text messages in a conversation. Each individual chat or groupchat mes-
`sage is like a sentence taken out of a telephone conversation; it doesn’t make sense unless you know
`what has been said before. The difference between chat and groupchat is that chat is a one-on-one con-
`versation while groupchat is a chat among more than two people.
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`6
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`CHAPTER 1
`Introduction to IM concepts
`
`systems similar to striking up conversations around the water cooler. You can eas-
`ily see who is available to talk, and casually start conversations.
`
`Presence:
`available
`
`Presence:
`available
`
`Presence:
`sleeping
`
`Presence:
`unavailable
`
`IM address
`
`IM address
`
`IM address
`
`IM address
`
`zzz
`
`Figure 1.2
`Presence shows at a
`glance who you can
`communicate with, and the
`best type of messaging to
`use (chat, messages, etc).
`
`Older chat systems like Unix talk work similarly to the phone system in the sense
`that they lack presence. When attempting to chat, you have to make blind calls to
`the person with whom you want to converse, hoping he or she is available to
`answer the call. Unlike the phone system, though, most people are not available
`to talk online as much as they are with the phone so the chance of finding some-
`one “at home” online is greatly reduced.
` The simplicity and integration of IM systems first caught on with the consumer
`market. The most successful of these consumer systems is AIM (AOL Instant Mes-
`senger) that introduced IM to the mainstream consumer and encouraged its large
`membership to embrace the technology. The consumer market for IM continues
`to grow and many opportunities exist for the enterprising developer.
` Despite the large consumer market for IM, most of the development commu-
`nity is interested in the opportunities for applying IM technologies in the enter-
`prise (table 1.1). First, enterprises live or die by their ability to communicate
`within the company and with partners and customers. IM provides new communi-
`cation channels that are well-suited to many messaging tasks.
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`Background on messaging
`
`11
`
`1.1.1 A brief history of Jabber
`The Jabber project began in early 1998 as the brainchild of Jeremie Miller.
`The project quickly grew and evolved. It garnered wide public attention when
`it was discussed on the popular developer discussion website Slashdot
`(www.Slashdot.org) in January 1999.
` The core Jabber protocols matured and the 1.0 release of the open source ref-
`erence Jabber server was released in May 2000. The core Jabber protocols that
`were implemented in the 1.0 release of the reference server have remained rela-
`tively unchanged to this day.
` From its beginnings, the Jabber development community has tried to create
`IM standards and encourage interoperability between IM systems. These cooper-
`ative efforts are in direct contrast to the behavior of other popular IM providers
`that actively work to keep their systems proprietary and isolated from other IM
`networks.
` As part of the Jabber IM standards effort, in June 2000 the Jabber community
`submitted the Jabber protocols as a Request for Comments (RFC) to the Inter-
`net Engineering Task Force (IETF)6 as part of its Instant Messaging and Pres-
`ence Protocol (IMPP) standard. The IETF maintains some of the most important
`Internet standards including those for email and Internet addresses. Unfortu-
`nately, the IMPP effort bogged down and as of this writing appears to be going
`nowhere fast. Jabber has also tried to participate in other standardization efforts
`like IMUnified (www.imunified.org) and the Presence and Availability Manage-
`ment Forum (www.pamforum.org) with varying degrees of success.7
` In May 2001 the Jabber community (www.jabber.org) and Jabber Inc. (www.jab-
`ber.com) created the Jabber Software Foundation (foundation.jabber.org).8 The
`Jabber Software Foundation is an organization similar to the successful Apache
`Foundation. Its charter clearly shows the Jabber community’s dedication to open
`standards and interoperability.
`
`
`
`
`6 The IETF website is at www.ietf.org.
`7 Jabber’s success at creating Internet standards for IM have mirrored that of the IM community in gen-
`eral, which is to say its success has been “zero.” I’m not sure if this is an indication of the immaturity and
`proprietary nature of IM today or a factor inherent to the IM community or its technology. I would haz-
`ard a guess that it’s the former, but a cynic would claim the latter. In any case, standards are desperately
`needed and work continues within the Jabber community to promote and push for standardization.
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`12
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`CHAPTER 1
`Introduction to IM concepts
`
`◆
`
`JABBER SOFTWARE FOUNDATION CHARTER
`The Jabber Software Foundation shall provide direct organizational assistance
`and indirect technical assistance to the Jabber Community in carrying out its mis-
`sion. Direct organizational assistance will include brand management, logistical
`support, legal assistance, press relations, and communication facilities. Indirect
`technical assistance will include project management, protocol specification,
`standards activities, documentation support, and site development. The Jabber
`Software Foundation will not make technical decisions for Jabber but will help
`the Jabber Community to make technical decisions more effectively. All activities
`and proceedings will be openly accessible and published. Jabber Software Foun-
`dation Announcement: http://jabber.org/?oid=1309.
`
`The Jabber Software Foundation is still in its infancy but I have great expectations
`for it. I’m particularly interested in using the Foundation to clarify and formalize
`the existing Jabber standards into a form that will allow Jabber technology to be
`rapidly adopted by the wider development community. This book is an effort
`toward that goal. This book at least begins to address the need for better docu-
`mentation.
` As Jabber moves forward, much work remains to be done. Further standard-
`ization work is still under way. In addition, new standards are being proposed to
`extend the Jabber protocols beyond simple IM to meet the wider needs for elec-
`tronic messaging of all forms. These changes include better security, and sup-
`port for enterprise requirements such as transactions, quality of service, and
`delivery guarantees.
`
`1.1.2 Goals of the Jabber project
`In many ways, the Jabber project’s goal is simply to build a better IM system sup-
`porting real-time presence and messaging. From my discussions with other Jabber
`developers, the underlying goals seem to really be about defining what “better”
`means. For most Jabber developers, better means:
`
`8 There is a tight link between the Jabber community and Jabber Inc. (usually called Jabber.com or jc).
`Jabber Inc. employs most of the “core” members of the Jabber community and developers including
`Jeremie Miller, the creator of Jabber. However Jabber Inc. has been earnest in its efforts to keep the
`Jabber community and Jabber standards open. The Foundation is primarily an effort to formalize Jab-
`ber Inc.’s commitment to open Jabber standards and provide a legal entity to represent the Jabber com-
`munity’s interests (especially when they don’t necessarily match that of Jabber Inc.). Many of the board
`members of the Foundation are Jabber Inc. employees; however there are enough outsiders on the
`board to keep Jabber commercial interests balanced against that of the community.
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`What is Jabber?
`
`13
`
`■
`
`■ A completely open system and standards—Unlike other systems, Jabber will be
`completely open, thus allowing anyone to create Jabber implementations at
`no cost and with no strings attached.
`■ Open, XML-based technology—Extensible Markup Language (XML is an
`enabling technology with many benefits with respect to flexibility, availabil-
`ity, and ease of use. The use of XML helps to “future proof” the Jabber pro-
`tocols. At the same time, XML is a well-known technology that people are
`interested in using. There is also an extremely wide selection of tools for
`modeling, analyzing, programming, and authoring XML.
`Interoperability with other IM systems—The value of a communication system
`increases with the number of people with whom you can communicate.
`Optimally, a user of an IM system should be able to IM with any other user
`regardless of the underlying IM system. Jabber will maximize its value by
`working with as many other IM systems as possible.
`■ Simple protocols—Simple protocols are easier to design and implement. In
`addition, for most Jabber developers, simple things have an inherent appeal
`that we strive for constantly.
`■ Simplifying the responsibilities of clients whenever possible—There will be many
`clients and few servers in any Jabber system. It makes sense to design the
`Jabber system so that writing clients is as simple as possible. In addition, cli-
`ents may run on resource-constrained systems so reducing their needs
`increases the possible types of clients.
`■ Control over the system—No organization, group, or service provider should
`be able to control the system. Jabber’s design allows anyone to create a Jab-
`ber server and run their own Jabber network as they see fit.
`
`These unwritten goals seem to be commonly held among Jabber community
`members. They represent an open, self-reliant community that sees the benefits
`in sharing technology and knowledge whether you are an open-source or com-
`mercial developer. From these basic goals, the Jabber community has produced a
`feature-rich IM framework.
`
`1.2 What is Jabber?
`
`Jabber IM means different things to different people. End users typically associate
`Jabber with the Jabber IM system as a whole, just as they consider the Web to mean
`the entire web system including web servers, web clients and the protocols and data
`structures that power the World Wide Web. Developers often confuse the Jabber
`
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`14
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`CHAPTER 1
`Introduction to IM concepts
`
`open source reference implementation of a Jabber server called jabberd9 for the
`Jabber protocols that it supports. For the purposes of this book, we’ll restrict the dis-
`cussion of Jabber IM to the Jabber protocols and messaging model.
` One of Jabber’s most discussed advantages is its truly open nature. The Jab-
`ber protocols and data formats are all documented either directly in the Jab-
`ber standards documents or as source code in the freely available Jabber
`reference server. This is in stark contrast to all other major IM systems that use
`proprietary standards.
` Jabber’s advantages don’t end there. Its use of XML-based data formats
`exploits the popularity and extensibility of XML. In addition, Jabber uses a simple,
`distributed client/server architecture. The combination of the simplicity of the
`basic client/server communication model with the scalability of distributed serv-
`ers makes it well-suited to the dynamic environments that IM systems will be used
`in the future.
` Let’s break down Jabber technology to get an overview of how it works.
`
`1.2.1 Jabber’s XML-based data formats
`If Jabber’s open standards are its primary political and business advantage, its use
`of XML as its standard data format is its crucial technical advantage. XML is a
`World Wide Web Consortium (W3C) standard that defines a standard, generic
`data format for documents. Its primary technical advantages are its simplicity,
`extensibility, and a compromise between easy human and machine readability.
` All Jabber communications involve the exchange of Jabber packets, each being
`a well-formed XML fragment. These XML fragments can be considered XML sub-
`documents within the Jabber communication stream. For example, to send a mes-
`sage to iain@shigeoka.com, you would send a Jabber packet that looks like this:
`
`<message to='iain@shigeoka.com'>
` <subject>How are you today?</subject>
` <body>Just thought I'd drop a line and see what you're up to.</body>
`</message>
`
`Even if you don’t know anything about XML you can probably figure out what
`each part of this XML fragment does.10 If you are XML-savvy, Jabber defines all its
`packets in standard XML Document Type Definitions (DTD) and uses XML
`namespaces extensively to define both its own standard packets, and provide
`
`9 The Jabberd reference server development is continuing along two separate paths. Jabberd will con-
`tinue to improve and expand for the foreseeable future. It provides the definitive reference implemen-
`tation of the current Jabber protocols that we'll cover in this book. A revised set of Jabber protocols,
`referred to as Jabber Next Generation (JNG), is in development. A new reference server named Jab-
`berd will be written to implement these protocols when they are settled upon.
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`18
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`CHAPTER 1
`Introduction to IM concepts
`
` The strong interest in XML and Java has created a huge market of tools, books,
`programming libraries, and third-party experts (consultants, contract program-
`ming firms, etc.). This market legitimizes both XML and Java, and helps to ensure
`that anything based on Java and XML will be compatible with future technologies.
`It appears that we’ll be using XML for all sorts of programming problems for a
`some time to come. However, even if it doesn’t work out that way, enough people
`are committed to XML that new technologies will almost certainly continue to
`offer XML support or conversion tools.
` The use of XML in the Jabber protocols is an excellent start in creating an
`accessible, flexible IM system. However XML alone is not enough to make Jabber
`truly interesting. The complementary breakthrough with Jabber is its simple, scal-
`able architecture.
`
`1.2.2 Jabber’s simple architecture
`The Jabber messaging model follows the well-understood client/server architec-
`ture. Jabber clients only communicate with Jabber ser vers in their Jabber
`domain.12 Jabber domains break the entire IM universe into separate zones of
`control, each supported and managed by a separate Jabber server. This is in con-
`trast with most IM systems that use one centralized server for the whole IM sys-
`tem.13 In Jabber, messages pass from the sender’s client, to the sender’s server, to
`the recipient’s server,14 to the recipient’s client.
`
`Jabber is client/server
`The basic Jabber communication model follows the simple and well-understood
`client/server architecture model. In client/server systems, the client displays
`information to the end user and handles user requests. Information is passed
`from the client to a server that offers well-defined services (figure 1.5).
`
`12 This statement is an over-simplification. Normal Jabber messaging occurs through the server. However,
`clients are always free to send data directly to each other through any means at their disposal. How to
`do so, though, is not covered by the Jabber standards. Jabber does provide an out-of-band packet to
`help clients arrange for these direct, client-to-client communications. However the client-to-client ex-
`change does not occur within the Jabber system.
`13 A central server is the IM system architecture for services like AIM, Microsoft Messenger, and Yahoo!
`Messenger.
`14 If the sender and receiver both use the same Jabber server, this step is not necessary.
`
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`What is Jabber?
`
`19
`
`
`Jabber client
`
`Jabber server
`
`Jabber client
`
`Jabber client
`
`Figure 1.5
`Jabber client/server
`architecture: clients use one
`connection and only
`communicate directly with
`their server.
`
`In Jabber, the client/server model is heavily weighted to favor the creation of sim-
`ple clients. Most of the processing and IM logic is carried out on the server. Mini-
`mal Jabber client responsibilities give Jabber client developers the most flexibility
`in creating Jabber IM clients that fit the needs of users. Embedded and other lim-
`ited resource environments can support simple Jabber clients. Heavyweight Jab-
`ber clients can concentrate on the user interface and ease of use issues. Finally,
`the simplicity of creating Jabber clients encourages people to write more clients
`on different platforms using different programming languages, helping to spread
`Jabber access far and wide.
` Unlike distributed systems, such as those based on peer-to-peer technology,15
`the simple Jabber client/server architecture provides an excellent opportunity to
`implement centralized control of Jabber domains and provides opportunities for
`enforcing quality of service guarantees. This is especially important for enterprises
`that may need to enforce corporate communication policies. In addition, as IM sys-
`tems are used for mission-critical messaging beyond direct user communication, it
`will become increasingly important to ensure certain levels of quality of service can
`be met.
`
`Jabber distributed servers build the Jabber network
`Just as the current email system allows separate, distributed email servers to manage
`email domains, Jabber servers manage a Jabber domain. Like email, Jabber
`domains are defined by an Internet domain name. So a Jabber server that manages
`
`15 Peer-to-peer describes a family of technologies that allows network systems to be created without the
`use of a central server.
`
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`What is Jabber?
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`users can communicate with other Jabber servers. These same advantages have
`been the primary reasons why the current Internet email system is so pervasive
`and maintains such a high degree of cohesiveness.
` In many ways, Jabber’s simple, client/server architecture with distributed serv-
`ers is old technology. There is nothing really innovative about Jabber’s messaging
`model. In my opinion, this is a major strength rather than a weakness. IM’s major
`innovation is the addition of presence to communication systems. Jabber’s inno-
`vation is the use of an open XML data format for the data being sent in the com-
`munication system. Adding any more innovations at the same time would have
`probably resulted in a much less elegant and easy to build system.
`
`1.2.3 Jabber’s four core concepts
`Four central Jabber concepts form the basis for Jabber systems. Before we can
`look at the Jabber protocols, we must understand these concepts as they exist in
`Jabber. They’re straightforward but it is important for us all to be working with
`the same set of messaging concepts before setting off into Jabber’s details. These
`four central concepts are:
`Jabber domains
`■ Users and resources
`Jabber IDs
`■ Presence
`
`■
`
`■
`
`Jabber systems organized into networks and domains
`The Jabber universe is broken down into several logical sets and subsets of enti-
`ties. From the largest to the smallest, we have a Jabber:
`■ Network—All Jabber domains that exchange messages. A network must con-
`tain at least one domain.
`■ Domain—A subset of the network containing all entities that handle or
`belong to a domain. Jabber domains provide local control over parts of the
`Jabber network while still communicating with users outside of the Jabber
`domain. A domain is defined by:
`■ A valid Internet domain name address.
`■ The server that handles connections to that address.
`■ Server—A logical entity that manages a Jabber domain.
`■ User—An entity representing a logical message delivery endpoint. Jabber
`data packets are usually addressed to users, but are always delivered to a
`resource. Users are managed on the server with user accounts.
`
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`CHAPTER 1
`Introduction to IM concepts
`
`■ Resource—An entity representing a particular message delivery endpoint for
`a user. All Jabber data packets are delivered to resources. Jabber clients play
`the role of Jabber resources.
`
`A minimal Jabber network is composed of a single Jabber server handling one
`Jabber domain, and the Jabber clients that use that server, as shown in figure
`1.7. Jabber servers can exchange messages using the jabber:server protocols.
`When servers do this, they are federating their Jabber domains to expand the
`Jabber network.16
`
`Jabber network
`
`shigeoka.com
`Domain
`
`Jabber client
`
`Jabber client
`
`shigeoka.com
`Jabber server
`
`Jabber client
`
`Figure 1.7
`A minimal Jabber network
`with a single domain
`managed by a Jabber server
`and zero or more clients.
`
`Server federations are the primary method for Jabber networks to grow. Many
`Jabber servers are configured to federate with any other Jabber server on Inter-
`net. This essentially creates a single Internet Jabber network containing thou-
`sands of users.
` It is also possible to bridge Jabber systems by using the client protocols to
`exchange messages between domains as shown in figure 1.8. I’ll refer to this pro-
`cess as bridging to differentiate it from the privileged server-side access that server
`federation requires.
`
`16 Jabber servers find each other by opening connections to the domain name address of other servers
`(see chapter 9 for more details). Jabber server administrators can play some “DNS tricks” such as allow-
`ing multiple physical machines to act as the same logical Jabber server. This trick, commonly referred
`to as round-robin DNS, creates server farms in advanced Jabber installations. Jabber servers are also free
`to develop their own proprietary server-to-server connections in addition to (or as a replacement for)
`support for standard Jabber server-to-server connections if needs demand.
`
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` In jabberd, the Jabber server reference implementation written in C, a hybrid
`bridging solution