Wiley Professional XMPP Programming with JavaScript and jQuery
Contents
1. s power and promise A BRIEF HISTORY OF XMPP The XMPP protocol is now more than 10 years old and it has come a long way from its humble begin nings Much of XMPP s design is due to the environment in which XMPP was created and the history of XMPP provides an interesting case study in how open protocols foster adoption and innovation In 1996 Mirabilis released ICQ which popularized rapid personal communication among Internet users Its use spread rapidly and before long other companies were releasing similar products In 1997 AOL launched AOL Instant Messenger Yahoo followed suit in 1998 with Yahoo Pager even tually renamed Yahoo Messenger and in 1999 Microsoft finally joined the competition with MSN Messenger now Windows Live Messenger Each of these instant messaging applications was tied to a proprietary protocol and network run by the companies that made them Users of ICQ could not talk to Yahoo users and vice versa It became common for users to run more than one of these applications to be able to talk to all of their contacts because no single vendor claimed 100 market share It didn t take long before developers desired to write their own clients for these proprietary IM net works Some wished to make multiprotocol clients that could unite two or more of the IM networks and others wanted to bring these applications to operating systems other than Microsoft Windows and Apple s Mac OS These developers ran into man2. an extended period A lt status gt element is a human readable string that the user can set to any value in order to com municate presence information This string is generally displayed next to the contact s name in the recipient s chat client Each connected resource of a user has a priority between 128 and 127 This priority is set to zero by default but can be manipulated by including a lt priority gt element in lt presence gt stanzas Users with multiple simultaneous connections may use this to indicate which resource should receive chat messages addressed to their bare JID The server will deliver such messages to the resource with the highest priority A negative priority has a special meaning resources with a negative priority will never have messages delivered to them that were addressed to the bare JID Negative priorities are extremely useful for automated applications that run on the same JID as a human is using for regu lar chat Extending Presence Stanzas It is tempting for developers to want to extend lt presence gt stanzas to include more detailed infor mation such as the song the user is currently listening to or the person s mood Because lt presence gt stanzas are broadcast to all contacts even those that may not have an interest in the information and constitute a large share of the network traffic in the XMPP network this practice is discouraged These kinds of extensions are handled by protocols that mor
3. iq gt stanza Each of the three basic stanzas has several possible values for the type attribute and these are explained when each stanza is covered in detail All three stanzas may have their type attribute set to a value of error This indicates that the stanza is an error response to a received stanza of the same kind You must not respond to a stanza with an error type to avoid feedback loops on the network Stanzas may be given an id attribute to aid in identifying responses For lt iq gt stanzas this attribute is required but for the other two it is optional If a stanza is generated in reply to a stanza with an id attribute the reply stanza must contain an id attribute with the same value The id attribute needs to be unique enough that the stanza s sender can use it to disambiguate responses Often it is easiest just to make these unique in a given stream to avoid any ambiguity Reply stanzas for lt message gt and lt presence gt stanzas are generally limited to reporting errors Reply stanzas for lt iq gt can signal successful operations acknowledge a command or return requested XMPP Stanzas 11 data In all these cases the client uses the id attribute of the reply stanza to identify which request stanza it is associated with In cases where many stanzas of the same type are sent in a short time frame this capability is essential because the replies may be delivered out of order Presence Stanzas The lt pres
4. or more accurately XMPP entities speaking the server to server protocol or the server end of the client to server protocol are the circulatory system of any XMPP network A server s job is to route stanzas whether they are internal from one user to another or from a local user to a user on a remote server The set of XMPP servers that can mutually communicate forms an XMPP network The set of public XMPP servers forms the global federated XMPP network If a server does not speak the server to server protocol it becomes an island unable to communicate with external servers An XMPP server will usually allow users to connect to it It is however also possible to write appli cations or services that speak the server to server protocol directly in order to improve efficiency by eliminating routing overhead Anyone can run an XMPP server and full featured servers are available for nearly every platform Ejabberd Openfire and Tigase are three popular open source choices that will work on Windows Mac OS X or Linux systems Several commercial XMPP servers are available as well including M Link and Jabber XCP The XMPP Network 7 Clients The majority of XMPP entities are clients which connect to XMPP servers via the client to server protocol Many of these entities are human driven traditional IM users but there are also auto mated services running as bots Clients must authenticate to an XMPP server somewhere The server rou
5. the type attribute set to error A lt message gt stanza with a type of normal has been sent outside the context of a one to one chat This type is rarely used in practice The groupchat type is used for messages sent from multi user chats It is used to disambiguate direct private messages from a multi user chat participant from the broadcast messages that participant sends to everyone in the room A private message has the type attribute set to chat whereas a mes sage sent to everyone in the room contains a type attribute set to groupchat XMPP Stanzas 15 The last lt message gt stanza type is headline These types of messages are used mostly by automated services that do not expect or support replies If automatically generated e mail had a type attri bute it would use a value of headline Message Contents Though lt message gt stanzas are allowed to contain arbitrary extension elements the lt body gt and lt thread gt elements are the normal mechanisms provided for adding content to messages Both of these child elements are optional The lt body gt element contains the human readable contents of the message More than one lt body gt element can be included as long as each of them contains a distinct xm1 lang attribute and this allows for lt message gt stanzas to be sent with content in multiple languages Conversations like e mail can form threads where each message in a thread is related to the same convers
6. xmlns jabber iq roster gt lt ig gt lt presence gt lt message to darcy pemberley 1lit from elizabaeth longbourn 1lit ballroom type chat gt lt body gt I cannot talk of books in a ball room my head is always full of something else lt body gt lt message gt lt presence type unavailable gt lt stream stream gt In this example Elizabeth created an XMPP stream by sending the opening lt stream stream gt tag With the stream open she sent her first stanza an lt iq gt element This lt ig gt element requested Elizabeth s roster the list of all her stored contacts Next she notified the server that she was online and available with a lt presence gt stanza After noticing that Mr Darcy was online she sent him a short lt message gt stanza thwarting his attempt at small talk Finally Elizabeth sent another lt presence gt stanza to inform the server she was unavailable and closed the lt stream stream gt ele ment ending the session You have now seen an example of each kind of XMPP stanza in action Each of these is explained in more detail but first you should learn about what properties they all share 10 CHAPTER1 GETTING TO KNOW XMPP Common Attributes All three stanzas support a set of common attributes Whether they are attributes of lt presence gt lt message gt or lt iq gt elements the following attributes all mean the same thing from to Stanzas almost always h
7. PART XMPP Protocol and Architecture gt CHAPTER 1 Getting to Know XMPP gt CHAPTER 2 Designing XMPP Applications Getting to Know XMPP WHAT S IN THIS CHAPTER gt The history of XMPP gt XMPP networks and connections gt XMPP s three building block stanzas The eXtensible Messaging and Presence Protocol XMPP is at its most basic level a protocol for moving small structured pieces of data between two places From this humble basis it has been used to build large scale instant messaging systems Internet gaming platforms search engines collaboration spaces and voice and video conferencing systems More unique appli cations appear every day further demonstrating how versatile and powerful XMPP can be XMPP is made of a few small building blocks and on top of these primitives many larger constructions have been made Within XMPP are systems for building publish subscribe ser vices multi user chat form retrieval and processing service discovery real time data transfer privacy control and remote procedure calls Often XMPP programmers create their own unique constructions that are fitted exactly for the problem at hand Most social media constructs that have propelled web sites like Facebook MySpace and Twitter into the forefront are also baked into XMPP Within XMPP you ll find rosters full of contacts that create a social graph with directed or undirected edges Presence notifications are sent automatic
8. ally when contacts come online and go offline and private and public messages are the bread and butter application of XMPP systems Developers will sometimes choose XMPP as the underlying technology layer simply because it gives them many social features for free leaving them to concentrate on the unique pieces of their application The possibilities are vast but before you can begin you need to know about XMPP s different pieces and how they fit together into a cohesive whole 4 CHAPTER1 GETTING TO KNOW XMPP WHAT IS XMPP XMPP like all protocols defines a format for moving data between two or more communicating entities In XMPP s case the entities are normally a client and a server although it also allows for peer to peer communication between two servers or two clients Many XMPP servers exist on the Internet accessible to all and form a federated network of interconnected systems Data exchanged over XMPP is in XML giving the communication a rich extensible structure Many modern protocols forgo the bandwidth savings of a binary encoding for the more practical feature of being human readable and therefore easily debugged XMPP s choice to piggyback on XML means that it can take advantage of the large amount of knowledge and supporting software for dealing with XML One major feature XMPP gets by using XML is XML s extensibility It is extremely easy to add new features to the protocol that are both backward and forwar
9. ation Threads are created by adding a lt thread gt element to a lt message gt stanza The con tent of the lt thread gt element is some unique identifier that distinguishes the thread A reply stanza should contain the same lt thread gt element as the one it is a reply to In IM contexts among others there are a few commonly used extensions to the message contents XHTML IM defined in XEP 0071 is used to provide formatting hyperlinking and rich media in messages Chapter 5 s microblogging client Arthur uses XHTML IM to provide enhanced message bodies Another extension Chat State Notifications KEP 0085 allows users to notify each other of when they are composing a message or have gone idle The Gab application in Chapter 6 uses these notifications to provide a nice user experience when one party is typing for a long time the recipient will have some indication that the other party is still actively engaged in the conversation IQ Stanzas The lt iq gt stanza stands for Info Query and provides a request and response mechanism for XMPP communication It is very similar to the basic workings of the HTTP protocol allowing both get and set queries similar to the GET and POST actions of HTTP Each lt ig gt stanza is required to have a response and as mentioned previously the stanza s required id attribute is used to associate a response with the request that caused it The lt iq gt stanza comes in four flavors differentiat
10. ave a from attribute This attribute identifies the JID of the stanza s origin Setting the from attribute on outgoing stanzas is not recommended the server adds the correct from attribute to all stanzas as they pass through and if you set the from attribute incorrectly the server may reject your stanza altogether If the from attribute is missing on a received stanza in a client to server stream this is interpreted to mean that the stanza originated from the server itself In the server to server protocol a missing from attribute is an error Note that the example stanzas in this book often include the from attribute This is done for clarity and disambiguation XMPP servers route your stanzas to the JID supplied in the to attribute Similarly to the from attri bute if the to attribute is missing in a client to server stream the server assumes it is a message intended for the server itself It is recommended that you omit the to attribute when you address the server itself If the JID specified in the to attribute is a user the server potentially handles the stanza on the user s behalf If the destination is a bare JID the server handles the stanza This behavior is different for the three stanza types and is explained alongside each type If a full JID is specified as the destination the server routes the stanza directly to the user type The type attribute specifies the specific kind of lt presence gt lt message gt or lt
11. cale deployments of XMPP technology were well underway highlighted by the launch of Google Talk Google s own XMPP based IM service Today the XMPP ecosystem is quite large Nearly 300 extensions have been accepted as XEPs and dozens of client and server implementations have been created both commercial and open source Software developers of virtually any programming language can find a library to speed their XMPP application development efforts XMPP applications started out very IM centric reflecting its origins but developers have found XMPP to be quite capable for a number of applications that weren t originally foreseen including search engines and synchronization software This utility is a testament to the power of an open system and open standardization process Most recently the IETF has formed a new XMPP working group to prepare the next versions of the XMPP specifications incorporating all the knowledge gained since the original RFCs were pub lished XMPP continues to be refined and extended so that application developers and Internet users will always have an open decentralized communications protocol THE XMPP NETWORK Any XMPP network is composed of a number of actors These actors can be categorized as servers clients components and server plug ins An XMPP developer will write code to create or modify one of these types of actors Each actor has its place on the XMPP network s stage Servers XMPP servers
12. connection with the remote server and forward the message there This communication between servers resembles the e mail network but unlike e mail servers XMPP servers always communicate directly with each other and not through intermediate servers This direct communication eliminates some common vectors for spam and unauthorized messages This is just one of the many ways in which XMPP is designed for security It also supports encrypted communications between endpoints through use of Transport Layer Security TLS and strong authentication mechanisms via Simple Authentication and Security Layers SASL XMPP is designed for the exchange of small bits of information not large blobs of binary data XMPP can however be used to negotiate and set up out of band or in band transports which can move large blocks from point to point For these kinds of transfers XMPP functions as a signaling layer A Brief History of XMPP 5 The focus on small structured bits of data gives the XMPP protocol extremely low latency and makes it extremely useful for real time applications These applications which include collaborative spaces games and synchronization are driving XMPP s growth in popularity as developers experi ment with the real time Web You will see how easy it is to make real time web applications through this book s examples By the end of the book you should have a thorough understanding of why so many people are excited about XMPP
13. d compatible This extensibility is put to great use in the more than 200 protocol extensions registered with the XMPP Standards Foundation and has provided developers with a rich and practically unlimited set of tools XML is known primarily as a document format but in XMPP XML data is organized as a pair of streams one stream for each direction of communication Each XML stream consists of an opening element followed by XMPP stanzas and other top level elements and then a closing ele ment Each XMPP stanza is a first level child element of the stream with all its descendent elements and attributes At the end of an XMPP connection the two streams form a pair of valid XML documents XMPP stanzas make up the core part of the protocol and XMPP applications are concerned with sending and responding to various kinds of stanzas Stanzas may contain information about other entities availability on the network personal messages similar to e mail or structured communica tion intended for computer processing An example stanza is shown here lt message to elizabeth longbourn 1lit from darcy pemberley lit dance type chat gt lt body gt What think you of books lt body gt lt message gt In a typical client server XMPP session a stanza such as this one from Elizabeth to Mr Darcy will travel from Elizabeth s client to her server Her server will notice that it is addressed to an entity on a remote server and will establish an XMPP
14. e tightly focus delivery of this extra information Presence Subscriptions The user s server automatically broadcasts presence information to contacts that have a presence subscription to the user Similarly users receive presence updates from all contacts for which they have a presence subscription Presence subscriptions are established and controlled by use of lt pres ence gt stanzas Unlike some social network and IM systems presence subscriptions in XMPP are directional If Elizabeth has a subscription to Mr Darcy s presence information this does not imply that Mr Darcy has a subscription to Elizabeth If a bidirectional subscription is desired a subscription must be separately established in both directions Bidirectional subscriptions are often the norm for human communicators but many services and even some users are interested in only one of the directions XMPP Stanzas 13 Presence subscription stanzas can be identified by a type attribute that has a value of subscribe unsubscribe subscribed or unsubscribed The first two values request that a new presence sub scription be established or an existing subscription be removed and the other two are the answers to such requests The following example shows Elizabeth and Mr Darcy establishing a mutual presence subscription lt presence from elizabeth longbourn 1lit outside to darcy pemberley 1lit type subscribe gt lt presence from darcy pemberley 1
15. ed by the stanza s type attribute There are two types of lt iq gt stanza requests get and set and two types of responses result and error Throughout the book these are often abbreviated as IO get IO set IO result and IOQ error Every IQ get or IQ set must receive a response IQ result or IQ error The following examples show some common lt ig gt stanzas and their possible responses Note that unlike lt message gt and lt presence gt stanzas which have defined children elements lt iq gt stanzas typically contain only extension elements relating to their function Also each pair of lt iq gt stanzas has a matching id attribute lt iq from jane longbourn 1lit garden type get id rosterl1 gt lt query xmlns jabber iq roster gt lt igq gt lt iq to jane longbourn 1lit garden 16 CHAPTER1 GETTING TO KNOW XMPP type error id roster1 gt lt query xmlns jabber iq roster gt lt error type cancel gt lt feature not implemented xmlns urn ietf params xml ns xmpp stanzas gt lt error gt lt ig gt Jane sent a malformed roster retrieval request to her server The server replied with an error Error stanzas are covered in detail later lt iq from jane longbourn 1lit garden type get id roster2 gt lt query xmlns jabber iq roster gt lt ig gt lt iq to jane longbourn 1lit garden type result id roster2 gt lt query xmlns jabber iq roster gt lt item jid elizabeth
16. ence gt stanza controls and reports the availability of an entity This availability can range from simple online and offline to the more complex away and do not disturb In addition lt presence gt stanzas are used to establish and terminate presence subscriptions to other entities In traditional instant messaging systems presence notifications are the main source of traffic To enable instant communication it is necessary to know when the other party is available to communi cate When you send an e mail you have no idea if the recipient is currently checking and responding to e mail but with instant messages and presence notifications you know before the message is sent if the recipient is around For applications in other domains presence notifications can be used to signal similar kinds of infor mation For example some developers have written bots that set their presence to do not disturb when they are too busy to accept more work The basic online and offline states can let applications know whether a service is currently functioning or down for maintenance Normal Presence Stanzas A normal lt presence gt stanza contains no type attribute or a type attribute that has the value unavailable or error These stanzas set or indicate an entity s presence or availability for communication There is no available value for the type attribute because this is indicated instead by the lack of a type attribute Users manipulate their ow
17. entity within the server and appears to the out side world as a sub server XMPP servers do not manage anything beyond basic stanza routing on behalf of connected components This allows great freedom to component developers to do things exactly as they want but places greater responsibility on them when they need functionality such as rosters and presence management The server also allows a component to internally route or manage stanzas for itself A component can therefore create separately addressable pieces to be used as rooms users or whatever the devel oper requires This is something that a client session cannot do and can be used to create really elegant services Finally because components do not have resources managed for them services that operate with many users or with a high amount of traffic can manage their own resources in a way that makes sense for their purpose Developers often create services as client bots only to discover later that the server s roster management capabilities often do not scale well to thousands upon thousands of con tacts Components can manage rosters if they have them at all in whichever way makes sense for the task and scale required 8 CHAPTER1 GETTING TO KNOW XMPP Plug ins Many XMPP servers can also be extended via plug ins These plug ins are usually written in the same programming language as the server itself and run inside the server s processes Their purpose overlaps to a la
18. gn XMPP applications in the next chapter
19. implies lt message gt stanzas are used to send messages from one entity to another These messages may be simple chat messages that you are familiar with from other IM systems but they can also be used to transport any kind of structured information For example the SketchCast application in Chapter 9 uses lt message gt stanzas to transport drawing instructions and in Chapter 11 lt message gt stanzas are used to communicate game state and new game moves 14 CHAPTER1 GETTING TO KNOW XMPP A lt message gt stanza is fire and forget there is no built in reliability similar to e mail messages Once the message has been sent the sender has no information on whether it was delivered or when it was received In some cases such as when sending to a non existent server the sender may receive an error stanza alerting them to the problem Reliable delivery can be achieved by layering acknowl edgments into your application s protocol see Message Receipts in XEP 0184 for an example of this Here are some example lt message gt stanzas lt message from bingley netherfield 1lit drawing_room to darcy pemberley 1lit type chat gt lt body gt Come Darcy I must have you dance lt body gt lt thread gt 4 d61b376 bc4950b9433 031a5595ab lt thread gt lt message gt lt message from bennets chat meryton 1lit mrs bennet to mr bennet longbourn 1lit study type groupchat gt lt body gt We have had a most delightful e
20. information on these please refer to Section 3 9 2 of RFC 3920 Note that each of these condition elements must be under the urn ietf params xml ns xmpp stanzas namespace TABLE 1 1 Common Defined Error Conditions CONDITION ELEMENT DESCRIPTION lt bad request gt The request was malformed or includes unexpected data lt Commlnicte Another resource or session exists with the same name lt feature not implemented gt The feature requested is not implemented by the service lt forbidden gt The client does not have authorization to make the request lt internal server error gt The server had an undefined internal error that prevented it from processing the request lt item not found gt The item involved in the request does not exist This error is equivalent to the HTTP 404 error lt recipient unavailable gt The intended recipient is temporarily unavailable lt remote server not found gt The remote server does not exist or could not be reached lt remote server timeout gt Communication with the remote server has been interrupted lt service unavailable gt The requested service is not provided 18 CHAPTER1 GETTING TO KNOW XMPP The following example IQ error stanza shows a fully constructed error response to a publish subscribe related lt iq gt stanza lt iq from pubsub pemberley 1lit to elizabeth longbourn 1lit sitting_room type error id subscribel gt lt pubsub xmlns http jabber org protoc
21. lit library to elizabeth longbourn 1lit outside type subscribed gt lt presence from darcy pemberley 1lit library to elizabeth longbourn 1lit type subscribe gt lt presence from elizabeth longbourn 1lit outside to darcy pemberley lit library type subscribed gt After this exchange of stanzas both Elizabeth and Mr Darcy will find each other in their rosters and be notified of each other s presence updates Chapter 6 explores a fairly traditional IM application with the ability to establish and remove sub scriptions as well as showing contacts presence statuses Directed Presence The final kind of lt presence gt stanza is directed presence A directed presence stanza is a normal lt presence gt stanza addressed directly to another user or some other entity These can be used to communicate presence to entities that do not have a presence subscription usually because the pres ence information is needed only temporarily One important feature of directed presence is that the recipient of the presence information is automatically notified when the sender becomes unavailable even if the sender forgets to notify the recipient explicitly Services can use directed presence to establish temporary knowledge of a user s availability that won t accidentally get out of date You see directed presence in action in Chapter 8 because it is quite important for multi user chat Message Stanzas As their name
22. longbourn 1lit name Elizabeth gt lt item jid bingley netherfield lit name Bingley gt lt query gt lt ig gt After resending a corrected request the server replied to Jane with her small roster You can see that Elizabeth and Bingley are both in Jane s contact list lt iq from jane longbourn 1lit garden type set id roster3 gt lt query xmlns jabber iq roster gt lt item jid darcy pemberley 1lit name Mr Darcy gt lt query gt lt ig gt lt iq to jane longbourn 1lit garden type result id roster3 gt Jane attempts to add Mr Darcy to her roster and the server indicates success with a blank 1Q result In the cases where the response is simply an acknowledgment of success the IQ result stanza will often be empty The lt iq gt stanza is quite useful in any case where result data or simple acknowledgment is required Most XMPP extension protocols use a mix of lt ig gt and lt message gt stanzas to accomplish their goals The lt iq gt stanzas are used for things like configuration and state changes whereas lt message gt stan zas are used for regular communication In some cases lt iq gt stanzas are used for communication because stanza acknowledgment can be used for rate limiting XMPP Stanzas 17 Error Stanzas All three of the XMPP stanzas have an error type and the contents of each type of error stanza are arranged in the same pattern Error stanzas have a well defined st
23. mature XMPP is great for writing IM applications but it also excels at any task that benefits from exchanging structured messages Servers clients components and plug ins are all parts of XMPP networks and have their special uses XMPP addresses called JIDs resemble e mail addresses and decompose into three parts the local part the domain and the resource Full JIDs are the most specific addresses for an entity for example darcy pemberley lit library Bare JIDs are the same as full JIDs without the resource for example darcy pemberley lit Servers will handle stanzas to a client s bare JID potentially routing them to one or more connected resources Stanzas addressed to a full JID are delivered directly to the given resource There are three stanzas in the main XMPP vocabulary lt message gt lt presence gt and lt iq gt The lt message gt stanza is used for fire and forget messages between two entities lt presence gt stanzas communicate presence status changes and are used to manipulate pres ence subscriptions The lt iq gt stanza provides request response semantics very similar to the GET and POST operations of the HTTP protocol Every XMPP session has a life cycle consisting of several phases connection stream set up authentication the session body and disconnection The basic concepts and protocol syntax of XMPP are only once piece of the puzzle You learn about how to use these ideas to desi
24. n presence status by sending lt presence gt stanzas without a to attribute addressing the server directly You ve seen two short examples of this already and these are included along with some longer examples here lt presence gt lt presence type unavailable gt lt presence gt lt show gt away lt show gt lt status gt at the ball lt status gt lt presence gt lt presence gt lt status gt touring the countryside lt status gt lt priority gt 10 lt priority gt lt presence gt lt presence gt lt priority gt 10 lt priority gt lt presence gt 12 CHAPTER1 GETTING TO KNOW XMPP The first two stanzas set a user s presence status to online or offline respectively These are also typically the first and last presence stanzas sent during an XMPP session The next two examples both show extra presence information in the form of lt show gt lt status gt and lt priority gt children The lt show gt element is used to communicate the nature of the user s availability The element is named show because it requests that the recipient s client use this information to update a visual indicator of the sender s presence Only one lt show gt child is allowed in a lt presence gt stanza and this element may only contain the following possible values away chat dnd and xa These values communicate that a user is away is interested in chatting does not wish to be disturbed or is away for
25. ndled by the server itself and potentially routed to a component or plug in The local part usually identifies a particular user at a domain It appears at the beginning of a JID before the domain and it is separated from the rest of the JID by the character just like the local part of an e mail address The local part can also be used to identify other objects a multi user chat service will expose each room as a JID where the local part references the room A JID s resource part most often identifies a particular XMPP connection of a client For XMPP clients each connection is assigned a resource If Mr Darcy whose JID is darcy pemberley 1lit is connected both from his study and his library his connections will be addressable as darcy pemberley lit study and darcy pemberley 1lit library Like the local part a resource can be used to identify other things on a multi user chat service the resource part of the JID is used to identify a particular user of a chat room JIDs are divided into two categories bare JIDs and full JIDs The full JID is always the most spe cific address for a particular entity and the bare JID is simply the full JID with any resource part removed For example if a client s full JID is darcy pemberley 1lit library its bare JID would be darcy pemberley 1it In some cases the bare JID and the full JID are the same such as when addressing a server or a specific multi user chat room XMPP Stanzas 9 Ba
26. ol pubsub gt lt subscribe node latest_books jid elizabeth longbourn 1lit gt lt pubsub gt lt error type cancel gt lt not allowed xmlns urn ietf params xml ns xmpp stanzas gt lt closed node xmlns http jabber org protocol pubsub errors gt lt text xmlns urn ietf params xml ns xmpp stanzas gt You must be on the whitelist to subscribe to this node lt text gt lt error gt lt ig gt The error s type is cancel indicating that this action should not be retried and the condition lt not allowed gt indicates the general failure The lt text gt child contains a description of the prob lem Finally the application condition element lt closed node gt gives the exact application error THE CONNECTION LIFE CYCLE The three stanzas you ve learned about can accomplish virtually any task in XMPP when combined properly However sending stanzas usually requires an authenticated XMPP session be established This section describes the other portions of an XMPP connection s life cycle connection stream set up authentication and disconnection Connection Before any stanzas are sent an XMPP stream is necessary Before an XMPP stream can exist a con nection must be made to an XMPP server XMPP includes some sophisticated support for establish ing connections to the right servers Typically clients and servers utilize the domain name system DNS to resolve a server s domain name into an address
27. ompression via zlib and several authentication mechanisms The XMPP streams set up between two servers look identical except that the top level namespace is jabber server instead of jabber client 20 CHAPTER1 GETTING TO KNOW XMPP Authentication XMPP allows for Transport Layer Security TLS encryption and most clients use this by default Once TLS support is advertised by the server the client starts the TLS connection and upgrades the current socket to an encrypted one without disconnecting Once TLS encryption is established a new pair of XMPP streams is created Authentication in XMPP uses the Simple Authentication and Security Layers SASL protocol and depending on the server involved can support a number of authentication mechanisms Normally servers provide plain text authentication and MDS digest based authentication but some servers support authenticating via Kerberos or special tokens These same encryption and authentication technologies are also used in many other protocols e mail and LDAP are two examples and common libraries exist for supporting TLS and SASL that can be used equally well for XMPP Once authentication is complete a client must bind a resource for the connection and start a session If you are watching XMPP traffic on the wire you will see lt bind gt and lt session gt elements inside lt iq gt stanzas being sent to do these jobs If the client does not provide a resource to bind
28. re JIDs for clients are somewhat special because the server itself will handle stanzas addressed to a client s bare JID For example a message sent to a client s bare JID will be forwarded to one or more connected resources of the user or if the user is offline stored for later delivery Stanzas sent to full JIDs however are usually routed directly to the client s connection for that resource You can think of bare JIDs as addressing the user s account as opposed to addressing one of the user s connected clients XMPP STANZAS Work is accomplished in XMPP by the sending and receiving of XMPP stanzas over an XMPP stream Three basic stanzas make up the core XMPP toolset These stanzas are lt presence gt lt message gt and lt iq gt Each type of stanza has its place and purpose and by composing the right kinds of quantities of these stanzas sophisticated behaviors can be achieved Remember that an XMPP stream is a set of two XML documents one for each direction of communication These documents have a root lt stream stream gt element The children of this lt stream stream gt element consist of routable stanzas and stream related top level children Each stanza is an XML element including its children The end points of XMPP communication process input and generate output on a stanza by stanza basis The following example shows a simplified and short XMPP session lt stream stream gt lt iq type get gt lt query
29. rge degree with external components but plug ins may also access internal server data structures and change core server behavior The virtually limitless abilities afforded to server plug ins come with a cost plug ins are not portable between different servers A different server may be written in a completely different language and its internal data structures may differ radically This cost aside plug ins are sometimes the only way to get a particular job done Plug ins have reduced overhead compared to components because they do not need to communicate over a network socket They also need not parse or serialize XML and can instead work directly with internal server representations of stanzas This can lead to much needed performance improve ments when the application must scale XMPP ADDRESSING Every entity on an XMPP network will have one or more addresses or JIDs JIDs short for jabber identifiers can take a variety of forms but they normally look just like e mail addresses darcy pemberley lit and elizabeth longbourn 1it are two examples of JIDs Each JID is made up of up to three pieces the local part the domain and the resource The domain portion is always required but the other two pieces are optional depending on their context The domain is the resolvable DNS name of the entity a server component or plug in A JID con sisting of just a domain is valid and addresses a server Stanzas addressed to a domain are ha
30. ructure often including the contents of the original offending stanza the generic error information and optionally an applica tion specific error condition and information All error stanzas must have a type attribute set to error and one lt error gt child element Many error stanzas also include the original stanza s contents but this is not required and in some cases not desirable The lt error gt child has a required type attribute of its own which can be one of cancel continue modify auth or wait The cancel value signifies that the action should not be retried because it will always fail A value of continue generally indicates a warning and is not frequently encoun tered An error type of modify communicates that the data sent needs some change in order to be accepted An auth error informs the entity that the action should be retried after authenticating in some way Finally the wait value reports that the server is temporarily having some problem and the original stanza should be resent unmodified after a short time An lt error gt child is also required to contain an error condition from a list of defined conditions as a child element It may also contain a lt text gt element giving further details about the error An application specific error condition can also be specified in a child element of the lt error gt element under its own namespace Table 1 1 lists the most common defined error conditions For more
31. ted features on the XMPP stream These mostly relate to encryption and authentication options that are available For example the server will specify if encryption TLS is available and whether or not anonymous logins are allowed You don t normally need to know much detail about this stage of an XMPP connection as the many libraries for XMPP development handle this work for you but the following example shows a typical exchange of lt stream stream gt elements as well as the server s feature list First the client sends the opening element to the server lt xml version 1 0 gt lt stream stream xmlns jabber client xmlns stream http etherx jabber org streams version 1 0 to pemberley lit gt The server replies lt xml version 1 0 gt lt stream stream xmlns jabber client xmlns stream http etherx jabber org streams version 1 0 from pemberley 1lit id 893ca401f5f 2ec29499984e9b7e8afc xml lang en gt lt stream features gt lt stream features gt lt starttls xmlns urn ietf params xml ns xmpp tls gt lt compression xmlns http jabber org features compress gt lt method gt zlib lt method gt lt compression gt lt mechanisms xmlns urn ietf params xml ns xmpp sasl gt lt mechanism gt DIGEST MD5 lt mechanism gt lt mechanism gt PLAIN lt mechanism gt lt mechanisms gt lt stream features gt From this exchange you know that the pemberley org server supports TLS stream c
32. tes all stanzas the client sends to the appropriate destination The server also manages several aspects of the clients sessions including their roster and their bare address which you see more of shortly All of the applications in this book are written as client applications This is typically the starting point of most XMPP development For applications without a user focus or with demanding needs it is often preferable to create a different kind of entity such as a server component Components Clients are not the only things that may connect to XMPP servers most servers also support exter nal server components These components augment the behavior of the server by adding some new service These components have their own identity and address within the server but run externally and communicate over a component protocol The component protocol defined in XEP 0114 enables developers to create server extensions in a server agnostic way Any component using the protocol can run on any server that speaks the component protocol assuming it doesn t use some special feature specific to a particular server A multi user chat service is a typical example of something that is often implemented as a component Components also authenticate to the server but this authentication is simpler than the full SASL authentication for clients Typically authentication is done with a simple password Each component becomes a separately addressable
33. the server chooses one for it usually randomly Also the server may alter the user s chosen resource even if the client provides one When two servers connect to each other the authentication steps are slightly different The servers exchange and verify TLS certificates or the recipient server uses a dialback protocol to verify the sender s identity via DNS Disconnection When users are done with their XMPP sessions they terminate the sessions and disconnect The most polite way to terminate a session is to first send unavailable presence and then close the lt stream stream gt element By sending a final unavailable presence the user s contacts can be informed about the reasons for the user s departure Closing the stream explicitly allows any in flight stanzas to arrive safely A polite disconnection would look like this lt presence type unavailable gt lt stream stream gt The server then terminates its stream to the client SUMMARY In this chapter you met the XMPP protocol and learned about its history use cases addressing vocabulary and the connection life cycle You ve also seen several example XMPP stanzas and learned about the different entities composing an XMPP network Summary 21 You should now understand gt Y vyv vyv y y XMPP is an open standardized protocol originally developed to replace proprietary IM networks The XMPP protocol is more than a decade old and quite
34. they can connect to E mail services in particular use mail exchange MX records to provide a list of servers that handle mail for a given domain so that one well known server address does not have to handle every service E mail being an early Internet application got special treat ment in DNS These days service records SRV are used to provide a similar function for arbitrary services The first thing an XMPP client or server does when connecting to another XMPP server is to query the appropriate SRV record at the server s domain The response may include multiple SRV records which can be used to load balance connections across multiple servers If an appropriate SRV record cannot be found the application tries to connect to the given domain directly as a fallback Most libraries also allow you to specify a server to connect to explicitly The Connection Life Cycle 19 Stream Set Up Once a connection is established to a given XMPP server an XMPP stream is started An XMPP stream is opened by sending the opening lt stream stream gt element to the server The server responds by sending the response stream s opening lt stream stream gt tag Once XMPP streams are open in both directions elements can be sent back and forth At this stage of the connection life cycle these elements will be related to the stream and the stream s features The server first sends a lt stream features gt element which details all the suppor
35. vening a most excellent ball lt body gt lt message gt The first example shows a typical lt message gt stanza for a private chat including a thread identifier The second example is a multi user chat message that Mrs Bennet has sent to the bennets chat meryton lit room received by Mr Bennet Message Types Several different types of lt message gt stanzas exist These types are indicated with the type attribute and this attribute can have the value chat error normal groupchat or headline Sometimes the message s type is used to inform a user s client how best to present the message but some XMPP extensions multi user chat being a prime example use the type attribute to disambiguate context The type attribute of a lt message gt stanza is optional but it is recommended that applications pro vide one Also any reply lt message gt stanza should mirror the type attribute received If no type attribute is specified the lt message gt stanza is interpreted as if it had a type attribute set to normal Messages of type chat are sent in the context of a one to one chat conversation This type is the most common in IM applications which are primarily concerned with private one to one communication The error type is used in reply to a message that generated an error These are commonly seen in response to malformed addressing sending a lt message gt stanza to a non existent domain or user results in a reply stanza with
36. y roadblocks they had to reverse engineer undocumented protocols and the IM networks aggressively changed the protocol to thwart third party developers It was in this climate that the idea for an open decentralized IM network and protocol was born Jeremie Miller announced the Jabber project in January of 1999 Jabber was a decentralized instant messaging protocol based on XML and a server implementation called jabberd A community immediately formed around the protocol and implementations spawning more clients and more ideas By May of 2000 the core protocols were stabilized and jabberd reached a production release The Jabber Software Foundation JSF was founded in 2001 to coordinate the efforts around the Jabber protocol and its implementations By late 2002 the JSF had submitted the core protocol spec ifications to the IETF process and an IETF working group was formed In October 2004 this stan dards process produced improved versions of the Jabber protocols renamed XMPP documented as RFCs 3920 3921 3922 and 3923 During the protocol s early life developers continued to expand its possibilities by submitting protocol extensions to the JSF These extensions were called Jabber Extension Proposals JEPs 6 CHAPTER1 GETTING TO KNOW XMPP Eventually the JSF and the extensions followed the naming change from Jabber to XMPP and became the XMPP Standards Foundation XSF and XMPP Extension Proposals XEPs By 2005 large s
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