<abstract>This document specifies XMPP semantics for using the publish-subscribe protocol to broadcast state change events associated with an instant messaging and presence account.</abstract>
<remark><p>Per a vote of the Jabber Council, advanced status to Draft.</p></remark>
</revision>
<revision>
<version>0.15</version>
<date>2006-08-30</date>
<initials>psa</initials>
<remark><p>Added the deliver_notifications and send_last_published_item configuration options to the recommended defaults.</p></remark>
</revision>
<revision>
<version>0.14</version>
<date>2006-08-02</date>
<initials>psa</initials>
<remark><p>Changed various recommended defaults from SHOULD to MUST; corrected several errors in the text and examples.</p></remark>
</revision>
<revision>
<version>0.13</version>
<date>2006-08-01</date>
<initials>psa</initials>
<remark><p>Recommended node creation with default configuration on initial publish; corrected several errors and clarified several points in the text.</p></remark>
</revision>
<revision>
<version>0.12</version>
<date>2006-08-01</date>
<initials>psa</initials>
<remark><p>Simplified the subscription process using XMPP presence and entity capabilities.</p></remark>
</revision>
<revision>
<version>0.11</version>
<date>2006-07-20</date>
<initials>psa</initials>
<remark><p>Clarified rules regarding number of notifications and when to generate notifications; corrected several errors in the text and examples.</p></remark>
<remark><p>Clarified terminology and defaults.</p></remark>
</revision>
<revision>
<version>0.7</version>
<date>2006-04-10</date>
<initials>psa</initials>
<remark><p>Specified that notifications are to be sent from bare JID, not full JID.</p></remark>
</revision>
<revision>
<version>0.6</version>
<date>2006-04-10</date>
<initials>psa</initials>
<remark><p>Updated to reflect pubsub changes; clarified business rules for generation of notifications and cancellation of subscriptions.</p></remark>
</revision>
<revision>
<version>0.5</version>
<date>2006-03-09</date>
<initials>psa</initials>
<remark><p>Modified roster groups example to use jabber:x:data; added note about advertising client support for PEP.</p></remark>
</revision>
<revision>
<version>0.4</version>
<date>2006-02-02</date>
<initials>psa/ks</initials>
<remark><p>Specified rules for generation of notifications, including use of presence in determining address of intended recipient for notifications and sending of last published item on receipt of presence information; changed name to Personal Eventing Protocol; specified service discovery identity of pubsub/pep; removed section on service types; added Kevin Smith as co-author.</p></remark>
</revision>
<revision>
<version>0.3</version>
<date>2006-01-30</date>
<initials>psa</initials>
<remark><p>Specified that a service may enforce additional privacy and security policies; specified that an account owner must always be allowed to subscribe and to retrieve items; specified that an implementation should enforce access modifications resulting from roster state changes.</p></remark>
<p>The XMPP &xep0060; extension ("pubsub") can be used to broadcast state change events associated with a Jabber/XMPP account or user, such as those described in &xep0080;, &xep0107;, &xep0108;, and &xep0118;. <note>Currently, many "extended presence" formats are sent using the &PRESENCE; stanza type; however, this overloads presence, results in unnecessary presence traffic, and does not provide fine-grained control over access. The use of publish-subscribe rather than presence is therefore preferable.</note> However, the full, generic pubsub protocol is often thought of as complicated and therefore has not been widely implemented. To make publish-subscribe functionality more accessible (especially to instant messaging and presence applications that conform to &xmppim;), this document defines simplified protocol semantics that can be followed by instant messaging client and server developers, hopefully resulting in the deployment of personal eventing services across the Jabber/XMPP network.</p>
<p><em>Note: This document does not show error flows related to the various publish-subscribe use cases referenced herein, since they are exhaustively defined in <cite>XEP-0060</cite>. The reader is referred to <cite>XEP-0060</cite> for all relevant protocol details related to the XMPP publish-subscribe extension.</em></p>
<li>Notifications are filtered based on expressed interests.</li>
<li>Smart defaults.</li>
</ol>
<p>These principles are described more fully below.</p>
<section2topic='Every Account a Pubsub Service'anchor='approach-everyjid'>
<p>When a user creates an account (or has an account provisioned) at a Jabber/XMPP server that supports PEP, the server associates a virtual pubsub service with the account. This greatly simplifies the task of discovering the account owner's personal pubsub nodes, since the root pubsub node simply is the account owner's bare JID (&BAREJID;). This assumption also simplifies publishing and subscribing.</p>
</section2>
<section2topic='One Publisher Per Node'anchor='approach-publisher'>
<p>There is no need for multiple publishers to a PEP service, since by definition the service generates information associated with only one entity. The owner-publisher for every node is the bare JID of the account owner.</p>
<section2topic='One Node Per Namespace'anchor='approach-onenode'>
<p>There is only one publish-subscribe node associated with any given payload type (XML namespace) for the account owner (e.g., there is one pubsub node for geolocation events, one node for tune events, and one node for mood events). This simplifies node creation, discovery, publishing, and subscribing.</p>
<p>Although generic publish-subscribe services do not necessarily have access to presence information about subscribers, PEP services are integrated with presence in the following ways:</p>
<ul>
<li>Each messaging and presence account simply <em>is</em> a virtual publish-subscribe service.</li>
<li>The default access model is "presence".</li>
<li>A contact's subscription to an account owner's personal eventing data is normally handled via the existence of an XMPP presence subscription.</li>
<li>Services take account of subscriber presence in the generation of notifications. <note>This works only if the subscription state is "both" (see <cite>RFC 3921</cite>).</note></li>
<p>By default, the existence of an XMPP presence subscription is used to establish a PEP subscription to the account owner's personal eventing data. In order to filter which notifications are sent by the PEP service, the contact's client includes extended &xep0115; information in the presence notifications it sends to the account owner, and the PEP service sends only those notifications that match the contact's expressed notification preferences.</p>
<p>Most pubsub configuration options and metadata are not needed for personal eventing. Instead, PEP services offer smart defaults to simplify node creation and management.</p>
<li>Geolocation information that only contacts in her "Friends" group may see (i.e., an access model of "roster" with a group of "Friends")</li>
<li>Bookmark information that only the account owner may see (i.e., an access model of "whitelist")</li>
</ol>
<p>Note: A PEP node with an access model of "whitelist" and no entities on the whitelist effectively results in a node that enables private data storage; for details, see the <linkurl='#privatedata'>Private Data Storage</link> section of this document.</p>
</li>
<li>
<p>Three users who have the following relationship to Juliet:</p>
<olstart='1'>
<li>benvolio@montague.net, who has no subscription to Juliet's presence</li>
<li>nurse@capulet.com, who has a bidirectional subscription to Juliet's presence and who is in the "Servants" group in Juliet's roster</li>
<li>romeo@montague.net, who has a bidirectional subscription to Juliet's presence and who is in the "Friends" group in Juliet's roster</li>
</ol>
</li>
</ol>
<p>The examples shown in the following sections walk through the protocol flows for node creation, discovery, publishing, and subscribing.</p>
</section1>
<section1topic='Account Owner Service Discovery'anchor='owner-disco'>
<p>Naturally, before an account owner attempts to complete any PEP use cases, its client SHOULD determine whether the account owner's server supports PEP; to do so, it MUST send a &xep0030; information request to the server:</p>
<p>If a server supports PEP, it MUST return an identity of "pubsub/pep" (as well as a list of the namespaces and other features it supports, including all supported <cite>XEP-0060</cite> features):</p>
<p>When an account owner attempts to publish an item to a PEP node and that node does not already exist, the PEP service MUST automatically create the node with default configuration. <note>This similar to the room creation process in <cite>XEP-0045: Multi-User Chat</cite>.</note> However, if the account owner wishes to create a node with a configuration other than the default (e.g., a node with an access model of "open", "roster", or "whitelist"), the account owner MUST follow the node creation protocol specified in <cite>XEP-0060</cite>.</p>
<section1topic='Contact Service Discovery'anchor='contact-disco'>
<p>A contact MAY send service discovery requests to the account owner's bare JID (&BAREJID;). Although this is not necessary in order to subscribe to the account owner's personal eventing data (as explained in the following section), it is shown here to further illustrate the role of access models.</p>
<p>First, benvolio@montague.net sends a disco#info request to juliet@capulet.com:</p>
<p>The account owner's server MUST check the access model for each of the account owner's PEP nodes and MUST return as service discovery items only those nodes to which the contact is allowed to subscribe or from which the contact is allowed to retrieve items without first subscribing.</p>
<p>Therefore, in this case, the server would return only the "http://jabber.org/protocol/tune" node (since it has an open access model and the contact does not have a presence subscription to the account owner's presence):</p>
<p>However, in this case, the server would return the "http://jabber.org/protocol/tune" node (open access model) <em>and</em> the "http://jabber.org/protocol/activity" node (presence access model):</p>
<p>In this case, the server would return the "http://jabber.org/protocol/tune" node (open access model) <em>and</em> the "http://jabber.org/protocol/activity" node (presence access model) <em>and</em> the "http://jabber.org/protocol/geoloc" node (roster access model):</p>
<p>If an entity is not subscribed to the account owner's presence, it MUST subscribe to a node using the protocol defined in <cite>XEP-0060</cite>. For instance, here is how benvolio@montague.net would subscribe Juliet's tune information:</p>
<examplecaption='Unaffiliated entity subscribes to an open node'><![CDATA[
<p>However, when a contact is affiliated with the account owner through a presence subscription, PEP greatly simplifies the subscription process. This is done by associating the presence subscription with a pubsub subscription to the account owner's root collection node (i.e., bare JID), with a subscription_type of "items" and a subscription_depth of "all".</p>
<p>Note: Automated pubsub subscriptions MUST be based on the JID contained in the 'from' address of the presence subscription request, which for IM contacts will be a bare JID (&BAREJID;).</p>
<source>Music for "Love's Labors Lost" (Suite for small orchestra)</source>
<track>1</track>
<length>255</length>
</tune>
</item>
</publish>
</pubsub>
</iq>
]]></example>
<p>As a result, the account owner's server generates notifications and sends them to all subscribers who have requested or are interested in the data as described in the <linkurl='#notifications'>Contact Notification Filtering</link> and <linkurl='#notifications'>Generating Notifications</link> sections of this document.</p>
<p>The server MUST set the 'from' address on the notification to the bare JID (&BAREJID;) of the account owner (in this example, "juliet@capulet.com"). When sending notifications to an entity that has a presence subscription to the account owner, the server SHOULD include an &xep0033; "replyto" extension specifying the publishing resource (in this example, "juliet@capulet.com/balcony"); this enables the subscriber's client to differentiate between information received from each of the account owner's resources (for example, different resources may be in different places and therefore may need to specify distinct geolocation data). However, a server MUST NOT include the "replyto" address when sending a notification to an entity that does not have a presence subscription to the account owner. In addition, any errors related to the notification MUST be directed to the JID of the 'from' address on the notification (i.e., the bare JID) so that bounce processing can be handled by the PEP service rather than by the publishing client.</p>
<p>Note the 'to' addresses: the notification to Benvolio is addressed to "benvolio@montague.net" (bare JID) since the PEP service does not have presence information about the subscriber, whereas the notifications to the Nurse and to Romeo are addressed to the full JIDs of those subscribers.</p>
<p>A contact may not want to receive notifications for all payload types. A contact SHOULD signal its preferences to the account owner's server by including <cite>XEP-0115</cite> information that specifies the namespaces for which the contact wishes to receive notifications (if any).</p>
<p>In order to make this possible, all possible payload namespaces can be appended with the string "+notify" to indicate that the contact wishes to receive notifications for the payload format. Thus if Romeo wants to receive notifications for activity data and geolocation data but not tune data, his client would advertise support for the following namespaces in the disco#info results it sends: <note>Including, say, the 'http://jabber.org/protocol/geoloc' namespace indicates that the client understands the geolocation namespace, whereas including the 'http://jabber.org/protocol/geoloc+notify' namespace indicates that the client wishes to receive notifications related to geolocation.</note></p>
<p>It is the responsibility of the account owner's server to cache <cite>XEP-0115</cite> information (including "ext" values and their associated namespaces). When the server receives presence from a contact, it MUST check that presence information for entity capabilities data and correlate that data with the desired namespaces for the contact's client. The server MUST NOT send notifications related to any data formats that the contact's client has not asked for via the relevant "namespace+notify" disco#info feature. This enables a client to turn off all notifications (e.g., because of bandwidth restrictions) and to easily receive all desired data formats simply by adding support for the appropriate "namespace+notify" combination in its disco#info results and client capabililies. However, it also implies that a client can request notifications only on a global basis and cannot request, say, mood information only from certain contacts in the user's roster. Community consensus is that this is an acceptable tradeoff. Also, note that this works only if the account owner has a presence subscription to the contact and the contact has a presence subscription to the account owner.</p>
<p>Some examples may help to illustrate the concept of notification filtering. Here we show presence generated by two of the contacts listed above (benvolio@montague.net does have any presence subscriptions to or from juliet@capulet.com and therefore is not involved in these protocol flows).</p>
<examplecaption='Presence with caps'><![CDATA[
<presencefrom='nurse@capulet.com/chamber'>
<cxmlns='http://jabber.org/protocol/caps'
node='http://exodus.jabberstudio.org/caps'
ver='0.9'
ext='asdf fdsa bar baz'/>
</presence>
<presencefrom='romeo@montague.net/orchard'>
<cxmlns='http://jabber.org/protocol/caps'
node='http://www.chatopus.com/ec'
ver='2.1'
ext='foobar pres+'/>
</presence>
]]></example>
<p>We assume that Juliet's server doesn't know anything about these capabilities, so it sends service discovery information requests to each of the clients on Juliet's behalf (realistically, the capulet.com server will quickly build up a cache of client capabilities, with the result that it will not need to send these service discovery requests):</p>
<examplecaption='Account server queries contact'><![CDATA[
<p>Now we revisit account owner publication and server generation of notifications, with filtering enabled because the server has caps information:</p>
<li><p>If Juliet publishes a tune item to the open-access "http://jabber.org/protocol/tune" node, her server will send notifications to <benvolio@montague.net> (bare JID) and to <nurse@capulet.com/chamber> (full JID) but not to <romeo@montague.net/orchard>.</p></li>
<li><p>If Juliet publishes an activity item to the presence-access "http://jabber.org/protocol/activity" node, her server will send notifications only to <nurse@capulet.com/chamber>.</p></li>
<li><p>If Juliet publishes a geolocation item to the roster-access "http://jabber.org/protocol/geoloc" node, her server will send notifications only to <romeo@montague.net/orchard>.</p></li>
<section2topic='Number of Notifications'anchor='notifications-num'>
<olstart='1'>
<li><p>If a subscriber subscribed using a full JID (&FULLJID;), domain identifier (&DOMAIN;), or domain plus resource (&DOMAINRES;), a PEP service MUST send one notification only, addressed to the subscribed JID.</p></li>
<li><p>If a subscriber subscribed using a bare JID (&BAREJID;) and a PEP service does not have appropriate presence information about the subscriber, a PEP service MUST send at most one notification, addressed to the bare JID (&BAREJID;) of the subscriber, and MAY choose not to send any notification. (By "appropriate presence information" is meant an available presence stanza with non-negative priority and <cite>XEP-0115</cite> data that indicates interest in the relevant data format.)</p></li>
<li><p>If a subscriber subscribed using a bare JID (&BAREJID;) and a PEP service has appropriate presence information about the subscriber, the PEP service MUST send one notification to the full JID (&FULLJID;) of each of the subscriber's available resources that have specified non-negative presence priority and included <cite>XEP-0115</cite> information that indicates an interest in the data format.</p></li>
<section2topic='When to Generate Notifications'anchor='notifications-when'>
<olstart='1'>
<li><p>When an account owner publishes an item to a node, a PEP service MUST generate a notification and send it to all appropriate subscribers (where the number of notifications is determined by the foregoing rules).</p></li>
<li><p>When a PEP service receives initial presence information from a subscriber's resource with a non-negative priority and including <cite>XEP-0115</cite> information that indicates an interest in the data format, it MUST generate a notification containing the last published item for that node and send it to the newly-available resource.</p></li>
<li><p>As an exception to the foregoing MUST rules, a PEP service MUST NOT send notifications to a subscriber if the user has blocked the subscriber from receiving all or any kinds of stanza (presence, message, IQ, or any combination thereof) using communiations blocking as specified in <cite>XMPP IM</cite>.</p></li>
</ol>
</section2>
</section1>
<section1topic='Sending the Last Published Item'anchor='last'>
<p>As described in the <linkurl='#notifications'>Generating Notifications</link> section of this document, a PEP service MUST send the last published item to all new subscribers and to all newly-available resources for each subscriber. <note>That is, the default value of the "pubsub#send_last_published_item" node configuration field must be "on_sub_and_presence"; this behavior essentially mimics the functionality of presence as defined in <cite>XMPP IM</cite>.</note></p>
<examplecaption='Subscriber sends presence from newly-available resource'><![CDATA[
<presencefrom='romeo@montague.net/orchard'>
<cxmlns='http://jabber.org/protocol/caps'
node='http://www.chatopus.com/ec'
ver='2.1'
ext='foobar pres+'/>
</presence>
]]></example>
<examplecaption='Subscriber's server sends presence from newly-available resource to publisher's bare JID (i.e., PEP service)'><