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<xep>
<header>
  <title>PubSub Chaining</title>
  <abstract>This specification defines a method for chaining pubsub nodes together, resulting in lightweight repeaters for pubsub notifications.</abstract>
  &LEGALNOTICE;
  <number>0253</number>
  <status>Deferred</status>
  <type>Standards Track</type>
  <sig>Standards</sig>
  <approver>Council</approver>
  <dependencies>
    <spec>XMPP Core</spec>
    <spec>XEP-0050</spec>
    <spec>XEP-0060</spec>
  </dependencies>
  <supersedes/>
  <supersededby/>
  <shortname>NOT_YET_ASSIGNED</shortname>
  &ralphm;
  &stpeter;
  <revision>
    <version>0.2</version>
    <date>2009-11-18</date>
    <initials>psa</initials>
    <remark><p>Specifed protocol flow for the chained subscription.</p></remark>
  </revision>
  <revision>
    <version>0.1</version>
    <date>2008-11-13</date>
    <initials>psa</initials>
    <remark><p>Initial published version.</p></remark>
  </revision>
  <revision>
    <version>0.0.2</version>
    <date>2008-10-07</date>
    <initials>psa</initials>
    <remark><p>Added ofrom header to notifications.</p></remark>
  </revision>
  <revision>
    <version>0.0.1</version>
    <date>2008-08-12</date>
    <initials>rm/psa</initials>
    <remark><p>First draft.</p></remark>
  </revision>
</header>

<section1 topic='Introduction' anchor='intro'>
  <p>To enable lightweight repeaters for &xep0060; notifications, we need the ability to subscribe one pubsub node to another pubsub node. This specification defines a method for doing so, using &xep0050;.</p>
</section1>

<section1 topic='How It Works' anchor='howitworks'>
  <p>The owner of a pubsub node can subscribe that "local" node to a "remote" node using the flow described below. In these examples, the node owner is admin@consumer.tld, the local node is weatherbot@consumer.tld/Chicagoland, and the remote node has a NodeID of "OHR" at the pubsub service notify.weather.tld.</p>
  <example caption='Owner requests chaining'><![CDATA[
<iq from='admin@consumer.tld/client'
    id='chaining-1'
    to='weatherbot@consumer.tld'
    type='set'
    xml:lang='en'>
  <command xmlns='http://jabber.org/protocol/commands'
           action='execute'
           node='http://jabber.org/protocol/pubsub#chaining'/>
</iq>
]]></example>
  <p>Unless an error occurs, the service SHOULD return the appropriate form.</p>
  <example caption='Service returns chaining form to node owner'><![CDATA[
<iq from='weatherbot@consumer.tld'
    id='chaining-1'
    to='admin@consumer.tld/client'
    type='result'
    xml:lang='en'>
  <command xmlns='http://jabber.org/protocol/commands'
           node='http://jabber.org/protocol/pubsub#chaining'
           sessionid='a73sjjvkla37jfea'
           status='executing'>
    <x xmlns='jabber:x:data' type='form'>
      <title>Chaining Request</title>
      <instructions>Fill out this form to complete your request.</instructions>
      <field type='hidden' var='FORM_TYPE'>
        <value>http://jabber.org/protocol/pubsub#chaining</value>
      </field>
      <field label='The Node ID of the local pubsub node'
             type='text-single'
             var='local-node'>
        <required/>
      </field>
      <field label='The Jabber ID of the remote pubsub service'
             type='jid-single'
             var='remote-service'>
        <required/>
      </field>
      <field label='The NodeID of the remote pubsub node'
             type='text-single'
             var='remote-node'>
        <required/>
      </field>
    </x>
  </command>
</iq>
]]></example>
  <example caption='Admin submits chaining form to service'><![CDATA[
<iq from='admin@consumer.tld/client'
    id='chaining-2'
    to='weatherbot@consumer.tld'
    type='submit'
    xml:lang='en'>
  <command xmlns='http://jabber.org/protocol/commands'
           node='http://jabber.org/protocol/pubsub#chaining'
           sessionid='a73sjjvkla37jfea'
           status='executing'>
    <x xmlns='jabber:x:data' type='submit'>
      <field type='hidden' var='FORM_TYPE'>
        <value>http://jabber.org/protocol/pubsub#chaining</value>
      </field>
      <field var='local-node'>
        <value>Chicagoland</value>
      </field>
      <field var='remote-service'>
        <value>notify.weather.tld</value>
      </field>
      <field var='remote-node'>
        <value>OHR</value>
      </field>
    </x>
  </command>
</iq>
]]></example>
  <example caption='Service informs admin of completion'><![CDATA[
<iq from='weatherbot@consumer.tld'
    id='chaining-2'
    to='admin@consumer.tld/client'
    type='result'
    xml:lang='en'>
  <command xmlns='http://jabber.org/protocol/commands'
           node='http://jabber.org/protocol/pubsub#chaining'
           sessionid='a73sjjvkla37jfea'
           status='completed'/>
</iq>
]]></example>
  <p>At this point, the service itself will subscribe to the remote node.</p>
  <example caption='Service subscribes to remote node for chaining'><![CDATA[
<iq type='set'
    from='weatherbot@consumer.tld/Chicagoland'
    to='notify.weather.tld'
    id='sub1'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <subscribe
        node='OHR'
        jid='weatherbot@consumer.tld/Chicagoland'/>
  </pubsub>
</iq>
]]></example>
  <p>Now subscribers who are local to the consumer.tld XMPP service can subscribe directly to weatherbot@consumer.tld/Chicagoland instead of the remote pubsub node at notify.weather.tld. We therefore refer to weatherbot@consumer.tld/Chicagoland as a "chaining node" and the remote node as a "chained node".</p>
</section1>

<section1 topic='Notifications' anchor='notifications'>
  <p>When a chaining node delivers a notification to its subscribers, it SHOULD include an &xep0033; header of "ofrom" to specify the chained node or service that generated the notification (note: this header is not yet defined in <cite>XEP-0033</cite>).</p>
  <example caption='Chaining node notifies subscribers'><![CDATA[
<message from='weatherbot@consumer.tld/bot'
         to='subscriber@consumer.tld'
         id='foo'>
  <event xmlns='http://jabber.org/protocol/pubsub#event'>
    <items node='Chicagoland'>
      <item id='ae890ac52d0df67ed7cfdf51b644e901'>
        <example xmlns='urn:xmpp:example'>message</example>
      </item>
    </items>
  </event>
  <addresses xmlns='http://jabber.org/protocol/address'>
    <address type='ofrom' jid='notify.weather.tld'/>
  </addresses>
</message>
]]></example>
</section1>

<section1 topic='Determining Support' anchor='support'>
  <p>If a pubsub service supports Ad-Hoc Commands, it MUST advertise the commands it supports via &xep0030; (as described in <cite>XEP-0050: Ad-Hoc Commands</cite>); such commands exist as well-defined discovery nodes associated with the service. In particular, if a pubsub service supports chaining it MUST advertise a command of "http://jabber.org/protocol/pubsub#chaining".</p>
</section1>

<section1 topic='Security Considerations' anchor='security'>
  <p>The ability to subscribe one node to another node introduces the possibility of exposing non-public information in a public way, since the access controls for the node that originates a notification might not be known or enforced by the downstream node. Therefore, the upstream node (or its owner) is advised to make a careful access decision before allowing a downstream node (or any other entity) to subscribe.</p>
  <p>Note: The upstream node can discover the identity of the downstream node by sending a service discovery information ("disco#info") request to the downstream node, and MAY cancel or decline the downstream node's subscription if it determines that the node has an identity of "pubsub/leaf" or "pubsub/collection".</p>
</section1>

<section1 topic='IANA Considerations' anchor='iana'>
  <p>This document requires no interaction with &IANA;.</p>
</section1>

<section1 topic='XMPP Registrar Considerations' anchor='registrar'>
  <p>The &REGISTRAR; shall include the following information in its registries.</p>
  <section2 topic='Protocol Namespaces' anchor='ns'>
    <p>The XMPP Registrar shall include 'http://jabber.org/protocol/pubsub#chaining' in its registry of protocol namespaces (see &NAMESPACES;).</p>
  </section2>
  <section2 topic='Field Standardization' anchor='registrar-formtype'>
    <p>&xep0068; defines a process for standardizing the fields used within &xep0004; scoped by a particular namespace (see &FORMTYPES;). The reserved fields for the 'http://jabber.org/protocol/pubsub#chaining' namespace are specified below.</p>
    <code caption='Registry Submission'><![CDATA[
<form_type>
  <name>http://jabber.org/protocol/pubsub#chaining</name>
  <doc>XEP-0253</doc>
  <desc>Forms used for chaining of pubsub nodes.</desc>
  <field var='local-node'
         type='text-single'
         label='The Node ID of the local node'/>
  <field var='remote-node'
         type='text-single'
         label='The NodeID of the remote node'/>
  <field var='remote-service'
         type='jid-single'
         label='The Jabber ID of the remote pubsub service'/>
</form_type>
]]></code>
  </section2>
</section1>

<section1 topic='Acknowledgements' anchor='ack'>
  <p>Thanks to Joe Hildebrand for his feedback.</p>
</section1>

</xep>
