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<header>
  <title>Reachability Addresses</title>
  <abstract>This document defines an XMPP protocol extension for communicating reachability information related to non-XMPP devices.</abstract>
  &LEGALNOTICE;
  <number>0152</number>
  <status>Experimental</status>
  <type>Standards Track</type>
  <sig>Standards</sig>
  <approver>Council</approver>
  <dependencies>
    <spec>XMPP Core</spec>
    <spec>XMPP IM</spec>
  </dependencies>
  <supersedes/>
  <supersededby/>
  <shortname>reach</shortname>
  &hildjj;
  &stpeter;
  <revision>
    <version>0.3</version>
    <date>2008-10-06</date>
    <initials>psa</initials>
    <remark>
      <ul>
        <li>Modified namespaces to incorporate namespace versioning.</li>
        <li>Removed IQ request-response method.</li>
        <li>Corrected PEP examples.</li>
        <li>Corrected XML schema.</li>
      </ul>
    </remark>
  </revision>
  <revision>
    <version>0.2</version>
    <date>2006-09-17</date>
    <initials>psa</initials>
    <remark><p>Defined PEP transport.</p></remark>
  </revision>
  <revision>
    <version>0.1</version>
    <date>2005-06-16</date>
    <initials>psa</initials>
    <remark><p>Initial version.</p></remark>
  </revision>
  <revision>
    <version>0.0.1</version>
    <date>2005-06-07</date>
    <initials>psa/jjh</initials>
    <remark><p>First draft.</p></remark>
  </revision>
</header>

<section1 topic='Introduction' anchor='intro'>
  <p>Sometimes it is desirable or necessary to switch from instant messaging (IM) to another real-time communications medium, such as a telephone conversation conducted over the traditional public switched telephone network (PSTN) or more recent Voice over Internet Protocol (VoIP) applications. In order to facilitate switching from IM to telephony or some other medium, a user needs to advertise the address(es) at which they can be reached. There are several possible ways to do this:</p>
  <ul>
    <li><p>Publish the reachability address(es) in the user's vCard (see &xep0054;); this is convenient, but is not very dynamic (e.g., reachability addresses might change when the user moves to a new conference room in an office building).</p></li>
    <li><p>Send the reachability address(es) within a &PRESENCE; stanza; this option is described in the <link url='#transport-presence'>Presence Broadcast</link> section of this document and is consistent with Section 5.1.2 of &rfc3921; since reachability is one aspect of a user's availability for communication.</p></li>
    <li><p>Send reachability address(es) to the appropriate &xep0163; node; this option is described in the <link url='#transport-pep'>PEP Transport</link> section of this document but may not be available at all service providers.</p></li>
  </ul>
</section1>

<section1 topic='Requirements' anchor='reqs'>
  <p>This document addresses the following requirements:</p>
  <ul>
    <li>Enable clients to dynamically publish reachability addresses.</li>
    <li>Minimize network traffic.</li>
  </ul>
</section1>

<section1 topic='Data Format' anchor='format'>
  <p>The following is an example of the basic data format for reachability addresses:</p>
  <example caption="Basic Data Format for Reachability Addresses"><![CDATA[
<reach xmlns='urn:xmpp:reach:0'>
  <addr uri='tel:+1-303-555-1212'/>
  <addr uri='sip:romeo@sipspeare.lit'/>
</reach>
  ]]></example>
  <p>When publishing reachability addresses, the &lt;reach/&gt; element MUST contain at least one &lt;addr/&gt; element. Each &lt;addr/&gt; element MUST possess a 'uri' attribute, whose value MUST be the Uniform Resource Identifier (&rfc3986;) or Internationalized Resource Identifier (&rfc3987;) of an alternate communications method for reaching the user.</p>
  <p>The &lt;addr/&gt; element MAY contain one or more &lt;desc/&gt; children whose XML character data is a natural-language description of the address; this element SHOULD possess an 'xml:lang' attribute whose value is a language tag that conforms to &rfc4646; (although the default language MAY be specified at the stanza level; see Section 9.1.5 of &rfc3920;). In order to preserve bandwidth, the &lt;desc/&gt; element SHOULD NOT be included when sending reachbility data via presence broadcast, but MAY be included when using the personal eventing protocol.</p>
  <example caption="Reachability Addresses With Descriptions"><![CDATA[
<reach xmlns='urn:xmpp:reach:0'>
  <addr uri='tel:+1-303-555-1212'>
    <desc xml:lang='en'>New conference room number</desc>
  </addr>
  <addr uri='sip:romeo@sipspeare.lit'>
    <desc xml:lang='en'>My softphone</desc>
  </addr>
</reach>
  ]]></example>
</section1>

<section1 topic='Data Transport' anchor='transport'>
  <p>As described below, this document specifies two methods of advertising reachability addresses:</p>
  <ul>
    <li>Presence broadcast</li>
    <li>Personal eventing via publish-subscribe (PEP)</li>
  </ul>
  <p>This document does not recommend one transport method over the other.</p>
  <p>In addition, a contact MAY request a user's reachability addresses using an &IQ; request-response sequence.</p>
  <section2 topic='Presence Broadcast' anchor='transport-presence'>
    <p>In order to broadcast reachability addresses in presence information, a user's client includes the &lt;reach/&gt; element in the &PRESENCE; stanza it sends to its server:</p>
    <example caption="User&apos;s Client Includes Reachability Addresses in Presence Broadcast"><![CDATA[
<presence from='romeo@montague.net'>
  <reach xmlns='urn:xmpp:reach:0'>
    <addr uri='tel:+1-303-555-1212'/>
    <addr uri='sip:romeo@sipspeare.lit'/>
  </reach>
</presence>
    ]]></example>
    <p>The user's server then broadcasts that presence stanza to all entities who are subscribed to the user's presence:</p>
    <example caption="User&apos;s Server Broadcasts Presence Information"><![CDATA[
<presence from='romeo@montague.net' to='juliet@capulet.com'>
  <reach xmlns='urn:xmpp:reach:0'>
    <addr uri='tel:+1-303-555-1212'/>
    <addr uri='sip:romeo@sipspeare.lit'/>
  </reach>
</presence>
    ]]></example>
  </section2>
  <section2 topic='Personal Eventing Protocol' anchor='transport-pep'>
    <p>In order to publish reachability addresses via the personal eventing protocol (<cite>XEP-0163</cite>), the entity simply publishes data to the "urn:xmpp:reach:0" node.</p>
    <example caption='Entity publishes reachability addresses'><![CDATA[
<iq type='set'
    from='romeo@montague.net'
    to='pubsub.shakespeare.lit'
    id='publish1'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <publish node='urn:xmpp:reach:0'>
      <item id='a1s2d3f4g5h6bjeh936'>
        <reach xmlns='urn:xmpp:reach:0'>
          <addr uri='tel:+1-303-555-1212'>
            <desc xml:lang='en'>My mobile number</desc>
          </addr>
          <addr uri='sip:romeo@sipspeare.lit'>
            <desc xml:lang='en'>My softphone</desc>
          </addr>
        </reach>
      </item>
    </publish>
  </pubsub>
</iq>
    ]]></example>
    <example caption='Subscriber receives event with payload'><![CDATA[
<message from='pubsub.shakespeare.lit'
         to='juliet@capulet.com'>
  <event xmlns='http://jabber.org/protocol/pubsub#event'>
    <items node='urn:xmpp:reach:0'>
      <item id='a1s2d3f4g5h6bjeh936'>
        <reach xmlns='urn:xmpp:reach:0'>
          <addr uri='tel:+1-303-555-1212'>
            <desc xml:lang='en'>My mobile number</desc>
          </addr>
          <addr uri='sip:romeo@sipspeare.lit'>
            <desc xml:lang='en'>My softphone</desc>
          </addr>
        </reach>
      </item>
    </items>
  </event>
</message>
    ]]></example>
  </section2>
</section1>

<section1 topic='Determining Support' anchor='support'>
  <p>If an entity supports reachability addresses, it MUST advertise that fact by returning a feature of "urn:xmpp:reach:0" &VNOTE; in response to a &xep0030; information request. The response MUST also include features for the application formats and transport methods supported by the responding entity, as described in the relevant specifications.</p>
  <example caption="Service Discovery Information Request"><![CDATA[
<iq from='juliet@capulet.com/balcony' 
    id='disco1'
    to='romeo@montague.net/orchard'
    type='get'>
  <query xmlns='http://jabber.org/protocol/disco#info'/>
</iq>
  ]]></example>
  <example caption="Service Discovery Information Response"><![CDATA[
<iq from='romeo@montague.net/orchard' 
    id='disco1'
    to='juliet@capulet.com/balcony' 
    type='result'>
  <query xmlns='http://jabber.org/protocol/disco#info'>
    <feature var='urn:xmpp:reach:0'/>
  </query>
</iq>
  ]]></example>
  <p>In order for an application to determine whether an entity supports this protocol, where possible it SHOULD use the dynamic, presence-based profile of service discovery defined in &xep0115;. However, if an application has not received entity capabilities information from an entity, it SHOULD use explicit service discovery instead.</p>
</section1>

<section1 topic='Implementation Notes' anchor='impl'>
  <p>To preserve network bandwidth, the sender SHOULD NOT include the &lt;desc/&gt; element unless that information is deemed necessary to enable communication.</p>
  <p>A recipient SHOULD attempt communications with reachability addresses in the order that the &lt;addr/&gt; elements appear within the &lt;reach/&gt; element.</p>
</section1>

<section1 topic='Internationalization Considerations' anchor='i18n'>
  <p>If included, the &lt;desc/&gt; element SHOULD possess an 'xml:lang' attribute specifying the language of the human-readable descriptive text for a particular address.</p>
</section1>

<section1 topic='Security Considerations' anchor='security'>
  <p>This document introduces no security considerations above and beyond those described in RFC 3920, RFC 3921, and (for the personal eventing transport) XEP-0163.</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'>
  <section2 topic='Protocol Namespaces' anchor='registrar-ns'>
    <p>This specification defines the following XML namespaces:</p>
    <ul>
      <li>urn:xmpp:reach:0</li>
    </ul>
    <p>Upon advancement of this specification from a status of Experimental to a status of Draft, the &REGISTRAR; shall add the foregoing namespaces to the registry located at &NAMESPACES;, as described in Section 4 of &xep0053;.</p>
  </section2>
  <section2 topic='Namespace Versioning' anchor='registrar-versioning'>
    <p>If the protocol defined in this specification undergoes a major revision that is not fully backward-compatible with an older version, or that contains significant new features, the XMPP Registrar shall increment the protocol version number found at the end of the XML namespaces defined herein, as described in Section 4 of <cite>XEP-0053</cite>.</p>
  </section2>
</section1>

<section1 topic='XML Schema' anchor='schema'>
  <code><![CDATA[
<?xml version='1.0' encoding='UTF-8'?>

<xs:schema
    xmlns:xs='http://www.w3.org/2001/XMLSchema'
    targetNamespace='urn:xmpp:reach:0'
    xmlns='urn:xmpp:reach:0'
    elementFormDefault='qualified'>

  <xs:element name='reach'>
    <xs:complexType>
      <xs:sequence>
        <xs:element name='addr' 
                    minOccurs='0'
                    maxOccurs='unbounded'
                    type='addrElementType'/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>

  <xs:complexType name='addrElementType'>
    <xs:sequence>
      <xs:element name='desc' 
                  minOccurs='0'
                  maxOccurs='unbounded'
                  type='xs:string'/>
    </xs:sequence>
    <xs:attribute name='uri' use='required' type='xs:anyURI'/>
  </xs:complexType>

</xs:schema>
  ]]></code>
</section1>
</xep>
