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<xep>
<header>
  <title>Invisible Command</title>
  <abstract>This document specifies an XMPP-compatible protocol for user invisibility.</abstract>
  &LEGALNOTICE;
  <number>0186</number>
  <status>Experimental</status>
  <type>Standards Track</type>
  <sig>Standards</sig>
  <dependencies>
    <spec>XMPP Core</spec>
    <spec>XMPP IM</spec>
    <spec>XEP-0030</spec>
  </dependencies>
  <supersedes>
    <spec>XEP-0018</spec>
  </supersedes>
  <supersededby>None</supersededby>
  <shortname>invisible</shortname>
  &stpeter;
  <revision>
    <version>0.9</version>
    <date>2008-10-07</date>
    <initials>psa</initials>
    <remark><p>Further clarified server and client handling of stanzas during an invisibility session.</p></remark>
  </revision>
  <revision>
    <version>0.8</version>
    <date>2008-10-06</date>
    <initials>psa</initials>
    <remark><p>Modified namespace to incorporate namespace versioning.</p></remark>
  </revision>
  <revision>
    <version>0.7</version>
    <date>2008-05-12</date>
    <initials>psa</initials>
    <remark><p>Added note about integration with privacy lists; removed XEP-0126 from list of protocols that this specification supersedes; changed temporary namespace to conform to XMPP Registrar procedures; copied several security considerations from XEP-0126.</p></remark>
  </revision>
  <revision>
    <version>0.6</version>
    <date>2007-09-05</date>
    <initials>psa</initials>
    <remark><p>Clarified that this specification is intended to supersede XEP-0018 and XEP-0126; added several additional examples.</p></remark>
  </revision>
  <revision>
    <version>0.5</version>
    <date>2007-01-30</date>
    <initials>psa</initials>
    <remark><p>Modified XML namespace name to conform to XEP-0053 processes.</p></remark>
  </revision>
  <revision>
    <version>0.4</version>
    <date>2006-08-09</date>
    <initials>psa</initials>
    <remark><p>Added XMPP Registrar considerations and XML schema.</p></remark>
  </revision>
  <revision>
    <version>0.3</version>
    <date>2006-08-02</date>
    <initials>psa</initials>
    <remark><p>Added inbound presence rule to server handling section.</p></remark>
  </revision>
  <revision>
    <version>0.2</version>
    <date>2006-07-07</date>
    <initials>psa</initials>
    <remark><p>Clarified that invisibility mode does not carry across sessions.</p></remark>
  </revision>
  <revision>
    <version>0.1</version>
    <date>2006-05-30</date>
    <initials>psa</initials>
    <remark><p>Initial version.</p></remark>
  </revision>
  <revision>
    <version>0.0.2</version>
    <date>2006-05-15</date>
    <initials>psa</initials>
    <remark><p>Recommended delivery of messages sent to bare JID.</p></remark>
  </revision>
  <revision>
    <version>0.0.1</version>
    <date>2006-05-11</date>
    <initials>psa</initials>
    <remark><p>First draft.</p></remark>
  </revision>
</header>

<section1 topic='Introduction' anchor='intro'>
  <p>Some instant messaging implementations of the Jabber/XMPP protocols have long supported the ability for IM users to be online but appear invisible. The existing protocols for doing so are:</p>
  <ul>
    <li><p>&xep0018; -- this protocol is not compatible with &xmppcore; and &xmppim;, and the specification does not provide reliable documentation of the protocol in use since many server implementations support presence of type "invisible" but not presence of type "visible".</p></li>
    <li><p>&xep0126; -- this protocol is a somewhat complicated use of &xep0016; for the temporary purpose of appearing invisible rather than the intended purpose of permanently blocking communications; however, the invisible command defined herein can provide a client-friendly interface to the same data store used for privacy lists.</p></li>
  </ul>
  <p>In order to provide a standards-compliant protocol that can be used in the long term, this document defines an IQ-based protocol that enables an IM user to become "invisible" and "visible" at will within the context of a given session. This protocol is intended to supersede the protocol described in <cite>XEP-0018</cite> and to provide a more client-friendly approach to invisibility than <cite>XEP-0126</cite>.</p>
</section1>

<section1 topic='Requirements' anchor='req'>
  <p>The requirements for invisible mode are straightforward:</p>
  <ol start='1'>
    <li>A user can become visible or invisible at any time within an XMPP session.</li>
    <li>Invisible mode is active only for the current session; if the user ends that session and starts another session, the invisibility mode set for the previous session does not carry over to the new session.</li>
    <li>When in invisible mode, a user can send directed presence to particular contacts.</li>
  </ol>
</section1>

<section1 topic='Use Cases' anchor='usecases'>
  <section2 topic='User Becomes Invisible' anchor='invisible'>
    <p>In order for a client to go invisible, it shall send an IQ-set with no 'to' address (thus handled by the user's server) containing an &lt;invisible/&gt; element qualified by the 'urn:xmpp:invisible:0' namespace &VNOTE;.</p>
    <example caption='Invisible command'><![CDATA[
<iq from='bilbo@tolkien.lit/shire'
    id='inv1'
    type='set'>
  <invisible xmlns='urn:xmpp:invisible:0'/>
</iq>
    ]]></example>
    <p>If the server can successfully process the invisibility command, it MUST return an IQ-result.</p>
    <example caption='Invisible command is successful'><![CDATA[
<iq to='bilbo@tolkien.lit/shire'
    id='inv1'
    type='result'/>
    ]]></example>
    <p>(Standard XMPP stanza errors apply; see <cite>RFC 3920</cite> and &xep0086;.)</p>
    <p>When the client enters invisible mode in the midst of a presence session (i.e., after having previously sent undirected presence with no 'type' attribute), the server MUST send &UNAVAILABLE; presence from the specified resource to all contacts who would receive unavailable presence if the client sent &UNAVAILABLE;.</p>
    <p>The following sections define how the server and the client shall handle inbound and outbound XML stanzas while the client is invisible.</p>
    <section3 topic='Server Handling' anchor='invisible-server'>
      <p>While the client is in invisible mode, the server:</p>
      <ol start='1'>
        <li><p>MUST NOT broadcast presence notifications as a result of receiving any subsequent undirected presence notifications from the client.</p></li>
        <li><p>MUST deliver directed presence stanzas generated by the client.</p></li>
        <li><p>MUST deliver inbound &PRESENCE; stanzas.</p></li>
        <li><p>SHOULD deliver inbound &MESSAGE; stanzas whose 'to' address is the bare JID &LOCALBARE; of the user (subject to standard XMPP stanza handling rules).</p></li>
        <li><p>MUST deliver inbound &MESSAGE; and &IQ; stanzas whose 'to' address is the full JID &LOCALFULL; corresponding to the resource of the client.</p></li>
        <li><p>MUST deliver outbound &MESSAGE; and &IQ; stanzas generated by the client (for an important note regarding presence leaks, see the <link url='#security'>Security Considerations</link> section of this document).</p></li>
        <li>
          <p>If there are no other available resources, MUST respond to all IQ-get requests and presence probes sent to the account's bare JID as if the account were offline; this includes but is not limited to the following:</p>
          <ul>
            <li>If the server responds to a presence probe, the last available presence MUST indicate that the user is unavailable, and if a time is indicated it MUST be the time when the client went invisibile.</li>
            <li>If the server responds to a &xep0012; request, the last activity time MUST be the time when the client went invisible.</li>
            <li>If the server responds to a &xep0030; items request, the response MUST NOT include the invisible resource as one of the account's available items.</li>
          </ul>
        </li>
        <li><p>If after sending directed presence the client then sends &UNAVAILABLE;, the server MUST deliver that unavailable presence only to the entities to which the client sent directed presence after going invisible.</p></li>
      </ol>
    </section3>
    <section3 topic='Client Handling' anchor='invisible-client'>
      <p>While the client is in invisible mode, the client:</p>
      <ul>
        <li><p>MUST maintain a temporary list of entities with which communication is allowed, and prompt the user before adding any entity to that "communicants list" for this invisibility session; the list MAY be auto-populated with trusted entities if so configured by the user.</p></li>
        <li><p>MUST prompt the user before sending any outbound traffic (message, presence, or IQ stanza) to a contact even if the user generated such traffic; upon receiving authorization from the user, the client SHOULD add the authorized entity to the communicants list for this invisibility session.</p></li>
      </ul>
    </section3>
  </section2>
  <section2 topic='User Becomes Visible' anchor='visible'>
    <p>In order for a client to become visible again, it shall send an IQ-set with no 'to' address (thus handled by the user's server) containing a &lt;visible/&gt; element qualified by the 'urn:xmpp:invisible:0' namespace &VNOTE;.</p>
    <example caption='Visible command'><![CDATA[
<iq from='bilbo@tolkien.lit/shire'
    id='vis1'
    type='set'>
  <visible xmlns='urn:xmpp:invisible:0'/>
</iq>
    ]]></example>
    <p>If the server can successfully process the visibility command, it MUST return an IQ-result.</p>
    <example caption='Visible command is successful'><![CDATA[
<iq to='bilbo@tolkien.lit/shire'
    id='vis1'
    type='result'/>
    ]]></example>
    <p>When the client becomes visible, the server MUST treat that state as equivalent to an active session before receiving initial presence from the client.</p>
    <p>It is the responsibility of the client to send an undirected presence notification to the server.</p>
    <example caption='Client sends presence'><![CDATA[
<presence/>
    ]]></example>
    <p>The server then MUST broadcast that presence to all entities who would normally receive presence broadcasts from the client (as well as any other entities to which the client sent directed presence while invisible).</p>
  </section2>
</section1>

<section1 topic='Discovering Support' anchor='support'>
  <p>In order for a client to discover whether its server supports the protocol defined herein, it MUST send a &xep0030; information request to the server:</p>
  <example caption='Service discovery request'><![CDATA[
<iq from='bilbo@tolkien.lit/shire'
    id='disco1'
    to='tolkien.lit'
    type='get'>
  <query xmlns='http://jabber.org/protocol/disco#info'/>
</iq>
  ]]></example>
  <p>If the server supports the protocol defined herein, it MUST return a feature of "urn:xmpp:invisible:0" &VNOTE;.</p>
  <example caption='Service discovery response'><![CDATA[
<iq from='bilbo@tolkien.lit/shire'
    id='disco1'
    to='tolkien.lit'
    type='result'>
  <query xmlns='http://jabber.org/protocol/disco#info'>
    <feature var='urn:xmpp:invisible:0'/>
  </query>
</iq>
  ]]></example>
  <p>A client SHOULD complete this service discovery process before sending initial presence to its server.</p>
</section1>

<section1 topic='Integration With Privacy Lists' anchor='priv'>
  <p>A server MAY use the same backend data store for invisibility mode as defined herein and &xep0016; as used for invisibility (see <cite>XEP-0126</cite>). If so, the server MUST update the relevant privacy lists on behalf of the user when the client requests initiation or termination of invisible mode.</p>
</section1>

<section1 topic='Security Considerations' anchor='security'>
  <p>No matter how it is implemented, invisibility can be defeated and presence leaks can occur without careful stanza handling on the part of the server and the client. Use of the protocol defined here does not guarantee that presence leaks will not occur, either technically or socially (e.g., if the user reveals his presence to one contact but not another and those contacts are in communication).</p>
</section1>

<section1 topic='IANA Considerations' anchor='iana'>
  <p>No interaction with &IANA; is required as a result of this document.</p>
</section1>

<section1 topic='XMPP Registrar Considerations' anchor='registrar'>
  <section2 topic='Protocol Namespaces' anchor='registrar-ns'>
    <p>This specification defines the following XML namespace:</p>
    <ul>
      <li>urn:xmpp:invisible: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 namespace to the registry located at &NAMESPACES;, as described in Section 4 of &xep0053;.</p>
  </section2>
  <section2 topic='Protocol 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:invisible:0'
    xmlns='urn:xmpp:invisible:0'
    elementFormDefault='qualified'>

  <xs:element name='invisible' type='empty'/>

  <xs:element name='visible' type='empty'/>

  <xs:simpleType name='empty'>
    <xs:restriction base='xs:string'>
      <xs:enumeration value=''/>
    </xs:restriction>
  </xs:simpleType>

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