xeps/xep-0283.xml

<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE xep SYSTEM 'xep.dtd' [
  <!ENTITY % ents SYSTEM 'xep.ent'>
  <!ENTITY MOVED "&lt;moved/&gt;">
%ents;
]>
<?xml-stylesheet type='text/xsl' href='xep.xsl'?>
<xep>
<header>
  <title>Moved</title>
  <abstract>This document defines an XMPP protocol extension that enables a user to inform its contacts about a change in JID.</abstract>
  &LEGALNOTICE;
  <number>0283</number>
  <status>Deferred</status>
  <type>Standards Track</type>
  <sig>Standards</sig>
  <approver>Council</approver>
  <dependencies>
    <spec>XMPP Core</spec>
  </dependencies>
  <supersedes>
  </supersedes>
  <supersededby/>
  <shortname>moved</shortname>
  <author>
    <firstname>Tory</firstname>
    <surname>Patnoe</surname>
    <email>tpatnoe@cisco.com</email>
    <jid>tpatnoe@cisco.com</jid>
  </author>
  <revision>
    <version>0.1</version>
    <date>2010-06-16</date>
    <initials>psa</initials>
    <remark><p>Initial published version.</p></remark>
  </revision>
  <revision>
    <version>0.0.7</version>
    <date>2010-06-09</date>
    <initials>tp</initials>
    <remark>
      <p>Modified syntax to use 'old' and 'new' attributes.</p>
    </remark>
  </revision>
  <revision>
    <version>0.0.6</version>
    <date>2010-06-07</date>
    <initials>tp</initials>
    <remark>
      <ul>
        <li>Update ordering so that it recommends sending the unsubscribe/unsubscribed, before, sending the subscribe</li>
        <li>Add section documenting what happens when a contact is offline when the unsubscribe/unsubscribed/subscribe stanzas are sent.</li>
        <li>Added an example about a hacker sending an unsolicited subscribe by guessing a roster entry.</li>
      </ul>
    </remark>
  </revision>
  <revision>
    <version>0.0.5</version>
    <date>2010-06-02</date>
    <initials>tp</initials>
    <remark>
      <p>Added one-way subscription section</p>
    </remark>
  </revision>
  <revision>
    <version>0.0.4</version>
    <date>2010-05-30</date>
    <initials>tp</initials>
    <remark>
      <p>Change MUST NOT to SHOULD NOT for clients auto-subscribing back; differentiate between inbound and outbound subscriptions.</p>
    </remark>
  </revision>
  <revision>
    <version>0.0.3</version>
    <date>2010-06-02</date>
    <initials>tp</initials>
    <remark><p>Add one-way subscription comments.</p></remark>
  </revision>
  <revision>
    <version>0.0.2</version>
    <date>2010-05-25</date>
    <initials>tp</initials>
    <remark><p>Minor tweaks.</p></remark>
  </revision>
  <revision>
    <version>0.0.1</version>
    <date>2010-05-22</date>
    <initials>tp</initials>
    <remark><p>First draft.</p></remark>
  </revision>
</header>
<section1 topic='Introduction' anchor='intro'>
  <p>There are a variety of reasons why a user may wish to change
  their JID. For example, a surname change because of marriage or simply
  an easier JID to remember.   
  </p>

  <p>This XEP defines an approach for communicating that your JID has 
  moved to a new JID, extending the existing subscription protocol documented
  in &xmppim;. The steps outlined here may be done either through a client 
  or automated by a server.
  </p>
</section1>

<section1 topic='Requirements' anchor='reqs'>
  <ul>
    <li>The methods described here maintain compatibility with &xmppcore; and <cite>RFC 6121</cite>.</li>
  </ul>
</section1>

<section1 topic='The Move'>
  <p>In this scenario user@example.com moves to user2@example2.com. 
    Both the user@example.com and user2@example2.com accounts have been
    created and still exist. The roster for user@example2.com is empty
    and the user wants to populate it with their entries from 
    user@example.com.</p>

  <dl>
    <di>
      <dt>original JID</dt>
      <dd>user@example.com</dd>
    </di>
    <di>
      <dt>new JID</dt>
      <dd>user2@example2.com</dd>
    </di>
  </dl>

  <section2 topic='Unsubscribing the original JID from outbound subscriptions'
            anchor='unsubscribe'>
    <p>Because the original JID is no longer going to be used, the user SHOULD 
    unsubscribe from all the outbound subscriptions user@example.com had.
    These can be identified as those in the 'to' or 'ask' states as 
    defined in the 'jabber:iq:roster' protocol in <cite>RFC 6121</cite>.</p>

    <p>To unsubscribe all outbound subscriptions for the original JID send an 
    unsubscribe &PRESENCE; stanza to all the old contacts with a &MOVED; 
    element containing the new JID.</p>

    <p>There is the potential for other users to send a malicious unsubscribe 
    containing a spoofed &MOVED; JID. Therefore, clients SHOULD NOT 
    automatically subscribe to the JID contained in the &MOVED; stanza when
    receiving a subscribe &PRESENCE; stanza without displaying the &MOVED;
    JID to the user. See the Security Considerations section for 
    details.</p>

    <example caption='Client unsubscribes outbound subscriptions from original JID'><![CDATA[
<presence from='user@example.com' to='contact@example.com' type='unsubscribe'>
    <status>I've changed JIDs from user@example.com to user2@example2.com</status>
    <moved xmlns='urn:xmpp:moved:0' new='user2@example2.com'/>
</presence>
]]></example>

  </section2>

  <section2 topic='Unsubscribing the original JID from inbound subscriptions'
            anchor='unsubscribed'>
    <p>Because the original JID is no longer going to be used, the user SHOULD 
    unsubscribe from all contacts the user@example.com had an inbound 
    subscription from. These can be identified as those in the 'from' 
    subscription state as defined in in the 'jabber:iq:roster' protocol 
    in <cite>RFC 6121</cite>.</p>

    <p>To unsubscribe all inbound subscriptions send an unsubscribed 
    &PRESENCE; stanza to all the old contacts with a &MOVED; element 
    containing the new JID.</p>

    <p>There is the potential for other users to send a malicious unsubscribed 
    containing a spoofed &MOVED; JID. Therefore, clients SHOULD NOT 
    automatically subscribe to the JID contained in the &MOVED; stanza 
    without displaying the &MOVED; JID to the user. See the Security 
    Considerations section for details.</p>

    <example caption='Client unsubscribes inbound subscriptions from original JID'>
<![CDATA[
<presence from='user@example.com' to='contact@example.com' type='unsubscribed'>
    <status>I've changed JIDs from user@example.com to user2@example2.com</status>
    <moved xmlns='urn:xmpp:moved:0' new='user2@example2.com'/>
</presence>
]]>
    </example>

  </section2>
  <section2 topic='Subscribing the new JID to all your existing contacts' 
            anchor='subscribe'>

      <p>Once the new JID has been created on a server it is possible for the
      new JID to subscribe to the contacts they had on the original JID's
      roster. This is done by sending a new subscription request with a 
      &MOVED; element containing the new JID.
      </p>

      <p>The new subscription MUST come from the new JID's server.</p>

      <p>There is the potential for other users to send a malicious subscribe
      request and spoof the content of the &MOVED; element identifying an 
      original JID. Therefore, clients SHOULD NOT automatically unsubscribe 
      an existing roster entry if is listed as the target in the &MOVED; 
      element when a subscribe is received. See the Security 
      Consideration section for details.</p>

      <p>Clients accepting the moved subscription SHOULD indicate to the
      user that that this subscription request was the result of a move 
      operation and because of potential malicious behavior SHOULD NOT 
      auto-accept the subscription without displaying the &MOVED; JID to the
      user.</p>

      <example caption='Client requests subscription from new JID'>
  <![CDATA[
  <presence from='user2@example2.com' to='contact@example.com' type='subscribe'>
      <status>I've changed JIDs from user@example.com to user2@example2.com</status>
      <moved xmlns='urn:xmpp:moved:0' old='user@example.com'/>
  </presence>
  ]]>
      </example>

    </section2>
    
  <section2 topic='Contacts Offline at the Time the Rename Occurs' 
   anchor='offline'>
    <p><cite>RFC 6120</cite> clarifies that an incoming subscribe &PRESENCE; stanza 
    MUST be preserved by the server and &PRESENCE; stanzas of type 
    unsubscribe and unsubscribed are not preserved on the server. 
    Therefore, for a contact who is offline, their servers MAY have 
    automatically removed the original roster entry when seeing the 
    unsubscribe and unsubscribed stanzas. At the time of writing this XEP, 
    NOT saving and forwarding the presence stanzas will be the default 
    behavior of most servers.
    </p>
    
    <p>What this means is that a contact coming online after the rename 
    outlined above MAY only see the &PRESENCE; of type 'subscribe' with 
    the &MOVED; element. Clients should be aware of this behavior.
    </p>
  </section2>

  <section2 topic='Presence Ordering' anchor='ordering'>
  <p>In following the principle of least surprise, it is considered good 
  practice to send the subscribe stanza after the unsubscribe and unsubscribed 
  stanzas.
  </p>
  </section2>

  <section2 topic='Preservation of Groups' anchor='grouping'>
    <p>One of the side effects of this scheme is the potential for a contact
    to lose the groups to which it had organized the original JID. Clients 
    aware of the &MOVED; element can mitigate this with the following rules.
    </p>

    <ul>
    <li>If the contacts client receives an unsubscribed with a &MOVED; 
    element, remove the subscription but initiate a roster push for the 
    original JID with the subscription set to 'none'. When the new 
    subscription is received the new JID MAY be placed into the roster 
    in the same groups as the original JID and the original JID can then be 
    removed from the roster.
    </li>

    <li>If a subscribe is received with a &MOVED; element, the client MAY
    automatically place the new JID into the same groups as the original JID.
    </li>
    </ul>

   <p>As discussed in 'Contacts Offline at the Time the Rename Occurs', a 
   server MAY automatically handle the unsubscribe and unsubscribed stanzas.
   If this occurs it will be impossible to preserve the original groups.
   </p>

  </section2>

  <section2 topic='One-way subcriptions and full roster state' anchor='one-way'>
  <section3 topic='One-way Inbound subscription' anchor='from_subs'>
    <p>If the original JID, user@example.com, had only an inbound subscription
    (from or pending in), then the contact will only receive an 
    unsubscribed &PRESENCE; stanza. The contact's client, knowing the 
    state of the subscription (which is 'to' or 'none' with 'ask='subscribe' 
    from the contact's perspective), at that point MAY choose to prompt the 
    user to subscribe to the new JID listed in the &MOVED; element.</p>

  <p>Because of the ability to spoof the &MOVED; element, the client SHOULD
     NOT automatically subscribe to the &MOVED; element target, but SHOULD
     present the new JID to the contact before sending out a subscription
     request.</p>
  </section3>

  <section3 topic='One-way Outbound subscription' anchor='to_subs'>
    <p>If the original JID, user@example.com, had only an outbound 
    subscription (to or ask), then the contact SHOULD only receive an 
    unsubscribe &PRESENCE; stanza. The contact's client, knowing the 
    state of the subscription (which is 'from' from the contact's perspective), 
    at that point MAY choose to prompt 
    the user to subscribe to the new JID listed in the &MOVED; element.</p>

  <p>Because of the ability to spoof the &MOVED; element, the client SHOULD
     NOT automatically subscribe to the &MOVED; element target.</p>
  </section3>

  <section3 topic='Roster state and action table' anchor='actions'>
    <table caption="Roster states and actios from the renamed user's perspective">  
      <tr>
        <td>Server state</td>
        <td>Client state (jabber:iq:roster)</td>
        <td>Send unsubscribe from original JID</td>
        <td>Send unsubscribed from original JID</td>
        <td>Send subscribe from new JID</td>
      </tr>
      <tr>
        <td>none</td>
        <td>none</td>
        <td></td>
        <td></td>
        <td></td>
      </tr>
      <tr>
        <td>none + pending out</td>
        <td>none + ask='subscribe'</td>
        <td>yes</td>
        <td></td>
        <td>yes</td>
      </tr>
      <tr>
        <td>none + pending in</td>
        <td>n/a</td>
        <td></td>
        <td>yes - server only</td>
        <td></td>
      </tr>
      <tr>
        <td>none + pending in/out</td>
        <td>none + ask='subscribe'</td>
        <td>yes</td>
        <td>yes - server only</td>
        <td>yes</td>
      </tr>
      <tr>
        <td>to</td>
        <td>to</td>
        <td>yes</td>
        <td></td>
        <td>yes</td>
      </tr>
      <tr>
        <td>to + pending in</td>
        <td>to</td>
        <td>yes</td>
        <td>yes - server only</td>
        <td>yes</td>
      </tr>
      <tr>
        <td>from</td>
        <td>from</td>
        <td></td>
        <td>yes</td>
        <td></td>
      </tr>
      <tr>
        <td>from + pending out</td>
        <td>from/none + ask='subscribe'</td>
        <td>yes</td>
        <td>yes</td>
        <td>yes</td>
      </tr>
      <tr>
        <td>both</td>
        <td>both</td>
        <td>yes</td>
        <td>yes</td>
        <td>yes</td>
      </tr>
     </table>
  </section3>
  </section2>
</section1>

<section1 topic='Security Considerations' anchor='security'>
  <p>It is not intended for servers to strip any &MOVED; elements from
  &PRESENCE; stanzas sent in from a client. This allows clients as well as
  servers to implement these same procedures.</p>
  
  <p>In order to prevent other users from maliciously altering contacts 
  the client SHOULD NOT automatically subscribe to a &MOVED; JID when it 
  receives an unsubscribe and SHOULD NOT automatically unsubscribe to
  a &MOVED; JID when it receives a subscribe.</p>

  <p>The following illustrates an example malicious attack.</p>

  <ol>
  <li>userA@example.com subscribes to userB@example.com</li>
  <li>The userB@example.com automatically accepts the subscription from 
      userA@example.com. (Automatically done by the client using a simple 
      domain trust).</li>
  <li>userA@example.com unsubscribes with the &MOVED; 'new' JID set to 
      companyCEO@example.com.</li>
  <li>The previous steps can be repeated and have userB@example.com subscribe
      to a large number of people.</li>
  </ol>

 <p>A similar attack can be done with a new subscribe request causing users
  by guessing which users are subscribed to a contact.</p>

  <ol>
  <li>hacker@example.com subscribes to userB@example.com guessing that
      userA@example.com is on userB's roster.
<![CDATA[
<presence from='hacker@example.com' to='userB@example.com' type='subscribe'>
  <status>Subscribe to me!</status>
  <moved xmlns='urn:xmpp:moved:0' old='userA@example.com'/>
</presence>
]]>    
  </li>
  <li>If userB's client automatically accepted the subscription without
  displaying at prompt to userB showing the new JID to be hacker@example.com,
  then the user has no idea that hacker@example.com was just added to
  the roster. 
  </li>
  </ol>

</section1>

<section1 topic='IANA Considerations' anchor='iana'>
  <p>This document requires no interaction with &IANA;.</p> 
</section1>
<section1 topic='XMPP Registrar Considerations' anchor='reg'>
  <section2 topic='Protocol Namespaces' anchor='registrar-ns'>
    <p>This specification defines the following XML namespace:</p>
    <ul>
      <li>urn:xmpp:moved: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'>
    &NSVER;
  </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:moved:0'
    xmlns='urn:xmpp:moved:0'
    elementFormDefault='qualified'>

  <xs:element name='moved'>
    <xs:complexType>
      <xs:simpleContent>
        <xs:extension base='xs:string'>
          <xs:attribute name='new' type='xs:string' use='optional'/>
          <xs:attribute name='old' type='xs:string' use='optional'/>
        </xs:extension>
      </xs:simpleContent>
    </xs:complexType>
  </xs:element>

</xs:schema>
  ]]></code>
</section1>
<section1 topic='Acknowledgements' anchor='ack'>
  <p>The author wishes to thank Doug Abbink, Mikhail Belov, Peter Saint-Andre, and Peter Sheu for their feedback.</p> 
</section1>
</xep>