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<!DOCTYPE xep SYSTEM 'xep.dtd' [
  <!ENTITY % ents SYSTEM 'xep.ent'>
  <!ENTITY rfc3921bis "<span class='ref'><link url='http://tools.ietf.org/html/draft-ietf-saintandre-rfc3921bis'>rfc3921bis</link></span> <note>RFC 3921: Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence &lt;<link url='http://tools.ietf.org/html/draft-ietf-saintandre-rfc3921bis'>http://tools.ietf.org/html/draft-ietf-saintandre-rfc3921bis</link>&gt;.</note>" >
%ents;
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<xep>
<header>
  <title>Message Carbons</title>
  <abstract>In order to keep all IM clients for a user engaged in a conversation, outbound messages are carbon-copied to all interested resources.</abstract>
  <legal>
    <copyright>This XMPP Extension Protocol is copyright (c) 1999 - 2010 by the XMPP Standards Foundation (XSF).</copyright>
    <permissions>Permission is hereby granted, free of charge, to any person obtaining a copy of this specification (the &quot;Specification&quot;), to make use of the Specification without restriction, including without limitation the rights to implement the Specification in a software program, deploy the Specification in a network service, and copy, modify, merge, publish, translate, distribute, sublicense, or sell copies of the Specification, and to permit persons to whom the Specification is furnished to do so, subject to the condition that the foregoing copyright notice and this permission notice shall be included in all copies or substantial portions of the Specification. Unless separate permission is granted, modified works that are redistributed shall not contain misleading information regarding the authors, title, number, or publisher of the Specification, and shall not claim endorsement of the modified works by the authors, any organization or project to which the authors belong, or the XMPP Standards Foundation.</permissions>
    <warranty>## NOTE WELL: This Specification is provided on an &quot;AS IS&quot; BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, express or implied, including, without limitation, any warranties or conditions of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A PARTICULAR PURPOSE. In no event shall the XMPP Standards Foundation or the authors of this Specification be liable for any claim, damages, or other liability, whether in an action of contract, tort, or otherwise, arising from, out of, or in connection with the Specification or the implementation, deployment, or other use of the Specification. ##</warranty>
    <liability>In no event and under no legal theory, whether in tort (including negligence), contract, or otherwise, unless required by applicable law (such as deliberate and grossly negligent acts) or agreed to in writing, shall the XMPP Standards Foundation or any author of this Specification be liable for damages, including any direct, indirect, special, incidental, or consequential damages of any character arising out of the use or inability to use the Specification (including but not limited to damages for loss of goodwill, work stoppage, computer failure or malfunction, or any and all other commercial damages or losses), even if the XMPP Standards Foundation or such author has been advised of the possibility of such damages.</liability>
    <conformance>This XMPP Extension Protocol has been contributed in full conformance with the XSF's Intellectual Property Rights Policy (a copy of which may be found at &lt;<link url='http://www.xmpp.org/extensions/ipr-policy.shtml'>http://www.xmpp.org/extensions/ipr-policy.shtml</link>&gt; or obtained by writing to XSF, P.O. Box 1641, Denver, CO 80201 USA).</conformance>
  </legal>
  <number>xxxx</number>
  <status>ProtoXEP</status>
  <type>Standards Track</type>
  <sig>Standards</sig>
  <approver>Council</approver>
  <dependencies>
    <spec>XMPP Core</spec>
    <spec>XEP-0001</spec>
    <spec>XEP-0030</spec>
    <spec>XEP-0085</spec>
  </dependencies>
  <supersedes>
    <spec>XEP-0259</spec>
  </supersedes>
  <supersededby/>
  <shortname>carbons</shortname>
  <author>
    <firstname>Joe</firstname>
    <surname>Hildebrand</surname>
    <email>jhildebr@cisco.com</email>
    <jid>jhildebr@cisco.com</jid>
  </author>
  <revision>
    <version>0.0.1</version>
    <date>2010-02-25</date>
    <initials>jjh</initials>
    <remark><p>First draft.</p></remark>
  </revision>
  <revision>
    <version>0.0.2</version>
    <date>2010-04-21</date>
    <initials>jjh</initials>
    <remark><p>Updated after further analysis of edge cases.</p></remark>
  </revision>
</header>
<section1 topic='Introduction' anchor='intro'>
  <p>At the time of original writing of this XEP, many XMPP servers
  handle message stanzas sent to a user@host (or "bare") JID with no
  resource by delivering that message only to the resource with the
  highest priority for the target user. Some server implementations,
  however, have chosen to send these messages to all of the online
  resources for the target user. If the target user is online with
  multiple resources when the original message is sent, a conversation
  ensues on one of the user's devices; if the user subsequently
  switches devices, parts of the conversation may end up on the
  alternate device, causing the user to be confused, misled, or
  annoyed.</p>

  <p>This XEP defines an approach for ensuring that all of my devices
  get both sides of all conversations in order to avoid user
  confusion.  As a pleasant side-effect, information about the current
  state of a conversation is shared between all of a user's clients
  that implement this protocol.</p>
</section1>

<section1 topic='Requirements' anchor='reqs'>
  <ul>
    <li>Large changes SHOULD NOT be required to existing servers</li>
    <li>Clients that do not implement the new protocol MUST be able
    participate in conversations</li>
    <li>Clients that do not implement the new protocol MUST NOT
    receive a large number of new partial conversations</li>
    <li>Clients that do not implement the new protocol MUST NOT
    receive protocol they do not expect</li>
    <li>All clients that turn on the new protocol MUST be able to see
    all inbound chat-type messages.</li>
    <li>All clients that turn on the new protocol MUST be able to see
    all outbound chat-type messages from all of the resources of the
    user, regardless of whether the clients for the other resources
    have implemented the new protocol.</li>
  </ul>
</section1>
<section1 topic='Use Cases' anchor='usecases'>
  <section2 topic='Discovering Support' anchor='disco'>
    <p>If a server implements the Message Carbons capability, it MUST specify the
    'urn:xmpp:carbons:0' feature in its service discovery
    information features as specified in &xep0030; or section 6.3 of &xep0115;.
    Clients MUST NOT enable or disable Carbons if their server does
    not support this feature.</p>
    <example caption='Client requests information about its own server'><![CDATA[
<iq type='get'
    from='romeo@montague.net/orchard'
    id='info1'>
  <query xmlns='http://jabber.org/protocol/disco#info'/>
</iq>]]></example>
    <example caption='Server responds with Carbons feature'><![CDATA[
<iq type='get'
    to='romeo@montague.net/home'
    from='montague.net'
    id='info1'>
  <query xmlns='http://jabber.org/protocol/disco#info'>
...
    <feature var='urn:xmpp:carbons:0'/>
...
  </query>
</iq>]]></example>
  </section2>

  <section2 topic='Enabling Carbons' anchor='enabling'>
    <p>Servers MUST NOT enable the Carbons protocol for a client by
    default, since unmodified clients might be confused by the new
    protocol.  When a client wants to participate in the Carbons
    protocol, it sends an IQ set to enable the protocol.</p>
    <example caption='Client enables Carbons'><![CDATA[
<iq type='set' id='enable1'>
  <carbons var='urn:xmpp:carbons:0' mode='enable'/>
</iq>]]></example>
    <p>Carbons will generally be enabled before the client sends the
    first undirected presence, to ensure that all inbound messages
    will be delivered according to the Carbon rules.  The server will
    respond with an IQ result when Carbons are enabled:</p>
    <example caption='Server acknowledges Carbons enablement'><![CDATA[
<iq type='result' id='enable1'/>]]></example>
  </section2>

  <section2 topic='Disabling Carbons' anchor='disabling'>
    <p>Some clients might want to disable Carbons.  An example of this
    might be a mobile client that wants Carbons when the application
    is in the foreground, and disabled when it is in the background.
    To disable Carbons, clients send an IQ set:</p>
    <example caption='Client disables Carbons'><![CDATA[
<iq type='set' id='disable1'>
  <carbons var='urn:xmpp:carbons:0' mode='disable'/>
</iq>]]></example>
    <p>The server will respond with an IQ result when Carbons are disabled:</p>
    <example caption='Server acknowledges Carbons enablement'><![CDATA[
<iq type='result' id='disable1'/>]]></example>
  </section2>

  <section2 topic='Error Cases' anchor='errors'>
    <p>Enabling or disabling Carbons may fail in the several ways.  If
    one of these errors is returned, the server MUST keep the previous
    state, where the initial state is Carbons disabled.  For example,
    if the first enable returns an error, the server MUST NOT enable
    Carbons.</p>
    <section3 topic='bad-request' anchor='bad-request'>
      <p>The sender has sent a stanza containing XML that does not
      conform to the appropriate schema or that cannot be processed.
      One example is an IQ stanza that includes an unrecognized value
      of the 'type' attribute.  Another is changing to the state that
      is already in effect (enabling Carbons a second time).</p>
      <example caption='Error: bad-request'><![CDATA[<iq id='enable1'
    type='error'>
  <error type='modify'>
    <bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
  </error>
</iq>]]></example>
    </section3>
    <section3 topic='feature-not-implemented' anchor='feature-not-implemented'>
      <p>The sender has sent an enable or disable request to a server
      that does not support the protocol.  This SHOULD NOT happen in
      practice, because clients MUST check for server support before
      sending their request.</p>
      <example caption='Error: feature-not-implemented'><![CDATA[<iq id='enable1'
    type='error'>
  <error type='cancel'>
    <feature-not-implemented xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
  </error>
</iq>]]></example>
    </section3>
    <section3 topic='forbidden'  anchor='forbidden'>
      <p>The sender does is forbidden by policy from enabling or
      disabling Carbons.</p>
      <example caption='Error: forbidden'><![CDATA[<iq id='enable1'
    type='error'>
  <error type='auth'>
    <forbidden xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
  </error>
</iq>]]></example>
    </section3>
    <section3 topic='not-allowed' anchor='not-allowed'>
      <p>The receiver does not allow any entity to turn on Carbons.
      This might occur in a multi-domain deployment, where
      administrators of one domain allow Carbons, but another does
      not.</p>
      <example caption='Error: not-allowed'><![CDATA[<iq id='enable1'
    type='error'>
  <error type='cancel'>
    <not-allowed xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
  </error>
</iq>]]></example>
    </section3>
  </section2>

  <section2 topic='Inbound Messages' anchor='inbound'>
    <p>Messages of type chat that are addressed to the bare JID
    (localpart@domain) MUST be copied by the receiving server to all of the
    resources for that user that have non-negative presence priority
    and have not filtered messages through some other means.  The
    process of making copies is known as "forking."  The receiving
    server SHOULD NOT modify the 'to' address of the forked
    messages.</p>

    <example caption='Juliet sends Romeo an undirected message, which is forked'><![CDATA[
<message
    from='juliet@example.com/balcony'
    to='romeo@example.net'
    type='chat'>
  <body>Wherefore art thou, Romeo?</body>
  <thread>0e3141cd80894871a68e6fe6b1ec56fa</thread>
</message>

... each of romeo@example.net's resources receives this stanza verbatim
]]></example>

    <p>Messages of type "chat" that are addressed to a full JID
    (localpart@domain/resource) MUST be sent by the receiving server to the
    addressed resource, and MUST also be sent to all of the
    Carbons-enabled resources for the receiving user.  The goal of
    the copies to Carbons-enabled resources is to allow those clients
    to have both halves of *all* IM conversations, including messages
    that are sent from clients that lock in to particular resources.</p>
  </section2>

  <section2 topic='Sending Messages' anchor='sending'>
    <p>Once most of the clients that are deployed have implemented
    Carbons, clients MAY choose to always send chat type messages to
    the bare JID.  Until then, traditional resource locking is
    RECOMMENDED. (Note: another XEP might be written to document
    traditional resource locking, if the documentation in &rfc3921bis;
    is not sufficient.)</p>

    <p>Also note that &xep0085; recommends sending chat state
    notifications as chat type messages, which means that they will be
    subject to Carbon-copying.  This is intentional.</p>
  </section2>

  <section2 topic='Outbound Message Carbon Copies' anchor='outbound'>
    <p>Carbons clients want to have copies of messages going in
    <em>both</em> directions for other resources associated with the
    same user.  To that end, messages of type chat that are sent from
    any resource MUST be copied by the sending server to each of the
    resources that have enabled Carbons, but are not the sending
    resource.  Note that the 'to' address will be the original target of
    the message (bare JID, as above), and the 'from' address will
    contain the full JID (localpart@domain/resource) of the sending
    resource.  The 'to' address not matching the JID of the session is
    somewhat unprecedented in XMPP, which is why Carbons must be
    explicitly enabled.</p>

    <p>Messages that have carbon copies sent back to Carbons-enabled
    resources MUST NOT be copied back to the originating client.  The
    copies MUST have the full JID (localpart@domain/resource) of the sender
    as the 'from' address.  The copies MUST include a sent element in
    the urn:xmpp:carbons:0 namespace.</p>

    <example caption='Romeo responds to Juliet, which is Carboned'><![CDATA[
<message
    to='juliet@example.com/balcony'
    from='romeo@example.net/home'
    type='chat'>
  <body>Neither, fair saint, if either thee dislike.</body>
  <thread>0e3141cd80894871a68e6fe6b1ec56fa</thread>
</message>]]></example>

    <example caption='Romeo&apos;s OTHER Carbons-enabled clients
                      receive a copy'><![CDATA[
<message
    to='juliet@example.com/balcony'
    from='romeo@example.net/home'
    type='chat'>
  <body>Neither, fair saint, if either thee dislike.</body>
  <thread>0e3141cd80894871a68e6fe6b1ec56fa</thread>
  <sent xmlns='urn:xmpp:carbons:0'/>
</message>]]></example>
  </section2>

  <section2 topic='Avoiding Carbons for a single message' anchor='avoiding'>
    <p>Some clients might want to avoid carbons on a single message,
    while still keeping all of the other semantics of Carbon support.
    This might be useful for clients sending end-to-end encrypted
    messages, for example.</p>

    <p>To avoid a message being Carbon-copied to its other resources,
    the sending client MUST add a private element in the
    urn:xmpp:carbons:0 namespace.  When the sending server receives
    the message, it MUST NOT make carbon copies to the other
    Carbons-enabled resources, and MUST remove the private element
    before forwarding the message to the intended recipient.</p>

    <p>Note: use of the private mechanism will lead to partial
    conversations on other devices.  This is the intended effect.</p>

    <example caption='Romeo sends to Juliet, avoiding Carbon copies'><![CDATA[
<message
    to='juliet@example.com'
    from='romeo@example.net/home'
    type='chat'>
  <body>Neither, fair saint, if either thee dislike.</body>
  <thread>0e3141cd80894871a68e6fe6b1ec56fa</thread>
  <private xmlns='urn:xmpp:carbons:0'/>
</message>]]></example>

    <example caption='Romeo&apos;s server removes the private tag before forwarding, and does NOT send carbon copies to Romeo&apos;s other resources'><![CDATA[
<message
    to='juliet@example.com'
    from='romeo@example.net/home'
    type='chat'>
  <body>Neither, fair saint, if either thee dislike.</body>
  <thread>0e3141cd80894871a68e6fe6b1ec56fa</thread>
</message>]]></example>
  </section2>
</section1>
<section1 topic='Business Rules' anchor='rules'>
  <section2 topic='Interaction with Chat States' anchor='chatstates'>
    <p>Clients that implement Carbons MAY take special use of
    &xep0085; notifications.</p>
    <p>It is RECOMMENDED that upon receiving an outbound <em>gone</em>
    chat state (as a carbon copy) for a given conversation, that
    conversation be removed from user display as if the user on the
    copied client had terminated the conversation.  In order to
    prevent unwanted termination of conversations on other resources,
    clients SHOULD NOT send <em>gone</em> chat states on logout, but
    instead SHOULD count on the unavailable presence to convey the change
    in attention.</p>
    <p>Upon receiving an outbound notification of any chat state other
    than <em>gone</em>, the copied client MAY conclude that the
    sending client has taken responsibility for the conversation, and
    make appropriate user interface modifications.  For example,
    notifications could be muted on non-primary devices.</p>
  </section2>
  <section2 topic='Handling of errors' anchor='errors'>
    <p>When a receiving server attempts to deliver a forked message,
    and that message bounces with an error for any reason, the
    receiving server MUST NOT forward that error back to the original
    sender.  The receiving server SHOULD use the sent element in the
    bounce to determine that an error is from a forked message.</p>
    <p>This rule is used to prevent some of the half-failure modes
    that have been an issue in other prototocols.</p>
  </section2>
  <section2 topic='Auto-responses' anchor='auto-responses'>
    <p>Clients that automatically respond to messages for any reason
    (e.g. when in DND presence state) MUST take adequate care when
    enabling Carbons in order to prevent storms or loops.  Carbon
    copies of outbound messages MUST NOT be auto-replied to under any
    circumstances.  Forked inbound messages SHOULD NOT be auto-replied
    to, unless the client has some way of knowing that the receiver
    will not receive more than one auto-reply from other similar
    clients for the same user.</p>
  </section2>
  <section2 topic='Mobile Considerations' anchor='mobile'>
    <p>Since mobile devices often must pay for network traffic based
    on usage, the enablement of Carbons for such devices should be
    undertaken advisedly.  More complicated mechanisms for controlling
    the Carbon-copying or forking of individual conversations may need
    to be added to deal with mobile clients in the future.</p>
  </section2>
</section1>
<section1 topic='Security Considerations' anchor='security'>
  <p>The security model assumed by this document is that all of the
  resources for a single user are in the same trust boundary.</p>
  <p>Outbound chat messages that are encrypted end-to-end are not often
  useful to receive on other resources.  As such, they should use the
  private element specified above to avoid such copying, unless the
  encryption mechanism is adjusted to have knowledge of Carbons.</p>
</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:carbons: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:carbons:0'
    xmlns='urn:xmpp:carbons:0'
    elementFormDefault='qualified'>

  <xs:element name='carbons'>
    <xs:complexType>
      <xs:simpleContent>
        <xs:extension base='empty'>
          <xs:attribute name='mode' use='required'>
            <xs:simpleType>
              <xs:restriction base="xs:string">
                <xs:enumeration value="enable"/>
                <xs:enumeration value="disable"/>
              </xs:restriction>
            </xs:simpleType>
          </xs:attribute>
        </xs:extension>
      </xs:simpleContent>
    </xs:complexType>
  </xs:element>

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

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

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

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