path: root/docs/tubes.xml

<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE xep [
  <!ENTITY NS_TUBES "http://telepathy.freedesktop.org/xmpp/tubes">
]>
<?xml-stylesheet type='text/xsl' href='xep.xsl'?>
<xep>
<header>
  <title>Tubes over XMPP</title>
  <abstract>A protocol for extensible collaboration over XMPP, designed
    specifically to implement Telepathy Tubes.</abstract>
  <legal>This document is copyright 2007 Collabora Ltd. and may be
    distributed under the same terms as the Telepathy specification.</legal>
  <number>proto-tubes</number>
  <status>ProtoXEP</status>
  <type>External extension</type>
  <sig>Telepathy project</sig>
  <approver>Telepathy project</approver>
  <dependencies>
    <spec>XMPP Core</spec>
    <spec>XEP-0045</spec>
    <spec>XEP-0047</spec>
    <spec>XEP-0095</spec>
    <spec>XEP-proto-muc-bytestream</spec>
  </dependencies>
  <supersedes/>
  <supersededby/>
  <shortname>NOT YET ASSIGNED</shortname>
  <author>
    <firstname>Simon</firstname>
    <surname>McVittie</surname>
    <email>simon.mcvittie@collabora.co.uk</email>
    <jid>simon.mcvittie@collabora.co.uk</jid>
  </author>
  <revision>
    <version>0.0.1</version>
    <date>2007-09-07</date>
    <initials>smcv</initials>
    <remark><p>First draft.</p></remark>
  </revision>
</header>
<section1 topic='Introduction' anchor='intro'>
  <p>The XML namespace defined here is &NS_TUBES; (NS_TUBES in the
    telepathy-gabble source code).</p>
</section1>
<section1 topic='Requirements' anchor='reqs'>
  <p>The Telepathy project defines an API called Tubes to be used by
    arbitrary collaborative applications to communicate with instant
    messaging contacts. This API is intended to be the same for several
    IM protocols.</p>

  <p>Tubes can be used in two types of context, referred to in Telepathy
    as Tubes channels:</p>

  <ul>
    <li>In a multi-user chat room (on XMPP this means XEP-0045) - each
      MUC may have a tubes channel</li>
    <li>On a peer-to-peer basis - each pair of contacts may have a Tubes
      channel</li>
  </ul>

  <p>Each tube has a service name, used to dispatch it to an appropriate
    client application, and some essentially arbitrary parameters understood
    by that client application. The tube's initiator (the contact who
    started it) must also be tracked.</p>

  <p>The following tube types are currently supported:</p>

  <ul>

    <li>D-Bus - a D-Bus message stream. In a MUC context this behaves like
      the D-Bus bus daemon, so participants can send each other private
      messages, or can send broadcast messages which are received by everyone
      in the MUC (including themselves). Each MUC participant has a D-Bus
      unique name. In a 1-1 context this behaves like a peer-to-peer
      D-Bus connection - arbitrary D-Bus messages can be sent by each
      participant and are received by the other participant.</li>

    <li>Stream - a TCP-like bytestream. The initiator exports a service
      using a listening socket. The other participant(s) can open one or more
      reliable stream connections which are forwarded through the instant
      messaging protocol as (possibly out-of-band) bytestreams.</li>
  </ul>

  <p>As well as sending and receiving data, the following operations are
    supported:</p>

  <ul>
    <li>List the tubes available in this Tubes channel, including their
      initiator, type, service, parameters and state</li>
    <li>List the unique-names corresponding to the contacts (JIDs) in a
      D-Bus tube, if it's in a MUC channel</li>
  </ul>
</section1>
<section1 topic='Use Cases' anchor='usecases'>
  <section2 topic='In a multi-user chat' anchor='usecases-muc'>
    <example caption='Offering a new D-Bus tube'>
      <![CDATA[
      <!-- The first witch offers a D-Bus tube to the service
      lit.macbeth.BubblingCauldron.

      The tube parameters provided via the Telepathy API are (in dbus-python
      notation):

      {
        's': 'hello',                     # string
        'ay': dbus.ByteArray('hello'),    # byte-array
        'u': dbus.UInt32(123),            # unsigned Int32
        'i': dbus.Int32(-123),            # signed Int32
      }

      See below for how the dbus-name is generated.
      -->
      <presence to='darkcave@conf.macbeth.lit'
          from='darkcave@conf.macbeth.lit/FirstWitch'>
        <x xmlns='http://jabber.org/protocol/muc'>
        </x>
        <tubes xmlns='http://telepathy.freedesktop.org/xmpp/tubes'>
          <tube type='dbus' initiator='darkcave@conf.macbeth.lit/FirstWitch'
            service='lit.macbeth.BubblingCauldron' stream-id='12345'
            id='54321' dbus-name=':2.Rmlyc3RXaXRjaAAA'>
            <parameters>
              <parameter name='s' type='str'>hello</parameter>
              <parameter name='ay' type='bytes'>aGVsbG8=</parameter>
              <parameter name='u' type='uint'>123</parameter>
              <parameter name='i' type='int'>-123</parameter>
            </parameters>
          </tube>
        </tubes>
      </presence>
      ]]>
    </example>

    <example caption='Accepting (joining) a D-Bus tube'>
      <![CDATA[
      <!-- The second witch accepts the tube by copying it into her own
      MUC presence, changing the dbus-name to an arbitrary value
      that nobody else is using. -->
      <presence to='darkcave@conf.macbeth.lit'
          from='darkcave@conf.macbeth.lit/Second witch'>
        <x xmlns='http://jabber.org/protocol/muc'>
        </x>
        <tubes xmlns='http://telepathy.freedesktop.org/xmpp/tubes'>
          <tube type='dbus' initiator='darkcave@conf.macbeth.lit/FirstWitch'
            service='lit.macbeth.BubblingCauldron' stream-id='12345'
            id='54321' dbus-name=':2.U2Vjb25kIHdpdGNo'>
            <parameters>
              <parameter name='s' type='str'>hello</parameter>
              <parameter name='ay' type='bytes'>aGVsbG8=</parameter>
              <parameter name='u' type='uint'>123</parameter>
              <parameter name='i' type='int'>-123</parameter>
            </parameters>
          </tube>
        </tubes>
      </presence>
      ]]>
    </example>

    <example caption='Sending data in a MUC D-Bus tube'>
      <![CDATA[
      <!-- The second witch sends a D-Bus signal to everyone.
      MUC D-Bus tubes always use MUC Bytestreams (XEP-proto-muc-bytestream).

      Messages can also go to a particular participant (for method calls and
      replies), and can be split up if too big - see the MUC Bytestreams
      document for details.

      In the serialized D-Bus message, the sender field will be
      ":2.U2Vjb25kIHdpdGNo". If the message was being sent to the first witch only
      rather than to all participants, its destination field would be
      ":2.Rmlyc3RXaXRjaAAA".

      The 'sid' here is the same as the 'stream-id' in the <tube> announcement.
      -->
      <message to='darkcave@conf.macbeth.lit'
          from='darkcave@conf.macbeth.lit/Second witch'>
          <data xmlns='http://telepathy.freedesktop.org/xmpp/protocol/muc-bytestreams'
            sid="12345">base64base64...</data>
      </message>
      ]]>
    </example>

    <example caption='Offering a new stream tube'>
      <![CDATA[
      <!-- The second witch also offers a stream tube to her WebDAV server. -->
      <presence to='darkcave@conf.macbeth.lit'
          from='darkcave@conf.macbeth.lit/Second witch'>
        <x xmlns='http://jabber.org/protocol/muc'>
        </x>
        <tubes xmlns='http://telepathy.freedesktop.org/xmpp/tubes'>
          <!-- This D-Bus tube is still here from the first example -
            it's not part of the stream tube -->
          <tube type='dbus' initiator='darkcave@conf.macbeth.lit/FirstWitch'
            service='lit.macbeth.BubblingCauldron' stream-id='12345'
            id='54321' dbus-name=':2.U2Vjb25kIHdpdGNo'>
            <parameters>
              <parameter name='s' type='str'>hello</parameter>
              <parameter name='ay' type='bytes'>aGVsbG8=</parameter>
              <parameter name='u' type='uint'>123</parameter>
              <parameter name='i' type='int'>-123</parameter>
            </parameters>
          </tube>

          <!-- This is the new stream tube. There is no initiator attribute
          because the initiator is implicitly the participant whose presence
          it's seen in -->
          <tube type='stream' service='webdav' id='666'>
            <parameters>
              <parameter name='u' type='str'>anonymous</parameter>
              <parameter name='p' type='str'>password</parameter>
              <parameter name='path' type='str'>/plots/macbeth</parameter>
            </parameters>
          </tube>
        </tubes>
      </presence>
      ]]>
    </example>

    <example caption='Accepting (joining) a stream tube'>
      <![CDATA[
      <!-- The first witch makes a connection to the second witch to
      open the stream. Data transfer proceeds as usual according to
      Stream Initiation - in this case, they'll use IBB.

      The child element is <muc-stream> and not <stream> in order to
      disambiguate between SI-based streams in a MUC Tubes channel, and
      SI-based streams in a 1-1 channel between two members of a MUC.

      The first witch MUST NOT copy the tube into her own presence
      in this case - only the initiator should put stream tubes in their
      presence, since stream tubes cannot continue after the initiator leaves.
      -->
      <iq from='darkcave@conf.macbeth.lit/FirstWitch'
          to='darkcave@conf.macbeth.lit/Second witch' type='set'>
        <si xmlns='http://jabber.org/protocol/si' id='55555'
          profile='http://telepathy.freedesktop.org/xmpp/tubes'>
          <muc-stream
            xmlns='http://telepathy.freedesktop.org/xmpp/tubes'
            tube='666' />
          <feature xmlns='http://jabber.org/protocol/feature-neg'>
            <x xmlns="jabber:x:data" type="form">
              <field var='stream-method' type='list-single'>
                <option>
                  <value>http://jabber.org/protocol/ibb</value>
                </option>
              </field>
            </x>
          </feature>
        </si>
      </iq>

      <!-- The second witch acknowledges the IQ and data transfer proceeds
      using stream ID 55555 (not shown here, but it's the same as for normal
      IBB-over-SI). -->
      ]]>
    </example>
    <example caption='Leaving a tube explicitly'>
      <![CDATA[
      <!-- The second witch stops offering the stream tube.
      She SHOULD close the stream first, if possible - assume this has
      already been done. -->
      <presence to='darkcave@conf.macbeth.lit'
          from='darkcave@conf.macbeth.lit/Second witch'>
        <x xmlns='http://jabber.org/protocol/muc'>
        </x>
        <tubes xmlns='http://telepathy.freedesktop.org/xmpp/tubes'>
          <tube type='dbus' initiator='darkcave@conf.macbeth.lit/FirstWitch'
            service='lit.macbeth.BubblingCauldron' stream-id='12345'
            id='54321' dbus-name=':2.Rmlyc3RXaXRjaAAA'>
            <parameters>
              <parameter name='s' type='str'>hello</parameter>
              <parameter name='ay' type='bytes'>aGVsbG8=</parameter>
              <parameter name='u' type='uint'>123</parameter>
              <parameter name='i' type='int'>-123</parameter>
            </parameters>
          </tube>
        </tubes>
      </presence>
      ]]>
    </example>
    <p>Leaving the MUC also implicitly implies leaving all tubes.</p>
  </section2>
  <section2 topic='Peer to peer' anchor='usecases-p2p'>
    <example caption='Offering a new D-Bus tube'>
      <![CDATA[
      <!-- Romeo offers Tybalt a D-Bus tube to the (hypothetical) service
      com.example.Chess.

      The tube parameters provided via the Telepathy API are (in dbus-python
      notation):

      {
        'theme': 'montagues-vs-capulets.conf'   # string
      }
      -->
      <iq to='tybalt@capulet.lit/Work'
          from='romeo@montague.lit/Home' type='set'>
        <si xmlns='http://jabber.org/protocol/si' id='chess-stream-1'
            profile='http://telepathy.freedesktop.org/xmpp/tubes'>
          <tube xmlns='http://telepathy.freedesktop.org/xmpp/tubes'>
              type='dbus' service='com.example.Chess' id='333'>
            <parameters>
              <parameter name='theme'
                type='str'>montagues-vs-capulets.conf</parameter>
            </parameters>
          </tube>
          <feature xmlns='http://jabber.org/protocol/feature-neg'>
            <x xmlns="jabber:x:data" type="form">
              <field var='stream-method' type='list-single'>
                <option>
                  <value>http://jabber.org/protocol/ibb</value>
                </option>
              </field>
            </x>
          </feature>
        </si>
      </iq>
      ]]>
    </example>
    <example caption='Accepting (joining) a D-Bus tube'>
      <![CDATA[
      <!-- Tybalt accepts the tube by accepting the SI -->
      <iq to='romeo@montague.lit/Home'
          from='tybalt@capulet.lit/Work' type='result'>
        <si xmlns='http://jabber.org/protocol/si'>
          <feature xmlns='http://jabber.org/protocol/feature-neg'>
            <x xmlns='jabber:x:data' type='submit'>
              <field var='stream=method'>
                <value>http://jabber.org/protocol/ibb</value>
              </field>
            </x>
          </feature>
        </si>
      </iq>

      <!-- Messages are now sent using whatever stream protocol was
      negotiated, in this case IBB. Implementations MUST NOT assume that
      message boundaries are preserved. Implementations MAY preserve message
      boundaries.
      -->
      ]]>
    </example>
    <example caption='Offering a new stream tube'>
      <![CDATA[
      <!-- Romeo offers Juliet a D-Bus tube to a private web server

      The tube parameters provided via the Telepathy API are (in dbus-python
      notation):

      {
        'path': '/poetry/for-juliet/'   # string
      }
      -->
      <message to='juliet@capulet.lit/Balcony'
          from='romeo@montague.lit/Home'>
        <tube xmlns='http://telepathy.freedesktop.org/xmpp/tubes'
            type='stream' service='http' id='1402'>
          <parameters>
            <parameter name='path' type='str'>/poetry/for-juliet/</parameter>
          </parameters>
        </tube>
      </message>
      ]]>
    </example>
    <example caption='Accepting (joining) a stream tube'>
      <![CDATA[
      <!-- Juliet's web browser connects to Juliet's XMPP implementation,
      causing a stream to be opened -->
      <iq to='romeo@montague.lit/Home'
          from='juliet@capulet.lit/Balcony' type='set'>
        <si xmlns='http://jabber.org/protocol/si' id='http-stream-1a'
            profile='http://telepathy.freedesktop.org/xmpp/tubes'>
          <stream xmlns='http://telepathy.freedesktop.org/xmpp/tubes'
            tube='1402'/>
          <feature xmlns='http://jabber.org/protocol/feature-neg'>
            <x xmlns="jabber:x:data" type="form">
              <field var='stream-method' type='list-single'>
                <option>
                  <value>http://jabber.org/protocol/ibb</value>
                </option>
              </field>
            </x>
          </feature>
        </si>
      </iq>
      <!-- Romeo's XMPP implementation accepts the incoming connection
      and routes it to his web server -->
      <iq to='juliet@capulet.lit/Balcony'
          from='romeo@montague.lit/Home' type='result'>
          <si xmlns='http://jabber.org/protocol/si'>
            <feature xmlns='http://jabber.org/protocol/feature-neg'>
              <x xmlns='jabber:x:data' type='submit'>
                <field var='stream=method'>
                  <value>http://jabber.org/protocol/ibb</value>
                </field>
              </x>
            </feature>
        </si>
      </iq>

      <!-- Messages are now sent using whatever stream protocol was
      negotiated, in this case IBB. -->
      ]]>
    </example>
    <p>If Juliet's web browser opens multiple parallel connections to her
      XMPP implementation, the implementation will open multiple parallel
      SI streams as shown above, and Romeo's XMPP implementation will open
      multiple parallel connections to his web server. This MUST be
      supported by implementations.</p>
    <example caption='Leaving a tube (initiator)'>
      <![CDATA[
      <!-- Romeo stops offering the stream tube.
      He SHOULD close all streams first. If not, Juliet SHOULD
      close all streams when this message is received. -->
      <message to='juliet@capulet.lit/Balcony'
          from='romeo@montague.lit/Home'>
        <close xmlns='http://telepathy.freedesktop.org/xmpp/tubes'
          tube='1402'/>
      </message>
      ]]>
    </example>
    <example caption='Leaving a tube (non-initiator)'>
      <![CDATA[
      <!-- Alternatively, Juliet could reject the stream tube.
      She SHOULD close all streams first. If not, Romeo SHOULD close all
      streams in response to this message. -->
      <message from='juliet@capulet.lit/Balcony'
          to='romeo@montague.lit/Home'>
        <close xmlns='http://telepathy.freedesktop.org/xmpp/tubes'
          tube='1402'/>
      </message>
      ]]>
    </example>
  </section2>
</section1>
<section1 topic='Business Rules' anchor='rules'>
  <section2 topic='D-Bus unique names in a MUC context'>
    <p>The D-Bus unique names used in the MUC SHOULD be ":2." followed by the
      MUC nick (i.e. the resource part of the MUC JID) processed with the
      following algorithm:</p>

    <ul>
      <li>Let <i>utf8nick</i> be the MUC nick encoded using UTF-8</li>
      <li>If <i>utf8nick</i> has 186 octets or fewer, encode it using
        Base64; otherwise encode a string consisting of the first 169
        octets of <i>utf8nick</i>, followed by the 20 octets of the
        SHA-1 of <i>utf8nick</i> (note that this is the whole of the raw
        binary SHA-1, and not the first half of the SHA-1 written in
        hexadecimal).</li>
      <li>Remove all whitespace</li>
      <li>Replace "+" with "_", "/" with "-" and "=" with "A"</li>
    </ul>

    <p>This produces collision-free unique names of length no more than 251
      octets (4/3 * 186 + 3) for nicknames no longer than 186 UTF-8 octets
      (replacing "=" with "A" is harmless here because XMPP resources cannot
      contain U+0000).</p>

    <p>For longer nicknames it makes collisions highly unlikely, and produces
      255-octet unique names (4/3 * 189 + 3), which still fit into D-Bus'
      255-octet limit.</p>

    <p>The choice of 186 and 169 for the magic numbers ensures that these long
      nicknames can only have unique-name collisions with other long
      nicknames, and that the Base64 encoding step when applied to long
      nicknames does not leave any trailing "=" characters.</p>

    <p>Implementations MUST NOT use unique names that start with ":2." but are
      not formed according to the above rules, and SHOULD treat tube elements
      containing such a dbus-name as if they were invalid.</p>

    <p>For backwards compatibility with earlier implementations, the dbus-name
      MAY also be ":1." followed by an arbitrary unique string of up to 252
      characters chosen from A-Z, a-z, 0-9, "-" or "_".</p>
  </section2>
  <section2 topic='Definition of the tubes SI profile'>
    <p>The Tubes SI profile is indicated by the profile string &NS_TUBES;.
      A Tubes SI IQ can be classified as follows:</p>
    <ul>
      <li>If the IQ contains a &lt;tube&gt; element with the namespace
        &NS_TUBES;, then the IQ represents a new tube being offered.</li>

      <li>If the IQ contains a &lt;stream&gt; element with the namespace
        &NS_TUBES;, then the IQ represents a new connection to an existing
        stream tube in a 1-1 Tubes channel.</li>

      <li>If the IQ contains a &lt;muc-stream&gt; element with the namespace
        &NS_TUBES;, then the IQ represents a new connection to an existing
        stream tube in a MUC Tubes channel - this message type is invalid
        and must be rejected unless received via a MUC.</li>

      <li>Otherwise, the SI is invalid and MUST be rejected.</li>
    </ul>
  </section2>
  <section2 topic='Parameter types'>
    <p>The allowed parameter types are:</p>
    <ul>
      <li>"str": A Unicode string containing no zero bytes '\0'. In this
          implementation it cannot contain any other character forbidden
          by XML either. The &lt;parameter&gt; character content is the
          string itself. The corresponding D-Bus signature is 's'.
          This corresponds to xs:string in XML Schema Part 2.</li>
      <li>"bytes": An array of bytes, represented in Base64. The corresponding
          D-Bus signature is 'ay'. This corresponds to xs:base64Binary in
          XML Schema Part 2.</li>
      <li>"uint": An unsigned integer representable in 32 bits. The
          corresponding D-Bus signature is 'u' or 'q' on input, 'u' on output.
          The element's character content is an ASCII unsigned
          decimal integer. This corresponds to xs:unsignedInt in XML
          Schema Part 2.</li>
      <li>"int": A signed integer representable in 32 bits using two's
          complement. The corresponding D-Bus signature is 'i' or 'h' on input,
          'i' on output, and the element's character content is an ASCII
          decimal integer, possibly starting with '-' or '+'. This corresponds
          to xs:int in XML Schema Part 2.</li>
        <li>"bool": A boolean value represented as one of the strings "false",
          "true", "0" or "1", as per xs:boolean in XML Schema Part 2.
          The corresponding D-Bus signature is 'b'. (Implementation note:
          early versions of Tubes in Gabble only accepted "0" or "1", so
          implementations that care about backwards compatibility SHOULD emit
          those representations).</li>
    </ul>
  </section2>
  <section2 topic='TODO'>
    <p>FIXME: how big do we want to allow a message to be? In the D-Bus
      specification the limit is 2**27 bytes, in the default dbus-daemon
      configuration (as used on the system bus) it's 32M.</p>
    <p>FIXME: limit how many messages are queued up somehow (the system bus
      limits it to 127M in total)</p>
  </section2>
</section1>
<section1 topic='Security Considerations' anchor='security'>
  <p>FIXME</p>
</section1>
<section1 topic='IANA Considerations' anchor='iana'>
  <p>None.</p>
</section1>
<section1 topic='XMPP Registrar Considerations' anchor='registrar'>
  <p>None, this isn't a real XEP (yet?).</p>
</section1>
<section1 topic='XML Schema' anchor='schema'>
  <code><![CDATA[
    <xs:schema
      xmlns:xs='http://www.w3.org/2001/XMLSchema'
      targetNamespace='http://telepathy.freedesktop.org/xmpp/protocol/tubes'
      xmlns='http://telepathy.freedesktop.org/xmpp/protocol/tubes'
      elementFormDefault='qualified'>

      <!-- Container for <tube> elements in MUC <presence> -->
      <xs:element name='tubes'>
        <xs:complexType>
          <xs:sequence minOccurs='0'>
            <xs:element ref='tube' minOccurs='0' maxOccurs='unbounded'/>
          </xs:sequence>
        </xs:complexType>
      </xs:element>

      <!-- A tube offer -->
      <xs:element name='tube'>
        <xs:complexType>
          <!-- Required attributes -->
          <xs:attribute name='id' type='xs:unsignedInt' use='required'/>
          <xs:attribute name='type' use='required'>
            <xs:simpleType>
              <xs:restriction base='xs:NCName'>
                <xs:enumeration value='dbus'/>
                <xs:enumeration value='stream'/>
              </xs:restriction>
            </xs:simpleType>
          </xs:attribute>
          <xs:attribute name='service' type='xs:string' use='required'/>

          <!-- MUC D-Bus tube attributes -->
          <!-- MUST be present for MUC D-Bus tubes, SHOULD NOT be
          emitted for other types of tube, MUST be ignored for other types
          of tube -->
          <xs:attribute name='initiator' type='xs:string' use='optional'/>
          <xs:attribute name='dbus-name' type='xs:string' use='optional'/>
          <xs:attribute name='stream-id' type='xs:string' use='optional'/>

          <!-- Parameters -->
          <xs:element name='parameters'>
            <xs:sequence minOccurs='0'>
              <xs:element name='parameter' minOccurs='0'
                maxOccurs='unbounded' type='name-value-pair'/>
              </xs:element>
            </xs:sequence>
          </xs:element>
        </xs:complexType>
      </xs:element>

      <!-- A name-value pair -->
      <xs:complexType name='name-value-pair'>
        <xs:simpleContent>
          <xs:extension base='xs:string'>
            <xs:attribute name='name' type='xs:string'/>
            <xs:attribute name='type'>
              <xs:simpleType>
                <xs:restriction base='xs:NCName'>
                  <xs:enumeration value='bool'/>
                  <xs:enumeration value='str'/>
                  <xs:enumeration value='bytes'/>
                  <xs:enumeration value='int'/>
                  <xs:enumeration value='uint'/>
                </xs:restriction>
              </xs:simpleType>
            </xs:attribute>
          </xs:extension>
        </xs:simpleContent>
      </xs:complexType>

      <xs:complexType name='tube-reference'>
        <xs:simpleContent>
          <xs:extension base='empty'>
            <xs:attribute name='tube' use='required' type='xs:unsignedInt'/>
          </xs:extension>
        </xs:simpleContent>
      </xs:complexType>

      <xs:element name='close' type='tube-reference'/>

      <xs:element name='stream' type='tube-reference'/>

      <!-- only valid in a MUC -->
      <xs:element name='muc-stream' type='tube-reference'/>

      <xs:simpleType name='empty'>
        <xs:restriction base='xs:string'>
          <xs:enumeration value=''/>
        </xs:restriction>
      </xs:simpleType>
    </xs:schema>
  ]]></code>
</section1>
<section1 topic='Appendix: test data for D-Bus unique name algorithm'>
  <p>The string "short" produces the unique name ":2.c2hvcnQA".</p>
  <p>The string "FirstWitch", as used in the examples, produces the
    unique name ":2.Rmlyc3RXaXRjaAAA".</p>
  <p>The string "Second witch", as used in the examples, produces the
    unique name ":2.U2Vjb25kIHdpdGNo".</p>

  <p>The string consisting of 18 repetitions of "0123456789" followed by
    "012345" (186 characters) produces a unique name consisting of ":2.",
    followed by 6 repetitions of the 40-character string
    "MDEyMzQ1Njc4OTAxMjM0NTY3ODkwMTIzNDU2Nzg5", followed by "MDEyMzQ1".</p>

  <p>The string consisting of 18 repetitions of "0123456789" followed by
    "0123456" (187 characters) produces a unique name consisting of ":2.",
    followed by 5 repetitions of the 40-character string above, followed by
    "MDEyMzQ1Njc4OTAxMjM0NTY3OEVd9C5NgmmRD6jp1ftG6XUEc11x".</p>

  <p>The string consisting of 20 repetitions of "0123456789" (200 characters)
    produces a unique name consisting of ":2.", followed by 5 repetitions of
    the 40-character string above, followed by
    "MDEyMzQ1Njc4OTAxMjM0NTY3OO-utwRnwcoUFhnJVMKg5pm9Hxal".</p>
</section1>
</xep>