xeps/xep-0180.xml

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<!DOCTYPE xep SYSTEM 'xep.dtd' [
  <!ENTITY % ents SYSTEM 'xep.ent'>
%ents;
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<?xml-stylesheet type='text/xsl' href='xep.xsl'?>
<xep>
<header>
  <title>Jingle Video via RTP</title>
  <abstract>This document defines methods for negotiating Jingle video sessions that use the Real-time Transport Protocol (RTP) for media exchange.</abstract>
  &LEGALNOTICE;
  <number>0180</number>
  <status>Proposed</status>
  <type>Standards Track</type>
  <sig>Standards</sig>
  <approver>Council</approver>
  <dependencies>
    <spec>XMPP Core</spec>
    <spec>XEP-0166</spec>
  </dependencies>
  <supersedes/>
  <supersededby/>
  <shortname>TO BE ASSIGNED</shortname>
  &stpeter;
  <author>
    <firstname>Milton</firstname>
    <surname>Chen</surname>
    <email>Milton.Chen@vseelab.com</email>
  </author>
  <revision>
    <version>0.9</version>
    <date>2007-11-15</date>
    <initials>psa</initials>
    <remark><p>Editorial review and consistency check.</p></remark>
  </revision>
  <revision>
    <version>0.8</version>
    <date>2007-05-23</date>
    <initials>psa</initials>
    <remark><p>Corrected examples to use video codecs; added clockrate attribute.</p></remark>
  </revision>
  <revision>
    <version>0.7</version>
    <date>2007-05-23</date>
    <initials>psa</initials>
    <remark><p>More completely specified how to include SDP parameters and codec-specific parameters (same approach as in XEP-0167); added and corrected Theora examples.</p></remark>
  </revision>
  <revision>
    <version>0.6</version>
    <date>2007-04-17</date>
    <initials>psa</initials>
    <remark><p>Specified Jingle conformance, including the need to use a lossy transport and the process of sending and receiving video content.</p></remark>
  </revision>
  <revision>
    <version>0.5</version>
    <date>2007-03-23</date>
    <initials>psa</initials>
    <remark><p>Added negotiation flow and SDP mapping; renamed to mention RTP as the associated transport; corrected negotiation flow to be consistent with SIP/SDP (each party specifies a list of the payload types it can receive); added profile attribute to content element in order to specify RTP profile in use.</p></remark>
  </revision>
  <revision>
    <version>0.4</version>
    <date>2006-12-21</date>
    <initials>psa</initials>
    <remark><p>Modified spec to use provisional namespace before advancement to Draft (per XEP-0053).</p></remark>
  </revision>
  <revision>
    <version>0.3</version>
    <date>2006-08-23</date>
    <initials>psa</initials>
    <remark><p>Modified namespace to track XEP-0166.</p></remark>
  </revision>
  <revision>
    <version>0.2</version>
    <date>2006-07-12</date>
    <initials>psa</initials>
    <remark><p>Updated to use content type instead of media type.</p></remark>
  </revision>
  <revision>
    <version>0.1</version>
    <date>2006-03-23</date>
    <initials>psa/mc</initials>
    <remark><p>Initial version.</p></remark>
  </revision>
  <revision>
    <version>0.0.1</version>
    <date>2006-03-20</date>
    <initials>psa/mc</initials>
    <remark><p>First draft.</p></remark>
  </revision>
</header>
<section1 topic='Introduction' anchor='intro'>
  <p>&xep0166; can be used to initiate and negotiate a wide range of peer-to-peer sessions. One session type of interest is video chat. This document specifies a format for describing Jingle video sessions, where the media exchange occurs using the Real-time Transport Protocol (see &rfc3550;).</p>
</section1>

<section1 topic='Requirements' anchor='reqs'>
  <p>The Jingle application format defined herein is designed to meet the following requirements:</p>
  <ol>
    <li>Enable negotiation of parameters necessary for video chat.</li>
    <li>Map these parameters to the Session Description Protocol (SDP; see &rfc4566;) to enable interoperability.</li>
    <li>Define informational messages related to video chat.</li>
  </ol>
</section1>

<section1 topic='Jingle Conformance' anchor='conformance'>
   <p>In accordance with Section 8 of <cite>XEP-0166</cite>, this document specifies the following information related to the Jingle Video via RTP application type:</p>
   <ol>
     <li><p>The application format negotiation process is defined in the <link url='#negotiation'>Negotiating a Jingle Video Session</link> section of this document.</p></li>
     <li><p>The semantics of the &DESCRIPTION; element are defined in the <link url='#format'>Application Format</link> section of this document.</p></li>
     <li><p>A mapping of Jingle semantics to the Session Description Protocol is provided in the <link url='#sdp'>Mapping to Session Description Protocol</link> section of this document.</p></li>
     <li><p>A Jingle video session MUST use a lossy transport method such as &xep0177; or the "ice-udp" method specified in &xep0176;.</p></li>
     <li>
       <p>Content is to be sent and received as follows:</p>
       <ul>
         <li><p>Outbound video content shall be encoded into RTP packets and each packet shall be sent individually over the transport. Each inbound packet received over the transport is an RTP packet.</p></li>
       </ul>
     </li>
   </ol>
</section1>

<section1 topic='Application Format' anchor='format'>
  <p>A Jingle video session is described by a content type that contains one application format and one transport method. The application format consists of one or more encodings contained within a wrapper &DESCRIPTION; element qualified by the 'http://www.xmpp.org/extensions/xep-0180.html#ns' namespace &NSNOTE;. In the language of <cite>RFC 4566</cite> these encodings are payload-types; therefore, each &lt;payload-type/&gt; child element specifies an encoding that can be used for the video stream. In Jingle Video, these encodings are used in the context of RTP. The most common encodings for the Audio/Video Profile (AVP) of RTP are listed in &rfc3551; (these "static" types are reserved from payload ID 0 through payload ID 95), although other encodings are allowed (these "dynamic" types use payload IDs 96 to 127) in accordance with the dynamic assignment rules described in Section 3 of <cite>RFC 3551</cite>.</p>
  <example caption="Video application format"><![CDATA[
    <description xmlns='http://www.xmpp.org/extensions/xep-0180.html#ns'>
      <payload-type id='96' name='theora' clockrate='90000' height='720' width='1280'>
        <parameter name='delivery-method' value='inline'/>
        <parameter name='configuration' value='somebase16string'/>
        <parameter name='sampling' value='YCbCr-4:2:2'/>
      </payload-type>
      <payload-type id='28' name='nv' clockrate='90000'/>
      <payload-type id='25' name='CelB' clockrate='90000'/>
      <payload-type id='32' name='MPV' clockrate='90000'/>
    </description>
  ]]></example>
  <p>The &DESCRIPTION; element is intended to be a child of a &CONTENT; element as specified in <cite>XEP-0166</cite>.</p>
  <p>The encodings SHOULD be provided in order of preference by placing the most-preferred &PAYLOADTYPE; element as the first child of the &DESCRIPTION; element (etc.).</p>
  <p>The defined attributes of the &PAYLOADTYPE; element are as follows:</p>
  <table caption='Video Description Attributes'>
    <tr>
      <th>Attribute</th>
      <th>Description</th>
      <th>Datatype/Units</th>
      <th>Inclusion</th>
    </tr>
    <tr>
      <td>channels</td>
      <td>The number of channels (e.g., 2 for stereoscopic video)</td>
      <td>positiveInteger (defaults to 1)</td>
      <td>OPTIONAL</td>
    </tr>
    <tr>
      <td>clockrate</td>
      <td>The sampling frequency in Hertz</td>
      <td>positiveInteger</td>
      <td>RECOMMENDED</td>
    </tr>
    <tr>
      <td>height</td>
      <td>The vertical extent of the displayed video, in pixels</td>
      <td>positiveInteger</td>
      <td>RECOMMENDED</td>
    </tr>
    <tr>
      <td>id</td>
      <td>A unique identifier for the payload type</td>
      <td>positiveInteger</td>
      <td>REQUIRED</td>
    </tr>
    <tr>
      <td>layer</td>
      <td>The relationship of a layer to the "bottom" of the stack, where 0 = bottom (the first layer)</td>
      <td>nonNegativeInteger</td>
      <td>OPTIONAL</td>
    </tr>
    <tr>
      <td>name</td>
      <td>A name for the payload type</td>
      <td>string</td>
      <td>RECOMMENDED for static payload types, REQUIRED for dynamic payload types</td>
    </tr>
    <tr>
      <td>transparent</td>
      <td>Whether or not a layer is transparent</td>
      <td>boolean</td>
      <td>OPTIONAL</td>
    </tr>
    <tr>
      <td>width</td>
      <td>The horizontal extent of the displayed video, in pixels</td>
      <td>positiveInteger</td>
      <td>RECOMMENDED</td>
    </tr>
    <tr>
      <td>x</td>
      <td>The horizontal starting point of a tile, in pixels from the origin point</td>
      <td>positiveInteger</td>
      <td>OPTIONAL</td>
    </tr>
    <tr>
      <td>y</td>
      <td>The vertical starting point of a tile, in pixels from the origin point</td>
      <td>positiveInteger</td>
      <td>OPTIONAL</td>
    </tr>
  </table>
  <p>Each &lt;payload-type/&gt; element MAY contain one or more child elements that specify particular parameters related to the payload. For example, as described in &rtptheora;, the "configuration", "configuration-uri", "delivery-method", and "sampling" parameters may be specified in relation to usage of the Theora <note>See &lt;<link url='http://www.theora.org/'>http://www.theora.org/</link>&gt;.</note> codec. Where such parameters are encoded via the "fmtp" SDP attribute, they shall be represented in Jingle via the following format, where the &lt;parameter/&gt; element is a child of the &PAYLOADTYPE; element:</p>
  <code><![CDATA[
<parameter name='foo' value='bar'/>
  ]]></code>
  <p>Note: The parameter names are effectively guaranteed to be unique, since &IANA; maintains a registry of SDP parameters (see &lt;<link url='http://www.iana.org/assignments/sdp-parameters'>http://www.iana.org/assignments/sdp-parameters</link>&gt;).</p>
</section1>

<section1 topic='Negotiating a Jingle Video Session' anchor='negotiation'>
  <p>When the initiator sends a session-initiate stanza to the receiver, the &DESCRIPTION; element includes all of the payload types that the initiator can receive for Jingle video (each one encapsulated in a separate &PAYLOADTYPE; element):</p>
  <example caption="Initiation"><![CDATA[
<iq from='romeo@montague.net/orchard' 
    to='juliet@capulet.com/balcony' 
    id='jinglevideo1' 
    type='set'>
  <jingle xmlns='http://www.xmpp.org/extensions/xep-0166.html#ns'>
          action='session-initiate'
          initiator='romeo@montague.net/orchard'
          sid='v1d30k1ll3dth3r4d10st4r'>
    <content content='initiator' name='this-is-the-video-content' profile='RTP/AVP'>
      <description xmlns='http://www.xmpp.org/extensions/xep-0180.html#ns'>
        <payload-type id='96' name='theora' clockrate='90000' height='720' width='1280'>
          <parameter name='delivery-method' value='inline'/>
          <parameter name='configuration' value='somebase16string'/>
          <parameter name='sampling' value='YCbCr-4:2:2'/>
        </payload-type>
        <payload-type id='28' name='nv' clockrate='90000'/>
        <payload-type id='25' name='CelB' clockrate='90000'/>
        <payload-type id='32' name='MPV' clockrate='90000'/>
      </description>
      <transport xmlns='http://www.xmpp.org/extensions/xep-0176.html#ns-udp'/>
    </content>
  </jingle>
</iq>
  ]]></example>
  <p>Upon receiving the session-initiate stanza, the receiver determines whether it can provisionally accept the session and proceed with the negotiation. The general Jingle error cases are specified in <cite>XEP-0166</cite> and illustrated &xep0167;. In addition, the receiver must determine if it supports any of the payload types advertised by the initiator; if it supports none of the offered payload types, it must reject the session by returning a &notacceptable; error with a Jingle-Video-specific condition of &lt;unsupported-codecs/&gt;:</p>
  <example caption="Receiver does not support any of the codecs"><![CDATA[
<iq type='error' 
    from='juliet@capulet.com/balcony' 
    to='romeo@montague.net/orchard' 
    id='jingleaudio1'>
  <error type='cancel'>
    <not-acceptable xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
    <unsupported-codecs xmlns='http://www.xmpp.org/extensions/xep-0180.html#ns-errors'/>
  </error>
</iq>
  ]]></example>
  <p>If there is no error, the receiver provisionally accepts the session:</p>
  <example caption="Receiver provisionally accepts session"><![CDATA[
  <iq from='juliet@capulet.com/balcony' 
      to='romeo@montague.net/orchard' 
      id='jinglevideo1' 
      type='result' />
  ]]></example>
  <p>The receiver then should send a list of the payload types that it can receive via a Jingle "content-accept" (or "session-accept") action. The list that the receiver sends MAY include any payload types (not a subset of the payload types sent by the initiator) but SHOULD retain the ID numbers specified by the initiator. The order of the &PAYLOADTYPE; elements indicates the receiver's preferences, with the most-preferred types first.</p>
  <example caption="Receiver accepts content type"><![CDATA[
<iq from='juliet@capulet.com/balcony' 
    to='romeo@montague.net/orchard' 
    id='jinglevideo2' 
    type='set'>
  <jingle xmlns='http://www.xmpp.org/extensions/xep-0166.html#ns'>
          action='content-accept'
          initiator='romeo@montague.net/orchard'
          sid='v1d30k1ll3dth3r4d10st4r'>
    <content content='initiator' name='this-is-the-video-content' profile='RTP/AVP'>
      <description xmlns='http://www.xmpp.org/extensions/xep-0180.html#ns'>
        <payload-type id='96' name='theora' height='720' width='1280'>
          <parameter name='delivery-method' value='inline'/>
          <parameter name='configuration' value='somebase16string'/>
          <parameter name='sampling' value='YCbCr-4:2:2'/>
        </payload-type>
        <payload-type id='32' name='MPV' clockrate='90000'/>
        <payload-type id='33' name='MP2T' clockrate='90000'/>
      </description> 
      <transport xmlns='http://www.xmpp.org/extensions/xep-0176.html#ns-udp'/>
    </content>
  </jingle>
</iq>
  ]]></example>
  <p>The initiator acknowledges the 'content-accept' with an empty IQ result:</p>
  <example caption="Initiator acknowledges modified application type"><![CDATA[
  <iq from='romeo@montegue.net/orchard' 
      to='juliet@capulet.com/balcony' 
      id='jinglevideo2' 
      type='result'/>
  ]]></example>
  <p>After successful transport negotiation (not shown here), the receiver then accepts the session:</p>
  <example caption="Receiver definitively accepts The session"><![CDATA[
<iq type='set' 
    from='juliet@capulet.com/balcony' 
    to='romeo@montague.net/orchard' 
    id='accept1'>
  <jingle xmlns='http://www.xmpp.org/extensions/xep-0166.html#ns'
          action='session-accept'
          initiator='romeo@montague.net/orchard'
          responder='juliet@capulet.com/balcony'
          sid='v1d30k1ll3dth3r4d10st4r'>
    <content creator='initiator' name='this-is-the-video-content' profile='RTP/AVP'>
      <description xmlns='http://www.xmpp.org/extensions/xep-0180.html#ns'/>
      <transport xmlns='http://www.xmpp.org/extensions/xep-0176.html#ns-udp'>
        <candidate component='2'
                   foundation='1'
                   generation='0' 
                   ip='192.0.2.3' 
                   network='1'
                   port='45664'
                   priority='1107821052'
                   protocol='udp'
                   pwd='asd88fgpdd777uzjYhagZg'
                   type='srflx'
                   ufrag='8hhy'/>
      </transport>
    </content>
  </jingle>
</iq>
  ]]></example>
  <p>And the initiator acknowledges session acceptance:</p>
  <example caption="Initiator acknowledges session acceptance"><![CDATA[
  <iq from='romeo@montague.net/orchard' 
      to='juliet@capulet.com/balcony' 
      id='accept1' 
      type='result' />
  ]]></example>
  <p>Note: Because a "session-accept" action implicitly indicates acceptance of the application format (i.e., "content-accept"), it is not necessary to send a separate "content-accept" action. This flow is shown for completeness only.</p>
</section1>

<section1 topic='Mapping to Session Description Protocol' anchor='sdp'>
  <p>If the payload type is static (payload-type IDs 0 through 95 inclusive), it MUST be mapped to a media field defined in <cite>RFC 4566</cite>. The generic format for the media field is as follows:</p>
  <code><![CDATA[
m=<media> <port> <transport> <fmt list>
  ]]></code>
  <p>In the context of Jingle video sessions, the &lt;media&gt; is "video", the &lt;port&gt; is the preferred port for such communications (which may be determined dynamically), the &lt;transport&gt; is whatever profile is negotiated via the 'profile' attribute of the &CONTENT; element in the Jingle negotiation (e.g., "RTP/AVT"), and the &lt;fmt list&gt; is the payload-type ID.</p>
  <p>For example, consider the following static payload-type:</p>
  <example caption="Jingle format for static payload-type"><![CDATA[
<payload-type id="28" name="nv"/>
  ]]></example>
  <p>That Jingle-formatted information would be mapped to SDP as follows:</p>
  <example caption="SDP mapping of static payload-type"><![CDATA[
m=video 9000 RTP/AVP 28
  ]]></example>
  <p>If the payload type is dynamic (payload-type IDs 96 through 127 inclusive), it SHOULD be mapped to an SDP media field plus an SDP attribute field named "rtpmap".</p>
  <p>For example, consider a VC-1 payload such as that described in &rfc4425;:</p>
  <example caption="Jingle format for dynamic payload-type"><![CDATA[
<payload-type id='98' name='vc1' height='288' width='352'/>
  ]]></example>
  <p>That Jingle-formatted information would be mapped to SDP as follows:</p>
  <example caption="SDP mapping of dynamic payload-type"><![CDATA[
m=video 49170 RTP/AVP 98
a=rtpmap:98 vc1/90000
a=fmtp:98 width=352;height=288;
  ]]></example>
  <p>As noted, if additional parameters are to be specified, they shall be represented as attributes of the &lt;payload-type/&gt; element or its child &lt;parameter/&gt; element, as in the following example.</p>
  <example caption="Jingle format for dynamic payload-type with parameters"><![CDATA[
<payload-type id='96' name='theora' height='720' width='1280'>
  <parameter name='delivery-method' value='inline'/>
  <parameter name='configuration' value='somebase16string'/>
  <parameter name='sampling' value='YCbCr-4:2:2'/>
</payload-type>
  ]]></example>
  <p>That Jingle-formatted information would be mapped to SDP as follows:</p>
  <example caption="SDP mapping of dynamic payload-type with parameters"><![CDATA[
m=video 49170 RTP/AVP 98
a=rtpmap:96 theora/90000
a=fmtp:96 sampling=YCbCr-4:2:2; width=1280; height=720; 
delivery-method=inline; configuration=somebase16string;
  ]]></example>
</section1>

<section1 topic='Error Handling' anchor='errors'>
  <p>The Jingle-Video-specific error conditions are as follows:</p>
  <table caption='Other Error Conditions'>
    <tr>
      <th>Jingle Video Condition</th>
      <th>XMPP Condition</th>
      <th>Description</th>
    </tr>
    <tr>
      <td>&lt;unsupported-codecs/&gt;</td>
      <td>&notacceptable;</td>
      <td>The recipient does not support any of the offered video encodings.</td>
    </tr>
  </table>
</section1>

<section1 topic='Determining Support' anchor='support'>
  <p>If an entity supports Jingle video exchanges via RTP, it MUST advertise that fact by returning a feature of "http://www.xmpp.org/extensions/xep-0180.html#ns" in response to &xep0030; information requests &NSNOTE;.</p>
  <example caption="Service discovery information request"><![CDATA[
<iq from='romeo@montague.net/orchard' 
    id='disco1'
    to='juliet@capulet.com/balcony' 
    type='get'>
  <query xmlns='http://jabber.org/protocol/disco#info'/>
</iq>
  ]]></example>
  <example caption="Service discovery information response"><![CDATA[
<iq from='juliet@capulet.com/balcony' 
    id='disco1'
    to='romeo@montague.net/orchard' 
    type='result'>
  <query xmlns='http://jabber.org/protocol/disco#info'>
    ...
    <feature var='http://www.xmpp.org/extensions/xep-0166.html#ns'/>
    <feature var='http://www.xmpp.org/extensions/xep-0180.html#ns'/>
    ...
  </query>
</iq>
  ]]></example>
</section1>

<section1 topic='Informational Messages' anchor='info'>
  <p>Informational messages may be sent by either party within the context of Jingle to communicate the status of a Jingle video session, device, or principal. The informational message MUST be an IQ-set containing a &JINGLE; element of type "session-info". No informational message payload elements have yet been defined, but they may be specified in a future version of this document.</p>
</section1>

<section1 topic='Implementation Notes' anchor='impl'>
  <section2 topic='Codecs' anchor='impl-codecs'>
    <p>Support for the Theora codec is RECOMMENDED.</p>
  </section2>
</section1>

<section1 topic='Security Considerations' anchor='security'>
  <p>In order to secure the data stream, implementations SHOULD use encryption methods appropriate to the transport method and media being exchanged; for example, in the case of UDP, that would include Datagram Transport Layer Security (DTLS) as specified in &rfc4347;. &sdpdtls; defines such methods for the Session Description Protocol; the relevant RTP profile (e.g., "UDP/TLS/RTP/AVP" for transporting the RTP stream over DTLS with UDP) shall be specified as the value of the &CONTENT; element's 'profile' attribute.</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='ns'>
    <p>Until this specification advances to a status of Draft, its associated namespaces shall be "http://www.xmpp.org/extensions/xep-0180.html#ns" and "http://www.xmpp.org/extensions/xep-0180.html#ns-errors"; upon advancement of this specification, the &REGISTRAR; shall issue permanent namespaces in accordance with the process defined in Section 4 of &xep0053;.</p>
  </section2>
  <section2 topic='Jingle Application Formats' anchor='registrar-content'>
    <p>The XMPP Registrar shall include "video-rtp" in its registry of Jingle application formats. The registry submission is as follows:</p>
    <code><![CDATA[
<application>
  <name>video-rtp</name>
  <desc>Jingle sessions that support video exchange via the Real-time Transport Protocol</desc>
  <transport>lossy</transport>
  <doc>XEP-0180</doc>
</application>
    ]]></code>
  </section2>
</section1>

<section1 topic='XML Schemas' anchor='schema'>
  <section2 topic='Application Format' anchor='schema-content'>
    <code><![CDATA[
<?xml version='1.0' encoding='UTF-8'?>

<xs:schema
    xmlns:xs='http://www.w3.org/2001/XMLSchema'
    targetNamespace='http://www.xmpp.org/extensions/xep-0180.html#ns'
    xmlns='http://www.xmpp.org/extensions/xep-0180.html#ns'
    elementFormDefault='qualified'>

  <xs:element name='description'>
    <xs:complexType>
      <xs:sequence>
        <xs:element ref='payload-type' minOccurs='0' maxOccurs='unbounded'/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>

  <xs:element name='payload-type'>
    <xs:complexType>
      <xs:sequence minOccurs='0' maxOccurs='unbounded'>
        <xs:element ref='parameter'/>
      </xs:sequence>
      <xs:attribute name='channels' type='xs:integer' use='optional' default='1'/>
      <xs:attribute name='height' type='xs:nonNegativeInteger' use='optional'/>
      <xs:attribute name='id' type='xs:unsignedByte' use='required'/>
      <xs:attribute name='layer' type='xs:nonNegativeInteger' use='optional'/>
      <xs:attribute name='name' type='xs:string' use='optional'/>
      <xs:attribute name='rate' type='xs:short' use='optional'/>
      <xs:attribute name='transparent' type='xs:boolean' use='optional'/>
      <xs:attribute name='width' type='xs:nonNegativeInteger' use='optional'/>
      <xs:attribute name='x' type='xs:integer' use='optional'/>
      <xs:attribute name='y' type='xs:integer' use='optional'/>
    </xs:complexType>
  </xs:element>

  <xs:element name='parameter'>
    <xs:complexType>
      <xs:simpleContent>
        <xs:extension base='empty'>
          <xs:attribute name='name' type='xs:string' use='required'/>
          <xs:attribute name='value' type='xs:string' use='required'/>
        </xs:extension>
      </xs:simpleContent>
    </xs:complexType>
  </xs:element>

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

</xs:schema>
    ]]></code>
  </section2>
  <section2 topic='Errors' anchor='schema-errors'>
    <code><![CDATA[
<?xml version='1.0' encoding='UTF-8'?>

<xs:schema
    xmlns:xs='http://www.w3.org/2001/XMLSchema'
    targetNamespace='http://www.xmpp.org/extensions/xep-0180.html#ns-errors'
    xmlns='http://www.xmpp.org/extensions/xep-0180.html#ns-errors'
    elementFormDefault='qualified'>

  <xs:element name='unsupported-codecs' type='empty'/>

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

</xs:schema>
    ]]></code>
  </section2>
</section1>
</xep>