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<header>
  <title>Use of ZRTP in Jingle RTP Sessions</title>
  <abstract>This specification defines a Jingle application type for negotiating one or more sessions that use the Real-time Transport Protocol (RTP) to exchange media such as voice or video. The application type includes a straightforward mapping to Session Description Protocol (SDP) for interworking with SIP media endpoints.</abstract>
  &LEGALNOTICE;
  <number>xxxx</number>
  <status>ProtoXEP</status>
  <type>Standards Track</type>
  <sig>Standards</sig>
  <approver>Council</approver>
  <dependencies>
    <spec>XMPP Core</spec>
    <spec>XEP-0166</spec>
    <spec>XEP-0167</spec>
    <spec>draft-zimmermann-avt-zrtp</spec>
  </dependencies>
  <supersedes/>
  <supersededby/>
  <shortname>N/A</shortname>
  <discuss>jingle</discuss>
  &stpeter;
  <revision>
    <version>0.0.2</version>
    <date>2009-02-24</date>
    <initials>psa</initials>
    <remark><p>Defined dedicated namespace for the zrtp-hash element to improve service discovery and handling of session-info message; clarified protocol flow and security considerations.</p></remark>
  </revision>
  <revision>
    <version>0.0.1</version>
    <date>2009-02-17</date>
    <initials>psa</initials>
    <remark><p>First draft, copied from XEP-0167.</p></remark>
  </revision>
</header>

<section1 topic='Protocol' anchor='protocol'>
  <p>&xep0167; recommends the use of the Secure Real-time Transport Protocol (SRTP) for end-to-end encryption of RTP sessions negotiated using &xep0166;. An alternative approach to end-to-end encryption of RTP traffic is provided by &rfc6189;. Although negotiation of ZRTP mainly occurs in the media channel rather than the signalling channel, the ZRTP specification defines one SDP attribute called "zrtp-hash" (this communicates the ZRTP version supported as well as a hash of the Hello message).</p>
  <p>The SDP format is shown below.</p>
  <code>
a=zrtp-hash:zrtp-version zrtp-hash-value
  </code>
  <p>An example follows.</p>
  <code>
a=zrtp-hash:1.10 fe30efd02423cb054e50efd0248742ac7a52c8f91bc2df881ae642c371ba46df
  </code>
  <p>This SDP attribute can be translated into Jingle as a &lt;zrtp-hash/&gt; element, as shown below.</p>
  <code><![CDATA[
<zrtp-hash version='zrtp-version'>zrtp-hash-value</zrtp-hash>
  ]]></code>
  <p>An example follows.</p>
  <code><![CDATA[
<zrtp-hash version='1.10'>fe30efd02423cb054e50efd0248742ac7a52c8f91bc2df881ae642c371ba46df</zrtp-hash>
  ]]></code>
  <p>If either party to a Jingle RTP session wishes to use ZRTP, the party SHOULD send a Jingle session-info message that includes a &lt;zrtp-hash/&gt; element qualified by the 'urn:xmpp:jingle:apps:rtp:zrtp:0' namespace, as shown in the following example.</p>
  <example caption="Initiator sends session-info message with zrtp-hash"><![CDATA[
<iq from='romeo@montague.lit/orchard'
    id='vna13b9a'
    from='juliet@capulet.lit/balcony'
    type='set'>
  <jingle xmlns='urn:xmpp:jingle:0'
          action='session-info'
          initiator='romeo@montague.lit/orchard'
          sid='a73sjjvkla37jfea'>
    <zrtp-hash xmlns='urn:xmpp:jingle:apps:rtp:zrtp:0'
               version='1.10'>fe30efd02423cb054e50efd0248742ac7a52c8f91bc2df881ae642c371ba46df</zrtp-hash>
  </jingle>
</iq>
    ]]></example>
    <p>In accordance with <cite>XEP-0166</cite>, if the receiving party does not understand the payload of the session-info message then it MUST return a &feature; error.</p>
    <example caption="Receiver returns feature-not-implemented error"><![CDATA[
<iq from='juliet@capulet.lit/balcony'
    id='ringing1'
    to='romeo@montague.lit/orchard'
    type='error'>
  <error type='modify'>
    <feature-not-implemented xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
    <unsupported-info xmlns='urn:xmpp:jingle:errors:0'/>
  </error>
</iq>
    ]]></example>
    <p>However, if the receiving party also supports and wishes to use ZRTP, it too SHOULD send a session-info message containing a zrtp-hash element.</p>
</section1>

<section1 topic='Determining Support' anchor='disco'>
  <p>If an entity supports the zrtp-hash session-info message, it MUST advertise that fact in its responses to &xep0030; information ("disco#info") requests by returning a feature of "urn:xmpp:jingle:apps:rtp:zrtp:0":</p>
  <example caption='A disco#info query'><![CDATA[
<iq type='get'
    from='calvin@usrobots.lit/lab'
    to='herbie@usrobots.lit/home'
    id='disco1'>
  <query xmlns='http://jabber.org/protocol/disco#info'/>
</iq>
  ]]></example>
  <example caption='A disco#info response'><![CDATA[
<iq type='result'
    from='herbie@usrobots.lit/home'
    to='calvin@usrobots.lit/lab'
    id='disco1'>
  <query xmlns='http://jabber.org/protocol/disco#info'>
    <feature var='urn:xmpp:jingle:apps:rtp:zrtp:0'/>
  </query>
</iq>
  ]]></example>
  <p>In order for an application to determine whether an entity supports this protocol, where possible it SHOULD use the dynamic, presence-based profile of service discovery defined in &xep0115;. However, if an application has not received entity capabilities information from an entity, it SHOULD use explicit service discovery instead.</p>
</section1>

<section1 topic='Security Considerations' anchor='security'>
  <p>Security considerations for ZRTP itself are provided in <cite>draft-zimmermann-avt-zrtp</cite>.</p>
  <p>XMPP stanzas such as Jingle session-info messages and service discovery exchanges are not encrypted or signed. As a result, it is possible for an attacker to intercept these stanzas and modify them, thus convincing one party that the other party does not support ZRTP and therefore denying the parties an opportunity to use ZRTP. However, because the zrtp-hash is mostly advisory, the parties could still use ZRTP even if the signalling channel is compromised.</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'>
  <p>See the <link url='#schema'>XML Schemas</link> section of this document.</p>
</section1>

<section1 topic='XML Schemas' anchor='schema'>
  <p>If this specification is advanced to a status of Draft, the XMPP Registrar shall add the following element definition to the schema for the 'urn:xmpp:jingle:apps:rtp:info:1' namespace defined in XEP-0167:</p>
  <code><![CDATA[
<?xml version='1.0' encoding='UTF-8'?>

<xs:schema
    xmlns:xs='http://www.w3.org/2001/XMLSchema'
    targetNamespace='urn:xmpp:jingle:apps:rtp:zrtp:0'
    xmlns='urn:xmpp:jingle:apps:rtp:zrtp:0'
    elementFormDefault='qualified'>

  <xs:element name='zrtp-hash'>
    <xs:complexType>
      <xs:simpleContent>
        <xs:extension base='xs:string'>
          <xs:attribute name='version' type='xs:string' use='required'/>
        </xs:extension>
      </xs:simpleContent>
    </xs:complexType>
  </xs:element>

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

</xep>
