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<header>
  <title>Best Practices for Use of SASL ANONYMOUS</title>
  <abstract>This document specifies best practices for use of the SASL ANONYMOUS mechanism in the context of client authentication with an XMPP server.</abstract>
  &LEGALNOTICE;
  <number>0175</number>
  <status>Active</status>
  <type>Informational</type>
  <sig>Standards</sig>
  <approver>Council</approver>
  <dependencies>
    <spec>XMPP Core</spec>
  </dependencies>
  <supersedes/>
  <supersededby/>
  <shortname>N/A</shortname>
  &stpeter;
  <revision>
    <version>1.1</version>
    <date>2007-11-07</date>
    <initials>psa</initials>
    <remark><p>Recommended that node identifier be a UUID; recommended that trace data not be included.</p></remark>
  </revision>
  <revision>
    <version>1.0</version>
    <date>2006-09-20</date>
    <initials>psa</initials>
    <remark><p>Per a vote of the Jabber Council, advanced status to Active.</p></remark>
  </revision>
  <revision>
    <version>0.1</version>
    <date>2006-02-09</date>
    <initials>psa</initials>
    <remark><p>Initial version; modified flow to remove unecessary challenge.</p></remark>
  </revision>
  <revision>
    <version>0.0.1</version>
    <date>2006-01-24</date>
    <initials>psa</initials>
    <remark><p>First draft.</p></remark>
  </revision>
</header>
<section1 topic='Introduction' anchor='intro'>
  &RFC3920BISNOTE;
  <p><cite>RFC 3920</cite> allows the use of any SASL mechanism (see &rfc4422;) in XMPP authentication, including the SASL ANONYMOUS mechanism (see &rfc4505;). This document specifies a recommended protocol flow for such use.</p>
  <p>Note: This document is provided for discussion purposes in order to clarify the usage of SASL ANONYMOUS in XMPP systems. It is not meant to supersede the text in <cite>RFC 3920</cite>, <cite>RFC 4422</cite>, or <cite>RFC 4505</cite>. However, the recommendations in this document may be folded into <cite>rfc3920bis</cite>.</p>
</section1>
<section1 topic='Recommendation' anchor='rec'>
  <p><cite>RFC 3920</cite> specifies that after an XMPP client authenticates with an XMPP server, it must bind a resource to the XML stream so that XML stanzas can be routed to the client. In essence there are three resource binding scenarios:</p>
  <ol>
    <li>The client specifies a desired resource identifier and the server accepts it.</li>
    <li>The client specifies a desired resource identifier but the server does not accept it, instead overruling the client and assigning a resource identifier.</li>
    <li>The client asks the server to assign a resource identifier and the server does so.</li>
  </ol>
  <p>No matter which scenario is enacted, at the end of the process the server informs the client of its full JID &LOCALFULL;. In particular, it might be helpful for an XMPP server to assign a full JID to the client (i.e., not just the resource identifier) if it authenticates with SASL ANONYMOUS, and to ensure that the "bare JID" portion &LOCALBARE; is unique in the context of the domain served by the server.</p>
  <p>The method for ensuring the uniqueness of the node identifier is a matter of implementation. It is RECOMMENDED for the node identifier to be a UUID as specified in &rfc4122;.</p>
  <p>Although <cite>RFC 4505</cite> allows the initiating entity (client) to provide so-called "trace data" when authenticating via SASL ANONYMOUS, it is NOT RECOMMENDED to include trace data as the XML character data of the &lt;auth/&gt; element (instead, the &lt;auth/&gt; element SHOULD be empty). However, if trace data is included, the server MUST NOT use it for any purpose other than tracing (e.g., in server logs).</p>
</section1>
<section1 topic='Protocol Flow' anchor='flow'>
  <p>The RECOMMENDED protocol flow following TLS negotiation (refer to <cite>RFC 3920</cite>) is as follows:</p>
  <ol>
    <li>
      <p>Client initiates stream to server.</p>
      <example caption="Stream initiation"><![CDATA[
<stream:stream 
        xmlns:stream='http://etherx.jabber.org/streams' 
        xmlns='jabber:client' 
        to='example.com' 
        version='1.0'>
      ]]></example>
    </li>
    <li>
      <p>Server replies with stream header.</p>
      <example caption="Stream header reply"><![CDATA[
<stream:stream 
        xmlns:stream='http://etherx.jabber.org/streams' 
        xmlns='jabber:client' 
        id='c2s_234' 
        from='example.com' 
        version='1.0'>
      ]]></example>
    </li>
    <li>
      <p>Server advertises stream features.</p>
      <example caption="Stream features advertisement"><![CDATA[
<stream:features>
  <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
    <mechanism>DIGEST-MD5<mechanism>
    <mechanism>ANONYMOUS<mechanism>
  </mechanisms>
</stream:features>
      ]]></example>
    </li>
    <li>
      <p>Client requests SASL ANONYMOUS mechanism.</p>
      <example caption="Requesting SASL ANONYMOUS"><![CDATA[
<auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl' mechanism='ANONYMOUS'/>
      ]]></example>
    </li>
    <li>
      <p>Server sends &lt;success/&gt;.</p>
      <example caption="Sending success"><![CDATA[
<success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
      ]]></example>
    </li>
    <li>
      <p>Client opens new stream.</p>
      <example caption="Initiating a new stream"><![CDATA[
<stream:stream
        xmlns:stream='http://etherx.jabber.org/streams'
        xmlns='jabber:client'
        to='example.com'
        version='1.0'>
      ]]></example>
    </li>
    <li>
      <p>Server tells client that resource binding is required.</p>
      <example caption="Stream header reply with features"><![CDATA[
<stream:stream
        xmlns:stream='http://etherx.jabber.org/streams'
        xmlns='jabber:client'
        id='c2s_345'
        from='example.com'
        version='1.0'>
<stream:features>
  <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
</stream:features>
      ]]></example>
    </li>
    <li>
      <p>Client requests that server create a resource for it.</p>
      <example caption="Requesting resource creation"><![CDATA[
<iq type='set' id='bind_1'>
  <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
</iq>
      ]]></example>
    </li>
    <li>
      <p>Server replies with full JID.</p>
      <example caption="Server informs client of full JID"><![CDATA[
<iq type='result' id='bind_1'>
  <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
    <jid>somenode@example.com/someresource</jid>
  </bind>
</iq>
      ]]></example>
    </li>
  </ol>
</section1>
<section1 topic='Security Considerations' anchor='security'>
  <p>This document introduces no security considerations or concerns above and beyond those discussed in <cite>RFC 3920</cite>.</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>This document requires no interaction with the &REGISTRAR;.</p>
</section1>
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