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<xep>
<header>
  <title>Component Connections</title>
  <abstract>This document specifies a standards-track XMPP protocol extension that enables server components to connect to XMPP servers.</abstract>
  &LEGALNOTICE;
  <number>0225</number>
  <status>Deferred</status>
  <type>Standards Track</type>
  <sig>Standards</sig>
  <approver>Council</approver>
  <dependencies>
    <spec>XMPP Core</spec>
  </dependencies>
  <supersedes/>
  <supersededby/>
  <shortname>component</shortname>
  &stpeter;
  <revision>
    <version>0.2</version>
    <date>2008-10-06</date>
    <initials>psa</initials>
    <remark><p>Modified namespace to incorporate namespace versioning; clarified that the value of the &lt;hostname/&gt; element can be either &lt;domain&gt; or &lt;domain/resource&gt;.</p></remark>
  </revision>
  <revision>
    <version>0.1</version>
    <date>2007-08-08</date>
    <initials>psa</initials>
    <remark><p>Initial published version.</p></remark>
  </revision>
  <revision>
    <version>0.0.1</version>
    <date>2007-07-31</date>
    <initials>psa</initials>
    <remark><p>First draft.</p></remark>
  </revision>
</header>
<section1 topic='Introduction' anchor='intro'>
  <p>&xep0114; defines a protocol that enables a server component to connect to an XMPP server. However, there are a number of perceived limitations with that protocol:</p>
  <ul>
    <li>It does not support Transport Layer Security (TLS; see &rfc5246;) for channel encryption.</li>
    <li>It does not support the Simple Authentication and Security Layer (SASL; see &rfc4422;) for authentication.</li>
    <li>It does not enable a component to bind multiple hostnames to one stream (as, for example, a client can bind multiple resource identifiers).</li>
    <li>It multiplies namespaces beyond necessity, adding the "jabber:component:accept" and "jabber:component:connect" namespaces to "jabber:client" and "jabber:server".</li>
  </ul>
  <p>This document specifies a standards-track protocol that addresses the basic requirements for component connections. In the future, additional documents may specify more advanced features on top of the protocol defined herein.</p>
</section1>
<section1 topic='Requirements' anchor='reqs'>
  <p>This document addresses the following requirements:</p>
  <ol>
    <li>Support Transport Layer Security for channel encryption.</li>
    <li>Support the Simple Authentication and Security Layer for authentication.</li>
    <li>Enable a component to bind multiple hostnames to one stream.</li>
    <li>Use one of the existing default namespaces for XML streams between components and servers.</li>
  </ol>
</section1>
<section1 topic='Stream Establishment' anchor='stream'>
  <p>XML streams are established between a component and a server exactly as they are between a client and a server as specified in &xmppcore;, with the following exceptions:</p>
  <ol>
    <li>The 'from' address of the initial stream header SHOULD be the "default" hostname of the component.</li>
    <li>The JID asserted by the end entity (in this case a component) during STARTTLS negotiation and SASL negotiation MUST be of the form &lt;domain&gt; in conformance with the definition of a domain identifier from XMPP Core.</li>
    <li>If a "simple user name" is included in accordance with the chosen SASL mechanism, it MUST be of the form &lt;domain&gt; in conformance with the definition of a domain identifier from XMPP Core.</li>
  </ol>
</section1>
<section1 topic='Hostname Binding' anchor='bind'>
  <p>The protocol defined in <cite>XEP-0114</cite> depended on use of the 'to' address in the stream header to specify the hostname of the component. By contrast, client-to-server connections use stream establishment is followed by binding of a resource to the stream (in fact multiple resources can be bound to the stream). This protocol emulates client-to-server connections by using a hostname binding process that is similar to the resource binding process specified in XMPP Core.</p>
  <p>If a server offers component binding over a stream, it MUST advertise a feature of "urn:xmpp:component:0".</p>
  <example caption='Stream Feature'><![CDATA[
S: <stream:stream
       from='example.com'
       id='gPybzaOzBmaADgxKXu9UClbprp0='
       to='chat.example.com'
       version='1.0'
       xml:lang='en'
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'>

S: <stream:features>
     <bind xmlns='urn:xmpp:component:0'>
       <required/>
     </bind>
   </stream:features>
]]></example>
  <p>In order to bind a hostname, the component sends a bind request to the server.</p>
  <example caption='Bind Request'><![CDATA[
C: <iq id='bind_1' type='set'>
     <bind xmlns='urn:xmpp:component:0'>
       <hostname>chat.example.com</hostname>
     </bind>
   </iq>
]]></example>
  <p>If the hostname can be bound, the server MUST return an IQ-result specifying the exact hostname that was bound.</p>
  <example caption='Bind Result'><![CDATA[
S: <iq id='bind_1' type='result'>
     <bind xmlns='urn:xmpp:component:0'>
       <hostname>chat.example.com</hostname>
     </bind>
   </iq>
]]></example>
  <p>If the hostname cannot be bound, the server MUST return an IQ-error, which SHOULD be &badrequest;, &conflict;, &notallowed;, or &constraint;, just as with client resource binding as specified in <cite>RFC 3920</cite>.</p>
  <p>Note: Although the JID asserted during STARTTLS and SASL negotiation MUST be of the form &lt;domain&gt; (i.e., an XMPP domain identifier), the &lt;hostname/&gt; element MAY be of the form &lt;domain/resource&gt;. This form can be used for application-specific functionality (e.g., load balancing), but such functionality is out of scope for this specification.</p>
  <p>A component can send a subsequent bind request to bind another hostname (a server MUST support binding of multiple hostnames).</p>
  <example caption='Another Bind Request'><![CDATA[
C: <iq id='bind_2' type='set'>
     <bind xmlns='urn:xmpp:component:0'>
       <hostname>foo.example.com</hostname>
     </bind>
   </iq>
]]></example>
  <p>If the server cannot process the bind request (e.g., because the component has already bound the desired hostname), the server MUST return an IQ-error (e.g., &conflict;).</p>
  <p>A component can also unbind a resource that has already been bound (a server MUST support unbinding).</p>
  <example caption='Unbind Request'><![CDATA[
C: <iq id='unbind_1' type='set'>
     <unbind xmlns='urn:xmpp:component:0'>
       <hostname>foo.example.com</hostname>
     </unbind>
   </iq>
]]></example>
  <p>If the hostname can be unbound, the server MUST return an IQ-result.</p>
  <example caption='Unbind Result'><![CDATA[
S: <iq id='unbind_1' type='result'/>
]]></example>
</section1>
<section1 topic='Security Considerations' anchor='security'>
  <p>This protocol improves upon the earlier component protocol defined in <cite>XEP-0114</cite> by specifying the use of Transport Layer Security (TLS) for channel encryption and the Simple Authentication and Security Layer (SASL) for authentication. Because this protocol re-uses the XML stream establishment processes defined in XMPP Core, the security considerations from RFC 3920 and RFC 6120 apply to this protocol as well.</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='registrar-ns'>
    <p>This specification defines the following XML namespace:</p>
    <ul>
      <li>urn:xmpp:component: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:component:0'
    xmlns='urn:xmpp:component:0'
    elementFormDefault='qualified'>

  <xs:element name='bind'>
    <xs:complexType>
      <xs:sequence>
        <xs:choice minOccurs='0' maxOccurs='1'>
          <xs:element name='hostname' type='xs:string'/>
        </xs:choice>
        <xs:element name='required'
                    minOccurs='0'
                    maxOccurs='1'
                    type='empty'/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>

  <xs:element name='unbind'>
    <xs:complexType>
      <xs:sequence minOccurs='0'>
        <xs:element name='hostname' type='xs:string'/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>

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