XEP-0225: Component Connections

WARNING: Consideration of this document has been Deferred by the XMPP Standards Foundation. Implementation of the protocol described herein is not recommended.

Table of Contents

1. Introduction

Jabber Component Protocol [1] 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:

• It does not support Transport Layer Security (TLS; see RFC 4346 [2]) for channel encryption.
• It does not support the Simple Authentication and Security Layer (SASL; see RFC 4422 [3]) for authentication.
• It does not enable a component to bind multiple hostnames to one stream (as, for example, a client can bind multiple resource identifiers).
• It multiplies namespaces beyond necessity, adding the "jabber:component:accept" and "jabber:component:connect" namespaces to "jabber:client" and "jabber:server".

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.

2. Requirements

This document addresses the following requirements:

1. Support Transport Layer Security for channel encryption.
2. Support the Simple Authentication and Security Layer for authentication.
3. Enable a component to bind multiple hostnames to one stream.
4. Use one of the existing default namespaces for XML streams between components and servers.

3. Stream Establishment

XML streams are established between a component and a server exactly as they are between a client and a server as specified in XMPP Core [4], with the following exceptions:

1. The 'from' address of the initial stream header SHOULD be the "default" hostname of the component.
2. The JID asserted by the end entity (in this case a component) during STARTTLS negotiation and SASL negotiation MUST be of the form <domain> in conformance with the definition of a domain identifier from XMPP Core.
3. If a "simple user name" is included in accordance with the chosen SASL mechanism, it MUST be of the form <domain> in conformance with the definition of a domain identifier from XMPP Core.

4. Hostname Binding

The protocol defined in XEP-0114 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.

If a server offers component binding over a stream, it MUST advertise a feature of "urn:xmpp:component:0".

Example 1. Stream Feature

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>
  

In order to bind a hostname, the component sends a bind request to the server.

Example 2. Bind Request

C: <iq id='bind_1' type='set'>
     <bind xmlns='urn:xmpp:component:0'>
       <hostname>chat.example.com</hostname>
     </bind>
   </iq>
  

If the hostname can be bound, the server MUST return an IQ-result specifying the exact hostname that was bound.

Example 3. Bind Result

S: <iq id='bind_1' type='result'>
     <bind xmlns='urn:xmpp:component:0'>
       <hostname>chat.example.com</hostname>
     </bind>
   </iq>
  

If the hostname cannot be bound, the server MUST return an IQ-error, which SHOULD be <bad-request/>, <conflict/>, <not-allowed/>, or <resource-constraint/>, just as with client resource binding as specified in RFC 3920.

Note: Although the JID asserted during STARTTLS and SASL negotiation MUST be of the form <domain> (i.e., an XMPP domain identifier), the <hostname/> element MAY be of the form <domain/resource>. This form can be used for application-specific functionality (e.g., load balancing), but such functionality is out of scope for this specification.

A component can send a subsequent bind request to bind another hostname (a server MUST support binding of multiple hostnames).

Example 4. Another Bind Request

C: <iq id='bind_2' type='set'>
     <bind xmlns='urn:xmpp:component:0'>
       <hostname>foo.example.com</hostname>
     </bind>
   </iq>
  

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/>).

A component can also unbind a resource that has already been bound (a server MUST support unbinding).

Example 5. Unbind Request

C: <iq id='unbind_1' type='set'>
     <unbind xmlns='urn:xmpp:component:0'>
       <hostname>foo.example.com</hostname>
     </unbind>
   </iq>
  

If the hostname can be unbound, the server MUST return an IQ-result.

Example 6. Unbind Result

S: <iq id='unbind_1' type='result'/>
  

5. Security Considerations

This protocol improves upon the earlier component protocol defined in XEP-0114 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 rfc3920bis apply to this protocol as well.

6. IANA Considerations

This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [5].

7. XMPP Registrar Considerations

8. XML Schema

<?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>
  

Appendices

Appendix A: Document Information

Series: XEP
Number: 0225
Publisher: XMPP Standards Foundation
Status: Deferred
Type: Standards Track
Version: 0.2
Last Updated: 2008-10-06
Approving Body: XMPP Council
Dependencies: XMPP Core
Supersedes: None
Superseded By: None
Short Name: component
Source Control: HTML  RSS
This document in other formats: XML  PDF

Appendix B: Author Information

Appendix C: Legal Notices

Appendix D: Relation to XMPP

The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 3920) and XMPP IM (RFC 3921) specifications contributed by the XMPP Standards Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this document has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.

Appendix E: Discussion Venue

The primary venue for discussion of XMPP Extension Protocols is the <standards@xmpp.org> discussion list.

Discussion on other xmpp.org discussion lists might also be appropriate; see <http://xmpp.org/about/discuss.shtml> for a complete list.

Errata can be sent to <editor@xmpp.org>.

Appendix F: Requirements Conformance

The following requirements keywords as used in this document are to be interpreted as described in RFC 2119: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".

Appendix G: Notes

Appendix H: Revision History

Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/

END