XEP-0168: Resource Application Priority

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

Within the Extensible Messaging and Presence Protocol (XMPP; see RFC 3920 [1]), presence indicates availability for communication. Specifically, in systems that bundle presence and instant messaging (see RFC 3921 [2]), the <priority/> child of the XMPP <presence/> stanza indicates availability for communications qualified by the "jabber:client" namespace, especially instant messaging. However, a wide variety of entities might provide XMPP presence, including entities that are not primarily focused on IM (e.g., phones) or even entities that do not support XMPP messaging at all.

Consider a scenario in which a contact wants to initiate a voice chat (see Jingle RTP Sessions [3]) with a user who has the following three XMPP resources:

If the contact chooses the resource with which it initiates a voice chat based on the standard XMPP <priority/> element, the resulting behavior could be misleading (i.e., initiating the voice chat with the "desktop" resource rather than the "mobile" resource).

What is needed is a way for the user's clients to indicate that the application priority for the three resources is different from the standard XMPP priority. This document defines such a mechanism via an optional XMPP presence extension.

As applications of that core use case, this document also defines:

• A way for an XMPP server to mark which resource it considers to be primary for any given application type, if it has information -- such as communication preferences -- that can help it determine the primary resource.
• A way for an XMPP server to use application priority data for more intelligent routing of specially-labelled XMPP <message/> stanzas directed to a user's bare JID <localpart@domain.tld>.

2. What Is An Application?

This specification deliberately leaves the meaning of the term "application" or "application type" fairly loose. Possible examples include:

• Messaging
• Voice chat
• Video chat
• Calendaring
• Whiteboarding
• Collaborative editing

A future version of this specificaiton might establish a registry for application types.

3. Application Priority

<rap xmlns='urn:xmpp:rap:0' 
     ns='urn:xmpp:jingle:apps:rtp:0' 
     num='5'/>
    

<presence from='juliet@capulet.lit/desktop' to='romeo@montague.lit/home'>
  <priority>10</priority>
  <rap xmlns='urn:xmpp:rap:0' 
       ns='urn:xmpp:jingle:apps:rtp:0' 
       num='5'/>
</presence>

<presence from='juliet@capulet.lit/pda' to='romeo@montague.lit/home'>
  <priority>5</priority>
  <rap xmlns='urn:xmpp:rap:0' 
       ns='urn:xmpp:jingle:apps:rtp:0' 
       num='-1'/>
</presence>

<presence from='juliet@capulet.lit/mobile' to='romeo@montague.lit/home'>
  <priority>-1</priority>
  <rap xmlns='urn:xmpp:rap:0' 
       ns='urn:xmpp:jingle:apps:rtp:0' 
       num='10'/>
</presence>
    

<message from='juliet@capulet.lit'
         id='rap1'
         to='romeo@montague.lit'
         type='headline'>
  <event xmlns='http://jabber.org/protocol/pubsub#event'>
    <items node='urn:xmpp:rap:0'>
      <item>
        <rap xmlns='urn:xmpp:rap:0' 
             ns='urn:xmpp:jingle:apps:rtp:0' 
             num='5'/>
      </item>
    </items>
  </event>
  <addresses xmlns='http://jabber.org/protocol/address'>
    <address type='replyto' jid='juliet@capulet.lit/desktop'/>
  </addresses>
</message>

<message from='juliet@capulet.lit'
         id='rap2'
         to='romeo@montague.lit'
         type='headline'>
  <event xmlns='http://jabber.org/protocol/pubsub#event'>
    <items node='urn:xmpp:rap:0'>
      <item>
      <rap xmlns='urn:xmpp:rap:0' 
           ns='urn:xmpp:jingle:apps:rtp:0' 
           num='-1'/>
      </item>
    </items>
  </event>
  <addresses xmlns='http://jabber.org/protocol/address'>
    <address type='replyto' jid='juliet@capulet.lit/pda'/>
  </addresses>
</message>

<message from='juliet@capulet.lit'
         id='rap3'
         to='romeo@montague.lit'
         type='headline'>
  <event xmlns='http://jabber.org/protocol/pubsub#event'>
    <items node='urn:xmpp:rap:0'>
      <item>
      <rap xmlns='urn:xmpp:rap:0' 
           ns='urn:xmpp:jingle:apps:rtp:0' 
           num='10'/>
      </item>
    </items>
  </event>
  <addresses xmlns='http://jabber.org/protocol/address'>
    <address type='replyto' jid='juliet@capulet.lit/mobile'/>
  </addresses>
</message>
    

4. Flagging the Primary Resource

The user's XMPP server might have special information that enables it to flag a resource as primary for a given application type. For instance, the server may include a communication policy service that enables the user to define (outside the context of any presence priorities) that she would prefer to be called at her desktop computer only between the hours of 9:00 AM and 5:00 PM local time, prefer to be called on her mobile phone at all other times, and so on.

To flag the primary resource related to a specific application type, the server shall add a <primary/> child to the relevant RAP element. Here is an example:

Example 4. Primary resource flag

<presence from='juliet@capulet.lit/mobile'>
  <priority>-1</priority>
  <rap xmlns='urn:xmpp:rap:0' 
       ns='urn:xmpp:jingle:apps:rtp:0' 
       num='10'>
    <primary/>
  </rap>
</presence>
  

The following business rules apply to primary resource flagging by the server:

1. A server MAY add the <primary/> element to RAP data generated by the resource it determines is "most available" for a given application type.

2. Because the default 'ns' is "jabber:client", to flag the primary resource for standard XMPP communications the server SHOULD NOT include a 'ns' attribute, SHOULD NOT include a 'num' attribute, and MUST include a <primary/> child.

3. An available resource that has specified a negative priority for an application type MUST NOT be flagged as the primary resource for that application type.

4. A client SHOULD NOT include the <primary/> element in RAP data that it generates; however, if a client includes a <primary/> element, the server SHOULD remove or overwrite the element.

5. In response to a presence probe, a server SHOULD send presence from the primary resource first (this enables the receiving client to skip any local "most-available-resource" algorithms it might implement) if the client includes RAP data in presence.

6. If the primary resource changes for a given application type and the client includes RAP data in presence, a server MUST broadcast updated presence information (including the <primary/> element) for the new primary resource. If the change in primary resource occurs because of a presence broadcast from the current primary resource, the server MUST push presence from the current primary resource (without the <primary/> element) before pushing presence from the new primary resource (including the <primary/> element).

5. RAP-Based Message Routing

A server MAY use the RAP data provided by a client in determining how to route incoming <message/> stanzas directed to the bare JID <localpart@domain.tld> of a registered account. In order to enable such routing, the sender MUST include an empty <route/> element qualified by the 'urn:xmpp:raproute:0' namespace (see Namespace Versioning regarding the possibility of incrementing the version number) including an 'ns' attribute corresponding to the desired application type.

For example, consider a Stanza Session Negotiation [7] request sent from one user (Romeo) to another (Juliet), where the users do not share presence. Romeo wants the request to be delivered to the highest-priority resource for the "urn:xmpp:jingle:apps:rtp:0" application type.

Example 5. User requests session

<message from='romeo@montague.lit/orchard'
         to='juliet@capulet.lit'
         type='headline'>
  <thread>ffd7076498744578d10edabfe7f4a866</thread>
  <feature xmlns='http://jabber.org/protocol/feature-neg'>
    <x xmlns='jabber:x:data' type='form'>
      <title>Open chat with Romeo?</title>
      <field var='FORM_TYPE' type='hidden'>
        <value>urn:xmpp:ssn</value>
      </field>
      <field label='Accept this session?' type='boolean' var='accept'>
        <value>true</value>
        <required/>
      </field>
    </x>
  </feature>
  <route xmlns='urn:xmpp:raproute:0'
         ns='urn:xmpp:jingle:apps:rtp:0'/>
</message>
    

If Juliet's server supports RAP routing, it would then deliver the message to whichever of Juliet's resources has the highest priority for the "urn:xmpp:jingle:apps:rtp:0" application type.

6. Determining Support

If an entity supports resource application priorities, it MUST advertise that fact by returning a feature of "urn:xmpp:rap:0", "urn:xmpp:raproute:0", or both (see Namespace Versioning regarding the possibility of incrementing the version number) in response to Service Discovery [8] information requests.

Example 6. Entity queries a server regarding protocol support

<iq from='juliet@capulet.lit/balcony'
    id='disco1'
    to='capulet.lit'
    type='get'>
  <query xmlns='http://jabber.org/protocol/disco#info'/>
</iq>
  

Example 7. Server communicates protocol support for RAP

<iq from='capulet.lit'
    id='disco1'
    to='juliet@capulet.lit/balcony'
    type='result'>
  <query xmlns='http://jabber.org/protocol/disco#info'>
    <feature var='urn:xmpp:rap:0'/>
    <feature var='urn:xmpp:raproute:0'/>
  </query>
</iq>
  

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 Entity Capabilities [9]. However, if an application has not received entity capabilities information from an entity, it SHOULD use explicit service discovery instead.

7. Security Considerations

When the pubsub transport is used, client publishing of resource application priority can result in a presence leak if the node access model is "open". Care should be taken in properly configuring the pubsub node so that unauthorized entities are not able to retrieve information about the user's available resources.

Server flagging of the primary resource is not known to introduce any vulnerabilities or compromises of user privacy.

8. IANA Considerations

This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [10].

9. XMPP Registrar Considerations

10. XML Schemas

<?xml version='1.0' encoding='UTF-8'?>

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

  <xs:element name='rap'>
    <xs:complexType>
      <xs:sequence>
        <xs:element name='primary' type='empty' minOccurs='0' maxOccurs='1'/>
      </xs:sequence>
      <xs:attribute name='ns' type='xs:string' default='jabber:client'/>
      <xs:attribute name='num' type='xs:byte'/>
    </xs:complexType>
  </xs:element>

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

    

<?xml version='1.0' encoding='UTF-8'?>

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

  <xs:element name='route'>
    <xs:complexType>
      <xs:simpleContent>
        <xs:extension base='empty'>
          <xs:attribute name='ns' type='xs:string' default='jabber:client'/>
        </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>

    

Appendices

Appendix A: Document Information

Series: XEP
Number: 0168
Publisher: XMPP Standards Foundation
Status: Deferred
Type: Standards Track
Version: 0.7
Last Updated: 2008-09-26
Approving Body: XMPP Council
Dependencies: XMPP Core, XMPP IM, XEP-0030
Supersedes: None
Superseded By: None
Short Name: NOT_YET_ASSIGNED
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