XEP-0168: Resource Application Priority

This document defines an XMPP protocol extension to indicate the presence priority of XMPP resources for applications other than standard XMPP messaging.

WARNING: This Standards-Track document is Experimental. Publication as an XMPP Extension Protocol does not imply approval of this proposal by the XMPP Standards Foundation. Implementation of the protocol described herein is encouraged in exploratory implementations, but production systems should not deploy implementations of this protocol until it advances to a status of Draft.

Document Information

Series: XEP
Number: 0168
Publisher: XMPP Standards Foundation
Status: Experimental
Type: Standards Track
Version: 0.6
Last Updated: 2007-11-19
Approving Body: XMPP Council
Dependencies: XMPP Core, XMPP IM, XEP-0030
Supersedes: None
Superseded By: None
Short Name: TO BE ASSIGNED
Wiki Page: <http://wiki.jabber.org/index.php/Resource Application Priority (XEP-0168)>

Author Information

Peter Saint-Andre

JabberID: stpeter@jabber.org
URI: https://stpeter.im/

Joe Hildebrand

Email: jhildebrand@jabber.com
JabberID: hildjj@jabber.org

Legal Notices

Copyright

This XMPP Extension Protocol is copyright (c) 1999 - 2008 by the XMPP Standards Foundation (XSF).

Permissions

Permission is hereby granted, free of charge, to any person obtaining a copy of this specification (the "Specification"), to make use of the Specification without restriction, including without limitation the rights to implement the Specification in a software program, deploy the Specification in a network service, and copy, modify, merge, publish, translate, distribute, sublicense, or sell copies of the Specification, and to permit persons to whom the Specification is furnished to do so, subject to the condition that the foregoing copyright notice and this permission notice shall be included in all copies or substantial portions of the Specification. Unless separate permission is granted, modified works that are redistributed shall not contain misleading information regarding the authors, title, number, or publisher of the Specification, and shall not claim endorsement of the modified works by the authors, any organization or project to which the authors belong, or the XMPP Standards Foundation.

Disclaimer of Warranty

## NOTE WELL: This Specification is provided on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, express or implied, including, without limitation, any warranties or conditions of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A PARTICULAR PURPOSE. In no event shall the XMPP Standards Foundation or the authors of this Specification be liable for any claim, damages, or other liability, whether in an action of contract, tort, or otherwise, arising from, out of, or in connection with the Specification or the implementation, deployment, or other use of the Specification. ##

Limitation of Liability

In no event and under no legal theory, whether in tort (including negligence), contract, or otherwise, unless required by applicable law (such as deliberate and grossly negligent acts) or agreed to in writing, shall the XMPP Standards Foundation or any author of this Specification be liable for damages, including any direct, indirect, special, incidental, or consequential damages of any character arising out of the use or inability to use the Specification (including but not limited to damages for loss of goodwill, work stoppage, computer failure or malfunction, or any and all other commercial damages or losses), even if the XMPP Standards Foundation or such author has been advised of the possibility of such damages.

IPR Conformance

This XMPP Extension Protocol has been contributed in full conformance with the XSF's Intellectual Property Rights Policy (a copy of which may be found at <http://www.xmpp.org/extensions/ipr-policy.shtml> or obtained by writing to XSF, P.O. Box 1641, Denver, CO 80201 USA).

Discussion Venue

The preferred venue for discussion of this document is the Standards discussion list: <http://mail.jabber.org/mailman/listinfo/standards>.

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

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.

Conformance Terms

The following 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".

Table of Contents


1. Introduction
2. Application Priority
    2.1. Format
    2.2. Generation
    2.3. Presence Transport
    2.4. Pubsub Transport
3. Flagging the Primary Resource
4. RAP-Based Message Routing
5. Determining Support
6. Security Considerations
7. IANA Considerations
8. XMPP Registrar Considerations
    8.1. Protocol Namespaces
9. XML Schemas
    9.1. RAP
    9.2. RAP Routing
Notes
Revision History

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 Audio via RTP [3]) with a user who has the following three XMPP resources:

Table 1: Application Presence

Resource Messaging Priority Voice Chat Priority
desktop 10 5
pda 5 -1
mobile -1 10

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 extensions to that core use case, this document also defines:

2. Application Priority

2.1 Format

Application priority is encapsulated by a <rap/> element qualified by the 'http://www.xmpp.org/extensions/xep-0168.html#ns' namespace (see Protocol Namespaces regarding issuance of one or more permanent namespaces). The attributes of the <rap/> element are as follows.

Table 2: RAP Attributes

Attribute Definition Inclusion
ns The primary namespace of the application type. REQUIRED
num The resource's priority for this application type. [4] REQUIRED

An example follows.

Example 1. Data format

<rap xmlns='http://www.xmpp.org/extensions/xep-0168.html#ns' 
     ns='http://www.xmpp.org/extensions/xep-0167#ns' 
     num='5'/>

2.2 Generation

The following business rules apply to the generation of resource application priority by the client:

  1. A client SHOULD include a <rap/> element for each application type for which it wishes to advertise a priority, but SHOULD NOT do so if the priority for that application is the same as the resource's standard XMPP priority.

  2. A client MUST NOT generate a <rap/> element that has a 'ns' attribute whose value is "jabber:client" or that has no 'ns' attribute (since the default 'ns' is "jabber:client").

  3. The <rap/> element SHOULD be empty.

As explained in the following sections, there are two possible transports for RAP data: standard XMPP presence and the XMPP publish-subscribe extension.

2.3 Presence Transport

RAP data MAY be included as extended content within a standard XMPP presence stanza. This is consistent with the rule that presence stanzas must be related to the network availability or communication preferences of the entity that provides presence information.

For the three resources ("desktop", "pda", and "mobile") mentioned above, the presence stanzas received by a contact would be as follows.

Example 2. Contact receives presence from user

<presence from='juliet@capulet.lit/desktop' to='romeo@montague.lit/home'>
  <priority>10</priority>
  <rap xmlns='http://www.xmpp.org/extensions/xep-0168.html#ns' 
       ns='http://www.xmpp.org/extensions/xep-0167#ns' 
       num='5'/>
</presence>

<presence from='juliet@capulet.lit/pda' to='romeo@montague.lit/home'>
  <priority>5</priority>
  <rap xmlns='http://www.xmpp.org/extensions/xep-0168.html#ns' 
       ns='http://www.xmpp.org/extensions/xep-0167#ns' 
       num='-1'/>
</presence>

<presence from='juliet@capulet.lit/mobile' to='romeo@montague.lit/home'>
  <priority>-1</priority>
  <rap xmlns='http://www.xmpp.org/extensions/xep-0168.html#ns' 
       ns='http://www.xmpp.org/extensions/xep-0167#ns' 
       num='10'/>
</presence>

2.4 Pubsub Transport

Alternatively, RAP data MAY be provided via the XMPP Publish-Subscribe [5] publish-subscribe extension, specifically the Personal Eventing via Pubsub [6] profile thereof.

For the three resources ("desktop", "pda", and "mobile") mentioned above, the pubsub notifications received by a contact would be as follows.

Example 3. Contact receives pubsub notifications from user

<message from='juliet@capulet.lit'
         id='rap1'
         to='romeo@montague.lit/orchard'
         type='headline'>
  <event xmlns='http://jabber.org/protocol/pubsub#event'>
    <items node='http://www.xmpp.org/extensions/xep-0168.html#ns'>
      <item>
        <rap xmlns='http://www.xmpp.org/extensions/xep-0168.html#ns' 
             ns='http://www.xmpp.org/extensions/xep-0167#ns' 
             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/orchard'
         type='headline'>
  <event xmlns='http://jabber.org/protocol/pubsub#event'>
    <items node='http://www.xmpp.org/extensions/xep-0168.html#ns'>
      <item>
      <rap xmlns='http://www.xmpp.org/extensions/xep-0168.html#ns' 
           ns='http://www.xmpp.org/extensions/xep-0167#ns' 
           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/orchard'
         type='headline'>
  <event xmlns='http://jabber.org/protocol/pubsub#event'>
    <items node='http://www.xmpp.org/extensions/xep-0168.html#ns'>
      <item>
      <rap xmlns='http://www.xmpp.org/extensions/xep-0168.html#ns' 
           ns='http://www.xmpp.org/extensions/xep-0167#ns' 
           num='10'/>
      </item>
    </items>
  </event>
  <addresses xmlns='http://jabber.org/protocol/address'>
    <address type='replyto' jid='juliet@capulet.lit/mobile'/>
  </addresses>
</message>

3. Flagging the Primary Resource

The user's XMPP server may 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='http://www.xmpp.org/extensions/xep-0168.html#ns' 
       ns='http://www.xmpp.org/extensions/xep-0167#ns' 
       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 <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).

4. RAP-Based Message Routing

A server MAY use the RAP data provided by a client in determing how to route incoming <message/> stanzas directed to the bare JID (<node@domain.tld>) of a registered account. In order to enable such routing, the sender MUST include an empty <route/> element qualified by the 'http://www.xmpp.org/extensions/xep-0168.html#ns-raproute' namespace (see Protocol Namespaces regarding issuance of one or more permanent namespaces) 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 "http://www.xmpp.org/extensions/xep-0167.html#ns" application type.

Example 5. User requests session

<message from='romeo@montague.lit/orchard'
         to='juliet@capulet.lit'
         type='headline'>
  <route xmlns='http://www.xmpp.org/extensions/xep-0168.html#ns-raproute'
         ns='http://www.xmpp.org/extensions/xep-0167.html#ns'/>
  <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>
      ...
  </feature>
</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 "http://www.xmpp.org/extensions/xep-0167.html#ns" application type.

5. Determining Support

In order to discover whether a server or other entity supports this protocol, an entity MUST use Service Discovery [8] or the dynamic profile of service discovery defined in Entity Capabilities [9].

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>

If the queried entity supports the functionality specified herein, it MUST return the following features (see Protocol Namespaces regarding issuance of one or more permanent namespaces):

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='http://www.xmpp.org/extensions/xep-0168.html#ns'/>
    <feature var='http://www.xmpp.org/extensions/xep-0168.html#ns-route'/>
    ...
  </query>
</iq>

6. Security Considerations

When the pubsub transport is used, client publishing of resource application priority may 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 know to introduce any vulnerabilities or compromises of user privacy.

7. IANA Considerations

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

8. XMPP Registrar Considerations

8.1 Protocol Namespaces

Until this specification advances to a status of Draft, its associated namespaces shall be:

Upon advancement of this specification, the XMPP Registrar [11] shall issue permanent namespaces in accordance with the process defined in Section 4 of XMPP Registrar Function [12].

The following namespaces are requested, and are thought to be unique per the XMPP Registrar's requirements:

9. XML Schemas

9.1 RAP

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

<xs:schema
    xmlns:xs='http://www.w3.org/2001/XMLSchema'
    targetNamespace='http://www.xmpp.org/extensions/xep-0168.html#ns'
    xmlns='http://www.xmpp.org/extensions/xep-0168.html#ns'
    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>

9.2 RAP Routing

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

<xs:schema
    xmlns:xs='http://www.w3.org/2001/XMLSchema'
    targetNamespace='http://www.xmpp.org/extensions/xep-0168.html#ns-route'
    xmlns='http://www.xmpp.org/extensions/xep-0168.html#ns-route'
    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>

Notes

1. RFC 3920: Extensible Messaging and Presence Protocol (XMPP): Core <http://tools.ietf.org/html/rfc3920>.

2. RFC 3921: Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence <http://tools.ietf.org/html/rfc3921>.

3. XEP-0167: Jingle Audio via RTP <http://www.xmpp.org/extensions/xep-0167.html>.

4. This protocol uses a 'num' attribute rather than a 'priority' attribute to reduce confusion with standard XMPP presence.)

5. XEP-0060: Publish-Subscribe <http://www.xmpp.org/extensions/xep-0060.html>.

6. XEP-0163: Personal Eventing via Pubsub <http://www.xmpp.org/extensions/xep-0163.html>.

7. XEP-0155: Stanza Session Negotiation <http://www.xmpp.org/extensions/xep-0155.html>.

8. XEP-0030: Service Discovery <http://www.xmpp.org/extensions/xep-0030.html>.

9. XEP-0115: Entity Capabilities <http://www.xmpp.org/extensions/xep-0115.html>.

10. The Internet Assigned Numbers Authority (IANA) is the central coordinator for the assignment of unique parameter values for Internet protocols, such as port numbers and URI schemes. For further information, see <http://www.iana.org/>.

11. The XMPP Registrar maintains a list of reserved protocol namespaces as well as registries of parameters used in the context of XMPP extension protocols approved by the XMPP Standards Foundation. For further information, see <http://www.xmpp.org/registrar/>.

12. XEP-0053: XMPP Registrar Function <http://www.xmpp.org/extensions/xep-0053.html>.

Revision History

Version 0.6 (2007-11-19)

Documented optional pubsub transport for RAP data.

(psa)

Version 0.5 (2007-11-15)

Editorial review and consistency check.

(psa)

Version 0.4 (2007-06-06)

Added section on RAP-based routing of messages sent to bare JIDs; removed RAP request protocol; changed app attribute to ns attribute; removed the application types registry since it is unnecessary if the ns attribute specifies the XML namespace of the data most closely associated with the application type; updated namespaces to conform to XMPP Registrar processes.

(psa)

Version 0.3 (2006-09-17)

Changed im application type to messaging; added jingle-video.

(psa)

Version 0.2 (2005-12-19)

Clarified structure of, and added schema for, RAP request namespace.

(psa)

Version 0.1 (2005-12-15)

Initial published version.

(psa)

Version 0.0.6 (2005-11-29)

Document cleanup.

(psa)

Version 0.0.5 (2005-11-17)

Added support for RAP requests via IQ.

(psa)

Version 0.0.4 (2005-10-27)

Defined registry of application types; clarified business rules; corrected schema.

(psa/jjh)

Version 0.0.3 (2005-10-24)

Broadened previous resource flagging proposal to include priority for applications other than messaging.

(psa/jjh)

Version 0.0.2 (2005-09-26)

Added more business rules and examples; defined service discovery guidelines.

(psa/jjh)

Version 0.0.1 (2005-09-23)

First draft.

(psa/jjh)

END