Within the Extensible Messaging and Presence Protocol (XMPP Core ), presence indicates availability for communication. Specifically, in systems that bundle presence and instant messaging (see XMPP IM ), 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 (XEP-0167) ) with a user who has the following three XMPP resources:
|Resource||Messaging Priority||Voice Chat Priority|
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:
This specification deliberately leaves the meaning of the term "application" or "application type" fairly loose. Possible examples include:
A future version of this specificaiton might establish a registry for application types.
Application priority is encapsulated by a <rap/> element qualified by the 'urn:xmpp:rap:0' namespace (see Namespace Versioning regarding the possibility of incrementing the version number). The attributes of the <rap/> element are as follows.
|ns||The primary namespace of the application type.||REQUIRED|
|num||The resource's priority for this application type. ||REQUIRED|
An example follows.
The following business rules apply to the generation of resource application priority by the client:
A client SHOULD include a <rap/> element for each application type it prioritizes, but SHOULD NOT do so if the priority for that application is the same as the resource's standard XMPP priority.
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").
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.
RAP data MAY be included as extended content within a standard XMPP presence stanza. This is consistent with the rule that presence stanzas need to 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.
For the three resources ("desktop", "pda", and "mobile") mentioned above, the pubsub notifications received by a contact would be as follows.
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:
The following business rules apply to primary resource flagging by the server:
A server MAY add the <primary/> element to RAP data generated by the resource it determines is "most available" for a given application type.
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.
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.
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.
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.
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).
A server MAY use the RAP data provided by a client in determining how to route incoming <message/> stanzas directed to the bare JID <firstname.lastname@example.org> 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 (XEP-0155)  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.
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.
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 (XEP-0030)  information requests.
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 (XEP-0115) . However, if an application has not received entity capabilities information from an entity, it SHOULD use explicit service discovery instead.
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.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) .
This specification defines the following XML namespaces:
Upon advancement of this specification from a status of Experimental to a status of Draft, the XMPP Registrar  shall add the foregoing namespaces to the registry located at <https://xmpp.org/registrar/namespaces.html>, as described in Section 4 of XMPP Registrar Function (XEP-0053) .
If the protocol defined in this specification undergoes a revision that is not fully backwards-compatible with an older version, the XMPP Registrar shall increment the protocol version number found at the end of the XML namespaces defined herein, as described in Section 4 of XEP-0053.
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The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 6120) and XMPP IM (RFC 6121) 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.
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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".
4. This protocol uses a 'num' attribute rather than a 'priority' attribute to reduce confusion with standard XMPP presence.)
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Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/
Documented optional pubsub transport for RAP data.
Editorial review and consistency check.
Changed im application type to messaging; added jingle-video.
Clarified structure of, and added schema for, RAP request namespace.
Initial published version.
Added support for RAP requests via IQ.
Defined registry of application types; clarified business rules; corrected schema.
Broadened previous resource flagging proposal to include priority for applications other than messaging.
Added more business rules and examples; defined service discovery guidelines.