XEP-0276: Presence Decloaking

1. Introduction¶

Various XMPP extensions, such as Jingle (XEP-0166) [1], require additional support from clients, advertised in presence via Entity Capabilities (XEP-0115) [2], or require that IQ stanzas are sent to a particular resource. For instance, Jingle calls can be made only by sending an IQ to a particular resource. However, two parties who wish to communicate do not always share presence information through subscriptions and therefore cannot use entity capabilities to determine the proper full JID for communication. Indeed, one of the parties might not even use XMPP: e.g., a remote user on the other side of a gateway to a network based on the Session Initiation Protocol (SIP; RFC 3261 [3]) or to the Public Switched Telephone Network (PSTN). It would be helpful if a user could make a call through such a gateway by typing the SIP URI or telephone number of an arbitrary contact, without first exchanging presence.

RFC 6121 [4] already defines a way to send directed presence to another entity. This document supplements RFC 6121 by defining an XML protocol extension enabling two parties to explicitly share presence with each other on a temporary basis through an "intentional presence leak"; we call this "decloaking".

Note: This protocol has already been implemented using an XML namespace of "http://telepathy.freedesktop.org/xmpp/protocol/decloak" but the XMPP Registrar [5] was requested to issue the XMPP URN "urn:xmpp:decloak:0" upon publication of this proposal in the XMPP Extension Protocols (XEP-0001) [6] series.

2. Requirements¶

An entity should be able to attempt to initiate a communication session that requires IQs and/or capability negotiation (Jingle, a file transfer, end-to-end encryption, or other similar communication modes) with an arbitrary entity.

If the receiving entity agrees to divulge their presence in this way, minimal presence (with no 'type' attribute, <show/> element, avatar hash, etc.) and capabilities should be communicated to the initiating entity, so that the initiating entity can continue to initiate the communication session.

3. Approach¶

The approach taken here is that the user who wishes to initiate presence sharing for the length of a communications session sends directed presence (including entity capabilities) to the bare JID <localpart@domain.tld> of the initiator's intended communication partner, including a special XMPP extension <decloak xmlns='urn:xmpp:decloak:0'/>. Upon receipt of this directed presence stanza, if configured to do so the recipient's sends directed presence (including entity capabilities) to the initiator's full JID <localpart@domain.tld/resource>. Once the parties complete their communications session, they can terminate presence sharing by sending directed <presence type='unavailable'/> to each other; alternatively, at any time they could "upgrade" their session-based presence sharing to a full XMPP presence subscription as described in XMPP IM [4].

Although the <decloak/> element could be sent in presence stanzas of type "subscribe" instead of in directed presence notifications, that behavior is discouraged because the "fall-through" case for subscription requests is a long-lived subscription, not temporary sharing of presence information for the life of a communication session.

4. Scenario¶

Suppose that Juliet wishes to make a media call to Tybalt, but the two parties do not share presence information in accordance with the core definition of XMPP. Suppose also that Juliet and Tybalt have the following presence, although neither can initially see the other's presence:

    
<presence from='juliet@shakespeare.lit/balcony'>
  <show>dnd</show>
  <status>on the phone</status>
  <c ver='juliet-caps-hash' .../>
</presence>

<presence from='tybalt@shakespeare.lit/library'>
  <show>dnd</show>
  <status>researching</status>
  <c ver='tybalt-caps-hash' .../>
</presence>

<presence from='tybalt@shakespeare.lit/garden'>
  <show>xa</show>
  <status>gone to the library</status>
  <c ver='tybalt-caps-hash' .../>
</presence>

Juliet requests that Tybalt divulge his availability and capabilities, by sending directed presence to his bare JID <tybalt@shakespeare.lit>, where the presence stanza contains a <decloak/> element.

<presence from='juliet@shakespeare.lit/balcony'
          to='tybalt@shakespeare.lit'>
  <c ver='juliet-caps-hash' .../>
  <decloak xmlns='urn:xmpp:decloak:0' reason='media'/>
</presence>

Tybalt MAY in response send session presence from one or more resources, by sending directed presence from those resource(s) to Juliet's bare JID.

    
<presence from='tybalt@shakespeare.lit/library'
          to='juliet@shakespeare.lit'>
  <c ver='tybalt-caps-hash' .../>
</presence>

<presence from='tybalt@shakespeare.lit/garden'
          to='juliet@shakespeare.lit'>
  <c ver='tybalt-caps-hash' .../>
</presence>

Once Juliet has received the session presence from Tybalt, if necessary she can perform service discovery to find out the meaning of the entity capabilities hashes (if unknown), then proceed to make a Jingle call, initiate a file transfer, or complete some other use case.

Naturally, it's also possible that Tybalt's client will ignore the request (in particular, this will happen for any resource that does not implement this specification). However, in this case the parties are no worse off than they were before Juliet requested decloaking.

5. Sharing Presence with a Gateway¶

Let us now suppose that Juliet wishes to make a media call to Romeo, who does not use XMPP but who has a SIP URI of sip:romeo@shakespeare.lit, which can be called via an XMPP-to-SIP gateway.

Juliet requests that the SIP contact representing Romeo on the gateway shall divulge its availability and capabilities, by sending directed presence to its bare JID at the gateway containing a <decloak/> element.

    
<presence from='juliet@shakespeare.lit/balcony'
          to='romeo%shakespeare.lit@sip.shakespeare.lit'>
  <c ver='juliet-caps-hash' .../>
  <decloak xmlns='urn:xmpp:decloak:0' reason='media'/>
</presence>

In response, the SIP gateway automatically shares session presence on behalf of that JID, in order to tell Juliet what the gateway's capabilities are.

    
<presence from='romeo%shakespeare.lit@sip.shakespeare.lit'
          to='juliet@shakespeare.lit'>
  <c ver='gateway-caps-hash' .../>
</presence>

As above, Juliet can now complete service discovery and any protocol-specific use cases.

6. The reason Attibute¶

To signal the type of communication that is desired, the entity that first shares session presence MAY include a 'reason' attribute on the <decloak/> element. The following values for the 'reason' attribute are defined:

Inclusion of the 'reason' attribute can be interpreted by the receiving client as a signal that communication is about to start; for instance, a call accept/reject dialog could double as a UI for accepting or rejecting a session presence request.

7. Business Rules¶

To limit the extent of the presence leak, the receiving entity SHOULD send only bare presence without the XMPP <priority/>, <show/>, or <status/> element. Unfortunately, this has two implications:

1. The initiating entity cannot know which of the receiving entity's resources is more likely to engage in communication. This might imply that the initiating entity will need to send a session initiation request or other communication to more than one of the receiving entity's resources (and then retract the session initiation requests that are not answered by the receiving entity). Solutions to that problem are out of scope for this specification.

2. Establishment of a session might be delayed (e.g., because in Jingle it is desirable to start negotiating candidates as soon as possible but a user interface that prompts the receiving entity to explicitly approve of divulging presence will tend to a delay in call setup). As a result, it may be advantageous to have a way to configure unconditional sharing of session presence in certain deployments, at least within the same trust domain.

8. Security Considerations¶

Because decloaking is a presence leak (albeit intentional), an XMPP client that implements the receiving side of this specification MUST disable sharing of session presence by default and MUST enable the feature only as a result of explicit user confirmation. Such confirmation can be provided per request, at the first request per requestor, by setting some "always decloak" configuration option (e.g., globally or per domain), or through some other suitable means as long as decloaking does not occur by default. (Gateways and other non-user entities MAY divulge their own presence and capabilities unconditionally, if that is appropriate for the service policy at the gateway.)

9. IANA Considerations¶

This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [13].

10. XMPP Registrar Considerations¶

The XMPP Registrar is requested to issue an initial namespace of "urn:xmpp:decloak:0".

11. XML Schema¶

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

  <xs:element name='decloak'>
    <xs:complexType>
      <xs:simpleContent>
        <xs:extension base='empty'>
          <xs:attribute name='reason' use='optional' type='xs:string'/>
        </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>

12. Acknowledgements¶

The need for this protocol extension, and a rough proposal for solving the problem, were originally determined at XMPP Summit 5 in the summer of 2007. Thanks to Diana Cionoiu, Justin Karneges, and Justin Uberti for their input to those discussions. Thanks also to Kurt Zeilenga for his feedback on the resulting specification.

Appendices

Appendix A: Document Information¶

Series

XEP

Number

0276

Publisher

XMPP Standards Foundation

Status

Deferred

Type

Standards Track

Version

0.3

Last Updated

2012-07-13

Approving Body

XMPP Council

Dependencies

None

Supersedes

None

Superseded By

None

Short Name

NOT YET ASSIGNED

Source Control

HTML

This document in other formats: XML  PDF

Appendix B: Author Information¶

Simon McVittie

Email

simon.mcvittie@collabora.co.uk

JabberID

simon.mcvittie@collabora.co.uk

Peter Saint-Andre

Email

stpeter@stpeter.im

JabberID

stpeter@jabber.org

URI

https://stpeter.im/

Robert McQueen

Email

robert.mcqueen@collabora.co.uk

JabberID

robert.mcqueen@collabora.co.uk

Appendix C: Legal Notices¶

Appendix D: Relation to XMPP¶

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.

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 <https://xmpp.org/community/> 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 https://xmpp.org/extensions/attic/

1. Version 0.3 (2012-07-13)¶

   Tweaked the security considerations.

   psa
2. Version 0.2 (2012-07-11)¶

   Back by popular demand; also changed namespace to decloak (again).

   psa
3. Version 0.1 (2010-01-26)¶

   Initial published version as accepted for publication by the XMPP Council; more fully described the reasons for using directed presence instead of presence subscriptions; changed requested namespace from decloak to temppres.

   psa
4. Version 0.0.2 (2010-01-17)¶

   Rewrote the introduction, clarified the security considerations, requested issuance of an appropriate URN from the XMPP Registrar.

   psa
5. Version 0.0.1 (2010-01-05)¶

   First draft.

   smcv/psa/rm

Appendix I: Bib(La)TeX Entry

@report{mcvittie2010not yet assigned,
  title = {Presence Decloaking},
  author = {McVittie, Simon and Saint-Andre, Peter and McQueen, Robert},
  type = {XEP},
  number = {0276},
  version = {0.3},
  institution = {XMPP Standards Foundation},
  url = {https://xmpp.org/extensions/xep-0276.html},
  date = {2010-01-05/2012-07-13},
}

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