Sometimes it is desirable to augment instant messaging (IM) with another real-time communications medium, such as a voice conversation conducted over the traditional public switched telephone network (PSTN) or Voice over Internet Protocol (VoIP) applications. In order to facilitate this functionality, a user needs to advertise the address(es) at which they can be reached. There are several possible ways to do this:
Publish the address(es) in the user's vCard (see vcard-temp (XEP-0054)  and vCard4 over XMPP (XEP-0292) ); this is convenient, but is not very dynamic (e.g., reachability addresses might change when the user moves to a new conference room in an office building).
Send the address(es) within a <presence/> stanza; this option is described in the Presence Transport section of this document and is consistent with RFC 6121  since reachability is one aspect of a user's availability for communication.
This document defines methods for publishing addresses at which a user can be reached temporarily, as opposed to semi-permanent addresses of the kind that are more appropriately communicated in a user's vCard. We illustrate this usage through the scenario of a user sending a reachability address upon entering a physical conference room and then sending an updated notification (without a reachability address) upon leaving the conference room. Similar scenarios might apply when an XMPP implementation is used with technologies such as WebRTC and in deployments of the combined use of SIP and XMPP ("CUSAX"; RFC 7081 ).
This document addresses the following requirements:
The following is an example of the data format for reachability addresses:
When publishing reachability addresses, the <reach/> element MUST contain at least one <addr/> element. Each <addr/> element MUST possess a 'uri' attribute, whose value MUST be the Uniform Resource Identifier (RFC 3986 ) or Internationalized Resource Identifier (RFC 3987 ) of an alternate communications method for reaching the user.
The <addr/> element MAY contain one or more <desc/> children whose XML character data is a natural-language description of the address; this element SHOULD possess an 'xml:lang' attribute whose value is a language tag that conforms to RFC 4646  (although the default language MAY be specified at the stanza level; see RFC 6120 ). In order to preserve bandwidth, the <desc/> element SHOULD NOT be included when sending reachability data via presence broadcast, but MAY be included when using directed presence or the personal eventing protocol.
This document specifies two methods of advertising reachability addresses:
This document does not recommend one transport method over the other.
In addition, a contact MAY request a user's reachability addresses in an XMPP <iq/> stanza of type "get" and a user MAY send reachability addresses in an XMPP <message/> stanza. However, the presence and PEP transport methods are preferred.
To broadcast reachability addresses in presence information, a user's client includes the <reach/> element in the <presence/> stanza it sends to its server.
For example, consider someone who walks into a conference room at the office. Via nearfield communication, the user's XMPP client might auto-discover a 'tel:' URI for the room audio system and a 'sip:' URI for the room video system.
The user's server then broadcasts that presence stanza to all entities who are subscribed to the user's presence:
(Naturally, a reachability address MAY alternatively be included in directed presence.)
Upon leaving the conference room, the user's client would send updated presence without the reachability extension.
To publish reachability addresses via the personal eventing protocol (XEP-0163), the entity publishes data to the "urn:xmpp:reach:0" node.
As above, when leaving the conference room the user's client would publish an updated payload indicating that it no longer has any temporary reachability addresses.
If an entity supports reachability addresses, it MUST advertise that fact by returning a feature of "urn:xmpp:reach:0" (see Namespace Versioning regarding the possibility of incrementing the version number) in response to a Service Discovery (XEP-0030)  information request.
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.
To preserve network bandwidth, the sender ought not include the <desc/> element unless that information is deemed necessary to enable communication.
A recipient ought to attempt communications with reachability addresses in the order that the <addr/> elements appear within the <reach/> element.
If included, the <desc/> element SHOULD possess an 'xml:lang' attribute specifying the language of the human-readable descriptive text for a particular address.
Security considerations for XMPP presence and PEP publication are described in RFC 6120, RFC 6121, XEP-0060, and XEP-0163.
Advertising a telephone number, SIP URI, or other real-time communication address to multiple contacts in an unencrypted way (e.g., via XMPP presence or PEP in cases where not all hops are TLS-protected) introduces the possibility of information leakage and subsequent attacks such as unsolicited phone calls. Clients are advised to appropriately warn users about the dangers of such attacks. Alternatively, if the address is especially sensitive (say, a hashname RFC 6920  for use in a system that enables direct private communication outside of XMPP), then a client could send it in a message that itself is end-to-end encrypted.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) .
This specification defines the following XML namespace:
Upon advancement of this specification from a status of Experimental to a status of Draft, the XMPP Registrar  shall add the foregoing namespace 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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Discussion on other xmpp.org discussion lists might also be appropriate; see <http://xmpp.org/about/discuss.shtml> for a complete list.
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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".
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Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/
Per a vote of the XMPP Council, advanced specification from Experimental to Draft.
Further clarified the intent of the protocol.
Modified the text and examples to better illustrate the primary use case.
Updated to reflect new XMPP RFCs; strengthened security considerations.
Defined PEP transport.
Initial published version.