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 &xep0054; and &xep0292;); 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 &rfc6121; since reachability is one aspect of a user's availability for communication.

• Send address(es) to the appropriate &xep0060; node; this option is described in the PEP Transport section of this document but might not be available at all service providers.

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"; &rfc7081;).

This document addresses the following requirements:

• Enable clients to dynamically publish reachability addresses.
• Minimize network traffic.

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 (&rfc3986;) or Internationalized Resource Identifier (&rfc3987;) 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 &rfc4646; (although the default language MAY be specified at the stanza level; see &rfc6120;). 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:

• Presence
• &xep0163; (PEP)

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.

If an entity supports reachability addresses, it MUST advertise that fact by returning a feature of "urn:xmpp:reach:0" &VNOTE; in response to a &xep0030; 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 &xep0115;. 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 &rfc6920; 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 &IANA;.