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 more recent 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 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 SHOULD NOT include the <desc/> element unless that information is deemed necessary to enable communication.

A recipient SHOULD 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.

This document requires no interaction with &IANA;.