The &xep0166; extensions to XMPP provide a technology for setup, management, and teardown of multimedia sessions between two entities, with an initial focus on voice over Internet Protocol (VoIP). By design, Jingle has been kept relatively simple and it does not cover the kind of advanced features that are available on the public switched telephone network (PSTN) and traditional private branch exchange (PBX) systems. However, because Jingle and XMPP itself provide an extensible technology for the real-time exchange of XML data, more advanced use cases can be defined through additional extensions. This document specifies one such extension, for the transfer of a session from one entity to another entity; although this extension will likely be used mainly in the context of VoIP interactions, it could also be used for any Jingle application type, such as video chat or screen sharing.

The session flow for negotiating an unattended transfer is as follows:

The protocol flow is shown below, where the caller is "caller@world.sun", the attendant is "attendant@office.biz", and the callee is "boss@bigdesk.biz".

First the caller initiates a normal call to the attendant.

The attendant's phone then acknowledges the session request.

Next the attendant answers the call.

The caller acknowledges the session-accept.

Now the attendant decides to transfer the call. It does this by sending a Jingle action of type session-info to the caller, specifying the address of the callee via an <transfer/> element qualified by the "urn:xmpp:jingle:transfer:0" namespace &VNOTE;.

If the caller understands the transfer request, it acknowledges the request (if not, it MUST return a &feature; error as specified in XEP-0166).

Now the caller puts the attendant on hold.

While the attendant is on hold, the caller initiates a new call to the callee. The session-initiation request includes an <transfer/> element that specifies the attendant's address.

The callee acknowledges the call.

Now the caller's phone detects the successful transfer, so it hangs up on the attendant:

The session flow for negotiating an unattended transfer is as follows:

The protocol flow is shown below, where the caller is "caller@world.sun", the attendant is "attendant@office.biz", and the callee is "boss@bigdesk.biz".

First the caller initiates a normal call to the attendant.

The attendant's phone then acknowledges the session request.

Next the attendant answers the call.

The caller acknowledges the session-accept.

Next the attendant makes a call to the callee for the purpose of making an attended transfer:

The callee acknowledges the session-initiate.

Now the attendant decides to transfer the call by sending a session-info action to the caller containing details about the attendant's session with the callee.

If the caller understands the transfer request, it acknowledges the request (if not, it MUST return a &feature; error as specified in XEP-0166).

Now the caller puts the attendant on hold.

While the attendant is on hold, the caller initiates a new call to the callee. The session-initiation request includes an <transfer/> element that specifies the attendant's address and the SessionID of the attendant's session with the callee.

The callee identifies an active session with the same from+to+sid and replaces that with the incoming call, so it hangs up on the existing session with the attendant.

The callee then acknowledges the session request from the caller.

Now the caller's phone detects the successful transfer, so it hangs up on the attendant:

If an entity supports session transfers, it MUST advertise that fact by returning a feature of "urn:xmpp:jingle:transfer:0" &VNOTE; in response to &xep0030; 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 &xep0115;. However, if an application has not received entity capabilities information from an entity, it SHOULD use explicit service discovery instead.

To follow.

This document requires no interaction with &IANA;.

Thanks to Robert McQueen for his feedback.