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 using either attended transfer or unattended transfer (for the difference between these scenarios, see for example &rfc5359;). 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@example.net", the attendant is "attendant@office.example.com", and the callee is "boss@execs.example.com".

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 a <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 a <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 attended transfer is as follows:

The protocol flow is shown below, where the caller is "caller@example.net", the attendant is "attendant@office.example.com", and the callee is "boss@execs.example.com".

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 completing an attended transfer. Before doing so, the attendant SHOULD verify that the callee supports Jingle session transfer, as described under Determining Support.

The callee acknowledges the session-initiate.

Now the attendant transfers 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 a <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.

In unattended transfer, the callee has no way to verify that the attendant specified in the session-initiate request received from the caller was actually involved in the transaction. This implies that:

1. A malicious caller could attribute its session-initiate request to an attendant, thus discrediting the attendant in the eyes of the callee.
2. A malicious attendant (or malicious code that has infected an attendant's legitimate client) could "transfer" all session requests it receives to the callee and disavow any responsibility.

This document requires no interaction with &IANA;.

Thanks to Robert McQueen for his feedback.