5.3.3. Appearance Numbers and Transfer

During a transfer operation, it is important that the appearance number not change during the operation. Consider the example of Alice, a member of a shared appearance group, who is talking to Carol, who is outside the shared appearance group. Carol transfers Alice to David, who is also outside the shared appearance group. For example, if Alice is using appearance 3 for the session with Carol, the resulting session with David should also use appearance number 3. Otherwise, an appearance number change can cause a "jump" on the UI and confusion to the user. There are two possible scenarios using the terminology of RFC 5589: Alice is the transferee in any type of transfer (receives the REFER) or the transfer target in an attended transfer (receives the INVITE with Replaces).

If Alice is the transferee, the triggered INVITE from the REFER is treated as a consultation call. Alice SHOULD publish requesting that the Appearance Agent not assign an appearance number for this INVITE. When the transfer completes, Alice SHOULD publish again to move the appearance number from the dialog with Carol to the dialog with David. If a PUBLISH is sent to move the appearance number, the publication MUST be sent prior to sending the BYE to Carol to avoid a race condition where the Appearance Agent reassigns the appearance number after seeing the BYE.

If Alice is the target, the incoming INVITE will contain a Replaces header field. As a result, the Appearance Agent will have reused the appearance number of the dialog with Carol, and this appearance number will continue to be used after the dialog with Carol has been terminated.