> From: Todd Huang > > Transferor Transferee Transfer > | | Target > | | | > dialog1 | INVITE/200 OK/ACK | > |<-------------------| | > dialog1 | INVITE (hold)/200 OK/ACK | > |------------------->| | > dialog2 | INVITE/200 OK/ACK | > |------------------------------------------->| > ==> dialog2 | INVITE (hold)/200 OK/ACK | > |------------------------------------------->| > dialog3 | REFER > (Refer-To:sips:TransferTarget?Replaces=dialog2) > |------------------->| | > dialog3 | 202 Accepted | | > |<-------------------| | > > > Please see the messages headed by the "==>" sign. Why the > Transferor needs to hold the > Transfer Target before sending REFER to the Transferee?
>From the SIP point of view, there is no *need* to put the original dialog on-hold. I suspect that this operation was included in the flow because in many phones, the first user action would be to place the call on-hold, followed by pushing a "transfer" button. > If the Attended Transfer is implemented by the above call flow, how > could we distinguish the > operation between Attended Transfer and 3-way conference? Or the hold > operation to the > Transfer Target has other specific meaning? What device is "we"? Presumably Transferor device knows which action it is performing. The Target device has no certain way to know what is being done by the Transferor, but in PSTN telophony, it doesn't have that knowledge, either. Dale _______________________________________________ Sip-implementors mailing list Sip-implementors@cs.columbia.edu http://lists.cs.columbia.edu/mailman/listinfo/sip-implementors
