ent location. Laura's UA INVITEs Bob using the new URL provided and terminates the previous session. Note that if Laura did not terminate the first call at this juncture, they could have easily established a three-party conference using the same mechanism. How It's Done To provide session transfer functionality, a new method was defined: REFER [draft-ietf-sip-cc-transfer] The idea behind this mechanism is that one SIP entity instructs another to perform a certain action. Namely, the REFER method instructs a server to send a specific request to a certain URL. Figure 6-17 illustrates the REFER mechanism. Laura calls Bob at his office and gets his secretary instead. Laura explains that she wants to speak to Bob. The secretary puts Laura on hold by send- ing a re-INVITE with a session description indicating call hold. Then she Extending SIP: The SIP Toolkit Figure 6-17 Please hold Bob's secretary Bob's while I transfer transfers the call Laura secretary your call. to Bob. (1) INVITE (2) 200 OK (3) ACK Conversation (4) INVITE (hold) (5) 200 OK (6) ACK (7) REFER Refer-To: SIP: Bob.Johnson@company.com Referred-By: SIP: Secretary@company.com (8) 202 Accepted (9) INVITE Referred-By: SIP: Secretary@company.com (10) 200 OK (11) ACK (12) NOTIFY (13) 200 OK (14) BYE (15) 200 OK Conversation sends a REFER to Laura instructing her to send a new INVITE to Bob's current location. The Refer-To header of the request contains the URL where Laura has to send her new INVITE. The Referred-By header con- tains the secretary's SIP URL. Laura copies the Requested-By header into her new INVITE and sends it to the SIP URL contained in the Request- To header. When Bob receives this INVITE, he knows it is the result of a transfer made by his secretary because the Referred-By header contains her SIP URL. Chapter 6 When the INVITE transaction is completed, Laura sends a NOTIFY indicating the result of the transfer. In our example, the transfer ended suc- cessfully, SO Bob's secretary sends a BYE to terminate her call with Laura. If the transfer