ion to INVITE the conference unit. Because this sec- ond INVITE contains the session description provided by Bob's computer, the conference unit will send the audio stream to Bob's computer. It responds with a 200 OK. Bob receives the session description of the con- ference unit in this 200 OK and sends it to his computer in an ACK. Now the media stream flows directly between the conference unit and Bob's com- puter while SIP signalling traverses Bob's SIP mobile. Chapter 6 The most important feature of this message flow is that it uses nothing but core SIP. Therefore, simple UAs with minimum SIP support can be con- trolled by a SIP controller. Session Transfer We have seen that third-party call control enables a SIP entity to be in con- trol of session signalling while the media is exchanged between other enti- ties. In some situations, the controller won't want to continue monitoring (controlling) the signalling of the session. Instead, he or she will want the other entities to continue the session independently. At this point, the con- troller needs a mechanism to transfer SIP sessions to another entity. The most common example of session transfer occurs when I make a phone call and it's answered by the callee's secretary. I tell her the purpose of my call and whom I want to contact. Then the secretary transfers my call to the covering person or voice mailbox. This is an example of session trans- fer because once the transfer has succeeded, the secretary immediately stops receiving any signalling or media related to that session. A SIP extension provides session transfer functionality. In our example, it enables the secretary to transfer the call. Furthermore, it enables her to check whether the transfer was successful or not. In case the transfer has failed, the secretary can regain control of the call for a second attempt. For example, Laura calls Bob's SIP Universal Resource Locator (URL), but his secretary picks up the phone. In order to transfer the call, she instructs Laura's UA to INVITE Bob at his curr