hapter 6 Conference Figure 6-14 Bob receives a Laura NOTIFY when (1) INVITE Laura joins the (2) 200 OK conference call. (3) ACK (4) SUBSCRIBE Number of participants (5) 200 OK (6) NOTIFY There are currently 6 participants (7) 200 OK (8) INVITE (9) 200 OK (10) ACK (11) NOTIFY There are currently 7 participants (12) 200 OK However, Bob only has his SIP mobile phone with him, and it has very limited storage capacity. He'll need to establish a session between the con- ference unit and his home PC. To resolve his problem, Bob resorts to third- party call control, which enables a SIP entity to manage sessions between other parties. A controller that establishes a session between two partici- pants will be involved in the SIP signalling, but not necessarily in the ses- sion media. In our example (Figure 6-15), Bob becomes the controller and establishes an audio session between the conference unit and his home desktop. How It's Done Third-party call control [draft-rosenberg-sip-3pcc] does not require any extension to the core SIP specification. The controller invites the participants and then exchanges the session descriptions between them. Figure 6-16 shows how this example is implemented. Bob sends an INVITE without any session description. Because Bob's computer receives an empty INVITE, it expects that the session description will be sent in the ACK instead. Bob's computer follows normal SIP procedures and returns a session description in a 200 OK response to Bob who then uses Extending SIP: The SIP Toolkit Figure 6-15 Bob establishes a session between the conference unit and SIP signalling between Bob his computer at and his computer at home SIP signalling between Bob home. and the conference unit Bob's Media session between computer his computer and the conference unit Conference Bob's Conference Figure 6-16 Third-party computer call control (1) INVITE message flow. no SDP (2) 200 OK SDP computer (3) )INVITE SDP computer (4) 200 OK SDP conference unit (5) ACK SDP conference unit (6) ACK no SDP this session descript