Internet Communications Using SIP : Delivering VoIP and Multimedia Services With Session Initiation Protocol {2Nd Ed.}

Basic Third-Party Call Control

The concept for basic third-party call control is shown in Figure 11.5, where a
controller will set up a call between two parties, A and B, without participat-
ing in the conversation in any way.
The operation is as follows:

1. The controller sets up a call with the first party (party A) by sending an
   INVITEmessage (Message 1 in Figure 11.5). This INVITEhas an SDP
   message body with no media lines. Party A responds with a 200 OK
   also containing SDP with no media lines. The controller then sends an
   ACKto complete the exchange. This sets up a SIP session or dialog but
   does not establish a media session.


2. The controller will now set up a call with the second party (party B) by
   sending a second INVITE(Message 4 in Figure 11.5) without any SDP
   present. This time, the controller stores the SDP data received from
   party B in the 200 OKmessage (Message 5). The controller holds off
   sending the ACKto party B until this media information is communi-
   cated to party A in the next step.


3. Next, the controller re-INVITEs party A using the SDP connection and
   media data supplied by party B in the previous 200 OKmessage (Mes-
   sage 5 in Figure 11.5). Party A responds with SDP media information in
   the 200 OK(Message 7). This SDP information is then passed back to
   party B in an ACK(Message 8). An ACKwithout SDP to A completes the
   session setup. Parties A and B can now exchange Real-Time Protocol
   (RTP) media, since they both have the required SDP connection and
   media data from each other.


4. From a signaling perspective, parties A and B are still communicating
   with the controller and not with each other. To terminate the call, any
   party can send a BYEmessage to terminate the connection to the con-
   troller (as shown in Message 11 in Figure 11.5). The controller will fol-
   low up by sending a BYEto the other party (Message 12). Both BYE
   messages are followed by 200 OKmessages (Messages 13 and 14), and
   the call is terminated.

Security for Third-Party Call Control

Third-party call control is simple to implement in well-secured IP networks
where no security risks are assumed within the trusted environment. In a
larger context, however, additional steps must be taken to authenticate the

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