Question: In an IMS session establishment, what is the primary purpose of the P-CSCF sending a Diameter AAR (Authentication and Authorization Request) to the PDF after receiving negotiated SDP parameters?
Option 1: To request IP address allocation for down-link media flows
Option 2: To initiate a new SIP signalling handshake with the UE
Option 3: To obtain an authorization token for the negotiated session resources (e.g., bandwidth, QoS)
Option 4: To notify the PDF of the termination of the IMS session
Explanation: The correct answer is Option 3 because, as outlined in the procedure, after identifying the necessary connection information for both up-link and down-link media flows (steps 2 and 5), the P-CSCF forwards this derived session information to the PDF and requests an authorization token via a Diameter AAR. This step is crucial for ensuring that the resources required for the IMS session (e.g., bandwidth, specific QoS parameters) are authorized by the network before the session is established with the UE. The authorization token subsequently sent by the PDF confirms that the requested resources are allocated and approved for use.