Question: In a UTRAN environment, when an RNC receives multiple MBMS SESSION START messages from different CN nodes for the same MBMS Bearer Service due to NNSF being active, but not all messages include the RA List of Idle Mode UEs IE, how does the RNC respond?
Option 1: The RNC sends an MBMS SESSION START RESPONSE to all CN nodes with a success indication.
Option 2: The RNC informs all CN nodes except one (towards which it confirmed successful MBMS RAB establishment) with an MBMS SESSION START FAILURE message, citing 'MBMS - Superseded Due To NNSF'.
Option 3: The RNC ignores the missing IE in subsequent messages and proceeds with the session start.
Option 4: The RNC sends a generic failure response to all CN nodes without specifying the cause.
Explanation: When NNSF is active and the RNC receives multiple MBMS SESSION START messages for the same MBMS Bearer Service with inconsistencies in the included IEs (e.g., missing RA List of Idle Mode UEs IE in some), it must inform the respective CN nodes about the inconsistency to ensure proper network operation. The correct response is to send an MBMS SESSION START FAILURE message to all but one CN node (the one with which successful RAB establishment was confirmed), specifying 'MBMS - Superseded Due To NNSF' as the cause, thereby providing a precise reason for the failure and enabling the core network to understand the outcome.