Question: In the context of Performance Management (PM) for IP Multimedia Subsystem (IMS), what triggers the measurement counter for 'Call release initiated by IM CN subsystem' in the MGCF, specifically for the `CC.RelCallsImInit.BYE` subcounter?
Option 1: Receipt of a SIP_BYE message by the MGCF
Option 2: Receipt of a SIP_INVITE message by the MGCF
Option 3: Successful call establishment between two IMS users
Option 4: Failure to establish a call due to insufficient network resources
Explanation: According to the specified performance measurement for 'Call release initiated by IM CN subsystem', the `CC.RelCallsImInit.BYE` subcounter is incremented upon the receipt of specific SIP messages by the MGCF. Among the provided options, only the receipt of a SIP_BYE message directly corresponds with the incrementing condition for this particular subcounter, as it signifies the initiation of call release. This highlights the importance of accurately tracking call termination events in IMS performance management to ensure network efficiency and diagnose potential issues.