Question: Upon receiving a ProSe Subscriber Information Retrieval Request, what action does the Home Subscriber Server (HSS) take if there is no ProSe subscription data for the existing IMSI?
Option 1: Return an Experimental-Result of DIAMETER_ERROR_USER_UNKNOWN
Option 2: Return an Experimental-Result of DIAMETER_ERROR_UNKNOWN_PROSE_SUBSCRIPTION
Option 3: Immediately update the subscription data for the IMSI before responding
Option 4: Deny access to ProSe services with a Result-Code of DIAMETER_ERROR_PROSE_NOT_ALLOWED
Explanation: According to the detailed behavior of the HSS, if the IMSI exists but there is no ProSe subscription data for the IMSI, the HSS specifically returns an Experimental-Result of DIAMETER_ERROR_UNKNOWN_PROSE_SUBSCRIPTION. This response indicates that while the user (identified by their IMSI) is known to the system, there is a lack of specific ProSe subscription information associated with them.