Question: When an Application Function (AF) subscribes to notifications of the status of the AF Signalling transmission path using an Rx Diameter session with a Policy and Charging Rules Function (PCRF), which AVP must be included in the Media-Component-Description AVP with a specific value for the subscription to be valid?
Option 1: Flow-Usage AVP set to "NORMAL_DATA"
Option 2: Media-Component-Number AVP set to "1"
Option 3: Flow-Usage AVP set to "AF_SIGNALLING"
Option 4: Framed-Ipv6-Prefix AVP with any valid IPv6 prefix
Explanation: For an AF to successfully subscribe to notifications of the status of the AF Signalling transmission path, it must provide specific attributes in the Diameter AA-Request command. Among these, setting the Flow-Usage AVP to "AF_SIGNALLING" within a single Media-Sub-Component AVP (contained in the Media-Component-Description AVP) is crucial. This specifies that the subscription is for AF signalling, distinguishing it from other types of data flows. Thus, Option 3 is the correct answer as it directly addresses the requirement for valid subscription to the specified notifications.