Question: How should a PFCP entity handle a received mandatory Information Element (IE) with a value listed as 'reserved' in a Request message?
Option 1: Silently discard the IE and continue processing the rest of the message.
Option 2: Treat the IE as valid but log a warning for potential future reference.
Option 3: Send a response with Cause set to 'Mandatory IE incorrect' along with the type and instance of the offending IE, and log the error.
Option 4: Ignore the message entirely and wait for a retransmission.
Explanation: According to the principles outlined for handling semantically invalid Information Elements in PFCP signalling messages, if a mandatory IE is received with a reserved value (treated as invalid), the entity should send a response indicating the error ('Mandatory IE incorrect') and log the issue. This ensures clear communication of the problem back to the sender, facilitating corrective action.