Question: What action does the network take when it receives a UL NAS TRANSPORT message with an unassigned or reserved PDU session identity value and the Request type IE is not set to "initial request" or "initial emergency request"?
Option 1: The network responds with a DL NAS TRANSPORT message with 5GMM cause #43 "invalid PDU session identity".
Option 2: The network responds with a DL NAS TRANSPORT message with 5GMM cause #90 "payload was not forwarded".
Option 3: The network ignores the message.
Option 4: The network sends a PDU SESSION RELEASE REJECT message.
Explanation: According to the specified procedures, when the network receives an UL NAS TRANSPORT message with an unassigned or reserved PDU session identity value and the Request type IE is not set to "initial request" or "initial emergency request", it responds with a DL NAS TRANSPORT message with 5GMM cause #90 "payload was not forwarded".