Question: When is the IP header compression configuration IE included in a message?
Option 1: Only when the UE and the network support user plane CIoT 5GS optimization.
Option 2: When the UE wishes to re-negotiate IP header compression configuration associated with a PDU session, or after an inter-system change from S1 mode to N1 mode, provided both the UE and the network support control plane CIoT 5GS optimization and IP header compression.
Option 3: Only when the UE is operating in dual-registration mode in the network supporting N26 interface.
Option 4: When the UE and the network do not support IP header compression.
Explanation: The correct answer is Option 2 because, according to the specification, the IP header compression configuration IE is included in a message under two conditions: (a) when the UE wishes to re-negotiate IP header compression configuration associated with a PDU session and both the UE and the network support control plane CIoT 5GS optimization and IP header compression; or (b) after an inter-system change from S1 mode to N1 mode, provided both the UE and the network support control plane CIoT 5GS optimization and IP header compression, and the UE is operating in single-registration mode in the network supporting N26 interface.