Question: What happens when a PDCP SDU's discardTimer expires or its successful delivery is confirmed, in relation to both the PDCP SDU and its corresponding PDCP PDU?
Option 1: The UE holds the PDCP SDU for potential retransmission while discarding the PDCP PDU.
Option 2: The UE only discards the PDCP SDU, keeping the corresponding PDCP PDU for further processing.
Option 3: If the PDCP PDU has been submitted to lower layers, the UE requests its return before discarding both.
Option 4: The UE discards both the PDCP SDU and its corresponding PDCP PDU, notifying lower layers if the PDU was already submitted.
Explanation: This is correct because when a PDCP SDU's discardTimer expires or its delivery is confirmed, the UE is required to discard both the SDU and its associated PDU. Additionally, if the PDU had been passed on to lower layers, the UE must indicate this discard to those layers, ensuring consistency across the protocol stack. This process ensures efficient resource management and minimizes unnecessary data retention.