Question: Upon receiving a PDCP Data PDU from lower layers, how does the receiving PDCP entity determine `RCVD_HFN` when `RCVD_SN` is greater than or equal to `SN(RX_DELIV) + Window_Size`?
Option 1: `RCVD_HFN` = `HFN(RX_DELIV)`
Option 2: `RCVD_HFN` = `HFN(RX_DELIV) - 1`
Option 3: `RCVD_HFN` is calculated based on the PDCP SN length and HFN length of the State Variable
Option 4: `RCVD_HFN` is directly extracted from the PDU header
Explanation: When `RCVD_SN >= SN(RX_DELIV) + Window_Size`, the receiving PDCP entity sets `RCVD_HFN` to `HFN(RX_DELIV) - 1`. This rule manages the High Frame Number (HFN) part of the State Variable, ensuring proper sequencing and integrity verification of received PDCP Data PDUs.