Question: Upon receiving a PDCP Data PDU for Signaling Radio Bearers (SRBs) with a Sequence Number (SN) less than Next_PDCP_RX_SN, how does the User Equipment (UE) handle the COUNT for deciphering and integrity verification?
Option 1: Using RX_HFN + 1 and the received PDCP SN
Option 2: Using RX_HFN and the received PDCP SN
Option 3: Based on the last successfully verified PDCP SN
Option 4: Randomly generated COUNT for enhanced security
Explanation: When the UE receives a PDCP Data PDU for SRBs with a Sequence Number (SN) less than Next_PDCP_RX_SN, it indicates an out-of-order delivery scenario where the UE needs to adjust its handling of the Hyper Frame Number (HFN) for security operations. Specifically, for deciphering and integrity verification in such cases, the UE uses COUNT based on RX_HFN + 1 combined with the received PDCP SN. This approach ensures secure processing of out-of-sequence packets without compromising the integrity of the communication.