The bold is my attempt at answering the question. Please correct as necessary.

In this part, you are given different scenarios for which the sender initially transmits some PDUs, and different situations occur after initial transmission. For each scenario, indicate a complete and numbered series of steps which describe exactly how the receiver ultimately successfully receives all transmitted PDUs, assuming continuous ARQ is being used with a sliding window of 2. Recall that continuous ARQ is capable of retransmitting PDUs when necessary. In your steps, the sender should successfully receive acknowledgment (ACK) from the receiver for all transmitted PDUs. The final step for each scenario will thus be receipt of the last ACK.

The sender transmits two PDUs without error, but both PDUs are lost before they arrive at the receiver.

1. Sender sends PDU1 and PDU2 2. Both PDU1 and PCU2 are lost 3. After timeout, sender resends PDU1 and PDU2 4. Receiver receives PDU1 and PDU2 and detects no errors 5. Receiver sends ACK1 and ACK2 6. Sender receives ACK3 and ACK4