Question: Upon expiration of t-Reassembly in a UM (Unacknowledged Mode) RLC entity, what action(s) are taken if RX_Next_Highest equals RX_Next_Reassembly + 1 and there's at least one missing byte segment before the last byte of all received segments of the associated RLC SDU?
Option 1: Only update RX_Next_Reassembly to the SN of the first unreassembled segment.
Option 2: Discard all segments with SN < updated RX_Next_Reassembly, but do not restart t-Reassembly.
Option 3: Start t-Reassembly, set RX_Timer_Trigger to RX_Next_Highest, and discard all segments with SN < updated RX_Next_Reassembly.
Option 4: Ignore the expiration and continue without updating any timers or sequence numbers.
Explanation: When t-Reassembly expires under specified conditions (RX_Next_Highest = RX_Next_Reassembly + 1 with at least one missing byte segment), the UM RLC entity takes comprehensive action: starting a new t-Reassembly timer, updating RX_Timer_Trigger to RX_Next_Highest, and discarding segments with SN less than updated RX_Next_Reassembly. This ensures seeking complete reassembly while eliminating obsolete data, optimizing reliability and throughput.