Method for handling state variables of a pdcp entity in wireless communication system and a device therefor

A receiving device operating in a wireless communication system, the UE comprising:

• a transceiver;
• at least one memory;
• and
• at least one processor operably connectable to the at least one memory and storing instructions that, when executed by the at least one processor, perform operations comprising:
• receiving a packet data convergence protocol (PDCP) data protocol data unit (PDU);
• when a reordering timer is not running, starting the reordering timer;
• and
• updating a first count value following a count value associated with the PDCP Data PDU which triggered the reordering timer, to a second count value of a next PDCP service data unit (SDU) expected to be received;
• when the reordering time expires, updating a third count value related to a lower edge of a reordering window, based on a count value of a first PDCP SDU which has not been delivered to upper layers, wherein the count value of the first PDCP SDU is equal to or higher than the first count value following the count value associated with the PDCP Data PDU which triggered the reordering timer.