A method performed by a terminal (la-35) in a wireless communication system, the method comprising: receiving (1n-05), from a base station (la-05), a message including first information indicating a packet data convergence protocol, PDCP, re-establishment for an acknowledged mode, AM, data radio bearer, DRB; storing (1n-01) PDCP packets, received from a radio link control, RLC, entity, in a reception buffer, based on a predefined condition being satisfied, wherein each PDCP packet is delivered from the RLC entity to the PDCP entity, based on all segments corresponding to the each PDCP packet being received and reassembled in the RLC entity; performing (1n-10) header decompression for the stored PDCP packets, in case that second information is not configured in the message, the second information indicating whether the PDCP entity for the AM DRB continues or resets a header compression protocol during the PDCP re-establishment; resetting (1n-15) the header compression protocol, after the header decompression for the stored PDCP packets; and in case that a PDCP packet received from a lower layer is corresponding to a first PDCP packet among PDCP packets which are not delivered to an upper layer of the PDCP entity, delivering at least one PDCP packet among the PDCP packets, the PDCP packets including the stored PDCP packets for which the header decompression is performed, wherein the first PDCP packet has an earliest count value among count values of the PDCP packets, and wherein each of the at least one PDCP packet with a consecutive count value starting from the earliest count value is delivered to the upper layer in an ascending order. receiving (1n-05), from a base station (la-05), a message including first information indicating a packet data convergence protocol, PDCP, re-establishment for an acknowledged mode, AM, data radio bearer, DRB; storing (1n-01) PDCP packets, received from a radio link control, RLC, entity, in a reception buffer, based on a predefined condition being satisfied, wherein each PDCP packet is delivered from the RLC entity to the PDCP entity, based on all segments corresponding to the each PDCP packet being received and reassembled in the RLC entity; performing (1n-10) header decompression for the stored PDCP packets, in case that second information is not configured in the message, the second information indicating whether the PDCP entity for the AM DRB continues or resets a header compression protocol during the PDCP re-establishment; resetting (1n-15) the header compression protocol, after the header decompression for the stored PDCP packets; and in case that a PDCP packet received from a lower layer is corresponding to a first PDCP packet among PDCP packets which are not delivered to an upper layer of the PDCP entity, delivering at least one PDCP packet among the PDCP packets, the PDCP packets including the stored PDCP packets for which the header decompression is performed, wherein the first PDCP packet has an earliest count value among count values of the PDCP packets, and wherein each of the at least one PDCP packet with a consecutive count value starting from the earliest count value is delivered to the upper layer in an ascending order. <EOS>