When a PDCP entity over a UM DRB is re-established, or when a cell handover occurs and a PDCP entity over a UM DRB uses a key used before the handover, the PDCP entity determines a first SDU, where the first SDU is an SDU that is associated with a sequence number by the PDCP entity but whose corresponding data has not been transmitted through an air interface; and delivers a PDU corresponding to the first SDU to an RLC entity.