5.3
SDU discard procedures
When indicated from upper layer (i.e. PDCP) to discard a particular RLC SDU, the transmitting side of an AM RLC entity or the transmitting UM RLC entity shall discard the indicated RLC SDU if no segment of the RLC SDU has been mapped to a RLC data PDU yet.
当上层指示RLC应丢弃某个SDU时，应如何实现这个过程呢，定义原语时是不是应该单独定义一条啊，还有RLC如何向上层报是否已经丢弃呢？