LTE中RLC关于状态报告触发的问题

romiyang

Triggers to initiate STATUS reporting include:
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Polling from its peer AM RLC entity:
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When a RLC data PDU with SN = x and the P field set to “1” is received from lower layer, the receiving side of an AM RLC entity shall:
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if the PDU is to be discarded as specified in subclause 5.1.3.2.2; or
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if x < VR(MS) or x >= VR(MR):
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trigger a STATUS report;
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else:
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delay triggering the STATUS report until x < VR(MS) or x >= VR(MR).
NOTE 1:
This ensures that the RLC Status report is transmitted after HARQ reordering.

上面这段话中讲明P字段设为“1”的序列号SN< VR(MS) or SN>= VR(MR)时会触发状态报告，并说这是为了保证HARQ排序后传送状态报告。
小弟不明白的有两点：1、为什么要保证HARQ排序后传送状态报告？2、为什么x < VR(MS) or x >= VR(MR)时会触发状态报告。希望有高手指点。