請問為什么PDCP下行數(shù)據(jù)傳輸時，Received PDCP SN已經(jīng)出窗，但卻不直接丟棄，而是先解壓縮解密后再丟棄。

if received PDCP SN – Last_Submitted_PDCP_RX_SN > Reordering_Window or 0 <= Last_Submitted_PDCP_RX_SN – received PDCP SN < Reordering_Window:
    -     if received PDCP SN > Next_PDCP_RX_SN:
        -     decipher the PDCP PDU as specified in the subclause 5.6, using COUNT based on RX_HFN - 1 and the received PDCP SN;
    -     else:
        -     decipher the PDCP PDU as specified in the subclause 5.6, using COUNT based on RX_HFN and the received PDCP SN;
-     perform header decompression (if configured) as specified in the subclause 5.5.5;
-     discard this PDCP SDU;

協(xié)議里寫的是解密后無條件丟棄，既然要無條件丟棄，還要解密做什么？

不把數(shù)據(jù)解出來，如何知道該數(shù)據(jù)是否有用那？