TCP RST的序列号

如何检查一个tcp rst包的序列号是否正确，tcp的RFC是这样规定的：
只要tcp rst的序列号在窗口之内，就是一个正确的tcp包。言下之意就是，如果当前的sequence是x，windown是y，而window scale是z，那么，只要rst的sequence是：
| x - rst_sequence | <= y << z

tcp的rfc的原文如下：
In all states except SYN-SENT, all reset (RST) segments are validated by checking their SEQ-fields. A reset is valid if its sequence number is in the window. In the SYN-SENT state (a RST received in response to an initial SYN), the RST is acceptable if the ACK field acknowledges the SYN.