计算ICMP头的校验和：
计算ICMP头的校验和：USHORT checksum(USHORT* buff, int nSize)
{
unsigned long cksum=0;
//将数据以字为单位加到cksum
while (nSize > 1)
{
cksum += *buff++;
nSize -= sizeof(USHORT);
}
//如果为奇数, 将最后一个字扩展到双字，再累加
if (nSize)
{
cksum += *(UCHAR*)buff;
}
//将cksum的高16位与低16位相加，取反后得到校验和
cksum = (cksum>>16) + (cksum&&0xffff);
cksum += (cksum>>16);
return (USHORT)(~cksum);
}
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