帮忙解释一下ICMP校验和算法(尤其是红色部分)
/*校验和算法*/
unsigned short cal_chksum(unsigned short *addr,int len)
{
int nleft = len;
int sum = 0;
unsigned short *w = addr;
unsigned short answer = 0;
/*把ICMP报头二进制数据以2字节为单位累加起来*/
while (nleft > 1)
{
sum += *w++;
nleft -= 2;
}
/*若ICMP报头为奇数个字节,会剩下最后一字节。把最后一个字节视为一个2字节数据的低字节,这个2字节数据的高字节为0,继续累加*/
if (nleft == 1)
{
//*(unsigned char *)(&answer)=*(unsigned char *)w;
//answer = (*(unsigned char *)w) << 8;
//printf("answer = %x\n", answer);
sum += *(unsigned char *)w;
printf("sum = %x\n", sum);
}
sum = (sum>>16) + (sum&0xffff);
sum += (sum>>16);
answer =~ sum;
return answer;
}