网际校验算法的代码的理解

C/C++ code

    unsigned short checksum(unsigned short *buf,int nword)    {        unsigned long sum;                for(sum=0;nword>0;nword--)            sum += *buf++;    sum = (sum>>16) + (sum&0xffff);        sum += (sum>>16);             return ~sum;    }

其中,以下代码，我没有理解，求高手详细解答,感谢分享！

C/C++ code

sum = (sum>>16) + (sum&0xffff);        sum += (sum>>16);   

[解决办法]
可以将unsigned long sum;看成两个unsigned short
设为unsigned short sum1;（高位），unsigned short sum2;（低位）
sum>>16即为sum1，sum&0xffff即为sum2
也就是说除了原来buf的加运算，每次得到的结果unsigned short溢出的部分也参与 加 运算