in_cksum(addr, len)u_short *addr;int len;{        register int nleft = len;        register u_short *w = addr;        register u_short answer;        register int sum = 0;        /*         *  Our algorithm is simple, using a 32 bit accumulator (sum),         *  we add sequential 16 bit words to it, and at the end, fold         *  back all the carry bits from the top 16 bits into the lower         *  16 bits.         */        while( nleft > 1 )  {                sum += *w++;                nleft -= 2;        }        /* mop up an odd byte, if necessary */        if( nleft == 1 ) {                u_short u = 0;                *(u_char *)(&u) = *(u_char *)w ;                sum += u;        }      /*         * add back carry outs from top 16 bits to low 16 bits         */       sum = (sum >> 16) + (sum & 0xffff); /* add hi 16 to low 16 */ （将高位的数值加到低位）

sum += (sum >> 16); /* add carry */ （如果超过16位的最大的情况也加起来） 

answer = ~sum; /* truncate to 16 bits */ （取反）

return (answer);}