IP包校验和

RFC 1071中定义了Checksum的算法：

   (1)  Adjacent octets to be checksummed are paired to form 16-bit
        integers, and the 1's complement sum of these 16-bit integers is
        formed.

   (2)  To generate a checksum, the checksum field itself is cleared,
        the 16-bit 1's complement sum is computed over the octets
        concerned, and the 1's complement of this sum is placed in the
        checksum field.

   (3)  To check a checksum, the 1's complement sum is computed over the
        same set of octets, including the checksum field.  If the result
        is all 1 bits (-0 in 1's complement arithmetic), the check
        succeeds.



算法的C语言实现：

   {
           /* Compute Internet Checksum for "count" bytes
            *         beginning at location "addr".
            */
       register long sum = 0;

       while( count > 1 )  {
           /*  This is the inner loop */
               sum += * (unsigned short) addr++;
               count -= 2;
       }

           /*  Add left-over byte, if any */
       if( count > 0 )
               sum += * (unsigned char *) addr;

           /*  Fold 32-bit sum to 16 bits */
       while (sum>>16)
           sum = (sum & 0xffff) + (sum >> 16);

       checksum = ~sum;

       return checksum;
   }

计算校验和的算法思路：

1. 将原Checksum位置全部置0，把报头中每16bit作为一组，当成无符号数相加得到sum，若报头字节数为单数，则最后一个字节直接当8bit无符号数加到前面的sum上去。

2. 对sum右移16位，并与原sum的低16位相加，得到结果重新赋给sum。

3. 直到sum的高16位为0时，再将sum取反（反码），返回sum值作为Checksum。


接收端校验过程：

接收到的结果再进行一次上述计算，得到返回的值应该为0。否则，报头在传输过程中出错。