IP校验和

   #include <stdio.h>
   #include <unistd.h>
   #include <linux/if_ether.h>
   #include <linux/ip.h>
   #include <linux/udp.h>
   #include <linux/types.h>

   unsigned short check_sum(unsigned char *data, int len);

  int main()
  {
          unsigned char data[] = {
          0x45, 0x00,
          0x00, 0x20, 0x00, 0x00, 0x40, 0x00, 0x40, 0x11,
          0x00, 0x00, 0xc0, 0xa8, 0x1f, 0x72, 0xc0, 0xa8,
          0x1f, 0x7a
          };

          unsigned short ret = ;
          ret = check_sum(data, );
          printf("check sum is %x\n", htons(ret));
  }

  unsigned short check_sum(unsigned char *data, int len)
  {
          unsigned short ret = ;
          int i = ;
          int sum = ;
          unsigned short *p = (short *)data;
          for(i=; i<len/; i++)
                  sum = sum + ntohs(p[i]);
          if(sum > 0xffff){
                  sum = (sum & 0x0ffff) + ((sum & 0xf0000) >> );
          }

          ret = (short)sum;
          return ~ret;
  }

1. 0x00 0x00
2. 按两字节相加完回滚（如果结果是2856D，则2要按0002 继续加 856D）
3. ～
4. htons()