IP首部校验和计算

根据RFC1071文档的计算方法,编写代码实现IP首部校验和的计算

计算步骤:

1.首先将IP首部中校验和字段置0

2.将IP首部每16bit进行相加,如果有进位产生,则将进位加到最低位。

3.将计算的结果取反即可

 unsigned short checksum(int count,unsigned short* addr)
 {
     long sum = ;

     while(count > )
     {
         sum   +=*addr++;
         count -= sizeof(unsigned short);
     }

     if(count > )
     {
         sum  +=*(unsigned char*)addr;
     }

     while(sum >> )
     {
         sum = (sum & 0xFFFF) + (sum >> );
     }

     return (unsigned short)(~sum);
 }
 int main(int argc,char* argv[])
 {
     unsigned char IpHeader[] = "\xa1\x00\x00\x45\x00\x00\x21\x2c\x00\x00\x11\x01\x2f\x0a\x1e\xac\xfa\xff\xff\xef"; //抓包实例  检验和是0xe6e3

     printf("0x%0x\n",checksum(sizeof(IpHeader)-,(unsigned short *)IpHeader));

     return ;
 }