Re: ip checksum (offtopic)

[Xeno Campanoli <xeno@eskimo.com>]

Tim locke wrote:
> 
> Hey, Can someone show me an working example on how ip
> checksum is computed? I'm having a hard time figuring
> it out...
> 
> thanks
> 
> please CC me if you guys don't mind
> 
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Here's one I pulled off Google:

/*
**************************************************************************
Function: ip_sum_calc
Description: Calculate the 16 bit IP sum.
***************************************************************************
*/
typedef unsigned short u16;
typedef unsigned long u32;

u16 ip_sum_calc(u16 len_ip_header, u16 buff[])
{
u16 word16;
u32 sum=0;
u16 i;
    
        // make 16 bit words out of every two adjacent 8 bit words in
the packet
        // and add them up
        for (i=0;i<len_ip_header;i=i+2){
                word16 =((buff[i]<<8)&0xFF00)+(buff[i+1]&0xFF);
                sum = sum + (u32) word16;       
        }
        
        // take only 16 bits out of the 32 bit sum
        while (sum>>16)
          sum = (sum & 0xFFFF)+(sum >> 16);

        // one's complement the result
        sum = ~sum;
        
return ((u16) sum);
}

Which has the URI:

http://www.netfor2.com/ipsum.htm
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