Re: [lwip-users] pinging

[Marcin Prusinski]

> Here's some code obtained from elsewhere:
>
> static int inet_cksum(u_short *addr, int len)
> {
>      register int nleft = len;
>      register u_short *w = addr;
>      register u_short answer;
>      register u_int sum = 0;
>      u_short odd_byte = 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_char *)(&odd_byte) = *(u_char *)w;
>          sum += odd_byte;
>      }
>
>      /*
>       * add back carry outs from top 16 bits to low 16 bits
>       */
>      sum = (sum >> 16) + (sum & 0x0000ffff); /* add hi 16 to low 16 */
>      sum += (sum >> 16);                     /* add carry */
>      answer = ~sum;                          /* truncate to 16 bits */
>      return (answer);
> }
Thanks for the reply,
I have one more question about this checksum. In ICMP ECHO header -
there are those fields:
type - 8 bits
code - 8 bits
checksum - 16 bits
id 16 bits
and seq number 16 bits
in exactly this order. Which fields are being computed in checksum ?
Should I use /inet_chksum/ like that:
/inet_chksum(iecho,1)/ ?  - where iecho is pointer to icmp_echo_hdr .
Then what's the point of 16 bit chksum field for 16 bit of data ?

Marcin