Hi Joakim,
On Mon, Sep 05, 2011 at 10:09:14PM +0200, Joakim Tjernlund wrote:
> > From: Baruch Siach <[email protected]>
> >
> > Use a single IP checksum routine for ping, traceroute and udhcp.
> >
> > Signed-off-by: Baruch Siach <[email protected]>
> > ---
> >
> > Changes from v1:
> > Fix inet_cksum() with odd length on big endian systems
> > Remove declaration of removed udhcp_checksum()
>
> This looks like it is from quagga. I cleaned that up/optimized it
> some years ago into this(you might want to fix the types):
What is the advantage of this implementation over the one I suggested (which I
took from the traceroute code)?
> int /* return checksum in low-order 16 bits */
> in_cksum(void *parg, int nbytes)
> {
> u_short *ptr = parg;
> register long sum; /* assumes long == 32 bits */
> register u_short answer; /* assumes u_short == 16 bits */
> register int count;
> /*
> * 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.
> */
> sum = 0;
> count = nbytes >> 1; /* div by 2 */
> for(ptr--; count; --count)
> sum += *++ptr;
>
> if (nbytes & 1) /* Odd */
> sum += *(u_char *)(++ptr); /* one byte only */
>
> /*
> * Add back carry outs from top 16 bits to low 16 bits.
> */
>
> sum = (sum >> 16) + (sum & 0xffff); /* add high-16 to low-16 */
> sum += (sum >> 16); /* add carry */
> answer = ~sum; /* ones-complement, then truncate to 16 bits */
> return(answer);
> }
baruch
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