Re: Incremental update of TCP Checksum

[Vishwas Raman]

Jamie Lokier wrote:
> Richard B. Johnson wrote:
>
> > If I were to modify a low byte somewhere by subtracting 1,
> > would I know that the new checksum, excluding the inversion,
> > was 0x0000? No. It could be 0xffff.
>
>
> You're right about information being thrown away, but wrong because IP
> checksums are more rigidly defined than that.
>
> RFC1624 was written because the earlier RFC actually got this wrong.
>
> As long as at least one of the checksummed words is known to be
> non-zero, 0x0000 is not a possible value.  This is true of all IP checksums.
>
> There is only one possible value of the non-complemented sum: 0xffff.
>
> So when you subtract 1 from 0x0001, you get 0xffff.
>
> To do this right, instead of subtracting a word, you add the
> complement of the word.  After carry-folding, this works out right.
>
> Vishwas Raman wrote:
>
> > Are you then suggesting that instead of trying to do an incremental 
> > update of the tcp checksum, I set it to 0 and recalculate it from 
> > scratch? But I thought that doing that was a big performance hit. Isn't it?
>
>
> You don't need to recalculate the sum.  All routers modify the IP
> header checksum when they decrement the TTL of a packet - it's a
> widely used algorithm.  Equation 3 from RFC1624 is correct :)

I was also under the belief that RFC1624 was handling this correctly.

> Your code looks fine to me.  Are you sure you're verifying the
> checksum correctly?


This is how I am verifying the checksum. It seems to work in other 
cases. (by the way, I am working with the 2.4.20 kernel src code)

/* I do this check for only packets that are less than or equal to 76 
bytes in length. And I make sure the packets that I am dealing with are 
less than this length */

int tcpFailoverVerifyChecksum(struct sk_buff* skb)
{
    int len = skb->len - sizeof(struct iphdr);
    retValue = tcp_v4_check(skb->h.th, len,
                            skb->nh.iph->saddr, skb->nh.iph->daddr,
                            csum_partial((char *)skb->h.th, len, 0));
    return retValue;
}

Is the above function right? If not, what is the right way to verify the 
checksum of a tcp packet?


> >    while (cksum >> 16)
> >    {
> >        cksum = (cksum & 0xffff) + (cksum >> 16);
> >    }
>
>
> In general you need to add back the carry bits at most twice, btw.
>
> 	cksum = (cksum & 0xffff) + (cksum >> 16);
> 	cksum += (cksum >> 16);

Ok...I will make the change... Thks...




-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/