Re: Incremental update of TCP Checksum

Vishwas Raman
2003-09-16T23:33:46Z

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...