Re: Incremental update of TCP Checksum

Raf D'Halleweyn
2003-09-17T03:28:34Z

This is what I wrote some time ago. But I haven't used it recently.

/* Incrementaly update a checksum, given old and new 32bit words */
static inline __u16 incr_check_l(__u16 old_check, __u32 old, __u32 new)
{ /* see RFC's 1624, 1141 and 1071 for incremental checksum updates */
__u32 l;
old_check = ~ntohs(old_check);
old = ~old;
l = (__u32)old_check + (old>>16) + (old&0xffff)
+ (new>>16) + (new&0xffff);
return htons(~( (__u16)(l>>16) + (l&0xffff) ));
}

I have a similar function that can be used when you only change a 16bit
word:

/* Incrementaly update a checksum, given old and new 16bit words */
static inline __u16 incr_check_s(__u16 old_check, __u16 old, __u16 new)
{ /* see RFC's 1624, 1141 and 1071 for incremental checksum updates
*/
__u32 l;
old_check = ~ntohs(old_check);
old = ~old;
l = (__u32)old_check + old + new;
return htons(~( (__u16)(l>>16) + (l&0xffff) ));
}

On Tue, 2003-09-16 at 16:32, Vishwas Raman wrote:
> Valdis.Kletnieks@vt.edu wrote:
> > On Tue, 16 Sep 2003 11:50:16 PDT, Vishwas Raman <vishwas@eternal-systems.com> said:
> >
> >
> >>Can anyone out there tell me the algorithm to update the checksum
> >>without having to recalculate it.
> >
> >
> > The canonical source is the RFCs:
> >
> > 1071 Computing the Internet checksum. R.T. Braden, D.A. Borman, C.
> > Partridge. Sep-01-1988. (Format: TXT=54941 bytes) (Updated by
> > RFC1141) (Status: UNKNOWN)
> >
> > 1141 Incremental updating of the Internet checksum. T. Mallory, A.
> > Kullberg. Jan-01-1990. (Format: TXT=3587 bytes) (Updates RFC1071)
> > (Updated by RFC1624) (Status: INFORMATIONAL)
> >
> > 1624 Computation of the Internet Checksum via Incremental Update. A.
> > Rijsinghani, Ed.. May 1994. (Format: TXT=9836 bytes) (Updates
> > RFC1141) (Status: INFORMATIONAL)
> >
> > http://www.ietf.org/rfc/rfc1071.txt
> > http://www.ietf.org/rfc/rfc1141.txt
> > http://www.ietf.org/rfc/rfc1624.txt
>
> As mentioned in RFC1624, I did the following.
> void changePacket(struct sk_buff* skb)
> {
> struct tcphdr *tcpHdr = skb->h.th;
>
> // Verifying the tcp checksum works here...
>
> __u16 oldDoff = tcpHeader->doff;
> tcpHeader->doff += 1;
>
> // Formula from RFC1624 is HC' = ~(C + (-m) + m')
> // where HC - old checksum in header
> // C - one's complement sum of old header
> // HC' - new checksum in header
> // C' - one's complement sum of new header
> // m - old value of a 16-bit field
> // m' - new value of a 16-bit field
>
> long cksum = (~(tcpHdr->check))&0xffff;
> cksum += (__u16)~oldDoff;
> cksum += tcpHeader->doff;
> while (cksum >> 16)
> {
> cksum = (cksum & 0xffff) + (cksum >> 16);
> }
> tcpHeader->check = ~cksum;
>
> // Verifying tcp checksum here fails with bad cksum
> }
>
> Is there any glaring mistake in the above code. If so, can someone
> please let me know what it is. It will be of great help.
>
> Thanks,
>
> -Vishwas.
>
>
>
>
>
>
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--
Raf D'Halleweyn <raf@noduck.net>