Re: Incremental update of TCP Checksum

[Vishwas Raman]

Valdis.Kletnieks@xxxxxx wrote:
> On Tue, 16 Sep 2003 11:50:16 PDT, Vishwas Raman <vishwas@xxxxxxxxxxxxxxxxxxx>  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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