Re: Incremental update of TCP Checksum

[Leo Mauro <lmauro@usb.ve>]

On Tuesday 16 September 2003 04:32 pm, Vishwas Raman wrote:

> 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;
                       ^  should be a -

>      cksum += tcpHeader->doff;
>      while (cksum >> 16)
>      {
>          cksum = (cksum & 0xffff) + (cksum >> 16);
>      }
>      tcpHeader->check = ~cksum;
>
>      // Verifying tcp checksum here fails with bad cksum
> }

-- 
Leo Mauro
Network Security
Simon Bolivar University
Caracas, Venezuela