Messages in this thread | | | Subject | Re: Incremental update of TCP Checksum | From | Raf D'Halleweyn <> | Date | Tue, 16 Sep 2003 23:28:26 -0400 |
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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. > > > > > > > - > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- Raf D'Halleweyn <raf@noduck.net>
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