Re: [IPX]: Fix checksum computation.

[Arnaldo Carvalho de Melo <acme@conectiva.com.br>]

Em Fri, Oct 31, 2003 at 04:38:43PM -0800, Stephen Hemminger escreveu:
> 
> Okay, here is the standard: (Inside Appletalk)
> 
> > The DDP checksum is provided to detect errors caused by faulty operation (such as memor
> > data bus errors) within routers on the internet. Implementers of DDP should treat generati
> > the checksum as an optional feature. The 16-bit DDP checksum is computed as follows:
> > CkSum := 0 ;
> > FOR each datagram byte starting with the byte immediately following th
> > Checksum field
> > REPEAT the following algorithm:
> >           CkSum := CkSum + byte; (unsigned addition)
> >           Rotate CkSum left one bit, rotating the most significant bit in
> >                               least significant bit;
> > IF, at the end, CkSum = 0 THEN
> >           CkSum := $FFFF (all ones).
> > Reception of a datagram with CkSum equal to 0 implies that a checksum is not performed.
> 
> 
> Here is the old loop:
> 
> 	while (len--) {
> 		sum += *data;
> 		sum <<=1;
> 		if (sum & 0x10000) {
> 			sum++;
> 			sum &= 0xffff;
> 		}
> 		data++;
> 	}
> 
> My buggy loop is:
> 
> 	while (len--) {
> 		sum += *data++;
> 		sum <<= 1;
> 		sum = ((sum >> 16) + sum) & 0xFFFF;
> 	}
> 
> The problem is the carry from the first addition needs to be dropped
> not folded back (like IP).  
> 
> Corrected fast code is:
> 
> 	while (len--) {
> 		sum += *data++;
> 		sum <<= 1;
> 		sum = (((sum & 0x10000) >> 16) + sum) & 0xffff;
> 	}
> 
> At least it is correct on the standalone random data test, and the
> new code is 30% faster for the cached memory case (13.7 clks/byte vs 18 clks/byte).

Testing...