[NUT-devel] Incomplete description of checksum algorithm

[Rich Felker]

On Fri, Feb 17, 2006 at 01:56:03PM +0200, Oded Shimon wrote:
> On Thu, Feb 16, 2006 at 04:31:59PM +0100, Michael Niedermayer wrote:
> > Hi
> > 
> > On Wed, Feb 15, 2006 at 10:43:25PM -0500, Rich Felker wrote:
> > > On Thu, Feb 16, 2006 at 01:10:41AM +0100, Michael Niedermayer wrote:
> > > > btw, why doesnt the syncpoint have a packet header / forward ptr !?
> > > 
> > > Because the damn thing is already way too big! :(
> > 
> > that makes it non-extendible which breaks one of nuts goals
> 
> IMO no.. frame headers can have arbitrary extendiblity, and syncpoints are 
> unique - their checksum doesn't come at the end of the syncpoint, but at 
> the following frame header...
> 
> As for forward ptr in checksum, it slightly annoys my coding because i do:
> 
> header -> temp buffer
> forward_ptr - length of header+4 -> file
> temp buffer -> file
> checksum of temp buffer -> file
> 
> So I'd have to somehow push the forward ptr to the begginning of the temp 
> buffer or some other solution. It doesn't matter badly though, I'm not 
> really against it.
> 
> But syncpoints don't need forward ptr though because they are special, they 
> carry themselves AND the following frame header... So you'd have to do some 
> weird thing where the forward ptr only points to the end of the syncpoint, 
> and then there's the frame header, and THEN there is the checksum. it's 
> ugly IMO...

This is all your implementation issue. If the header is enclosed in
the checksum, it's natural to consider it part of the syncpoint packet
anyway. Also there's no reason a checksum needs to come from just one
buffer. As long as you pass a state pointer to your checksum function
you can checksum arbitrarily many different buffers as if they were
all connected..

Rich