Date: Tue, 9 Apr 2002 17:28:07 -0700 (PDT) From: Julian Elischer <julian@elischer.org> To: Bakul Shah <bakul@bitblocks.com> Cc: net@freebsd.org Subject: Re: IP checksup update Message-ID: <Pine.BSF.4.21.0204091727110.57350-100000@InterJet.elischer.org> In-Reply-To: <200204100018.UAA13907@sheffield.cnchost.com>
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just looking at this.... (it's been a while....)
On Tue, 9 Apr 2002, Bakul Shah wrote:
> > to update a checksum when changing a word
> >
> > #define FIXSUM16(c, op, np) \
> > do { \
> > (c) -= (u_int16_t) ~*((u_int16_t *) (op)); \
> > if ((c) < 0) { \
> > (c) += 0xffff; \
> > } \
> > (c) -= (u_int16_t) *((u_int16_t *) (np)); \
^^ hmm should that be "+=" ?
> > if ((c) < 0) { \
> > (c) += 0xffff; \
> > } \
> > } while (0)
> >
> > The question is:
> > apparently there are several "tricks" with this checksum
> > and + and - 0. Does anyone know how to test if this macro gets it
> > right?
>
> Looks like a straight forward implementation of rfc1624 eqn 4.
> "if (c < 0) c += 0xffff;" is needed to turn two's complement
> subtraction to one's complement.
>
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