Date: Tue, 09 Apr 2002 17:18:47 -0700 From: Bakul Shah <bakul@bitblocks.com> To: Julian Elischer <julian@elischer.org> Cc: net@freebsd.org Subject: Re: IP checksup update Message-ID: <200204100018.UAA13907@sheffield.cnchost.com> In-Reply-To: Your message of "Tue, 09 Apr 2002 16:02:01 PDT." <Pine.BSF.4.21.0204091557570.57350-100000@InterJet.elischer.org>
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> 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)); \
> 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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