Date: Wed, 15 Nov 2000 16:26:15 +0200 From: Ruslan Ermilov <ru@FreeBSD.ORG> To: Julian Elischer <julian@elischer.org> Cc: Charles Mott <cmott@scientech.com>, Archie Cobbs <archie@dellroad.org>, net@FreeBSD.ORG, Ari Suutari <ari@suutari.iki.fi> Subject: Re: libalias: Incremental Update of Internet Checksum Message-ID: <20001115162615.A3145@sunbay.com> In-Reply-To: <3A1295DE.6930D125@elischer.org>; from julian@elischer.org on Wed, Nov 15, 2000 at 05:55:42AM -0800 References: <Pine.BSF.4.21.0011122254240.50684-100000@carcassonne.scientech.com> <Pine.BSF.4.21.0011130015100.50906-100000@carcassonne.scientech.com> <20001113103852.E34671@sunbay.com> <3A126F63.8EB0D49@elischer.org> <20001115150433.A98014@sunbay.com> <3A1295DE.6930D125@elischer.org>
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On Wed, Nov 15, 2000 at 05:55:42AM -0800, Julian Elischer wrote:
> Ruslan Ermilov wrote:
> >
> > On Wed, Nov 15, 2000 at 03:11:31AM -0800, Julian Elischer wrote:
> > > Ruslan Ermilov wrote:
> > > >
> > > Here's the version I wrote some years ago..
> > >
> > > #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)
> > >
> > > it replaces the 16 bit word at *op with the new value at *np and updates
> > > the checksum c
> > >
> > The above is the implementation of 4th equation from RFC 1624, right?
> > But we are talking about the version that allows updating of N 16-bit
> > words, not exactly one word.
>
> I can't remember which it was, but I also had a version that did N
> words..
> the same math applies though..
> you just do it in a loop :-)
>
> I wrote this myself but read the RFCs
> does it pass your test cases?
>
I will try your one-word version, but it would be really nice if you
have found the N-word version, so I could test it further. I am
mostly interested in how to convert two's complement subtraction to
one's complement subtraction.
I went further, and wrote another test program that demonstrates that
both implementation for formula 4 I posted at the very beginning, and
the current version in libalias(3) produce wrong results in some cases.
By "wrong" here I mean really wrong results, not the 0 -> 0xffff case.
See my next posting...
--
Ruslan Ermilov Oracle Developer/DBA,
ru@sunbay.com Sunbay Software AG,
ru@FreeBSD.org FreeBSD committer,
+380.652.512.251 Simferopol, Ukraine
http://www.FreeBSD.org The Power To Serve
http://www.oracle.com Enabling The Information Age
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