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From: "Brian Somers" <brian@utell.co.uk>
To: "Charles Mott" <cmott@srv.net>
Cc: "Julian Elischer" <julian@whistle.com>,
        "Eivind Eklund" <eivind@dimaga.com>,
        "Brian Somers" <brian@awfulhak.demon.co.uk>,
        "Ari Suutari" <ari.suutari@ps.carel.fi>, <hackers@freebsd.org>
Subject: Re: Single socket version of natd
Date: Tue, 4 Feb 1997 18:37:30 -0000
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I don't know specifically without checking the code, but I recall seeing a
comment in the -current sources (I've just checked SNAP-961014, nothing
about it there) about using the ip_sum to differentiate between incoming
and outgoing packets.....  It *WAS* commented as being a HACK :(

I'll check the code out this evening when I get home (-current machine)
and tell you.

Brian <brian@awfulhak.demon.co.uk> <brian@freebsd.org> <brian@utell.co.uk>
      <http://www.awfulhak.demon.co.uk>
Don't _EVER_ lose your sense of humour

----------
> From: Charles Mott <cmott@srv.net>
> To: Brian Somers <brian@utell.co.uk>
> Cc: Julian Elischer <julian@whistle.com>; Eivind Eklund
<eivind@dimaga.com>; Brian Somers <brian@awfulhak.demon.co.uk>; Ari Suutari
<ari.suutari@ps.carel.fi>; hackers@freebsd.org
> Subject: Re: Single socket version of natd
> Date: 04 February 1997 17:17
> 
> > If we leave it in both ppp & natd, can we add a (third) arg to
> > PacketAlias{In,Out}() that tells it not to do anything with the
> > ip_sum ?  If it's reading raw packets from a wire (with ppp),
> > the sum may be wrong (this has been discussed before) and
> > we want to leave incorrect the sum alone, but with natd, it's a
> > terrible waste to have the kernel code zero the sum, have natd
> > re-calculate it, have PacketAliasIn() check it, and then probably
> > re-calculate it again (if anything's changed).
> > 
> > With a "leave the sum alone option", natd could pass the packet
> > with the zero'd ip_sum to PacketAliasIn() and know that it has
> > to calculate it itself afterwards....
> 
> Why does the kernel zero the checksum?
> 
> IP checksum has never been much of a problem, because it is only 20 bytes
> or so.  It is the TCP, UDP and ICMP layers where efficiency pays off. 
> 
> In any event, all IP checksums modifications in alias*.c are now
> differential, so calculations are only made for packet data which has
been
> altered.  Unlike older versions, there is no checking and recomputation
of
> the complete IP header. 
> 
> Charles Mott