Date: Tue, 27 Jul 1999 16:48:06 +0100 From: Brian Somers <brian@FreeBSD.org.uk> To: Mike Smith <mike@smith.net.au> Cc: Matthew Dillon <dillon@apollo.backplane.com>, "Justin T. Gibbs" <gibbs@caspian.plutotech.com>, cvs-all@FreeBSD.org, cvs-committers@FreeBSD.org Subject: Re: cvs commit: src/release/sysinstall tcpip.c Message-ID: <199907271548.QAA04985@keep.lan.Awfulhak.org> In-Reply-To: Your message of "Tue, 27 Jul 1999 01:07:17 PDT." <199907270807.BAA01251@dingo.cdrom.com>
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> > You can blame the 6-byte MAC header for most of the headaches.
>
> Actually, I blame the way that layered protocol information is prefixed
> rather than postfixed in the datagram's wire format. It doesn't strike
> me as being at all well designed from a performance perspective
> (although mbuf chaining helps a lot). It would be possible to reduce
> both copying and checksum overheads by preallocating trailing space in
> the buffer based on the down-stack path (or even just a worst-case
> assessment of potential buffer growth), and adding an incremental
> checksum field.
>
> But this would make you totally incompatible with everything else, so
> let's just forget that line of thought.
Of course there's nothing wrong with guessing how much preceeding
space might be required and setting up the mbuf with an appropriate
unused gap at the front. The user-land mbuf code in ppp does this so
that it already has room for the protocol and address & control
fields and doesn't have to go off and find another mbuf. It gets a
bit muddy on I-must-be-aligned type architectures, but not really
much more than it would anyway.
> --
> \\ The mind's the standard \\ Mike Smith
> \\ of the man. \\ msmith@freebsd.org
> \\ -- Joseph Merrick \\ msmith@cdrom.com
--
Brian <brian@Awfulhak.org> <brian@FreeBSD.org>
<http://www.Awfulhak.org>; <brian@OpenBSD.org>
Don't _EVER_ lose your sense of humour ! <brian@FreeBSD.org.uk>
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