Date: Fri, 27 Feb 2004 20:26:38 +0100 From: "Poul-Henning Kamp" <phk@phk.freebsd.dk> To: "Kevin Oberman" <oberman@es.net> Cc: Sam Leffler <sam@errno.com> Subject: Re: cvs commit: src/sys/contrib/pf/net if_pflog.c if_pflog.h if_pfsync.c if_pfsync.h pf.c pf_ioctl.c pf_norm.c pf_osfp.c pf_table.c pfvar.h src/sys/contrib/pf/netinet in4_cksum.c Message-ID: <14680.1077909998@critter.freebsd.dk> In-Reply-To: Your message of "Fri, 27 Feb 2004 11:16:02 PST." <20040227191602.2A2045D07@ptavv.es.net>
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In message <20040227191602.2A2045D07@ptavv.es.net>, "Kevin Oberman" writes: >The very existence of ICMP is a layering violation! > >TCP/IP pre-dates the OSI reference model and really doesn't fit it. You >can't build a stack that runs reasonably without "layering violations". >These are NOT bugs! More to the point, when people tried to design the protocols to end all data transmission according to the OSI protocol reference model banning all layering violations, they hit some very recognizable problems which they couldn't solve or couldn't solve sanely. My personal favourite is the network management (CMIP etc) which runs at layer 7 and therefore reliably informs you of your network being in trouble by ceasing to report anything at all. In the two "The science of discworld" books, the authors introduce the concept of "lies to children" to explain how you have to progressively add detail if you want to teach children and students things: You simply cannot give the right explanation until the child or student has accumulated enough knowledge to be able to understand what you're talking about in the first place. You start out with simple mechanics, then you add acceleration an forces, then you drop Newton in there, then Keppler and only then can you pull Einstein out: You have to have more than one step on your ladder. The OSI reference model is such a "lies to children" and it is not how you get data communication to work in practice. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 phk@FreeBSD.ORG | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence.
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