Date: Sun, 28 Mar 1999 12:36:09 -0800 From: "Justin C. Walker" <justin@apple.com> To: net@freebsd.org Subject: Re: OSI layering Query.. Please Help ME Message-ID: <199903282036.MAA00636@walker3.apple.com>
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> From: Gerald Heinig <heinig@hdz-ima.rwth-aachen.de>
> Date: 1999-03-28 01:57:47 -0800
> To: justin@apple.com
> Subject: Re: OSI layering Query.. Please Help ME
>
> "Justin C. Walker" wrote:
>
> > find detailed discussions of the model, and its applicabiliity to IP,
> > in many places. See, e.g., Stallings, "Handbook of Computer
> > Communications Standards, V. 1, The Open Systems Interconnect model
> > ..." (MacMillan/Stallings); Stevens' "TCP/IP Illustrated, V1"
> > (Addison Wesley), discusses layering as it applies specifically to
> > TCP and IP (i.e., up through what OSI calls the transport layer).
> >
> > > And what exactly are PDU?
> > Protocol Data Units - this is OSI-speak for "packets",
> > although it allows you to talk about packets at the various protocol
> > layers (e.g., an IP packet as distinct from a TCP packet or an
> > ethernet packet).
> >
> > > What exactly is SAP??
> > Service Access Point - it's a protocol addressing term.
> > E.g., in IP terms, an SAP at the network layer would be a port (and
> > is known as an NSAP). You'd need to delve more deeply into the OSI
> > model to appreciate the subtlety of the concept :-}.
>
> Hmmm. Yet another interpretation. Did you get this out of the books
> you mentioned above? The books I took my definitions out of are "Data
> communications, computer networks and OSI" by Fred Halsall and
> "Internetworking with TCP/IP volume 1" by Douglas Comer. The Halsall
> book is very abstract and hardly mentions TCP/IP, but the Comer book
> does. It says, for example:
>
> "Level 3: A reference to transport level communication derived from the
> ISO 7-layer reference model. For TCP/IP internets, level 3 refers to IP
> and the IP datagram format. Thus, a level 3 address is an IP address."
>
> Since level 3 is defined as the network layer, the point at which you
> access this layer is the logical network interface and you use a
network
> service access point address to do this. Or so the logic goes.
Well, yes; what the I meant to say is that the IP address is
an NSAP (Network SAP); the port is a TSAP. A bit of "fingers
outrunning brain"...
> The Halsall book is very hazy on this and seems to contradict itself at
> several points. Worse, nobody seems to be able to agree on this...
All of which indicates (to me, at least) that applying OSI
terminology where it doesn't belong merely serves to confuse and
cloud.
The layering ideas from OSI (or SNA, ...) are a convenient
framework, but the OSI terminology need not survive its protocols
:-}.
Regards,
Justin
Justin C. Walker, Curmudgeon-At-Large *
Institute for General Semantics |
Manager, CoreOS Networking | Men are from Earth.
Apple Computer, Inc. | Women are from Earth.
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