RE: Re: Much More Idle Chatter About Reference Models


Jim Stevens (Stevens@A.ISI.EDU)
Sat 19 Dec 87 14:37:23-EST


Phil,

I tried to address 2 problems in my note about protocol reference models:

(1) where do things like network management protocols belong in
     general and

(2) identification of a common problem in charts showing the
     packet radio network protocol layering.

It is true that the answer to the first problem depends upon a
persons acceptance of the different protocol reference models.
However the second problem is independent of whether there are
always 7 layers in a network or whether there are N layers in a
network or whether layers can be skipped (i.e. null layers). In
fact, I have seen this second problem in many different papers using
both the ISO/OSI reference model and any of the several slightly
different DoD reference models.

The second problem is caused when people confuse the host/terminal
interface to a network as being the protocols which run within the
network. This has especially been true for radio networks (such as
Packet Radio and Packet Satellite).

I can create an alternate example of this confusion for the Ethernet.
My PC is connected to a server on the Ethernet via an RS232 link. If
I were to confuse my interface to the Ethernet as being the actual
protocols used within the Ethernet, then I would say that the
physical protocol used in the Ethernet is RS232 instead of IEEE
802.3. However, most of us would agree that this is incorrect.

The reason why the confusion with radio networks continues is because
there are many more people who use other networks such as Ethernet
than Packet Radio. Thus when most people write a paper and create a
chart, they do like I do. They (and I) look at earlier papers and
assume that the information in the areas that they do not know is
correct.

Thus the goal of my earlier message with respect to this second
problem was to

   provide the correct information about what Link and Physical Layer
   protocols are used within the DARPA Packet Radio Networks, and

   explain that there is confusion between interface protocols and
   network protocols so that this type of error is repeated less
   often.

Jim

P.S. I do not intend to argue about the absolute correctness of the
OSI model for all types of actual networks. (I myself can list
examples of things like networks that appear to be just a link to
another network which then appears to be just a link to yet another
higher network.) But the most important thing about the OSI model is
that is has become ubiquitous and is a useful way to describe concepts.

I would classify all of the existing network management protocols as
being of 2 types: (a) lying within the network layer (ex. ICMP) or
(b) lying above the network layer (ex. EGP). This is the same type
of structure breakdown that was decided upon in the protocol model
work that I referenced in my earlier message.

Thus the goal of my earlier message with respect to the this first
problem was to

    indicate that while I agree that EGP, GGP, and HMP lie above the
    Internet Protocol, I do not consider them to be transport
    protocols, and

    provide reference to the good work that is being performed on
    network management architectures. In particular, I feel that
    the CCITT I.320 Recommendation on the ISDN Protocol Reference
    Model contains many useful ideas.

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