I have been having a hard time grasping the meaning of the top 3 upper
layers of the OSI Model (Application, Presentation, Session). Can
someone explain them to me in plain english. Thanks, CREAM

In article <1189388421.643146.175870@y42g2000hsy.googlegroups .com>,
fbalbert@gmail.com wrote:

> I have been having a hard time grasping the meaning of the top 3 upper
> layers of the OSI Model (Application, Presentation, Session). Can
> someone explain them to me in plain english. Thanks, CREAM

The presentation and session layers don't really exist in any practical
sense. Unless you're interested in them for purely pedagogical purposes
(e.g. you're taking a class and this will be on the exam) you can pretty
much ignore them.

--
Barry Margolin, barmar@alum.mit.edu
Arlington, MA
*** PLEASE post questions in newsgroups, not directly to me ***
*** PLEASE don't copy me on replies, I'll read them in the group ***

<fbalbert@gmail.com> wrote:

> I have been having a hard time grasping the meaning of the top 3 upper
> layers of the OSI Model (Application, Presentation, Session). Can
> someone explain them to me in plain english.

I think this article gives understandable explanations:

http://en.wikipedia.org/wiki/OSI_model

Bert

On Sep 10, 9:40 am, fbalb...@gmail.com wrote:
> I have been having a hard time grasping the meaning of the top 3 upper
> layers of the OSI Model (Application, Presentation, Session). Can
> someone explain them to me in plain english. Thanks, CREAM

Something concrete:

Application layer: HTTP.
Presentation layer: HTML.
Session layer: .

On Sep 9, 9:12 pm, Barry Margolin <bar...@alum.mit.edu> wrote:
> In article <1189388421.643146.175...@y42g2000hsy.googlegroups .com>,
>
> fbalb...@gmail.com wrote:
> > I have been having a hard time grasping the meaning of the top 3 upper
> > layers of the OSI Model (Application, Presentation, Session). Can
> > someone explain them to me in plain english. Thanks, CREAM
>
> The presentation and session layers don't really exist in any practical
> sense. Unless you're interested in them for purely pedagogical purposes
> (e.g. you're taking a class and this will be on the exam) you can pretty
> much ignore them.

So true.

Be wary of the OSI model. For years, I had been using it as a
foundation to conceptualize how a network protocol stack should be.
Later I realized that it a really low meat-to-fat ratio. That which
makes OIS appealing, IMO,is that it conveys a different type of model
within it, which *does* have high meat-to-fat ratio: the idea of
layering at the physical, link, network, and even transport layers.
But beyond that, the utility of the model drops off quickly. So, the
principle of "layering" in network could be summed up in one page.
The rest is of questionable value.

Another thing to be wary of: there are people who insist on taking the
concept of layering literally. When they go to implement an "OSI
protocol stack", they will actually try to structure the software in
layers, right up to Layer 7. They often end up with, or demand to end
up with, clumps of functions between each later, where data units are
passed between the layers. This works well with lower layers but gets
more difficult in upper layers, when it is not clear what those layers
should be.

And finally: there are people who would swear that there is floating
around somewhere a physical embodiment of OSI protocol stack. They
claim that there have been many OSI "implementations." I spent two
days reading OSI documentation, and it was one of the most nauseating
experiences I have ever had reading a "technical" document, even more
nauseating than the time I had to read a 100-page document on the
various dimensions of HVAC pipes. Paramedics should carry around full
copies to induce vomiting in poison patients.

-Le Chaud Lapin-

Le Chaud Lapin <jaibuduvin@gmail.com> writes:

> Another thing to be wary of: there are people who insist on taking the
> concept of layering literally. When they go to implement an "OSI
> protocol stack", they will actually try to structure the software in
> layers, right up to Layer 7.

Very true. Even the OSI protocol development effort ran into this
snag. It took them a couple of years to realize they needed to turn
the upper layers on their side, with application "layer" using
both presentation and application services to , but with the
application itself being the one that dealt with the lower layers
directly, since "presentation" really has nothing to do with the
communication path.

> And finally: there are people who would swear that there is floating
> around somewhere a physical embodiment of OSI protocol stack.

I'm not sure what this means. There were several physical OSI protocol
implementations available (even if you don't include things like X400
and X500 that didn't follow the OSI model in any reasonable sense, but
were blessed as "OSI protocols" because the OSI community had
political needs for working protocols). There were even testing
companies that made sure the implementations conformed to the defined
protocols. No one used those protocol stacks for anything other than
checking boxes and, maybe a little, playing around, but I can assure
you they existed, for all they were worth.
-don