Application, Prestination, Session - TCP-IP

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  1. Application, Prestination, Session

    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


  2. Re: Application, Prestination, Session

    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 ***

  3. Re: Application, Prestination, Session

    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


  4. Re: Application, Prestination, Session

    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: Cookies.


  5. Re: Application, Prestination, Session

    On Sep 9, 9:12 pm, Barry Margolin 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-


  6. Re: Application, Prestination, Session

    Le Chaud Lapin 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 help, 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

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