I don't quite understand the session layer - TCP-IP

This is a discussion on I don't quite understand the session layer - TCP-IP ; hello I'm sorry for making two threads in such a short time span, but this next question is rally bugging me 1) One of the session layer ( layer 5 in OSI model ) services is synchronization. In short, session ...

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Thread: I don't quite understand the session layer

  1. I don't quite understand the session layer

    hello

    I'm sorry for making two threads in such a short time span, but this
    next question is rally bugging me

    1)
    One of the session layer ( layer 5 in OSI model ) services is
    synchronization. In short, session layer inserts checkpoints into data
    stream, so that if the connection crashes, only the data after the
    last checkpoint has to be repeated.
    I don't understand the need for that, since TCP connection
    acknowledges the received packets and if connection crashes, why
    couldn't TCP make sure that only the data after the last acknowledged
    packet would have to be send. So why duplicate the work ( which
    transport layer already performs ) in session layer?


    2)
    Session layer also performs dialogue control. Sessions can allow
    traffic to go in both directions at the same time, or in only one
    direction at a time. If traffic can only go one way at a time, the
    session layer can help keep track of whose turn it is.

    Can't TCP connection perform the same function - meaning, can't it
    somehow tell the other PC ( besides aborting connection ) when to send
    data and when to just listen?



    BTW - I realise that most protocol stacks implement session layer into
    application layer


    thank you for your help

    cheers


  2. Re: I don't quite understand the session layer

    wrote:

    > 1)
    > One of the session layer ( layer 5 in OSI model ) services is
    > synchronization. In short, session layer inserts checkpoints into data
    > stream, so that if the connection crashes, only the data after the
    > last checkpoint has to be repeated.
    > I don't understand the need for that, since TCP connection
    > acknowledges the received packets and if connection crashes, why
    > couldn't TCP make sure that only the data after the last acknowledged
    > packet would have to be send. So why duplicate the work ( which
    > transport layer already performs ) in session layer?


    Okay, TCP does incorporate some of the functions of Layer 5. But here's
    an example of when that might not be enough. Back in the bad old days of
    dial-up networking, you'd sometimes (often) get dropped off by the
    telco, just when you had downloaded the first zillion of three zillion
    bytes. Some good applications allowed you, a long time later, to dial
    back in and pick up where you had left off. This was typically a
    completely different TCP session.

    To me, that's a Layer 5 function. No matter in what shrink-wrapped
    package it was included.

    > 2)
    > Session layer also performs dialogue control. Sessions can allow
    > traffic to go in both directions at the same time, or in only one
    > direction at a time. If traffic can only go one way at a time, the
    > session layer can help keep track of whose turn it is.
    >
    > Can't TCP connection perform the same function - meaning, can't it
    > somehow tell the other PC ( besides aborting connection ) when to send
    > data and when to just listen?


    You mean, can't TCP be rewritten to do this? Maybe, but it shouldn't be.
    Because it's up to the application to decide whether this timing is
    important or not. Something higher than TCP should, and does, make that
    call.

    Bert


  3. Re: I don't quite understand the session layer

    In article <1182974839.125083.291130@k79g2000hse.googlegroups. com>,
    kaja_love160@yahoo.com wrote:

    > hello
    >
    > I'm sorry for making two threads in such a short time span, but this
    > next question is rally bugging me
    >
    > 1)
    > One of the session layer ( layer 5 in OSI model ) services is
    > synchronization. In short, session layer inserts checkpoints into data
    > stream, so that if the connection crashes, only the data after the
    > last checkpoint has to be repeated.
    > I don't understand the need for that, since TCP connection
    > acknowledges the received packets and if connection crashes, why
    > couldn't TCP make sure that only the data after the last acknowledged
    > packet would have to be send. So why duplicate the work ( which
    > transport layer already performs ) in session layer?


    Acknowledgements tell you that the data was received by the destination
    MACHINE and processed by its network stack. They don't tell you that
    the data was received by the destination PROCESS. If the process
    crashes, it won't read data that was received and acknowledged by TCP.

    --
    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 ***

  4. Re: I don't quite understand the session layer

    Bonjour Kaja,

    For evolving in the Telecom understanding, you need to drop OSI model.
    Today, the more adapted model is the TCP/IP model which existed prior
    to the OSI model. It doesn't not mean that OSI was not useful!

    In the TCP/IP model, a TCP/IP application is not an OSI application
    (and, even, an application in short, as Word can be). The client of
    the TCP protocol is generally a TCP/IP application protocol, or an
    emulation of it.

    For the lower layers, the ITU-T talks about, and more and more, of
    layer networks, and not of network layers. In the layer networks,
    there is a relation of client-server between adjacent networks (layer
    networks). The differentiation with OSI is clear.

    Best regards,
    Michelot


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