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I have been searching out for in depth knowledge of OSI layer model. I want to know that what information does a Application, Presentation and session layer header contains. Moreover where do these layer exists? the protocols which work at top three layers are present inside OS or they are just built-in inside that specific application which the user is using.

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    – Ron Trunk
    Jun 15 '20 at 11:57
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    Dec 17 '20 at 18:46
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Moreover where do these layer exists?

Nowhere. There's no current operating system that is compliant with the OSI model. All existing computers and networking enabled devices use the TCP/IP model.

Although in literature (including this site), we use a mix of the two models, keeping the layer 1/2/3 from the OSI model, and layer 4/5 from the TCP/IP model. To be really consistent we should refer as IPv4 or IPv6 as layer 2 protocols and TCP, UDP, etc.. as layer 3, but there's really old habit about this and its not gonna change.

So the answer to:

I want to know that what information does a Application, Presentation and session layer header contains

is: nothing, since they don't actually exist.

Even with the TCP/IP model, keep in mind that it is just a model, i.e. a guideline on how to build a networking stack, and real implementation sometimes have to make compromises with the model.

What is really used are:

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The physical layer (L1) and the data link layer (L2) are usually implemented in hardware: switches, WAPs, NICs. L1 is the only layer actually moving data and L2 controls its flow inside a local network.

IEEE explicitly state that their 802 family of protocols uses a layering model derived from the OSI model, emphasizing the lower two layers (see IEEE 802: Overview and Architecture).

The network layer (L3) and the transport layer (L4) are usually implemented in an operating system's "network stack" - most prominently IPv4, IPv6, and TCP, UDP. L3 takes care of global routing and L4 provides transport for actual data for an application.

The application layer (L7) sits inside an application. An email client speaks SMTP and IMAP, a browser speaks HTTP/S. Various base protocols like DNS are commonly implemented by the operating system but that varies.

The session layer (L5) and the presentation layer (L6) aren't usually implemented separately but - if implemented at all - also sit inside the application.

Most protocols in each layer use prepended headers to organize their work but those headers can vary massively, so there's no general format.

Each layer has a specific task, and all together they form a modular "stack" with an extreme range of functionality and adaptability. After all, that modularity has enabled TCP/IP to become the most popular and powerful set of protocols on the planet.

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  • True except for limiting layer 2 to "hardware". Bridges can be very complicated, much like routers. There aren't any that don't involve a good bit of software! Repeaters are hardware, L1. Have you ever looked at the STP specs? (IEEE 802.1D) Yeesh. It works so well most of the time we forget about it but no WAY that lives in hardware! Dec 18 '20 at 3:19
  • L2 is usually implemented in hardware. The basic (self-learning) bridge functions are in hardware (e.g. in a simple, unmanaged switch), but there are loads of additional functions that require software, that's true. However, there's a slow but constant migration of (complex) software functions to hardware (offloading), e.g. from software routing to layer-3 switching.
    – Zac67
    Dec 18 '20 at 9:19
  • Even much of the "hardware" is "software" at some level. I've written that kind of code. The hardware/software divider is really a red herring. On the hardware I'm currently coding, ARP entries get created in hardware without CPU intervention, but that doesn't mean ARP is in hardware. Dec 18 '20 at 17:52
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    True - in a given, complex scenario you might not be able to draw a sharp line. But I was generalizing.
    – Zac67
    Dec 18 '20 at 17:59

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