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I was studiyng those protocols, and I even understand the basis of each layer they have, but I can't understand how they work in practice.

For example: When an application make a request, isn't it the thing that fill all those informations (like the destination, the port, the protocols used, etc.)? In other words, when my browser make a request to a server, isn't the browser - the application - that fill the entire request layer by layer?

With that in mind I can't see when the application layer gets separated from the other ones, could you explain?

Rafael Santiago Altoé
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In practice layer up to 4 (TCP and UDP) are implemented inside an OS. The mechanism is called sockets. The application provides IP address and port, and selects transport protocols. Then it provides data, and OS is handling all the filling out. So, separating layers 2, 3, and 4 from the rest makes sense. Separating 2 and 3 from 4 is necessary for the network in between to work.

The rest IMHO does not. In TCP/IP model for example there is only one application layer on top of the transport layer. I do not know if anyone truly understands the intended functionality of session and presentation layer in OSI model. We all learn them and some protocols that are attributed to these layers for some unknown reason (e.g., i don't know why is TLS layer 5 and not 6). IMHO, these layers could make sense if you are designing particular class of applications, but if you consider all current Internet applications, I don't think these generalization makes sense.

Effie
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  • I don't know why they are still teaching OSI nearly 30 years after its demise. In fact I don't know why they ever taught it. It was never intended as a pedagogical tool or as a universal model: it was only written as an aid to describe the layering of the OSI protocols themselves. – user207421 Aug 26 '22 at 01:05
  • @user207421 you know, the infamous "we have always done it this way" – Effie Aug 26 '22 at 10:25