Hey.

Who can give a good analogy (from life) of the Open System Interconnection (OSI) reference model? So that this analogy fully reflects the work of OSI from the application layer to the physical. When you open any book, everything is described as somehow confused. As bud specially deliberately write. And after reading more questions than before you open the book. That is, to shift the “dry” (incomprehensible, abstract) book model of OSI to something (analogy) from real life.

I want to understand the OSI model, but not stupidly to memorize, like a verse. Understand how this model was conceived.

I only understand that the model was invented in order to break up a complex task (for example, I enter a website’s legal entity in a browser and enter it) into simpler pieces. It is easier to develop and easier to catch a mistake. Each level has its own “rules” for processing incoming (from a higher level or lower level) - protocols.

I can’t get a complete picture of the OSI model in my head. For example, I enter in the browser " http://google.ru ", I press Enter, the data went to the application level, something happens to them ..... At the very end, I received a series of impulses-bits at the output of the network card . I want to understand the details.

What are some programs that allow you to "touch" hands the exchange of data between the client and the server? That it was possible to "transfer" the transferred data pieces hands.

Closed due to the fact that it is necessary to reformulate the question so that it was possible to give an objectively correct answer by the participants Vladimir Martyanov , pavel , D-side , aleksandr barakin , user207618 Sep 9 '16 at 7:24 .

The question gives rise to endless debates and discussions based not on knowledge, but on opinions. To get an answer, rephrase your question so that it can be given an unambiguously correct answer, or delete the question altogether. If the question can be reformulated according to the rules set out in the certificate , edit it .

  • one
    If biological sciences are close to you, then roughly speaking, ecology studies populations, morphology - organisms, histology - tissues, cytology - cells, further molecular biology, and there is not far to the atomic structure :) That is, ecology does not matter how a separate organism works, then similarly. - Ponio
  • one
    @Dimon: Maybe this is because your question is offtopic? - VladD
  • 3
    I see no reason to give an analogy. It can be understood "as is", from the top (from the application level) down, the analysis of levels one by one. Because at each level you can sit, more or less forgetting about the levels below, they work for themselves and work, and we see only the results. Most part of time. And the remaining cases can be considered, already descending. - D-side
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    @all: Once the question has already been given a good answer, it makes sense to rediscover it. - VladD
  • 2
    "What programs are there that allow you to" touch "with your hands the exchange of data between the client and the server? So that you can touch the pieces of data with your hands." - I did not immediately notice this, you need a wireshark, of course. It will easily show the packets enclosed in each other. - etki

1 answer 1

Bob, a handsome and handsome young man, comes to the company for fulfilling desires and delivering all sorts of things in Brooklyn’s Russian quarter, puts his feet on the table and demands that a certain “Alice from St. Petersburg” necessarily convey a message, which he immediately dictates to the company’s manager. The message fits on two hundred A4 sheets, after which the manager swears that everything will be delivered and goes to the next room to send the message.

This is the application level at which the user uses an interface that allows him to send data to the service. In addition to direct contact with the manager, Bob could call the company or prepare two hundred A4 sheets.

After that, the manager lowers the application to the clearance department. In this department, experts take a handwritten text, reprint it, observing all the formatting standards, and slap an additional piece of paper on top, which they write with the address, so that any postman can at least say it out loud from America to Russia. After that, the responsible manager gently shakes the resulting stack of sheets and carries it along with others to the delivery department.

This is a presentation layer that packs heterogeneous user data into clear structures that the service operates on.

The shipping department accepts a weighty pile of sheets, including Bob’s letter. Since Alice lives in another country, the letter must comply with the documents defining the transfer of items from the US branch to the Russian branch. If there is no such document, the heads of the delivery department begin to correspond using the protocols described below in order to reach a consensus before transferring the real items.

This is the session layer, which is responsible for establishing communication between different instances of the application services.

After the contract for the transfer of shipments has been concluded, the delivery department decides exactly how best to deliver the sheets of which the shipments consist. In order not to send Bob’s entire letter in case of losses, it is divided into several separate stacks of sheets, another one additional sheet is put at the beginning of each page, on which is written to whom (box 9042), from whom (box 65535) the order number of the pile and the number of sheets in a pile - in this way the receiving party will be able to receive stacks of sheets in random order and still collect them in the original order.

This is the transport level. For what he is responsible, I will not write, so as not to disgrace; I think in the comments prompt more precisely.

Dividing the letter into separate piles, the delivery department sends them to the post office, ordering them to be sent to St. Petersburg. The mail adds on top another sheet that says "From: Brooklyn, To: St. Petersburg, Liszt: N". Mail would be glad to go all the way in one fell swoop, but the ocean shares the shipping address, and there is no direct message.
Therefore, mail delivers to the next office in the direction of St. Petersburg. The post office knows that this office will transfer the shipments to another node closer as soon as possible, and sooner or later the shipments will reach the ocean, then cross it, and in St. Petersburg they will see the coincidence of the final address and transfer it from the warehouse to the customer delivery department, which already and will deal with a / i.

This is the network layer, which is responsible for the interaction of nodes as a single organism. The network layer does not know anything about the population of nodes, but it can arrange the delivery of shipments from one node to another, so that the node itself will figure out to whom it was sent.

Delivery between separate offices provide truck drivers. Truck drivers do not know the exact addresses of offices in other countries, but they know the exact addresses of neighboring offices: for example, that office No. 1 is located on Stremyannaya Street. In addition, truck drivers know the protocol of acceptance and transfer of items and what forms you need to fill out.

This is the link layer that provides communication between individual nodes.

Only the physical level remains: when the driver delivers a stack of sheets from point A to point B, they physically move from one place to another.

When the stacks physically swim to Russia, the truck driver will receive them.
The truck driver will transport them from the port to the ASC "St. Petersburg".
ASC "St. Petersburg" will understand what exactly the address specified in the mailings.
At this address will begin to receive stacks of sheets that make up the letter.
As soon as the letter is fully assembled, it will go to the registration department.
The design department translates the address back and sends the letter to the responsible reader.
The responsible reader with the expression and according to the scheme adopted by the company will read the received letter to Alice.

It turns out the following scheme:

  • Random user data is transmitted.
  • Data is converted to well-defined structures for transmission.
  • For the transfer of structures the connection of application services is organized
  • Data transmitted inside the connection is wrapped in transport-level packets to ensure data transfer requirements.
  • Transport layer packets are transmitted over the network with the end address specified.
  • Each node of the network forwards packets to a friend located closer to the final address.
  • At the physical level, this is converted into a change in voltage at the connectors, into radio waves or another data transfer format.

I probably rather lied, sorry, if that. Feel free to edit if you see a pack.

  • The transport level is responsible for guaranteed delivery, and not for clearance. - Pavel Mayorov
  • thanks for the answer. as far as I know, at the presentation level the address (ip, mac or port) is not "hung up". you probably have to figure out how EACH of the protocols works in order to understand how these models (OIS and TCP / IP) work. - Dimon
  • @PavelMayorov UDP is not responsible for guaranteed delivery - etki
  • @Dimon did not mean anything like that here. In a real application at this stage, the user's ID would be substituted, and not the address of the database where the record should go - etki
  • @Etki: Megacruto, you saved the question. - VladD