I’m not a computer science student. But I want to learn it and grasp these two models in very simple language and terminology.
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Let’s say you want to play a game of chess with someone by mail. First, you’d write something like “I got your last move for Turn #3. Now I move my pawn from h7 to h5.” Then you’d put that piece of paper in an envelope with your opponent’s name and address on it. Then you’d take the envelope outside and put it in a mailbox. The postal worker would take it to a sorting facility, then to your opponent’s house, then your opponent would open it rather than another member of his family, and then he’d move the piece on his copy of the chessboard after reading your letter.
The postal worker is the physical layer, actually moving the data from one computing device to another. The sealed envelope itself represents the networking layer with information like the city, state, and postal code to guide the postman in where it should go. The information about the turn number and acknowledgement are the session layer, letting the person who opens the envelope know where this information fits into the overall series of messages, and whether a lost message needs to be re-sent. The text of the chess move is part of the presentation layer, representing a physical piece with a predetermined code for the locations on the board that needs to be interpreted by the reader. Finally, the piece is actually moved, doing something useful to the human player, which is the application layer.
TCP is represented by acknowledging the prior communication and numbering the present communication, so that if any packet is dropped, it will be noticed and re-transmitted. IP is just a means of transmitting packets to a particular destination, regardless of what their contents are, just like an envelope is in the mail system.
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