Today I learn about the OSI and TCP/IP stack, the foundation of networking. The Open Systems Interconnection (OSI) is a conceptual model to explain how two networks talks to each other, and the Transmission Control Protocol/Internet Protocol (TCP/IP stack) is the stack 99.9% the internet actually runs on.
Please do not take sales person’s advice is the mnemonic devise for the name of the OSI stack: Physical, DataLink, Network, Transport, Session, Presentation, Application.
The International Standards Organization created the conceptual OSI model, whereas another group, the Internet Engineering Task Force worked on the TCP/IP model.
To understand each model, the folks at CBT Nuggets used a great analogy…
A Tale of Two Kingdoms: How the OSI and TCP/IP works
Imagine there are two kingdoms. The King of Kingdom A wants to deliver a message to Kingdom B. (The king is our internet users). If the king wants to send a message, an invitation for dinner to Kingdom B, he doesn’t get on his horse and send the message himself. There is a chain of command the message needs to go through.
First, king tells the Scribe, the second highest seat of the land, to write the message. The scribe replays the message to our translator to correctly formats and encrypt the message. Then the translator replays the message to the lawyer. It’s the lawyer’s job to negociate the converstation, gives the message an official number before he hands the message to the middle manager.
The middle manager is a busy person, she is in charge of making sure the message gets delivered. She can use to deliver the message the fast way, which is unreliable, or the reliable, but slower way. If the message is too big, she makes sure it gets broken down into smaller chunks–into multiple letters. Next, our manager drops the message in the mailroom. In the mailroom, the staff adds a label with the specific street and houue numbers (the logical address). The mailroom is like the Datalink Layer of our stack. Lastly, the mail is ready for the delivery wagon.
When the delivery wagon arrives the gates of Kingdom B, the steps are in reversed order: the mail goes through the mailroom, where the staff checks if the physical address matches; then through the middle manager, who sends an acknowledgement to Kingdom A that she receives the message; on the desk of the laywer, who checks if Kingdom A and B are on good terms; then to the translator to decrypt the message; and to the scribe who finally reads the mesage to the king.
The OSI model has 7 layers, while the TCP/IP only has 4. Layer 5,6,7 or Session, Presentation, and Application are consolidated to the Application Layer in the TCP/IP model. The transport layer (our middle manager), is the same. The network layer in OSI corresponds to the internet layer in TCP/IP. The last two layer: the Datalink and Physical Layer is combined as a Network Interface layer in the TCP/IP model.
Vocabulary:
- encapsulation — when data is being moved down the protocol stack to send to another network
- decapsulation — when data is moved to the toher side, and it moves up the protocol stack
NOTES
