# OSI 7 Layers
> The English parts are content studied initially & the Korean parts are content studied again from books
>
> Reference: \[Book] Network Introduction for IT Engineers
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### Before getting started
* In the past, communication protocols were not standardized, so there were many incompatible systems and applications, and communication was impossible
* The effort to **unify** these into a single **standard** remains as today's `OSI 7 Layers`!
* Although `OSI 7 Layers` is utilized as the main network reference model since it helps in understanding and developing network operations by dividing them,
* Currently most protocols are based on the **TCP/IP protocol stack**
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## What is OSI 7 Layers?
### OSI 7 Layers
* OSI (= Open Systems Interconnection) is a **7 layer** architecture with each layer having specific functionality to perform.
* All these 7 layers work *collaboratively* to **transmit the data** from one person to another across the globe.
The OSI 7 Layers can be divided into two categories based on the **role** and **purpose** of each layer.
#### Layers 1\~4
* `Lower Layer`
* `Data Flow Layer`
* Responsible for **delivering** data to the other party
#### Layers 5\~7
* `Upper Layer`
* `Application Layer`
* Responsible for **creating** data
* **Application developers**
* Focus on **representing** data without considering the data flow layer
* View the network in a `Top-down` manner
* **Network engineers**
* Since the application layer is the domain application developers should consider, network engineers generally do not worry seriously about this area
* View the network in a `Bottom-up` manner
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## Details
### 1. Physical Layer (Layer 1)
* The lowest layer of the OSI reference model is the physical layer.
* It is responsible for the actual physical connection between the devices.
* The physical layer contains information in the form of **bits.**
* It is responsible for transmitting individual bits from one node to the next.
* When receiving data, this layer will get the signal received and convert it into 0s and 1s and send them to the Data Link layer, which will put the frame back together.
* Defines information related to **physical connections**
* Mainly focused on **transmitting electrical signals**
* Since the goal is to **transmit incoming electrical signals as-is**, when an electrical signal enters a `Layer 1 device`, it **regenerates** the electrical signal and sends it out
* Layer 1 devices **have no concept of addresses**!
* They transmit the **same electrical signal** to **all ports** except the port the signal came in on
* In other words, source and destination cannot be distinguished
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### 2. Data Link Layer (DLL) (Layer 2)
* The data link layer is responsible for the node to node delivery of the message.
* The main function of this layer is to make sure data transfer is error-free from one node to another, over the physical layer.
* When a `packet` arrives in a network, it is the responsibility of DLL to transmit it to the Host using its `MAC address`.
* Data Link Layer is divided into two sub layers :
1. `Logical Link Control (LLC)`
2. `Media Access Control (MAC)`
* The packet received from Network layer is further divided into frames depending on the frame size of `NIC(Network Interface Card)`.
* DLL encapsulates Sender and Receiver's MAC address in the header.
* The Receiver's MAC address is obtained by placing an `ARP(Address Resolution Protocol)` request onto the wire asking *"Who has that IP address?"* and the destination host will reply with its MAC address. 
* Collects electrical signals and **processes** them into a recognizable **data form**
* Unlike Layer 1, rather than precisely **transmitting** electrical signals, the focus is on **defining** `address information` and **communicating** to the `correct address`
* Checks the `source` and `destination` addresses, verifies whether it was sent to me or whether I need to process it, and then performs **data processing**
* In Layer 2, as the `address system` is introduced, functions for **distinguishing** between multiple simultaneous communications are primarily defined
* Since it collects `electrical signals` and processes them into data form, it can perform the role of **detecting** or **correcting** **errors** in data
* In Ethernet-based networks, Layer 2 only performs the role of detecting errors
* Having an address system means communicating with **multiple parties** simultaneously rather than just one, so instead of blindly sending data, **checking whether the recipient can currently receive data** must be done first
* This role is called `Flow Control`
* **Flow Control**
1. Server sends data to the switch
2. Congestion occurs at the switch. The switch sends a Pause frame to the server
3. The server waits after receiving the Pause frame
* The network components of Layer 2 are `Network Interface Card` and `Switch`
* The most important feature of Layer 2 is the **MAC address** system
* Both the `Network Interface Card` and `Switch` operating in Layer 2 can understand **MAC addresses**, and the `Switch` has the ability to specify the port to communicate with based on the MAC address
* **Network Interface Card operation**
1. Converts electrical signals into data form
2. Checks the destination MAC address and source MAC address
3. Checks the MAC address of the Network Interface Card
4. If the destination MAC address **matches** the Network Interface Card's MAC address, it **processes** the data; if they **differ**, it **discards** the data
* **Switch operation**
* The Switch can determine what MAC address a terminal has and which port it is connected to through the **Address learning** process
* Based on this data, when terminals communicate, the switch appropriately **filters** ports and **forwards** to the correct port
* With the switch's proper filtering and forwarding functions, only the needed ports are used for communication, and unnecessary processing across the entire network decreases
* This greatly improved Ethernet network efficiency,
* And became the catalyst for the rapid increase in Ethernet-based networks!
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### 3. Network Layer (Layer 3)
* Network layer works for the transmission of data from one host to the other located in different networks.
* It also takes care of `packet routing` i.e. selection of the shortest path to transmit the packet, from the number of routes available.
* The sender & receiver's IP address are placed in the header by the network layer.
* The functions of the Network layer are :
1. **Routing**
* The network layer protocols determine which route is suitable from source to destination.
* This function of network layer is known as routing.
2. **Logical Addressing**
* In order to identify each device on internetwork uniquely, network layer defines an addressing scheme.
* The sender & receiver's IP address are placed in the header by network layer.
* Such an address distinguishes each device uniquely and universally.
* Segment in Network layer is referred as **Packet**. 
* Network layer is implemented by networking devices such as routers
* In Layer 3, **logical addresses** such as `IP addresses` are defined
* Two types of addresses are used in data communication
1. Layer 2's **physical MAC address**
2. Layer 3's **logical IP address**
* Unlike MAC addresses, IP addresses can be **changed** to suit the user's environment
* IP addresses are divided into a `network address part` and a `host address part`
* Devices or terminals that can understand Layer 3 can use **network address information** to
1. Distinguish between `their own network` and `remote networks`
2. Determine the **path** to reach a remote network
* The device operating at Layer 3 is the **Router**
* The Router can understand IP addresses defined in Layer 3
* The Router finds the **optimal path** using IP addresses and forwards `packets` along that path
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### 4. Transport Layer (Layer 4)
* Transport layer provides services to application layer and takes services from network layer.
* The data in the transport layer is referred to as *`Segments`*.
* It is responsible for the End to End Delivery of the complete message.
* The transport layer also provides the acknowledgement of the successful data transmission and re-transmits the data if an error is found.
* Transport layer is operated by the `Operating System`.
* It is a part of the OS and communicates with the Application Layer (Layer 7) by making system calls.
* Transport Layer is called as **Heart of OSI** model.
* Layer 4 plays a different role from Layers 1\~3
* The lower layers (Layer 1\~4) split data, attach information, and deliver it to the destination,
* Layers 1\~3 focus on sending signals and data to the **correct location** and **properly generating and sending** actual signals
* In contrast, Layer 4 plays the role of **verifying** that the data is actually being sent correctly
* In `Packet networks`, since data is **split** and **loaded into packets** for transmission, packets may be **lost** or arrive **out of order** in transit
* Layer 4 is responsible for correcting these issues
* When Layer 4 splits packets, it writes the **sending order** and **receiving order** in the `packet header`, so
* If a packet is lost, retransmission can be requested,
* Even if the order is mixed up, it can be corrected
* The **sending order** noted in the Packet is the `Sequence Number`,
* The **receiving order** noted in the Packet is the `ACK Number (Acknowledgement Number)`
* `Port Numbers` are used to distinguish among many applications within a device
* The devices operating at Layer 4 are **Load Balancers** and **Firewalls**
* These devices use the application identifier (`Port Number`) visible at Layer 4 and `Sequence`, `ACK Number` information to
* **Distribute load** or **establish security policies** to **pass** or **block** packets
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### 5. Session Layer (Layer 5)
* The Session layer is responsible for establishment of connection, maintenance of sessions, authentication and also ensures security.
* The functions of the session layer are :
1. **Session establishment, maintenance and termination**
* The layer allows the two processes to establish, use and terminate a connection.
2. **Synchronization**
* This layer allows a process to add checkpoints which are considered as `synchronization points` into the data.
* These synchronization point help to identify the error so that the data is re-synchronized properly, and ends of the messages are not cut prematurely and data loss is avoided.
3. **Dialog Controller**
* The session layer allows two systems to start communication with each other in half-duplex or full-duplex.
* The **Session Layer**, Layer 5, helps application processes at both ends
* **Establish** connections,
* **Manage** connections to keep them **stable**,
* And **terminate** connections after work is complete
* The main role of the session layer is to **manage** `Sessions`, creating and destroying **TCP/IP sessions**
* It also performs **recovery** and **retransmission** for communications **interrupted** by **errors**
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### 6. Presentation Layer (Layer 6)
* Presentation layer is also called the **Translation layer**.
* The data from the application layer is extracted here and manipulated as per the required format to transmit over the network.
* The functions of the presentation layer are :
1. **Translation**
* For example, ASCII to EBCDIC.
2. **Encryption/ Decryption**
* Data encryption translates the data into another form or code.
3. **Compression**
* Reduces the number of bits that need to be transmitted on the network.
* The Presentation Layer, Layer 6, performs the function of **converting** data into a unified **syntax format** to assist communication between applications or systems with **different representation methods**
* It acts as a kind of `translator` or `converter`,
* And this function relieves the application layer of the user system from the burden of dealing with **format differences** in data
* Operations such as `MIME encoding`, `encryption`, `compression`, and `code conversion` take place at this layer
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### 7. Application Layer (Layer 7)
* At the very top of the OSI Reference Model stack of layers, we find Application layer which is implemented by the network applications.
* These applications produce the data, which has to be transferred over the network.
* This layer also serves as a window for the application services to access the network and for displaying the received information to the user.
* Ex: Application – Browsers, Skype Messenger etc.
* Application Layer is also called as Desktop Layer.
* The Application Layer, the topmost Layer 7 of OSI 7 Layers, defines application processes and performs application services
* Defining the **UI parts** or **user I/O parts** of network software is the role of the application layer
* There are an enormous number of application layer protocols, but representative ones include `FTP`, `SMTP`, `HTTP`, and `TELNET`
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## Summary
### Major Protocols and Devices by Layer
| Layer | Protocol | Device |
| ------------------ | --------------------------------------------------- | ------------------- |
| Application Layer | HTTP, SMP, SMTP, STUN, TFTP, TELNET | ADC, NGFW, WAF |
| Presentation Layer | TLS, AFP, SSH | |
| Session Layer | L2TP, PPTP, NFS, RPC, RTCP, SIP, SSH | |
| Transport Layer | TCP, UDP, SCTP, DCCP, AH, AEP | LB, Firewall |
| Network Layer | ARP, IPv4, IPv6, NAT, IPSec, VRRP, Routing protocol | Router, L3 Switch |
| Datalink Layer | IEEE 802.2, FDDI | Switch, Bridge, NIC |
| Physical Layer | RS-232, RS-449, V.35, S and other cables | Cable, Hub, TAP |