What is OSI Model | Real World Examples

Summary

This video explains how data is sent over the internet, focusing on the OSI model and TCP/IP. It describes the OSI model's seven layers, from physical to application, and their functions. The video details how data is encapsulated with headers at each layer before transmission and decapsulated at the destination. It emphasizes the educational value of the OSI model, despite its theoretical nature, and highlights practical applications like L4 and L7 load balancers. Viewers are encouraged to subscribe for more insights into system design.

Takeaways

• 🌐 The OSI model is a theoretical framework used to understand network communication, dividing it into seven abstraction layers.
• 🔩 The physical layer deals with the transmission of raw bits of data across a physical connection.
• 🔗 The data link layer organizes raw bits into frames and ensures they reach the correct destination, with Ethernet primarily operating here.
• 🔄 The network layer is responsible for routing data frames across different networks, with IP from TCP/IP being a key protocol.
• 🔌 The transport layer manages end-to-end communication, featuring protocols like TCP for reliable communication and UDP for simpler, faster transmission.
• 🔢 TCP segments data into manageable parts, assigns sequence numbers, and checks for errors, ensuring data integrity.
• 🚫 UDP, in contrast, does not offer the same level of error-checking and leaves reliability to the receiving end.
• 📚 The session, presentation, and application layers are considered less practical and are often collapsed into a single layer in real-world applications.
• 🌐 Application protocols like HTTP are considered part of the top layer of the OSI model.
• 🔄 Data transmission involves adding headers at each OSI layer, from the application down to the physical, and then reversing the process at the receiving end.
• 🌐 In practice, MAC addresses used in data link layer headers are typically not of the sending/receiving devices but of routing devices in the next network hop.
• 📚 The OSI model serves an educational purpose and is used by networking vendors and cloud providers to describe where their products fit in the model, such as L4 and L7 load balancers.

Q & A

What is the OSI model and why is it important?

The OSI model, or Open Systems Interconnect model, is a theoretical framework for understanding networking. It divides network communication into seven abstraction layers, which helps in conceptualizing and standardizing how data is transmitted and received between devices.

What is the role of the physical layer in the OSI model?

The physical layer is the bottom-most layer of the OSI model and is responsible for transmitting raw bits of data across a physical connection, such as through cables or wireless signals.

How does the data link layer contribute to network communication?

The data link layer takes the raw bits from the physical layer and organizes them into frames, ensuring that the frames are delivered to the correct destination. It is also where protocols like Ethernet operate.

What is the primary function of the network layer?

The network layer is responsible for routing data frames across different networks. It determines the path that data takes from the source to the destination, with the Internet Protocol (IP) being a key example of this layer.

How does the transport layer facilitate communication between two nodes?

The transport layer handles end-to-end communication between two nodes, ensuring that data is sent and received reliably. It includes protocols like TCP, which provides reliable communication by segmenting data and using sequence numbers, and UDP, which is faster but less reliable.

What is the difference between TCP and UDP in the transport layer?

TCP (Transmission Control Protocol) provides reliable, end-to-end communication with error-checking and data reassembly based on sequence numbers. UDP (User Datagram Protocol), on the other hand, is simpler and faster but does not offer the same level of reliability or error-checking.

Why are the session, presentation, and application layers considered less useful in practice?

The session, presentation, and application layers are considered less useful in practice because they are too fine-grained and do not reflect the real-world applications as accurately. They are often collapsed into a single layer in practical terms.

How does an HTTP request move through the OSI model layers?

An HTTP request starts at the application layer where the HTTP header is added. It then moves to the transport layer where a TCP header is added, creating TCP segments. At the network layer, an IP header is added, and at the data link layer, a MAC header is added. Finally, it is transmitted as raw bits through the physical layer.

What happens when a web server receives a request at the physical layer?

When a web server receives raw bits from the network, it reverses the process by removing headers layer by layer, starting from the physical layer up to the application layer, where it finally processes the HTTP request.

How are the OSI model layers related to real-world networking products?

The OSI model layers are used by networking vendors and cloud providers to describe where their networking products operate. For example, L4 and L7 load balancers refer to products operating at the transport layer (TCP level) and the application protocol layer (HTTP/HTTPS), respectively.

What is the educational purpose of the OSI model?

The OSI model's primary purpose is educational, providing a framework to understand and teach how different networking components interact. Even though it may not perfectly fit real-world scenarios, it is still widely used for conceptual understanding in networking.

Outlines

00:00

🌐 Understanding the OSI Model and Networking Layers

This paragraph introduces the OSI model as a theoretical framework for understanding network communication, dividing it into seven layers. It explains the role of each layer, from the physical layer responsible for transmitting raw data bits, to the data link layer organizing them into frames and ensuring delivery. The network layer is highlighted for its role in routing data across networks, with IP being a key protocol. The transport layer is further elaborated, with TCP providing reliable communication by segmenting data and using sequence numbers for reassembly, while UDP offers a faster, less reliable alternative. The session, presentation, and application layers are mentioned but noted as less practical, often combined into a single layer for simplicity. An example of data transmission using HTTP over the network is provided, detailing the process of data encapsulation and decapsulation through the layers. The OSI model's educational value and its use by networking vendors and cloud providers for product categorization is also discussed.

Keywords

💡OSI model

The OSI model, or Open Systems Interconnect model, is a conceptual framework used to understand the workings of a communication network by breaking it down into seven distinct layers. In the video, it is the central theme that helps explain how data is sent over the internet. Each layer has a specific role, from the physical transmission of bits to the application layer where user interactions occur.

💡TCP/IP

TCP/IP stands for Transmission Control Protocol/Internet Protocol and is a set of communication protocols used to connect hosts on the internet. In the script, it is mentioned as fitting into the OSI model, particularly with the IP protocol being an example of the network layer and TCP being part of the transport layer, which is crucial for data transmission reliability.

💡Physical layer

The physical layer is the first and bottom-most layer of the OSI model, responsible for the actual transmission of raw bits over a physical medium like wires or airwaves. It is foundational to the entire networking process as it deals with the physical aspects of data transmission, as explained in the script.

💡Data link layer

The data link layer is the second layer in the OSI model, which organizes raw bits into frames and ensures their delivery to the correct destination. Ethernet operates primarily in this layer, as mentioned in the script, which is essential for local area network communications.

💡Network layer

The network layer, the third layer of the OSI model, is responsible for routing data frames across different networks. The script highlights the IP protocol as a key component of this layer, which is vital for directing data packets to their destination on the internet.

💡Transport layer

The transport layer, the fourth layer in the OSI model, manages end-to-end communication between network nodes. The script explains that TCP and UDP, two protocols that operate at this layer, handle the sending of data but differ in their approach to reliability and error checking.

💡TCP

Transmission Control Protocol (TCP) is a protocol in the transport layer that ensures reliable communication between devices. The script describes how TCP divides data into segments, assigns sequence numbers for reassembly, and includes error checking to ensure data integrity during transmission.

💡UDP

User Datagram Protocol (UDP) is another protocol in the transport layer, mentioned in the script as being simpler and faster than TCP. Unlike TCP, UDP does not provide extensive error-checking or reliability, it merely sends data packets without ensuring their successful arrival.

💡Session layer

The session layer is one of the upper layers of the OSI model, which is involved in establishing, managing, and terminating sessions between applications. The script notes that this layer, along with the presentation and application layers, is less practical and often collapsed into a single layer in real-world applications.

💡Encapsulation

In the context of the OSI model and networking, encapsulation refers to the process of adding header information to data at each layer before it is sent over the network. The script describes how data is encapsulated with headers at various layers, such as TCP, IP, and MAC, before being transmitted at the physical layer.

💡MAC address

A MAC address, or Media Access Control address, is a unique identifier assigned to network interfaces for communications on a physical network segment. The script explains that at the data link layer, a MAC header with source and destination MAC addresses is added, which is crucial for local network routing, although the actual MAC addresses used may belong to routing devices rather than the end devices.

Highlights

The OSI model is a theoretical framework for understanding network communication.

Network communication is divided into seven abstraction layers in the OSI model.

The physical layer transmits raw bits of data across a physical connection.

The data link layer organizes raw bits into frames and ensures delivery to the correct destination.

Ethernet primarily operates at the data link layer.

The network layer routes data frames across different networks, with IP being a key example.

The transport layer manages end-to-end communication, where TCP and UDP protocols reside.

TCP provides reliable communication by segmenting data and using sequence numbers.

UDP is a faster, simpler protocol without the reliability and error-checking of TCP.

The session, presentation, and application layers are often collapsed into a single layer in practice.

Application protocols like HTTP are considered layer 7 protocols in the simplified OSI model.

An example of data transmission involves adding HTTP and TCP headers at different layers.

IP headers at the network layer contain source and destination IP addresses.

MAC headers at the data link layer include MAC addresses of routing devices in the next hop.

The physical layer sends the encapsulated frames as raw bits over the network.

Web servers reverse the process by removing headers layer by layer to process HTTP requests.

The OSI model serves an educational purpose and is used by networking vendors and cloud providers.

Cloud load balancers are categorized as L4 or L7 based on the OSI model layer they operate on.

An L7 load balancer operates at the application protocol layer, while an L4 operates at the TCP level.

Transcripts

How is data sent over the internet?

What does that have to do with the OSI model?

How does TCP/IP fit into this?

Let’s take a look.

The OSI model, or the Open  Systems Interconnect model,  

is a theoretical framework that provides  one way of thinking about networking.

It splits the network communication between two  

devices on a network into  seven abstraction layers.

The physical layer is the bottom-most layer.

It is responsible for transmitting raw  bits of data across a physical connection.

The data link layer is the second  layer. It takes the raw bits from  

the physical layer and organizes them into frames.

It ensures that the frames are  delivered to the correct destination.

The Ethernet primarily lives in this layer.

The network layer is the third layer.

It is responsible for routing data  frames across different networks.

The IP part of TCP/IP is a  well-known example of this layer.

The transport layer is the fourth layer.

It handles end-to-end  communication between two nodes.

This is the layer where TCP and UDP live.

TCP provides reliable, end-to-end  communication between devices.

It does this by dividing the data into small,  

manageable segments and sending  each segment individually.

Each segment has a sequence number attached to it.

The receiving end uses the sequence numbers  to reassemble the data in the correct order.

TCP also provides error checking to make sure that  the data was not corrupted during transmission.

UDP is another popular protocol  in the transport layer.

It is similar to TCP, but  it is simpler and faster.

Unlike TCP, UDP does not provide the same  level of error-checking and reliability.

It simply sends packets of data  from one device to another.

The receiving end is responsible for determining  whether the packets were received correctly.

If an error is detected, the receiving  end simply discards the packet.

The remaining layers are the session,  presentation, and application layers.

This is where the OSI model  loses its usefulness in practice.

They are too fine-grained and  do not really reflect reality.

In general, it is sufficient  to collapse them into a single  

layer and consider application protocols  like HTTP as simply layer 7 protocols.

Let’s go through an example and examine how data  

moves through the layers when  transmitting over the network.

When a user sends an HTTP request  to a web server over the network,  

the HTTP header is added to the  data at the application layer.

Then, a TCP header is added to the data.

It is encapsulated into TCP  segments at the transport layer.  

The header contains the source port,  destination port, and sequence number.

The segments are then encapsulated  with an IP header at the network layer.

The IP header contains the source  and destination IP addresses.

A MAC header is added at the data link layer,  with the source and destination MAC addresses.

It is worth noting that in the  real world this is a bit nuanced.

The MAC addresses are usually not the MAC  address of the sending and receiving ends.

They are the MAC address of the routing devices in  

the next hop of a usually long  journey across the internet.

The encapsulated frames are sent over the  network in raw bits in the physical layer.

When the web server receives the raw bits  from the network, it reverses the process.

The headers are removed layer by layer,  

and eventually, the web server  processes the HTTP request.

To conclude, the OSI model is one  way of thinking about networks.

Its primary purpose is educational.

Even though the layers don’t fit  the real-world use cases perfectly,  

they are still widely used by networking  vendors and cloud providers as a shorthand  

to describe where their networking  products sit in the OSI model.

For example, cloud load balancers are broadly  divided into two categories - L4 or L7.

An L7 load balancer is a shorthand to  mean that the load balancer operates at  

the application protocol layer like HTTP or HTTPS.

An L4 load balancer, on the other  hand, operates at the TCP level.

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