What is the OSI Model?

The Open Systems Interconnection model (OSI model) is a conceptualmodel that characterizses and standardizses the communication functions of a telecommunication or computing system without regard to its underlying internal structure and technology. Its goal is the interoperability of diverse communication systems with standard communication protocols. 

In the OSI model, network communication is categorized into 7 different layers, each with its own specific functions. This helps to standardize how different computer systems and applications interact with each other and exchange data. Each layer has a responsibility to perform specific tasks concerning sending and receiving data. All of the layers are needed for a message to reach its destination.

Why is the OSI model important? 

At the simplest level, the OSI model is important because it enables different computer systems to communicate with one another. More specifically it provides the following advantages: 

• It helps speed up innovation: Developers can tailor their efforts to improve communication in specific layers of the model, without impacting others. It also allows different teams to work on different challenges in the communication layers, at the same time, removing the need for linear improvements. 

• Faster troubleshooting: With a universal language for networking, organizations can operate from a shared understanding of their complex systems. The division into 7 layers makes it even easier for troubleshooting to occur: teams are speaking the same language and can perform improvements in isolated targeted portions of the layers. 
  

• Flexible standardization: The beauty of the OSI model is that in standardizing network communication, teams can easily understand, build, and improve on complex systems, without needing extensive training on the system to bring them up to speed. The standardization essentially places everyone on a level playing field. The layered nature of the model means technologies can be implemented without disrupting other layers or the overall structure of the network. All of this combined results in an assurance of sustainably scalable network infrastructure. 

What are the 7 layers of the OSI Model? 

1. Physical Layer

The Physical Layer of the OSI model refers to the physical equipment needed to transfer data -  think of switches and cables. Raw data bits are transferred over these physical mediums. 

The physical layer is responsible for establishing a connection between physical devices.

2. Data Link Layer

The Data Link layer of the OSI model refers to the technology used to connect two machines across a network. 

The data link layer is further divided into two sublayers: the Logical Link Control (LLC) layer - responsible for flow and error control - and the Media Access Control (MAC) layer - responsible for providing access to the physical layer. These two sub-layers are controlled by the Network Interface Card (NIC) to ensure that the delivery of data from one computer node to another is completed successfully. 

In simpler terms, this layer of the OSI model is responsible for maintaining transmission of data between two nodes within the same network segment. 

3. Network Layer

The Network Layer of the OSI model is responsible for enabling the transfer of data from a source host on a network to a destination host on a different network. It enables packet forwarding and routing between different networks, or from one host computer to another. 

The network layer routes data through the fastest (shortest) physical path: think of this like traffic. It works around congestion, service priority, broken links, and many other ‘roadblocks’. Key activities at this layer include routing, forwarding and addressing across networks and connected networks of nodes or machines. 

4. Transport Layer

The Transport Layer of the OSI model is responsible for sending and delivering reliable or ‘correct’ data from one device to another, either through a network or between two networks. In order for a data pack to arrive correctly it must be: in the correct order, with no data losses or errors, and easily recovered. 

It is common for the Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) to be used at this level. 

5. Session Layer

The Session Layer of the OSI model is responsible for managing connections between applications: this includes setting up, coordinating and breaking down these connections. 

The Session Layer helps to establish a link to start a session, authentication efforts to verify senders and receivers, authorization of shared communications, work to maintain sessions once they have been established, and finally termination of the connection upon completion. 

A real life example involves internet traffic: a user might wish to browse a webpage for a short period of time. The activities that occur behind the scenes to establish, authorize, maintain and then terminate this user’s session to their desired webpage are all performed on the Session Layer of the OSI model. 

6. Presentation Layer

The Presentation Layer of the OSI model is responsible for presenting data (packets of code) in a useful and usable format. Data packets flow through networks, often encrypted. When a packet arrives at layer 6, it must be presented in a usable format. 

In this layer data may be converted, character code might be translated, data might be compressed, and data could be encrypted or decrypted - all activities that help present data in a ‘usable’ format. 

7. Application Layer

The Application Layer of the OSI model involves the way in which a user application (think browser, email, website) interfaces with the network. Layer 7 functions as the essential interface between the applications a user interacts with, and the underlying network its data travels through. 

As the top layer of the OSI model, layer 7 involves data processing just beneath the virtual surface of the applications a user interacts with.  Data is presented in a way that user-facing applications can actually use it. A common example is an HTTP request used to load a webpage. 

Security and the OSI Model

Fastly’s AppSec solution suite can help secure your organization’s applications across the layers of the OSI model. We provide multi-layered protection that keeps you secure from the network layer to the application layer. 

Fastly’s AppSec offerings include: 

Fastly CDN: We’ve built security into the fabric of our platform. Fastly's real-time log streaming offers instant visibility into traffic, performance, threats, and troubleshooting. 

Fastly Next-Gen WAF: The Fastly Next-Gen WAF provides advanced protection for your applications, APIs, and microservices, wherever they live, from a single unified solution

API Security: Stop API abuse by monitoring for unexpected values and parameters submitted by endpoints and blocking unauthorized requests. Fastly can detect and block attacks in SOAP, REST, gRPC, WebSockets, and GraphQL APIs.

DDoS Protection: Deploy rapidly and immediately protect against application DDoS attacks. Leveraging our network’s massive bandwidth and adaptive techniques, Fastly DDoS Protection automatically keeps you performant and available without any required configuration.

Bot Management: Fastly Bot Management quickly identifies and mitigates unwanted bot activity, protecting your applications against a variety of automated attacks.