Working in the software field, it’s not uncommon for engineers to refer to different “layers.” Perhaps you’re working with protocols at the “networking layer” or evaluating a solution that sits at “Layer 4” or “Layer 7.” Although these concepts are obvious to some, not everyone knows what these layers refer to. To get you up to speed, below, we’ll define the seven layers of the OSI Model.
What Is the OSI Model?
The Open Systems Interconnection model (OSI model) is a conceptual model to help visualize a computerized network. Made up of seven layers, it represents the distinct levels that make up an end-to-end computing system. In the spirit of promoting open interoperability, the model is intended to represent a universal standard that’s agnostic to a specific technology or vendor.
The OSI Model was co-conceived by International Organization for Standardization (ISO) and Internet Engineering Task Force (IETF) and first published in 1984. It has since been redefined as ISO/IEC 7498-1:1994. Although the OSI Model has been around for decades, it’s still referenced quite often.
The Seven OSI Model Layers
The OSI Model is split into seven abstraction layers: Physical, data link, network, transport, session, presentation and application. You can think of the bottom one, Layer 1 (the physical layer), as the closest to the most rudimentary electrical connections. The farther up you rise, the closer you get to Layer 7 (application layer), which is the most user-facing category.
1. Physical Layer
The physical layer, Layer 1, represents the lowest level of data transmission. This category refers to raw unstructured data bits—and the process of converting them into electrical signals for a device to read. “Physical” hardware like network hubs, modems, adapters, cabling and network controllers work in this layer. Bluetooth, Ethernet and USB describe physical layer specifications.
2. Data Link Layer
The data link layer deals with data packaged into frames. Technologies in this layer assist in node-to-node data transfer, and the protocols here describe how to establish and terminate a connection and how the connection should flow. The data link layer is further divided into two sublayers: Media access control (MAC), responsible for how nodes connect with one another, and logical link control (LLC), which checks for errors and orchestrates the frame flow and synchronization. An example data link layer protocol is Point-to-Point Protocol (PPP). MACsec also can work to apply encryption at this layer.
3. Network Layer
The network layer is responsible for sending and receiving data frames structured in packets. Technologies in the networking layer use routers to send packets to nodes on different networks. The network layer might split or fragment the message if it exceeds the maximum network packet sizes. You’re probably familiar with the network layer if you’ve looked up your IP address—the network layer uses the IP protocol (or other logical protocols) to find locations. Such protocols specify a message along with the address of the intended recipient node.
4. Transport Layer
The transport layer deals with the transmission of data segments, which are sequences of data at variable lengths. The goal of the transport layer is to optimize data transmission by performing data segmentation and adjusting the size of the package or the rate of transmission. Transport layer protocols include TCP and UDP. Tunneling protocols also operate at the transport layer. The OSI Model also defines five classes of the connection-mode transport protocol.
5. Session Layer
The session layer handles the communications between two or more computers. Protocols here are used to create a “session” between entities, which is common in applications that use remote procedure calls. The session layer handles the connection and authentication between a client or server, including actions like logon, look up, log off or session termination. DNS, along with name resolution protocols, operate in the session layer.
6. Presentation Layer
The presentation layer is about data translation and formatting. In this layer, protocols handle things like encryption, decryption, compression and decompression. The goal of the presentation layer is to transform data in such a way that it can be sent over a network in syntax that fits the constructs specified by the application layer. For example, technologies that serialize data structures to XML or JSON can be thought of as working for the presentation layer. This sort of data translates into what is graphically displayed for the end-user.
7. Application Layer
The application layer is the top layer of the OSI Model and sits closest to the end-user application. User-facing software directly interacts with the application layer through functions like file sharing, message handling or database access. High-level protocols such as HTTP and FTP are used in this layer to share resources. Web browsers and email clients are examples of applications that interact with the application layer.
The OSI Models: A Helpful Taxonomy
There you have it—a primer or refresher of all seven OSI Model layers. It’s important to note that the model does not intend to serve as an implementation specification itself—it’s purely a conceptual framework. But it does help provide context into where tools operate and how they interact with elements of a distributed computing system.
In the context of DevOps technologies, Envoy-based service mesh is often described as working on Layer 4 (the transport layer) and Layer 7 (the application layer). Or, since eBPF filters network frames, it can be said to work at Layer 2 of the OSI Model.
