5. Session
4. Transport
3. Network
2. Data Link
1. Physical
The seven layers are taught zealously in most introductory networking courses. You may have had them permanently drilled into your head with the help of one or two fun little mnemonics! (My favorite is “All people seem to need data processing.”) As we discuss the functions of the different layers, keep in mind that the layers of the OSI model are arbitrary. They're not written on stone tablets, nor are they the result of a rigorous scientific process that conclusively proved that the perfect network has these seven layers. The ISO arrived at each layer by attempting to group similar network functions together in a layer and then organizing the layers in a hierarchical fashion so that each layer of functions is dependent on the one below it. This led to impressive results in layers 1–4 (the lower layers) and utter confusion in layers 5–7 (the upper layers).
Table 1.1 shows what common protocols fall into each of the lower layers.
Table 1.1 The lower layers and their associated protocols
| Layer | Name | Example protocols |
| 1 | Physical | Thicknet (10BASE5) |
| Thinnet (10BASE2) | ||
| 1000BASE-T | ||
| T1/E1 | ||
| 2 | Data Link | IEEE 802.3/Ethernet II (DIX) |
| Point-to-Point Protocol (PPP) | ||
| High-Level Data Link Control (HDLC) | ||
| 3 | Network | IPv4 |
| IPv6 | ||
| 4 | Transport | TCP |
| UDP |
The Upper Layers: Application, Presentation, and Session
One thing that has always been clear about the OSI model is that the Application layer includes application data and application protocols. The Hypertext Transfer Protocol (HTTP) is an application protocol that a web browser uses for communicating with web servers. Application data would be an HTTP GET request that the browser sends to a web server. Likewise, the web page that the server sends in response would also be application data. In short, application data is whatever the application sends or receives over the network.
Incidentally, an application can use more than one protocol. For example, when a web browser uses the Hypertext Transfer Protocol Secure (HTTPS) protocol to send a request to a web server, it's making use of two protocols: HTTP and Transport Layer Security (TLS). Despite the latter's confusing name, both are application protocols.
For all practical purposes, the upper layers (Session, Presentation, and Application) are one layer: the Application layer. The actual functions of the Session and Presentation layers—things like authentication and negotiating an application protocol—occur in the application anyway. They don't include any network functions and are concerned only with application data and application protocols.
Making Sense of Layers
The ISO never clearly defined what a layer is. The closest they came was a circular definition. But we can infer from the OSI reference model what they had in mind.
The concept of layering comes straight from software development (many of the OSI folks were operating system developers). The idea was that applications would treat the network as a software abstraction, somewhat like a filesystem. A filesystem acts as a layer that sits between the application and physical storage (e.g., disks). When the application needs to store some data, it just sends that data to the filesystem layer, which in turn takes care of the specifics of writing it to disk.
The OSI folks thought that in the same way that an application can store data on a filesystem without having to know anything about the underlying disks, so could it also send data over a network without requiring any network-specific coding or knowing anything about the network's infrastructure. Each layer would consist of a set of network-related functions implemented by the operating system or some middleware that would sit between the application and the host's physical network interface. Collectively, these layers would handle all the mechanisms of getting the application data onto the network and giving the network enough information to make sure the data got to its destination.
With the exception of the Physical layer, the layers of the OSI model are purely imaginary. Just as a filesystem is a software abstraction that hides the details of physical storage, the layers of the OSI model are just collections of software functions that hide the details of the network from applications and users. You can't see a filesystem with your eyes in the same way that you can see a hard drive, and you can't see the Data Link layer in the same way that you can see a switch. Layers are software abstractions and nothing more.
Figure 1.1 illustrates the concept of how layering might work using the Transmission Control Protocol (TCP) and Internet Protocol (IP), which are both included in the kernels of modern operating systems (Linux, Unix, and Windows). Keep in mind that the only real objects in this figure are the host and the physical network interface.
Figure 1.1 How layers abstract the network from an application
In this high-level example, when an application needs to send data it places the data