The OSI Reference Model Pt 3 (Upper Layers & The TCP/IP Model)

We've talked about what the OSI model is, we've discussed the lower layers of the OSI model, now we're going to discuss the upper layers of the OSI model.

L5 - SESSION LAYER

Layers five to seven are upper layers. They're the layers that are more important to the programmer than they are to the network engineer. Also, from layers 4 - 7 are handled by the hosts (the end devices like your phone, laptop, etc.) and layers 1 - 3 are handled by the network devices (routers, switches, hubs, firewalls, access points, etc.).

The session layer establishes connection between hosts. It's basically like calling someone on the phone. When you call someone, any thing you guys say to each other in that call is considered all part of one conversation. When you're done, you hang up. To talk again, you have to start another conversation. This is what the session layer does. It establishes sessions so that any message passed between two particular hosts while the session is up is considered part of that session. When the communication is over, it terminates the session.

Some session layer protocols include Layer 2 Tunneling Protocol (L2TP), Remote Transport Control Protocol (RTCP) and Socket Secure (SOCKS).?

L6 - PRESENTATION LAYER

This layer is concerned with the formatting of data in a way it can be sent over the network. For example, some devices may not support ASCII characters for text. This layer converts them to a format in which they can be sent over the network and re-converted to what the device understands. It's the layer that decides the format the image or video or text is going to be sent in. It also handles encryption. Protocols like Secure Sockets Layer (SSL) and File Transfer Protocol (FTP) fall into this layer.

L7 - APPLICATION LAYER

This is the topmost protocol. The "application" in this layer doesn't refer to specific apps. It refers to the use or function of a particular protocol. Just like the application of a car is in transport to convey people and stuff. This layer, however, provides services to the applications (software) you use. For example Hypertext Transfer Protocol (HTTP) helps you request and get hypermedia documents like HTML pages from servers. Simple Mail Transfer Protocol (SMTP) helps you send mails. File Transfer Protocol (FTP) helps you transfer files from one device to another. Other protocols like Dynamic Host Control Protocol (DHCP), Trivial File Transfer Protocol (TFTP) and Domain Name System (DNS) operate at this layer.

Now, it may confuse you that FTP appeared twice. Some protocols exist on multiple layers at once. In fact, from layer 5 upwards, things aren't as clear cut as the lower layers. That's probably why they were all grouped together in the TCP/IP model (which you will learn about shortly).

HOW DO I REMEMBER ALL THESE LAYERS?

Hopefully by now, some of the layers have stuck in your head, but there's still a way to remember them easily if you need to. To memorize the layers from top to bottom, you can use "All People Seem To Need Data Processing".

To memorize the PDUs from bottom to top, you can use "Bacon Frying Produces Salivation".

THE TCP/IP MODEL

There's another model very similar to the OSI reference model, but is slightly different. The TCP/IP Model. The difference between the TCP/IP Model and the OSI Model is that the TCP/IP Model collapses layers 5, 6 and 7 into one, so we only have five layers; physical layer, data-link layer, network layer, transport layer and application layer.

I don't yet know the reason why anyone should choose the TCP/IP Model over the OSI Model, but I know that they're both standards in networking (that is to say frameworks in which we think of networking), and the OSI Model is better to teach newbies. The TCP/IP stack, however is a bit more realistic in the sense that at layers 5, 6 and 7, things get a bit mushy. You can't really draw lines and categorize protocols. If they fall into two or more of those (hard to differentiate) layers, why not just collapse the layers into one?

In the TCP/IP Model, the application layer is still referred to most times as layer 7. If you have any more information, don't forget to leave it in the comments below. Also, like this article and subscribe to the newsletter for more.