What is OSI model?

Today I tweeted some thoughts about how the OSI model doesn't really match up to how TCP/IP actually works, and it got me thinking - what exactly is the OSI model? Reading some of the responses on Twitter, it seems there are at least three different ways of thinking about it:

• A literal description of how TCP/IP works
• An abstract model that can be used to describe and compare many different network protocols
• A literal description of some computer network protocols from the 1980s, most of which are no longer in use today.

In this article, I'm not going to try to argue which of the above is the "real" OSI model - different people seem to think about it in all of these ways. It doesn't matter.

The OSI model has seven layers

Before we discuss what the OSI model means, let's discuss what it is in general. It is an abstract model that describes how a network works on seven numbered layers:

• Layer 1: Physical Layer
• Layer 2: Data Link Layer
• Layer 3: Network Layer
• Layer 4: Transport Layer
• Layer 5: Session Layer
• Layer 6: Presentation Layer
• Layer 7: Application Layer

I won't waste time explaining what each layer means, there are thousands of explanations available online.

The OSI Model: A Literal Description of How TCP/IP Works

First, I want to talk about a common way people use the OSI model in practice: as a literal description of how TCP/IP works. Some layers of the OSI model map very easily to TCP/IP:

• Layer 2 corresponds to Ethernet
• The third layer corresponds to IP
• The fourth layer corresponds to TCP or UDP (or ICMP, etc.)
• Layer 7 corresponds to anything inside a TCP or UDP packet (such as a DNS query)

This mapping makes sense for layers 2, 3, and 4—a TCP packet has three headers corresponding to these three layers (Ethernet header, IP header, and TCP header).

It's useful to describe the different headers in a TCP packet with numbers - if you say "layer two", it's obvious that it's "below" layer three, because two is smaller than three.

The weird thing about the "OSI model as a literal description" is that layers 5 and 6 don't really correspond to anything in TCP/IP - I've heard lots of different interpretations of what layer 5 or 6 might be (you could say layer 5 is TLS or something!) but there's no clear correspondence like there is with layers 2, 3, and 4 where "each layer has a corresponding header in the TCP packet".

Furthermore, some parts of TCP/IP don't fit neatly into the OSI model even at layers 2 to 4 - for example, which layer are ARP packets? ARP packets send some data with an Ethernet header, so does that mean they're layer 3? Or layer 2? The Wikipedia article listing the different OSI layers categorizes them as "layer 2.5", which is not very satisfying.

Because the OSI model is sometimes used to teach TCP/IP, it can be confusing to understand which parts of it map to TCP/IP and which parts don't. This is the real problem.

The OSI model: an abstraction for comparing network protocols

Another way I’ve heard of thinking about OSI is that it’s an abstraction that can be used to draw analogies between many different network protocols. For example, if you want to understand how the Bluetooth protocol works, maybe you could use the OSI model to help you — here’s a diagram I found on this page that shows how the Bluetooth protocol fits into the OSI model.

The OSI model: a literal description of some obsolete protocols

Some very brief research on Wikipedia shows that in addition to the abstract description of these seven layers, the OSI model also includes a set of specific protocols that implement these layers. Apparently this happened during the Protocol Wars of the 70s and 80s, where the OSI model lost and TCP/IP won.

This explains why the OSI model does not correspond well to TCP/IP, because if the OSI protocol had "won" at the time, then the OSI model would correspond exactly to how Internet networks actually work.

Conclusion

I wrote this post because when I first learned about the OSI model, I found it very confusing (What are all these layers? Are they real? Is this how networking actually works? What's going on?) I wish someone had told me that as someone who only uses TCP/IP networking protocols, I only need to understand how OSI model layers 2, 3, 4, and 7 relate to TCP/IP and then ignore everything else about it. So I hope this post helps someone!