So I had some time to kill recently and I was on YouTube checking out channel of a colleague and there
was such an interesting discussion going on over there and that was about should we be teaching the
OSSI model anymore?
And they were, you know, discussing this in any technical trainer type of a context, right?
If you're teaching cybersecurity, should you teach the OSCE model?
If you're teaching just basic networking, should you teach it?
Should you teach it if you're teaching a plus?
And I really couldn't even believe it was being discussed because I think we have to teach the OSCE
model for a whole bunch of important reasons.
And I really do mean it when I say we have to teach it.
One of the reasons I believe this is true is because of the fact that this terminology of layers is
just so ingrained in technology today that will still see it presented that way.
And we don't want to be at a loss when folks start talking about layers, but I'm getting ahead of myself,
I guess.
Why is the OSCE model such a great thing?
Well, it was originally created for stuff to have a chance to be able to communicate together.
You see, when networking and all of this amazing technology with compute really started taking fire
in the sixties and seventies and eighties.
One of the challenges in those decades was that you would have equipment from one manufacturer like
IBM that couldn't communicate with like NBC or DEC, you know.
So there was all these vendors that had great stuff, but it didn't interoperate.
So obviously we needed interoperability so that we could have the incredible Internet filled with its
diverse systems that we have today.
So the reason the OSSI model was created is one thing that's kind of historical, but how can it help
us today?
Well, one of the ways it's going to help us today is in troubleshooting.
I mean, think about it.
When we break down the network around us into these layers and we recognize there's a problem at layer
three, the network layer.
So we number from the bottom up.
Layer one is the physical layer two is data link, layer three is network and layer four is transport.
When we talk about there being a problem at layer three, we know that this is most likely causing the
issue at layer four, layer five, layer six, layer seven, because those layers depend on the network.
Layer a fun, totally like in-your-face example of this would be if I unplug your laptop and there's
no battery inside it, so I unplug your computer and it loses power.
Yeah.
So you're not going to be able to get email.
The physical layer is dead and therefore so is layer two, layer three, layer four, layer five, layer
six, and the application layer layer seven.
Now, one of the first skills that you're going to need is just memorizing the layers.
And there's two famous ways to do that, if you stay with the bottom up, are also popular in pubs,
by the way.
Bottoms up anyways.
If you stay at the physical and work up it's people are oh no it's please do not throw sausage pizza
away.
And that's the one I always like because I'm Italian and I love sausage pizza.
So please do not throw sausage pizza away if you're a top down person.
It's all people seem to need data processing, but you just obviously would have these memorized.
I don't have to think about these anymore as far as you know.
Please do not throw sausage pizza away.
I don't need to play games like that.
When you've been in this industry for any amount of time, these are just ingrained inside you.
Now, what do they actually mean?
What's actually happening happening here?
Well, what's happening is encapsulation of information.
That's really what's happening.
What what we do is we are wanting to send data from one network device to another network device.
And there's going to be a bunch of encapsulation that takes place.
And what this means is additional information that's going to be added to our data so that the community.
Stations actually works.
Let me walk you through this with some examples.
So here we can see the OCI model.
Often called the OCI stack because of course we're stacking these layers on top of each other and think
about us transmitting information from one workstation to another workstation in an organization.
And what they're communicating with is copper.
Yeah, the physical media is copper that is inside of Ethernet cables.
So the physical layer consists of copper.
And the physical layer kind of encapsulation involves chopping up our data into the appropriate bits
for transmission on to the copper wire.
Then what happens is we are about to send this over an Ethernet network.
So at the next layer of the OCI model, the data link layer, we will encapsulate and we don't do it.
The machine does it for us.
Thank goodness the computer will encapsulate onto our data information about our MAC address because
the MAC address is the identifier of a workstation in an Ethernet network.
So now that the MAC address has been appended to the data, it can be handled by the network layer.
This is going to be responsible for taking what we call a packet and sending it on its way throughout
the entire network, maybe even the Internet, the global Internet.
Once again, addressing is needed here.
So we had a layer two data link, layer example of a mac address.
And at the network layer, with the TCP IP protocol, we have an IP address that meet that would be
appended or encapsulated is the best way to say it to our data.
At the transport layer, we have options for how this data will be transported through the network.
For example, if we use transmission control protocol, we'll be transporting the data reliably.
If we use user datagram protocol will be transporting it unreliable.
I'll do videos on this for you to make sure you understand TCP and UDP fully, but for right now we
note that the TCP or UDP information would be encapsulated on our data.
So the data is picking up little ingredients, picking up additional addressing parameters, picking
up additional pieces of information so that we can successfully send it through the network.
The next layer is the session layer, and an example would be the when protocol of X25 or the secure
copy protocol for securely transferring files through a network.
The session layer does just what it sounds like it is, making sure that sessions often secure sessions
can be established.
The presentation layer is doing just what it sounds like, ensuring that things are formatted in an
understandable way by the next layer up the application layer.
So we might have ASCII for text or mime for email messaging, and then there's the application layer
and it's going to be speaking the application layer protocols like the language of love for the Internet.
The World Wide Web is, of course, powered by the hypertext transfer protocol or HTP, just like the
world of email is handled by the simple mail transfer protocol.
We're file transfers by the file transfer protocol.
So notice there is additional information appended to our data for the application layer to consume.
Each layer encapsulates additional information that's going to be required in our environment for proper
communications.
It's really a glorious invention, and it wasn't all that unique.
As a matter of fact, we're going to see in the next video how the Department of Defense here in the
United States had actually created a model that gave rise to this OSA model.
Years later, we'll take a look at the amazing TCP IP architecture model that the DOD came up with and
that really helped fuel the push towards the OSA model.
Now, before I wrap up this video, we also need to realize, and you heard me mentioning these, that
we give specific names to the.
Bottom four layers of the encapsulated data with its you know whatever was encapsulated do it right
so at the physical layer we save the data in any encapsulation information are just bits at the data
link layer.
When we encapsulate, let's say, the Mac address information, we call this a frame at the network
layer, we call the data and the encapsulated information a packet and at the transport layer, layer
four, we call it a segment.
Now, I've never heard a, you know, mnemonic device that will help you memorize these.
So I invented one and I invented it decades ago, and I wrote it in a couple of my books and it actually
caught fire.
So I think if you hear this from someone else, they got it from me.
And it's some people fear birthdays and notice I use the top down approach for segment packet frame
in bits.
Some people fear birthdays, so that's an easy way to remember those protocol data units and you will
really want to practice with that and practise thinking in those terms.
Because when you are talking to networking, cyber security analysts and specialists that are your peers,
they're going to be talking in terms of, hey, what did you find in the packet?
And they want you to know that they're talking about the IP network layer information or they'll say,
What did you find out for the source address in the frame and you need to write away, go.
Okay.
They're talking about a mac address at the data link layer.
So this terminology is super important.
By the way, do we call the information of the session presentation and application layer something
special?
Now we just tend to call that data.
Okay.
So the PD you for all three of those layered layers would just be considered data.
It's the lower four layers where we have the specialized terms that I want you to memorize those layer
four lower layers.
Those are all so specific in what they do that they get big time scrutiny by a cyber security people
because the stuff of the session presentation and application layers that is much less related to networking
than these lower layers, as you might expect, you know, pure networking and pure communications,
the abstraction increases as you go up is another way to think about it.
All right.
Well, enough about this.
Let's turn our attention to the next video where I just wanted to make sure you were clear on the model
that actually predated.
That's right.
Came before this one.
It's that TCP IP architecture model.