welcome back to the CCNA journey with me, And this would be part 2 of the OSI model, Focusing on the upper layers. For those who don't know, You can contact me here on EveDumps, Facebook, Twitter or you can connect on LinkedIn. If this article series has been helpful, Please do like and subscribe, And it's also worth noting that this article was pre-recorded for the sea scent or the icnd1, And it's being reused for this new CCNA course, Because the content of the OSI model is still relevant and Required in order to pass your CCNA
let's get started , So here we are, We've got the overview of the open system, Interconnection, Which is the OSI and we're going to start at layer 7 and work our way down discussing each layer and a little more Detail to give you an understanding once you've discussed it, And we understand what each layer, The OSI model actually does, And briefly how it interacts was the layer above below which referred to as adjacent layers. What we then do is have a discussion around the encapsulation and decapsulation process that may be in this article, But it will most likely be in the next, Because I'm sure that this article will take up a lot of time going through the open system into connector.
So, Starting with the application layer, Then why is it essentially? It provides the initial network connection for user applications and it allows the applications to connect into the network. This is where your end users actually sit. So we've previously discussed this concept of networking applications and we agreed that a networking application is an application that uses some protocol in this case, Let's say, For example, HTTP or SMTP to interact with the network.
So we said a browser is a good example of networking application, Whether that's a Firefox, Chrome, SafarI or IE. Essentially, It uses a protocol in this case. Well, In this example, Let's say it's HTTP hypertext Transfer Protocol and it uses that to interact with the network. We also said that there are many other networking applications like email, Clients and email clients may use protocols like The Simple Mail Transfer Protocol SMTP to send emails.
It may also use things like pop free post office protocol to receive e-mails. All IMAP, For example, Regards what is. We all agreed that the networking application is an application that interacts with the network and therefore those networking applications sit at layer 7.
We also said was: there are certain applications that do not require American interaction. Applications like notepad and pane and those type of applications do not sit at layer 7 of the OSI model. For up to this, We then had a brief discussion about presentation. Now. Presentations are very simple one when we think about presentation. What we need to think about is how to present the data.
Now we also have, With this term last article called upper layers and lower layers, And we said that anything in these three layers here I refer to as the upper layer, And anything here is referred to as the lower layer. Now, Essentially, As network engineers, We primarily focus on the transport network datalink in physical and there's no way, Ultimately, As a network engineer to control what happens at the session presentation and application or we need to be aware of, Is what actually happening there and what are The responsibilities of those layers, So we know the application layers where the users interact. The presentation layer is how the application layer actually presents the data. So maybe we have a HTTP protocol interact in and requesting a haitch tml page and in that HTML page, Maybe it uses asciI or maybe it's got pictures in there.
So it's just jpg and it pulls down those ones and zeros from a remote web server. And ultimately, The presentation ensures that the information that is poor God is able to be presented both when it's received at this end and when it's requested at the other end. This is also where two very important elements it. This is where compression and encryption also sits again. Here we think about compression and encryption.
It changes the way the data is presented when two hosts received traffic from one another. So moving further down the stack become against the session layer. The session layer is layer.
Five and quite a few things here, Hhappens at the session layer. The session layer helps establish sessions with reserved ports, Helps track connections between remote hosts, And it also allows us to negotiate some duplex with the remote host. So, To give you an example, Let's say we have our PC and our PC is communicating with very remote server and t're going to be using the hypertext Transfer Protocol, Because this is ultimately a web server and on this PC he has e Y. We have our favored web browser open and we have tab 1.
Then we have tab 2. Now, In this kind of scenario, There's actually a few things. The session layer and layer 5 helps us with the first thing it does is a duplex negotiation with the remote host. The remote host in this sense happens to be a web server. It could have been another PC and essentially, What happened is it negotiates to see whether the communication flow can either be full or whether it be half-full, Allows you to send and receive bitter half allows you to send all receive data Mart, But at the same time, Once it negotiates the duplex settings and, Let's say, For example, It agrees on full what the session layer session layer also does is it puts unique port numbers or unique pulse, Unique source port numbers against every session to a remote host? Now, This actual unique port number is actually a bit of a weird name: it's called a ephemeral port and that's not an Arthur, That's actually an M.
So that's EP h, E m e r, Al, A ephemeral port and essentially any ephemeral port, Is a randomly generated port number between a particular range to allow a remote server to keep these sessions coming in from your particular IP unique. Now, What we'll do is we'll discuss this a bit more when we talk about transport, But essentially this session layer reserves what ports to be used and give those reports downstream to the transport layer and the transport layer populates. Its PDU is header, With the port's that the session layer supplies it.
Let's say, For example, These two tabs up here one may get three three three three as a ephemeral port and the other may get three three four four. Maybe you might even get 60000 344. It's just a random number between.
I think it's between a thousand and twenty four and up to 65 thousand three hundred and something I think, It's probably 35 or 65. But ultimately it's a man deport within this range. The reason is not up to 1024 is because 1 to 1024 is something called the well-known ports and some of these ports, Which I'll get to in a moment on the transport layer, A port numbers that you have to remember like for HTTP.
We know it's port. 80 SMTP port 25 pop port, 110 and so forth, But ultimately the session layer assigns unique ports to each session that you have. So when the receiving host receives your data, It knows what port to send it back to in order to make sure that the sessions are not overlapping or bleeding between one another.
So, As you can see, The session is obviously just as important is any other layer. So moving forever down the stack to layer, For this is our transport layer. This is the first lower layer, Lower layer that we're going to discuss because we've now finished discussing the upper layers now at the transport layer, T're two protocols that mainly use, Which is the transmission control protocol and the user Datagram protocol and we'll have a separate article Discussing the differences between those but very high level, TCP is something that allows us to have meshes acknowledgement message message: segmentation allows us to essentially deliver traffic error-free in sequence, With no loss or duplication. Udp, On the other hand, Is very much shoveled out the front door and hopefully it receives.
Hopefully it's received by the remote host. There's no error correcting there, There's no retransmission and there's no tracking of information and will come later on we're coming to actually understanding the differences between them and why you may use one over the other. What also happens at the transport layer is, As I previously just mentioned.
This is where the session layer populates the transport layers PDU with port numbers. So we will think about where we are. At the moment we have data. The data was given to us from the application layer and as part of the application layer, It identified how that data should be presented and what ports to use both the destination and the source port and the source will be in that ephemeral port. It gives it down to the transport layer, The transport layer, Bpdu comes on and depending on the application, It made you to use TCP or it may choose use UDP.
Let's say in this case it's decided to use TCP the session layer will give us the source port number - let's say that's: 60000 311 and the destination port number. Let's say that's 80, Because we're using TCP. We know that the information is going to be reliable. It's going to get there and if it doesn't get there, It's going to be retransmitted and there's going to be a what's called a three three-way handshake with a remote host and the freeway handshake is essentially before a house sounds traffic to another host.
It's and using TCP it does it through a handshake where the whole sends what's called a sync. The remote house sends a sync ACK, And then this guy sends an ACK back to say. I'Ve received your sync ACK and then these two devices, Whether t're on the same segment or whether t're multiple networks away, Is able to communicate with one another to ensure that the sequencing of traffic is correct and any traffic that's missing is able to resend and essentially Correct any errors that may have happened along the path, Whether may be a Reuters dropped it because it was too busy or buffle was full wherever it may be. Tcp adds that layer of have that layer essentially off the reliability, Which is why it's initially called a connection orientated protocol.
Rather then, It's it's partner-in-crime, If you like, Which is a connectionless protocol which does not go through this handshake process and in fact, Just completely sends the UDP s or the traffic. If you like, Directly to the remote host without any reliability or without any error, Correcting or flow control - and there are some reasons why we might want to use one of the other and, As I said we'll come on to that as we get through our vision Series, But for now we know our data, And we know that this particular example is using TCP and transmission control protocol. We know the ephemeral port that the session has given us and we also know the destination port.
I said to you previously that these things called the well-known ports. These well-known ports are things like port 80, Be an HTTP 25 for for the Simple Mail Transfer Protocol, SMTP and other things, Like 110 being pop 23, Which is in town at 4:43, Which is HTTP and a few others, And depending on what reading material go through There be a list most likely of 10 or probably more ports that you have to actually remember and you'll have to remember them. There is no way around it and you will not only just remember the name of the ports, But you'll definitely remember what protocol that layer 4 actually uses so HTTP, SMTP, Pop town, HTTP or use TCP, Whereas things like des primarily uses UDP.
But in some answers can use TCP, So you need to know most of be, I would say most of the world home ports because there's a thousand 24 of them, But the most that I've outlined in the actual book that you've gone through so make sure you Put them down, Maybe have them on flash cards or something, So it helps you through learning all that information. Ok, What I want to do is actually just end the lesson there. The reason I want end of that is because the OSI model is a big chunk of the CCENT and it's extremely important that you understand it in depth. So what I don't want to do is have a long, Laborious article I want to cover up.
So it's easier to digest in this particular article we took based on the application layer. We understand that this is where end users interact with the network or interact with the OSI stack. We had a discussion around networking applications and what t actually are and what are not networking applications things like paint and notepad. We then moved on to the presentation layer and we explained that the presentation layer is how the information should be presented. So we talked about things like compression working here and decrypt.
Anything now would essentially modify the way the information should be perceived. We also said things like ASCII and JPEG work at the presentation layer. We then went on to the session layer. We had a brief discussion around why this essay is so important and how it actually does the negotiation of duplex with a remote host and how it uses the ephemeral, Ports and destination ports to keep the session separate and how it also pulls that information downstream and Gives it to the transport layer in order to populate its PDU with the port numbers that are required, And then we finished up talking about the transport layer.
We had a very, Very brief overview of what is tcp and what is UDP, Why you may use one over the other. As I said, There'll, Be more articles to come on just the transport layer itself, Because it's so big, As well as the layers to come. So all the lower layers will have or see more information than the upper layers in the next article we're going to continue with the open system, Interconnection model and we're going to finish up with understanding the network, Data link and physical layers.
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