Seven Layer OSI Model


Video Script

As we saw in the video, the work of Paul Bran led to the creation of packet switch networks, a packet switch network might look something like this diagram, we have three computers. But we actually have six network devices that are creating a complicated network infrastructure that connects these three devices. So looking at this diagram, can you see some advantages to this network setup. For example, if one node goes down, can we still find a way to get a message to all the other nodes, it looks like we can in a lot of cases. So that makes this network very fault tolerance. In addition, if we want to add more computers to the network, we just have to plug them in at one of these open spots and they’ll be able to communicate as well. So packet switch network is really important. One of the key concepts in packet switch networking is routing. And routing is figuring out a way to create a path in the network to get from point A to point B using the shortest path which means Be the fewest number of hops, it really depends. And so with networking, if any one particular system goes down, our routing can actually figure out a way to get around that problem without any issues.

When we talk about modern computer networking, we can think of it in terms of the seven layer OSI network model, sometimes referred to as the network stack. Each of these layers performs a particular task in turn involved in getting data sent from one computer application to another application on a different computer. The top three layers application presentation and session are usually handled in the software itself. So we’re not going to talk about those too much in this video, but we’ll take a look at the bottom four layers and how they are actually used to get data from one computer to another computer using the modern internets. The big concept here is encapsulation. Each of those layers adds a little bit of data around the data that we want to send, allowing it to get where it needs to go. So we start with data from the application. Then we go down to the transport layer and we add a little bit of data to it. Then we go down to the internet layer, we add some more data. And finally we get to the link layer and add even more data.

This may seem really confusing, but let’s use a little metaphor to try and understand it better. For example, think about a letter you’re sending through the postal system. We have the letter, we might put it in an envelope, and that envelope might have a name on it, such as mom and dad, then we can take that nice envelope and put it in another envelope. And that’s the actual mailing envelope that has the address information on the back and the stamp and the return address. And then we put that letter in the mailbox. And when that letter gets picked up by the Postal Service, it might go into a box full of letters going to the same destination making up the frame. Then when the letter gets to the destination post office, it’s taken out of that box, they read the address, it gets to the location where it needs to go. They open the outs envelope, they look at the inside envelope, they see who it’s intended for. And then that person can open that envelope and get their data back out. And so encapsulation is really the important concept around this layered model of networking.

And we’ll take a look at each layer and what it does in particular, just to see a little bit more about how it works. The very bottom layer of the network model is the physical layer. And the physical layer handles the actual sending of individual bits and bytes across the network wires. He uses technologies such as hundred 10, or 1000, base T or gigabit And the important thing about the physical layer is it doesn’t have any knowledge of the packet whatsoever, just the next hop the next destination that it needs to be sent to on the network itself.

The next layer up is the data link layer. The DataLink layer or the Ethernet layer deals with different frames of packets and it deals with things such as congestion. One of the most important things that happens at the DataLink layer is Establishing the rules of the road for each packets, it assures that a packet has a location and an address. And so the data link layer really handles the way the data is routed across individual Ethernet cables between different networking devices. You can think of it kind of like working with a zip code.

Next, we go up to the network layer. And the network layer is the Internet Protocol layer. And this is actually what defines the end points of the packet where the packet starts at one computer and the destination of the packet at the other end. And so in the network layer, we have this packet structure that has a lot of different information in it. But the most important thing that it has is the IP address of the computer that sends it and the IP address of the destination computer that should receive it. You can think of an IP address like a phone number. It’s a unique number that identifies a single computer on the internet itself. And it tells us where it’s coming from and where it’s going to. This is the packet structure for IP version four but it is Quickly being replaced by IP version six.

Let’s take a look at the difference between IP version four and IP version six. IP version four uses addresses that have 32 bits of data. And so if you think about it two to the 32nd is about 4.2 billion. So really big number. And for a long time, we thought that that was going to be enough IP addresses on the internet. Of course, the internet has become much, much larger of the last few years. And so it’s very easy to imagine that there well more than 4.2 billion devices in the world. I mean, there are over 7 billion people in the world. So we don’t even have enough IP addresses for each person to have just one device. And if you’re anything like me, you probably have multiple devices that you’re using. So we’re looking at moving toward IP version six, which uses 128 bit addresses. It’s four times as many bits but we get to the point where we can have 340 undecillion IP addresses. Another way to think of it is Each atom in the basically each grain of sand on the earth can be assigned its entire ipv4 address space of 4 billion addresses and we’d still have plenty of them leftover. So I’m hoping that ipv6 is probably enough, at least for now. So of course, the network packet at the ipv6 layer looks very similar with just a larger source address and a destination address. And so hopefully this works for a long time to come.

This graphic taken from an XKCD comic shows the IP address space as it looked like a few years ago, in 2006, when this comment was written, all of the greenspaces were still available as open IP address space that could be taken on the internet. But of course, it’s all now gone. And so we’re running out of address spaces on the ipv4 internets. One interesting thing to note is originally on the internet, the 256. First octets of IP addresses were originally assigned to individual companies. For example, IBM was the assigned any IP address starting in nine. The K-State IP address range if you’re interested, is 129.130. And so we have one 256 of one square of this particular diagram was assigned to K-State originally.

The fourth layer is the transport layer. And the transport layer is where the transmission control protocol or TCP resides. The biggest thing that TCP does is it adds ports to the IP address. And so on a computer we can have many different programs that are all connecting via the internet at the same time, and we need a way to tell which data goes with which program. That’s what the transport layer does for us by assigning ports to programs on a computer. As we receive packets or as we send packets, we can mark them with a certain port that is associated with a certain program that should be able to interpret that data. Computers today use a set of ports, the US 65,535 unique ports. And some of them you might be familiar with. For example, the internet. The World Wide Web uses Port 80 for TCP or for HTTP connections.

So there are two different things that operate at the transport layer. There’s TCP the Transmission Control Protocol. And the other protocol at the transport layer is UDP the User Datagram Protocol. And you can see here the UDP packet structure has a lot less information. This is because unlike TCP, UDP has no technology to guarantee transmission of data. As we’ve discussed earlier, TCP relies on verification and retransmission of missing data so that TCP can guarantee every single packet is delivered if possible, whereas UDP does nothing like that. It just sends the packets and hopes they received, but it doesn’t have any way of controlling that. TCP of course is very useful for when we need to make sure every single bit of data gets there, but UDP also has its uses. For example, think of streaming data or store streaming video, or streaming video game data. If you don’t need every particular packet, instead of trying to retransmit those packets that have been missed, UDP will just skip them entirely and keep moving on.

This, of course leads to the two worst jokes in the history of computer science. Do you want to hear a TCP joke? I could tell it to you. But then I’d have to keep repeating it until I was sure you got it. Do you want to hear a UDP joke? I could tell it to you. But I’d really never be sure if you got it or not. See what I mean. It’s really two different technologies trying to do two different things. But they all have their very important uses in the internet today.

So to summarize, TCP is a connection orientated, reliable protocol that uses acknowledgments to verify the data has been sent and received properly. UDP on the other hand, is a connectionless protocol that is not really reliable, because it doesn’t use that system of acknowledgments to make sure the data has been received properly. These two texts analogies are two of the core technologies that operate at the transport layer on the internet in that seven layer OSI model that allows different computer programs to talk to each other using the internet.