Network Layers Model (Networking Basics) - Computerphile

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Some of your videos began with, "There's a new paper out last week." This is the opposite end of that spectrum because if I was doing this video in around about 1979, it would look very much the same. When I'm talking about networking, sometimes I'll say something like, "Oh, this is at the transport layer or this is about an IP packet or this is this is a layer two thing." A lot of your audience, they'll know what I mean, but some people might be hard on. Transport layer? Layer seven? What are we talking about? So, this is going to be the basics of the internet layers model. Um so, we're going to talk a little bit how that works. If people like it, we'll do a detailed thing on each of the different layers. Whenever this is taught, there's two rival things. So, there is there's the OSI ISO and there's TCP IP. And they're very, very closely related. So, OSI ISO, open system interconnection model. And ISO, I was talking to you this before we started filming. What what do we think ISO stands for? I thought it was obviously International Standards Organization. >> And it's International Organization for Standardization. I'm infuriated. Um I wonder if that's a language thing. Sometimes languages put things in different orders, don't they? I don't know. No, but but I I checked that and in French it's something normal. So, it's not even that it's in the wrong language. So, as they say, the good things about standards, there are so many of them. We're in the 1970s, so we've got a good old chunky 1970s style computer with a big built-in keyboard. Da da da da da. And it wants to send some data to some other big old chunky 1970s computer with a built-in keyboard. So, here's our data. And our problem is we're going to send it via some system of hardware. So, these things I always draw as routers like this. Don't know why actually they make them with a little cross, but everybody does. I'm a bit crossroads for data. >> Oh, yeah, could be. Could be. So, we want to get the data between these computers. And that's quite a big problem. So, if we've got a big problem in computer science, what do we do? >> Uh break it down. Break it down. We're going to break it down in a number of smaller problems. Now, I'm going to do this this top-down. We start with an application. So, maybe this is email. Not good planning. Application. The application layer. We're going to call this layer seven for reasons that will become apparent. So, let's say we're sending an email from here. So, we've got a program there. But, we've probably got a few other programs running on the computer. So, we've got our our web browser. And here, we've got another email. We've got maybe a web server. Would it be Usenet or something back in the '70s? >> Usenet, right. But, we might have lots of different programs here. So, there's a couple of layers I'm not even going to mention. So, the OSI model had a presentation layer. Got to be careful or I'm going to run out of space. And then a session layer. The first layer I'm actually going to talk about is transport at layer four. And the reason is that while they were developing their lovely, beautiful standard, which is what we should have had and in ideal world this is what we'd get, on this side, people were kind of just building things. So, they were not without principle, of course, but so they have the transport layer as well. And these people were implementing. And by the time this beautiful standard was completed, there was so much of this TCP IP model out there, we couldn't get to here. So, our next layer down, the transport layer. So, we've got our applications, the programs running on the computer. They want to send or receive data. And they want interface to do so. We don't want our programmer to have to write this code. So, this should be part of our operating system. So, this transport layer sits only on the end computer. So, if we make the transport layer blue here, the transport layer only exists as end points on these end devices, yeah? And it's got a few responsibilities. If we need reliability, it's going to make sure that data is resent. So, if we notice, "Oh, I've seen the first bit of the email and I've seen the third bit of the email. I've not seen the second bit." It's the transport layer's job to make sure that happens. It's the transport layer's job to make sure that anything passed to an application doesn't have errors because it's a bit of a wild west. There could be anything in here and that could lose data, corrupt data. So, the transport layer needs to make sure that anything passed to an application doesn't have errors. But, that's living on the end points. That's living on our big old 1970s computer here, our big old 1970s computer here. So, the next layer we have down is the network. I think this has it's a bit unfair on the poor old network layer. I think this has the most difficult job of all. Cuz it's got to get the data. So, we'll color this one green. And it's got to get the data from any machine on the planet to any other machine on the planet. That's That's a huge amount of different things. So, if this machine's in Berlin and this machine's in New York, it's got to navigate this huge tangle of routers. It's got to work out that this is the way to go. It's got to figure out the paths to take. It's got to realize how to get an address for this computer, how to get an address for this computer, and how to send the data to and from for the whole journey. And that's our network layer. Next one down, layer two, is the data link or link layer. Now, I said our network layer is doing the whole journey. It's getting the data from New York to Berlin, from London to Beijing, whatever it is. You've got a number. And the data link, if the network is the whole journey, the data link is one step of that journey, yeah? So, it's got to get the data from this router to the next router. From this router to the next router. From this router into your computer. So, it's been more like the nuts and bolts or the Yeah, yeah, yeah. >> The cab ride to the airport, the Yes, yeah. Well, actually that's a very good analogy. In the in the textbook I teach from, they break it down like a journey like that. So, they say, "This is you walking to the tube. This is the tube to the airport. This is the flight to the other airport. This is the the limousine you're taking at the other end cuz you're on expenses." So, yeah, that we could think of it as that. And the data link layer has to be pretty flexible. There's loads of technologies down here because and here's where I've painted myself into a bit of a corner here, is where we have the physical layer. Don't like to talk about the physical layer too much. Messy, messy. It's all physics and wires and radio waves and things that Well, things that I haven't studied since I was a teenager studying physics. But, the physical layer, obviously, we can't live without the physical layer. We'd love to, but we do need the wires and the radio waves. Um and it's the physical layer that means the data link layer has to be really flexible because this might be a radio mast sending 5G. It might be going to a satellite. There's actually a specification for doing this with with pigeons as your data carrier medium. And people have carried that out because of course they have. So, you know, you're tying a little message to the leg of a pigeon. It's flapping about with a message. And if we do it right, these other layers, the network layer, should work independently of whether this is a radio wave, an optical fiber, or a pigeon. So, this is the OSI ISO and this is the Transmission Control Protocol Internet Protocol. So, this is what we actually have. So, when we're talking about layers, it's almost mysterious if you don't know the secret that the application layer says, "Oh, that's layer seven." And then this is layer four. And these ones are still not implemented. They're missing. Uh there's another thing that uh it's a common joke amongst us internet people. If we say, Uh well, the network isn't working, but I think the problem's at layer eight, Sean. What am I telling you? It's the user, maybe? >> It is the user. The problem exists between the keyboard and the chair. That's layer eight. Um so, that. There's good things about this layers model and there's bad thing. So, there's something we call the narrow waist. The at the application layer, if I start to ask you to name applications, we'd be here till the end of the day, right? We've got our email, we've got our web, we've got our World of Warcraft, we've got whatever whatever whatever. We can name loads and loads of things that um are at the application layer. Physical layer, I've I've named lots of things. And the data link layer needs to have a protocol to interface with all of those different physical layer technologies. So, it's quite easy to innovate at the application layer. We can create new things. At the transport layer, I can think of four things, but the only ones people usually talk about are TCP and UDP, transmission control protocol and user datagram protocol. I did that video about quick, but that's really abusing UDP. So, there's two technologies there. And at the network layer, internet protocol version four, internet protocol version six, and internet protocol version six has been it's about to show up, it's nearly ready for all of the time I've been teaching about networks. So, it's really hard to to um innovate there. So, we call it the narrow waist because there's loads and loads of things up here, but by the time we get down to transport and network, it's really hard to innovate. But then, so there's this narrow waist. Now, at the data link layer, you know, if you came up with a way of um sending messages via cameras or whatever, yeah, we can you've got a physical layer, we can knock up a data link layer, it'll interface to network layer, and it'll work. Actually, and also some people say layering considered harmful. I say, "Okay, that's great, but what if we find that this application would be a lot faster if we optimize the data link layer?" And you can't do it cuz you've got to go through these other layers. But, that said, it's been incredibly successful. This same model, as I said at the start of the video, if I was teaching this in 1981, this diagram would look more or less the same. And it's So, in terms of computer science, that is sort of deep magic from the dawn of time. It's been there forever. It's barely changed. Foundational stuff. >> Foundational stuff, exactly. Exactly. The 3 4 is going to index into this table, and then again, we might discover, "No, this memory is not mapped. End of story." Or, it may say, "No, turn to page 22." Page 22 is going to be over here. And we follow our chip here. And then finally, we're going to look up page 50