This principle continues with the Internet, and it is material that will be covered to some extent in this course. It's called layers on layers, and it explores one of the fundamental concepts behind the internet, which is that of layering. Now we have different layers and each of which gets allocated different functions which they have to perform. And these functions are things that the internet needs to do in order to make it work. So let's look at a few examples here to get the idea. If you're sitting at your end host computer, the first thing that you need to do is you need to get your message onto the internet in the first place. And so this is the first link, wherever that may be. You actually have to physically get your message onto that link, and then send it across that link. Now we have a layer that deals with this, which is called the physical and link layer. And frankly those are two different layers and we can view them as one for here. So the physical and link layer take care of the process of getting data onto the link and then actually sending it across the link. Now once your packet hits the end right there, it's going to get to a router. And the router, once the packet gets inside. It will examine it's IP address, as we looked at before, and it will determine which of its egress ports it should send the packet out depending upon the IP address. And once it does that, it will send the packet out, and each router along the path will repeat that process until it actually gets to the destination. Notice on two routers along the path here. But naturally there are many more. This process of routing is allocated to the network layer. And this works hop by hop, meaning one router at a time. So each router looks at the IP address and determines which length to send it off to. Then this router will look for the IP address again and determine which link to send it off to, and so on until it actually gets to the destination. Hop by hop means one step at a time. But now, you're not the only session that's happening on the internet, there are others as well and each of you is sending data simulatneously. There's a layer that takes into account these end-to-end sessions. And we call that the transport layer. It takes care of end-to-end connectivity. Whereas the network layer does things hop-by-hop, this is actually looking at end-to-end connectivity. One of the main functions of the transport layer is that of congestion control. So, if at some point, this transport layer detects that there's too much congestion on the internet, like right now, then it will tell the host to maybe send only half of the packets next time. And each session has their own instance of the transport layer, so each of them is monitoring congestion on their own. Now, you're probably saying well this is great and all but how come me sitting at my computer never sees any of this. Well, there's a seperate layer for things like Facebook and Twitter which are applications and they sit up at the application there which is directly visible to the end user. So it's important to note that different network devices only process or really understand up to different layers in this protocol stack as we call it. So this layering right here is called a protocol stack. For instance. A modem [SOUND] can only understand up to the physical link layer. You have a modem in your house. Now, a router can process up to the network layer, as we've said, it's managing these hop by hop decisions. And your cell phone [SOUND] can understand all the way up to the application layers since it runs things like Facebook and Twitter. So what this means is that your phone can understand everything and decode all of these layers, all the way directly down to the physical layer, whereas the router can only understand up to the network layer and the modem can only still understand up to the physical layer. The eighth and final principle is not something we'll have time to cover in this course unfortunately, but it is in the book and it's called Bigger and Bigger. It revolves around the fact that the internet keeps getting larger and sometimes its size can be hard to comprehend. If you just look at the number of connected devices as opposed to the world's population, sometime between 2003 and 2010 we actually had more connected devices than people and that ratio continues to get larger and larger. By 2010 it was 1.8 which means that every person on average has 1.8 connected or between 1 and 2 devices that connect to the internet. And if you think that's cool, well by 2015, that's expected to go up to 3.5, and by 2020 it's expected to be between six and seven, which means that every person in 2020 will have between six and seven devices that can connect to the internet. So there's many implications of the fact that the Internet keeps getting so large, one of which we can just think about for a second. So I'm imagine you on some social networking site. And try to think about if you took some random person in the world, where ever that person may be. How many steps it would take for you to get to that person, and by step I mean that you can go to one of your friends, who can go to one of their friends, who can go to one of their friends and so on. And each one of those is the step so if you Found one of your friends, that would be one step, who could then find one of their friends, which would be another step, and so on. How many steps do you think it would take to get to this random person, on average. You may think, well, probably a lot, maybe like 50, or 60, or something, some really large number. Well, it turns out that, on average, that it's been experimentally proven to be around six. And that's where the notorious six degrees of separation comes from. Even though there's so many people on the Internet, that on average, it should take you about six steps to get to this person. Something like this. You find one of your friends, who then finds one of their friends, then one of their friends, one of their friends, and so on. And it'll take 1, 2, 3, 4, 5, 6 steps on average. Now there's a lot to this because we didn't say anything about how you might go about finding this path, for instance you have to choose your right friend in order to make that the smallest number of people possible, then your friend has to choose the right person and so on... And in the book we look at ways you can go about describing such networks, and what properties these types of networks have. Now another thing that we've seen from day to day now is the idea of the cloud, which is this mysterious place where you can put all of your data, and you can go back and access it. And the idea is that rather than having to have external storage, you can actually go and store all of your data or run processes within this cloud, and then you can come back and request those at any time that you want. But if you ever stop to think what's actually inside of that cloud, what is that cloud actually doing, because it's pretty amazing that it can keep operating as efficiently as it does. With the amount of people now that have internet connected devices, and especially by 2020 the amount of people that will have these devices and being able to keep cloud storage going. Well, in order to do that we have to actually open up the cloud and see what's inside of it. Well, the cloud actually consists of physically a number of data centers. And within each of these data centers we have a lot of servers, which store and perform these tasks that we need. And so, the cloud keeps getting more and more servers as it needs to have more and more users. But if you've ever heard of the concept, you're only as strong as your weakest link, that definitely applies here. And that every single component of this Cloud has to be strong or else we're going to be limited by the capacity of one of those links. And to really see how that's done, we have to go into understanding the fundamental building blocks are behind the cloud. And they come into things that are even smaller than these individual. Servers, and to do that we can consider it an analogy to building a lego set for instance, and each of these lego sets consists of one of these really small building block legos, and without these small legos we can never build something as big as this large house right here. Just like... Without these devices called switches, which are very small, but there's very, very many of them, we could never connect all these servers together, so that we can operate and make best use of our available capacity on each of the servers. And operate the cloud as efficiently as possible.
