So, returning back to the first question we asked ourselves in this was, how does the Internet get your messages where they need to go? And that's really the idea behind routing, and routing Internet traffic. And so we'll look at that now, and we'll stay on routing for really the rest of this lecture. So the first thing is, what is a router? Well, here is a picture of a router, it's large Cisco router. you have a router in your house. the router, the router in your house is probably a lot smaller than this, but the basic idea is the same thing; it directs traffic. Right, so, it takes packets of data in, and it spits them back out as fast as it possibly can. So that's the whole idea behind a router, really, it's just moving packets of data from an input port to an output port. And so they direct packets of data where they need to go, it's kind of like when you're sending a letter to someone. It's like all the Post Office on the intermediates routes between your house and wherever it's going. They look at the outgoing address when they send it out to go to the next post office and the next route. and it's done, this is done hop by hop. Okay? This is a very, very important point. Right? So, if I have two routers, say I'm just going to go into circles. Right? And they're connected like this. this is one hop. Right, so this router has an incoming data coming into it, it looks at it, and determines and it sends it out to the next hop. So it just looks hop-by-hop, and every router can modify the path that it's going to take depending upon where it's going. So each router only knows where the packet of data is going to go next, not where it's going to go. This router doesn't, for instance, doesn't know that this router is going to send it over here, for instance. So it's all just done one hop at a time. So now routers perform the function of routing, which really we just talked about, and that's selecting the actual paths along which to send traffic. Kind of like on a roadway. We call roadways routes sometimes, so were you taking route 1, or you're taking route 95 when you're traveling, what street are you driving down? That's your carrying out a different route, but when you're following a GPS it already has the route calculated. so it's not that because again it's, so, the idea of routing would be like if your GPS just told you a new street, where to turn next, right? And, it wouldn't you the overall route that you were looking at. Packets don't know that. so if you're car is a packet, you wouldn't, for instance, know the entire route. That's getting a little beside the point here. But so think of, roadways again as being routes, Now when we talk about routing, notice that each ISP forms a domain. we talked about ISPs before, right, so if this is one ISP this is ISP, this is another ISP, each of them forms a domain that we call an autonomous system. So each of these are autonomous systems, they're kind of functioning in of themselves. So when we talk about routing, we can either be talking about Inter-AS routing, which means routing between ASs. Right, so, we're routing packets of data between ISPs. Or Intra-AS routing. And that's going to be our focus of interest. So, we're going to really look when we talk about routing is, what's actually happening inside of the ASs themselves. Right, so, the routing, Inter-AS routing gets a lot more complicated because it starts to deal with things like for instance not necessarily, you're not trying to just minimize some like Overall Path Distance or something. they actually look and they say, okay, well, I'm going to see who's going to charge me the least amount of money to send, right? So if this AS right here if this is a tier tier one isp, this is a tier two isp this is a tier two isp. If there were like 60 tier two isp's to a destination or you could have just gone up to tier one and you could have sent it right to the destination you'll choose this path to go through all the ASs. Because you don't have to pay any money rather than if you just go up to the tier one then you have to pay money. so there's three main features of routing. and we'll look at each of these, one by one. the first 1 is Addressing, right, so how do you address packets? The second one is Routing itself, so what routing protocol you're actually using to determine the paths? And the third 1 is Forwarding, so how does the router actually determine where to send the data next?
