Routing Traffic

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En provenance du cours de Princeton University

Networks Illustrated: Principles without Calculus

43 notes

Princeton University

Networks Illustrated: Principles without Calculus

43 notes

What makes WiFi faster at home than at a coffee shop? How does Google order its search results from the trillions of webpages on the Internet? Why does Verizon charge $15 for every GB of data we use? Is it really true that we are connected in six social steps or less? These are just a few of the many intriguing questions we can ask about the social and technical networks that form integral parts of our daily lives. This course is about exploring the answers, using a language that anyone can understand. We will focus on fundamental principles like “sharing is hard”, “crowds are wise”, and “network of networks” that have guided the design and sustainability of today’s networks, and summarize the theories behind everything from the social connections we make on platforms like Facebook to the technology upon which these websites run. Unlike other networking courses, the mathematics included here are no more complicated than adding and multiplying numbers. While mathematical details are necessary to fully specify the algorithms and systems we investigate, they are not required to understand the main ideas. We use illustrations, analogies, and anecdotes about networks as pedagogical tools in lieu of detailed equations. All the features of this course are available for free. It does not offer a certificate upon completion.

À partir de la leçon

Routing Traffic through the Internet

It is hard to overstate the impact that the Internet has had on society. In this lesson, we will overview the fundamental concepts behind the way the Internet is designed. We will also take a look at routing, which is the process of determining how packets of information are transported.

Rencontrer les enseignants

Christopher Brinton
Lecturer
Electrical Engineering
Mung Chiang
Professor
Electrical Engineering

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?

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