Distributed Hierarchy

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Networks Illustrated: Principles without Calculus

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Princeton University

Networks Illustrated: Principles without Calculus

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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.

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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.

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Christopher Brinton
Lecturer
Electrical Engineering
Mung Chiang
Professor
Electrical Engineering

So, Packet Switching is, one of the fundamental concepts, behind the internet.

Distributed Hierarchy, we're going to look at

now, that's the second fundamental concept.

And then the third fundamental concept is the idea

of Layering, which we'll look at in the next section.

That'll really be the focus of going

in specifically on different layers of the Internet.

so, the internet is really large, geographically, as you can tell.

And it has not only just a large number

of users, but it's also expanded to every portion

of the world, basically.

So different ISPs, as we said, own different portions of the Internet.

And that's their commercial venture ISPs, profit seeking ISPs,

we talked about them also in the lecture on pricing.

We talked about different pricing models for mobile data, in that case.

But each ISP owns a different portion of the internet.

As this is how it looks.

it really becomes a three-tiered structure, and

this is how the hierarchy is built.

The spatial hierarchy

of the Internet, right, all the way down at the bottom.

And notice how it's similar again to the

NCF's structure where they go from campus to

residential to to the highest tier which forms,

as we said, the backbone of the Internet.

So, at the bottom down, we have, all of our tier 3's.

then, we have tier 2's in the middle and tier 1's up at the top.

So, let's, let's look at each of these, and see kind of what they are.

So, on top is the tier 1's.

those are like AT&T and Verizon, level 3 communications, and

you've heard of them. Those are all, tier 1 ISP's.

Those are the big shots.

they, they always have global footprints, so, they, they extend, you know,

outside of different countries, and they, they can go all around the world.

they formed, they have full mesh peering with one another.

Basically means that any tier 1 ISP can get to any

other tier 1 ISP in terms of sending traffic between them.

so they're all connected, and they form the internet backbone, it's

called.

so, this is really what we, when we refer to the internet.

Backbone are really talking about all of the huge ISPs

connected together to one node, than rather at the core.

And the, each of the, each of the tier

1's peer with one another, they have their peer relationships.

Which means that they actually pass they pass

traffic to one another without having to pay each.

Right, so tier-1 ISPs don't have to pay, anyone to send their traffic anywhere.

Now once we

go down and we get onto the tier 2

ends, then tier 2's one step below tier one clearly.

And it's hard, typically it's very hard to draw the line between 1

and tier 1 or tier 2, tier 3 ISP's, they're blurred a little bit.

Just, these are just the main ideas that you would use to characterize one or

the other, the main one with the tier one is that you're not paying anyone, right?

So you don't have to pay any tier 1 ISP's, clearly

not for your tier 2 ISP's, and you're part of that backbone.

Tier 2 ISP is like Vodafone, Comcast, or different tier 2's.

They may also peer with one another.

They're not going to peer with the tier ones.

They form what is called

a customer-provider relationship with tier 1,

so if you're at this tier two right here, you have some

traffic and need to get to another tier two or a

tier one, you have to send traffic upstream to a tier one.

And then you have to pay the tier-1 for sending your traffic up to the tier-1.

And so that's that,

so in order to reach the internet backbone you automatically

have to pay because you have to enter into mode tier-1.

Now tier-3 is all the way down at the bottom, or below tier-2s.

Those are like campus networks, like if you're on

an academic campus, they'd like, corporations will have tier-3 ISPs.

residential areas have tier-3 ISPs.

And tier-3s are focused with taking traffic

to and from from their customers, right.

So, you may have, you have your customers in, either inside

your tier-3 ISPs which you're sending your traffic to.

And you're always going to be paying someone to do that.

Right?

So, you perform the customer-provider relationship with

tier two ISPs, Tier threes pay money to

tier twos, and tier twos pay money to tier ones in order to send the traffic.

Right?

So tier threes have to go all the way up

the ladder to get to the backbone of the network.

So, this idea, this is the idea of

the distributive hierarchy, and this really should give the

idea of a network of networks, which is really the main recurring theme here.

The Internet is really a network composed of many different sub-networks, and

in this case, we're saying the

sub-networks are really different geographical ISPs.

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