Forwarding

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Del curso dictado por Princeton University

Networks Illustrated: Principles without Calculus

40 calificaciones

Princeton University

Networks Illustrated: Principles without Calculus

40 calificaciones

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.

De la lección

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.

Conoce a los instructores

Christopher Brinton
Lecturer
Electrical Engineering
Mung Chiang
Professor
Electrical Engineering

So now the actual forwarding that's occurring and going on with these routers.

the forwarding method implements the routing policy.

so and this is done one hop at a time right.

So the basic idea is that when a packet arrives at

a router's input ports, it's going to examine the destination IP in

the packet and it may only look at the prefix, it

depends where you are in the overall in the scheme of things.

If you're really close, if you're already within the subnet, then you're going

to look at the host ID as well.

But you may only be looking at the prefix, and then the routing

table is going to tell us which port to send the packet out, right.

So routers have a number of ports right?

So if something comes into port two for instance,

and it has an IP address, we'll say 10.1.2.300.

then the router's going to look up, and it's going to

say okay, well I have a match for 10.1.2.

Alright, so I'm only looking

at the first three decimal numbers, or the first 24 bits in IPV4.

And then I'm going to say, okay 10.1.2 matches out to

port three, so that would be sent out to port three.

Or if it was 10.8.4.something it would get out port one as well,

so it's going to search and it's going to look and see the match.

And again, it's only going to try to match the

amount of digits that it has in here that it's storing.

The zero indicate like a,

really like a don't care, not looking at that bit.

So the best way to think about this is to think about the way the mail system works.

The zip code is what you use to get to the town.

And we said this before, but let's just

highlight, the zip code is like the prefix, and

that's what we use to get to the specific town in question which is like the subnet.

A subnet is just a group of devices, just like

a town could be a group of houses we'll say.

Then the house address, or the host ID is used

to get to the final destination, or the end host.

Just to highlight again, the router needs to do

this function as quick as possible, right, because you could

hit tens or dozens of routers on the path between

where you are and where your packets has to go.

So router really need, they need to be designed to be able to

do this look up as quick as possible and, to be able to have

the electronics within them, that they can send out to

the, the right alpha port just as fast as possible.

And, so, there's, there's lots of issues and things

with router design In terms of what components to use.

That's a whole abundant field that we won't look into

here, but just suffice it to say that routers have

routing tables, and it uses the routing table to look

up and figure out which port to send off next.

And we will look at how routing tables

became popular to some extent, in trying to figure out

the shortest path distance between routers, or from a router to

a destination and that will drive, basically, where things get

forwarded next when they want to go to a specific destination.

So, basically look at, we'll say, okay, for

one destination, in the example in this book.

If everyone is trying to go to one destination, then we'll say, okay.

Well, how are we going to determine which alpha port to send this out.

And we'll look at that in a few minutes.

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