The Internet: Crash Course Computer Science #29 CrashCourse English English (auto-generated)

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Hi, I’m Carrie Anne, and welcome to CrashCourse Computer Science!

hi I'm Kerry Ann and welcome to crash

As we talked about last episode, your computer is connected to a large, distributed network,

course computer science as we talked about last episode your computer is connected to a large distributed network

called The Internet.

called the Internet I know this because

I know this because you’re watching a youtube video, which is being streamed over that very

you're watching a YouTube video which is

being streamed over that very Internet

It’s arranged as an ever-enlarging web of interconnected devices.

it's arranged as an ever enlarging web

For your computer to get this video, the first connection is to your local area network,

of interconnected devices for your computer to get this video the first connection is to your local area network

or LAN, which might be every device in your house that’s connected to your wifi router.

or LAN which might be every device in your house has connected to your Wi-Fi

This then connects to a Wide Area Network, or WAN, which is likely to be a router run

router this then connects to a wide area network called one which is likely to be

by your Internet Service Provider, or ISP – companies like Comcast, AT&T or Verizon.

a Rooter run by your internet service provider or ISP companies like Comcast AT&T or Verizon at first this will be a

At first, this will be a regional router, like one for your neighborhood, and then that

regional Rooter like one for your

router connects to an even bigger WAN, maybe one for your whole city or town.

neighborhood and then that Rooter connects to an even bigger one maybe one for your whole city or town there might

There might be a couple more hops, but ultimately you’ll connect to the backbone of the internet

be a couple more hops but ultimately you'll connect to the backbone of the

made up of gigantic routers with super high-bandwidth connections running between them.

internet made up of gigantic reuters with super high bandwidth connections

To request this video file from youtube, a packet had to work its way up to the backbone,

running between them to request this video file from YouTube a packet had to work its way up to the backbone travel

travel along that for a bit, and then work its way back down to a youtube server that

along that for a bit and then work its way back down to a YouTube server that

had the file there might be four hops up

That might be four hops up, two hops across the backbone, and four hops down, for a total

two hops across the backbone and four

hops down for a total of ten hops if

If you’re running Windows, MacOS or Linux, you can see the route data takes to different

you're running Windows Mac or Linux you can see the root data takes two

places on the internet by using the traceroute program on your computer.

different places on the Internet by using the traceroute program on your

Instructions in the Doobly Doo.

computer instructions are in the

For us here at the Chad & Stacey Emigholz Studio in Indianapolis, the route to the DFTBA

doobly-doo for us here at the chad and stacey emigholz studio in indianapolis the roots of the dftba server in

server in California goes through 11 stops.

California goes through 11 stops we

We start at 192.168.0.1 -- thats the IP address for my computer on our LAN.

start a 192.168.0.1 that's the IP address for my computer on our land

Then there’s the wifi router here at the studio, then a series of regional routers,

then there's the Wi-Fi Rooter here at the studio then a series of regional

then we get onto the backbone, and then we start working back down to the computer hosting

reuters and then we get on to the backbone and then we start working back down to the computer hosting dftba.com

“DFTBA dot com”, which has the IP address 104.24.109.186.

which has the IP address 1 0 4 2 4 2 1 0

But how does a packet actually get there?

9.1 86 but how does a packet actually

What happens if a packet gets lost along the way?

get there what happens if a packet gets

If I type “DFTBA dot com” into my web browser, how does it know the server’s address?

lost along the way if I type dftba.com into my web browser how does it know the server's address

Those are our topics for today!

these are our topics for today

As we discussed last episode, the internet is a huge distributed network that sends data

as we discussed last episode the Internet is a huge distributed network

around as little packets.

that sends data around as little packets

If your data is big enough, like an email attachment, it might get broken up into many

if your data is big enough like an email attachment it might get broken up into

many packets for example this video

For example, this video stream is arriving to your computer right now as a series of

stream is arriving to your computer right now as a series of packets and not

packets, and not one gigantic file.

one gigantic file internet packets have

Internet packets have to conform to a standard called the Internet Protocol, or IP.

to conform to a standard called the Internet Protocol or IP it's a lot like

It’s a lot like sending physical mail through the postal system – every letter needs a

sending physical mail through the postal system every letter needs a unique and

unique and legible address written on it, and there are limits to the size and weight

legible address written on it and there

are limits to the size and weight of

Violate this, and your letter won’t get through.

packages violate this and your letter

IP packets are very similar.

won't get through IP packets are very

However, IP is a very low level protocol – there isn’t much more than a destination address

similar however IP is very low-level protocol there isn't much more than a

in a packet’s header, which is the metadata that’s stored in front of the data payload.

destination address in a packets header which is the metadata that's stored in front of the data payload this means

This means that a packet can show up at a computer, but the computer may not know which

that a packet can show up at a computer

application to give the data to; Skype or Call of Duty.

but the computer may not know which application to give the data to Skype or

For this reason, more advanced protocols were developed that sit on top of IP.

call of duty for this reason more advanced protocols were developed the sit on top of IP one of the simplest and

One of the simplest and most common is the User Datagram Protocol, or UDP.

most common is the user Datagram protocol or UDP UDP has its own header

UDP has its own header, which sits inside the data payload.

which sits inside the day's payload

Inside of the UDP header is some useful, extra information.

inside of the UDP header is some useful extra information one of them is a port

One of them is a port number.

number every program wanting to access

Every program wanting to access the internet will ask its host computer’s Operating System

the Internet will ask its host computer's operating system to be given

to be given a unique port.

a unique ports like Skype might ask for

Like Skype might ask for port number 3478.

the port number 3478

When a packet arrives to the computer, the Operating System will look inside the UDP

when a packet arrives to the computer the operating system will look inside

header and read the port number.

the UDP header and read the port number

Then, if it sees, for example, 3478, it will give the packet to Skype.

then if it sees for example 3478 it will give the packet to Skype so to

So to review, IP gets the packet to the right computer, but UDP gets the packet to the right

review IP gets the packet for the right computer but UDP gets the packet to the

program running on that computer.

right program running on that computer

UDP headers also include something called a checksum, which allows the data to be verified

UDP headers also include something called a checksum which allows the data

for correctness.

to be verified for correctness as the

As the name suggests, it does this by checking the sum of the data.

name suggests it does this by checking

Here’s a simplified version of how this works.

the sum of the data here's a simplified version of how this works let's imagine

Lets imagine the raw data in our UDP packet is 89 111 33 32 58 and 41.

the raw data in our UDP packet is 89 111

Before the packet is sent, the transmitting computer calculates the checksum by adding

33 32 58 and 41 before the packet is sent the transmitting computer calculates the checksum by adding all

all the data together: 89 plus 111 plus 33 and so on.

the data together 89 plus 111 plus 33

In our example, this adds up to a checksum of 364.

and so on in our example this adds up to a checksum of 364 in UDP the checksum

In UDP, the checksum value is stored in 16 bits.

value is stored in 16 bits if the sum

If the sum exceeds the maximum possible value, the upper-most bits overflow, and only the

exceeds the maximum possible value the uppermost bits

lower bits are used.

overflow and only the lower bits are

Now, when the receiving computer gets this packet, it repeats the process, adding up

used now when the receiving computer gets this packet it repeats the process

all the data. 89 plus 111 plus 33 and so on.

adding up all the data 89 plus 111 plus 33 and so on if that sum is the same as

If that sum is the same as the checksum sent in the header, all is well.

the checksum sent in the header all is

But, if the numbers don’t match, you know that the data got corrupted at some point

well but if the numbers don't match you know that the data got corrupted at some

in transit, maybe because of a power fluctuation or faulty cable.

point in transit mainly because of a power fluctuation or 40 cable

Unfortunately, UDP doesn’t offer any mechanisms to fix the data, or request a new copy – receiving

unfortunately UDP doesn't offer any mechanisms to fix the data or request a

programs are alerted to the corruption, but typically just discard the packet.

new copy receiving programs are alerted to the corruption but typically just

Also, UDP provides no mechanisms to know if packets are getting through – a sending

discard the packet also UDP provides no mechanisms to know if packets are

computer shoots the UDP packet off, but has no confirmation it ever gets to its destination

getting through a sending computer shoots the UDP packet off but has no confirmation it ever gets to its

destination successfully both of these

Both of these properties sound pretty catastrophic, but some applications are ok with this, because

properties sound pretty catastrophic but some applications are okay with this

UDP is also really simple and fast.

because UDP is also really simple fast

Skype, for example, which uses UDP for video chat, can handle corrupt or missing packets.

Skype for example which uses UDP for video chat can handle corrupt or missing

That’s why sometimes if you’re on a bad internet connection,

packets that's why sometimes if you're

Skype gets all glitchy – only some of the UDP packets are making it to your computer.

on a bad internet connection this guy gets all glitchy only some of the UDP

Skype does the best it can with the data it does receive correctly.

packets are making it to your computer

But this approach doesn’t work for many other types of data transmission.

but this approach doesn't work for many other types of data transmission like it

Like, it doesn’t really work if you send an email, and it shows up with the middle

doesn't really work if you send an email and it shows up with the middle missing

the whole message really needs to get

The whole message really needs to get there correctly!

there correctly when it absolutely

When it “absolutely, positively needs to get there”, programs use the Transmission

positively needs to get there programs

Control Protocol, or TCP, which like UDP, rides inside the data payload of IP packets.

use the transmission control protocol or TCP which like UDP right inside the data payload of IP packets for this reason

For this reason, people refer to this combination of protocols as TCP/IP.

people refer to this combination of protocols as TCP IP like UDP the TCP

Like UDP, the TCP header contains a destination port and checksum.

header contains a destination port and

But, it also contains fancier features, and we’ll focus on the key ones.

checksum but it also contains fancier features and will focus on the key ones

First off, TCP packets are given sequential numbers.

first off TCP packets are given

So packet 15 is followed by packet 16, which is followed by 17, and so on... for potentially

sequential numbers so packet 15 is followed by packet 16 which is followed by 17 and so on for potentially millions

millions of packets sent during that session.

of packets sent during that session

These sequence numbers allow a receiving computer to put the packets into the correct order,

these sequence numbers allow a receiving computer to put the packets into the

even if they arrive at different times across the network.

correct order even if they arrive at

So if an email comes in all scrambled, the TCP implementation in your computer’s operating

different times across the network so if an email comes in all scrambled the TCP implementation in your computer's

system will piece it all together correctly.

operating system will piece it all

Second, TCP requires that once a computer has correctly received a packet – and the

together correctly second TCP requires that once the computers correctly

data passes the checksum – that it send back an acknowledgement, or “ACK” as the

received a packet and the data passes to checksum that is sent back an acknowledgment or ACK as the cool kids

cool kids say, to the sending computer.

say to the sending computer knowing the

Knowing the packet made it successfully, the sender can now transmit the next packet.

packet made it successfully the sender can now transmit the next packet but

But this time, let’s say, it waits, and doesn’t get an acknowledgement packet back.

this time let's say it waits and doesn't get an acknowledgment packet

Something must be wrong If enough time elapses, the sender will go ahead and just retransmit

something must be wrong if enough timing that says the sender will go ahead and

the same packet.

just retransmit the same packets it's

It’s worth noting that the original packet might have actually gotten there, but the

worth noting here that the original packet might have actually gotten there

acknowledgment is just really delayed.

but the acknowledgment is just really

Or perhaps it was the acknowledgment that was lost.

delayed or perhaps it was the

Either way, it doesn’t matter, because the receiver has those sequence numbers, and if

acknowledgment that was lost either way it doesn't matter because the receiver has those sequence numbers and if a

a duplicate packet arrives, it can be discarded.

duplicate packet arrives it can be

Also, TCP isn’t limited to a back and forth conversation – it can send many packets,

discarded also TCP isn't limited to a back and forth conversation it can send

and have many outstanding ACKs, which increases bandwidth significantly, since you aren’t

many packets and have many outstanding acts which increases bandwidth significantly since you aren't wasting

wasting time waiting for acknowledgment packets to return.

time waiting for acknowledgement packets

Interestingly, the success rate of ACKs, and also the round trip time between sending and

to return interestingly the success rate of acts and also the round-trip time between sending and acknowledging can be

acknowledging, can be used to infer network congestion.

used to infer network congestion TCP

TCP uses this information to adjust how aggressively it sends packets – a mechanism for congestion

uses this information to adjust how aggressively it sends packets a mechanism for congestion control

control. So, basically, TCP can handle out-of-order packet delivery, dropped packets – including

so basically TCP can handle out of order packet delivery dropped packets

retransmission – and even throttle its transmission rate according to available bandwidth.

including retransmission and even throttle it's transmission rate according to available bandwidth pretty

Pretty awesome!

awesome you might wonder why anyone

You might wonder why anyone would use UDP when TCP has all these nifty features.

would use UDP when TCP has all those

The single biggest downside are all those acknowledgment packets – it doubles the

nifty features the single biggest downside are all those acknowledgment

number of messages on the network, and yet, you're not transmitting any more data.

packets it doubles the number of messages on the network and yet you're not transmitting any more data that

That overhead, including associated delays, is sometimes not worth the improved robustness,

overhead including associated delays is sometimes not worth the improved

especially for time-critical applications, like Multiplayer First Person Shooters.

robustness especially for time critical applications like multiplayer first-person shooters and if it's you

And if it’s you getting lag-fragged you’ll definitely agree!

getting lag frags you're definitely

When your computer wants to make a connection to a website, you need two things - an IP

agreed when your computer wants to make a connection to a website you need two things an IP address and a port like

address and a port. Like port 80, at 172.217.7.238.

port 80 at 172 to 17.7

This example is the IP address and port for the Google web server.

to 3/8 this example is the IP address and port for the Google web server in

In fact, you can enter this into your browser’s address bar, like so, and you’ll end up

fact you can enter this into your browser's address bar like so and you'll

on the google homepage.

end up on the Google home page this gets

This gets you to the right destination, but remembering that long string of digits would

you to the right destination but remembering that long string of digits

be really annoying.

would be really annoying it's much

It’s much easier to remember: google.com.

easier to remember Google com so the

So the internet has a special service that maps these domain names to addresses.

Internet has a special service that map's these domain names to addresses

It’s like the phone book for the internet.

it's like the phonebook for the internet

And it’s called the Domain Name System, or DNS for short.

and it's called the domain name system or DNS for short you can probably guess

You can probably guess how it works.

how it works when you type something

When you type something like “youtube.com” into your web browser, it goes and asks a

like youtube.com/ into your web browser it goes and after DNS server usually one

DNS server – usually one provided by your ISP – to lookup the address.

provided by your ISP to look up the

DNS consults its huge registry, and replies with the address... if one exists.

address DNS consults it's huge registry and replies with the address if one

In fact, if you try mashing your keyboard, adding “.com”, and then hit enter in your

exists in fact if you try mashing your keyboard adding dot

browser, you’ll likely be presented with an error that says DNS failed.

and then hit enter in your browser you'll likely be presented with an error

That’s because that site doesn’t exist, so DNS couldn’t give your browser an address.

that says DNS failed that's because the site doesn't exist so DNS couldn't give

But, if DNS returns a valid address, which it should for “youtube.com”, then your

your browser an address but if DNS returns a valid address which it should for youtube.com then your browser shoots

browser shoots off a request over TCP for the website’s data.

off a request over TCP for the website's

There’s over 300 million registered domain names, so to make that DNS Lookup a little

data there's over 300 million registered domain names so to make our DNS lookup a

more manageable, it’s not stored as one gigantically long list, but rather in a tree

little bit more manageable it's not stored as one gigantically long list but

data structure.

rather in a tree data structure what

What are called Top Level Domains, or TLDs, are at the very top.

called top-level domains or tio DS are at the very top these are huge

These are huge categories like .com and .gov.

categories like com and gov then there

Then, there are lower level domains that sit below that, called second level domains; Examples

are lower level domains that sit below that called second-level domains

under .com include google.com and dftba.com.

examples under com include google calm

Then, there are even lower level domains, called subdomains, like images.google.com,

and dftba.com then there are even lower level domains called sub domains like

store.dftba.com.

images google comm and stored at dftba.com and this tree is absolutely

And this tree is absolutely HUGE! Like I said, more than 300 million domain names, and that's just second level domain

huge like I said more than 300 million domain names and that's just

names, not all the sub domains.

second-level domain names not all the

For this reason, this data is distributed across many DNS servers, which are authorities

subdomains for this reason the data is distributed across many DNS servers

for different parts of the tree.

which are authorities for different

Okay, I know you’ve been waiting for it...

parts of the tree ok I know you've been

We’ve reached a new level of abstraction!

waiting for it we've reached a new level of abstraction

Over the past two episodes, we’ve worked up from electrical signals on wires, or radio

over the past two episodes we've worked up from electrical signals on wires or

signals transmitted through the air in the case of wireless networks.

radio signals transmitted through the air in the case of wireless networks

This is called the Physical Layer.

this is called the physical layer MAC

MAC addresses, collision detection, exponential backoff and similar low level protocols that

addresses collision detection exponential back-off and similar

mediate access to the physical layer are part of the Data Link Layer.

low-level protocols that mediate access to the physical layer are part of the

Above this is the Network Layer, which is where all the switching and routing technologies

data link layer above this is the network layer which is where all the switching and routing technologies that

that we discussed operate.

we discussed operates and today we've

And today, we mostly covered the Transport layer, protocols like UDP and TCP, which are

mostly covered the transport layer protocols like UDP and TCP which are

responsible for point to point data transfer between computers, and also things like error

responsible for point-to-point data transfer between computers and also

detection and recovery when possible.

things like error detection and recovery

We’ve also grazed the Session Layer – where protocols like TCP and UDP are used to open

when possible we've also grazed the session layer where protocols like TCP and UDP are

a connection, pass information back and forth, and then close the connection when finished

used to open a connection pass information back and forth and then close the connection when finished

– what’s called a session.

what's called a session this is exactly

This is exactly what happens when you, for example, do a DNS Lookup, or request a webpage.

what happens when you for example do a DNS lookup or request the web page these

These are the bottom five layers of the Open System Interconnection (OSI) model, a conceptual

are the bottom five layers of the open system interconnection OSI model a

framework for compartmentalizing all these different network processes.

conceptual framework for compartmentalizing all these different network processes each level has

Each level has different things to worry about and solve, and it would be impossible to build

different things to worry about and solve and it would be impossible to

one huge networking implementation.

build one huge networking implementation

As we’ve talked about all series, abstraction allows computer scientists and engineers to

as we've talked about all series abstraction allows computer scientists

be improving all these different levels of the stack simultaneously, without being overwhelmed

and engineers to be improving all these different levels of the stack simultaneously without being overwhelmed

by the full complexity.

by the full complexity and amazingly

And amazingly, we’re not quite done yet…

we're not quite done yet

The OSI model has two more layers, the Presentation Layer and the Application Layer, which include

the OSI model has two more layers the presentation layer and the application

things like web browsers, Skype, HTML decoding, streaming movies and more.

layer which include things like web browsers Skype HTML decoding streaming

Which we’ll talk about next week.

movies are more which we'll talk about

next week see you then crash course computer science is produced in association with PBS Digital Studios at their channel you can check out a

playlist of shows like physics girl deep look and PBS space time this episode was filmed at the chad and stacey ever got studio in indianapolis indiana and it

was made with the help of all these nice people and our wonderful graphics team is thought cafe that's where we're going to have to halt and catch fire see you

next week [Music]

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