The Internet: Packets, Routing and Reliability

Spotify engineer Lynn Root and Vint Cerf, Father of the Internet, explain what keeps the Internet running and how information is broken down into packets.

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[song counts down: 7, 6, 5, 4, 3, 2, 1] The
Internet: Packets, Routing, and Reliability

Hi, my name is Lynn. I'm a software engineer
here at Spotify and I'll be the first to admit

that I often take for granted the reliability
of the internet. The sheer amount of information

zooming around the internet is astonishing.
But how is it possible for every piece of

data to be delivered to you reliably? Say
you want to play a song from Spotify. It seems

like your computer connects directly to Spotify
servers and Spotify sends you a song on a

direct, dedicated line. But actually, that's
not how the internet works. If the internet

were made of direct, dedicated connections
it would be impossible to keep things working

as millions of users join. Especially since
there is no guarantee that every wire and

computer is working all the time. Instead,
data travels on the internet in a much less

direct fashion. Many many years ago, in the
early 1970s my partner Bob Kahn and I began

working on the design of what we now call
the internet. Bob and I had the responsibility

and the opportunity to design the internet's
protocols and its architecture. So we persisted

in participating in the internet's growth
and evolution for all of this time up to and

including the present. The way information
gets transferred from one computer to another

is pretty interesting. It need not follow
a fixed path, in fact, your path may change

in the midst of a computer to computer conversation.
Information on the internet goes from one

computer to another in what we call a packet
of information and a packet travels from one

place to another on the internet a lot like
how you might get from one place to another

in a car. Depending on traffic congestion
or road conditions, you might choose or be

forced to take a different route to get to
the same place each time you travel. And just

as you can transport all sorts of stuff inside
a car, many kinds of digital information can

be sent with IP packets but there are some
limits. What if for example you need to move

a space shuttle from where it was built to
where it will be launched. The shuttle won't

fit in one truck so it needs to be broken
down into pieces, transported using a fleet

of trucks. They could all take different routes
and might get to the destination at different

times. But once all the pieces are there,
you can reassemble the pieces into the complete

shuttle and it will be ready for launch. On
the internet the details work similarly. If

you have a very large image that you want
to send to a friend or upload to a website,

that image might be made up of 10s of millions
of bits of 1s and 0s, too many to send along

in one packet. Since it's data on a computer,
the computer sending the image can quickly

break it into hundreds or even thousands of
smaller parts called packets. Unlike cars

or trucks these packets don't have drivers
and they don't choose their route. Each packet

has the internet address of where it came
from and where it's going. Special computers

on the internet called routers act like traffic
managers to keep the packets moving through

the networks smoothly. If one route is congested,
individual packets may travel different routes

through the internet and they may arrive at
the destination at slightly different times

or even out of order. Let's talk about how
this works. As part of the internet protocol,

every router keeps track of multiple paths
for sending packets, and it chooses the cheapest

available path for each piece of data based
on destination IP address for the packet.

Cheapest in this case doesn't mean cost, but
time and non-technical factors such as politics

and relationships between companies. Often
the best route for data to travel isn't necessarily

the most direct. Having options for paths
makes the network fault tolerant. Which means

the network can keep sending packets even
if something goes horribly, horribly wrong.

This is the basis for a key principle of the
internet: reliability. Now, what if you want

to request some data and not everything is
delivered? Say you want to listen to a song.

How can you be 100% sure all the data will
be delivered so the song plays perfectly?

Introducing your new best friend, TCP (transmission
control protocol). TCP manages the sending

and receiving of all your data as packets.
Think of it like a guaranteed mail service.

When you request a song on your device, Spotify
sends a song broken up into many packets.

When your packets arrive, TCP does a full
inventory and sends back acknowledgements

of each packet received. If all packets are
there, TCP signs for your delivery and you're

done. (song plays) If TCP finds some packets
are missing, it won't sign, otherwise your

song won't sound as good or portions of the
song could be missing. For each missing or

incomplete packet, Spotify will resend them.
Once TCP verifies the delivery of many packets

for that one song request, your song will
start to play. What's great about the TCP

and router systems is they're scalable. They
can work with 8 or 8 billion devices. In fact,

because of these principles of fault tolerance
and redundancy, the more routers we add the

more reliable the internet becomes. What's
also great is we can grow and scale the internet

without interrupting service for anybody using
it. The internet is made of hundreds of thousands

of networks and billions of computers and
devices connected physically. These different

systems that make up the internet connect
to each other, communicate with each other,

and work together because of agreed upon standards
for how data is sent around on the internet.

Computing devices, or routers along the internet,
help all the packets make their way to the

destination where they're reassembled, if
necessary, in order. This happens billions

of times a day, whether you and others are
sending an email, visiting a web page, doing

a video chat, using a mobile app, or when
sensors or devices on the internet talk to

each other.