[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.