The Internet: Packets, Routing & Reliability

Seven, six, five, four, three, two, one [Music] Hi, my name is Lynn Root. I am 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 Khan 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 and 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’ll 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 tens  of millions of bits or ones and zeros, 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. So 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 nontechnical 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 percent 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 acknowledgments of each packet received. If all packets are there, TCP  signs for your delivery and you’re done. [Music] If TCP finds some packets are missing, it won’t  sign.

Otherwise, your song wouldn’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.

[Music] What’s great about the TCP and router systems is they’re scalable. They can work  with eight devices or eight 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. [Music] 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.