WebRTC Basics: How (and Why) WebRTC Uses your Browser’s IP Address

To reach out to you.

I've been asked recently to write a few more on the topic of WebRTC basics - explaining how it works. This is one of these posts. There's been a recent frenzy around with the NY Times use of WebRTC. The fraud detection mechanism for the ads there used WebRTC to find local addresses and to determine if the user is real or a bot. Being a cat and mouse game over ad money means this will continue with every piece of arsenal both sides have at their disposal, and WebRTC plays an interesting role in it. The question was raised though - why does WebRTC needs the browser's IP address to begin with? What does it use it for? To answer, this question, we need to define first how the web normally operates (that is before WebRTC came to be).

The illustration above explains it all. There's a web server somewhere in the cloud. You reach it by knowing its IP address, but more often than not you reach it by knowing its domain name and obtaining its IP address from that domain name. The browser then goes on to send its requests to the server and all is good in the world. Now, assume this is a social network of sorts, and one user wants to interact with another. The one and only way to achieve that with browsers is by having the web server proxy all of these messages - whatever is being sent from A to B is routed through the web server. This is true even if the web server has no real wish to store the messages or even know about them. WebRTC allows working differently. It uses peer-to-peer technology, also known as P2P.

The illustration above is not new to VoIP developers, but it has a very important difference than how the web worked until the introduction of WebRTC. That line running directly between the two web browsers? That's the first time that a web browser using HTML could communicate with another web browser directly without needing to go through a web server. This is what makes all the difference in the need for IP addresses. When you communicate with a web server, you browser is the one initiating the communication. It sends a request to the server, when will then respond through that same connection your browser creates. So there's no real need for your browser to announce its IP address in any way. But when one browser needs to send messages to another - how can it do that without an IP address? So IP addresses need to be exchanged between browsers. The web server in the illustration does pass messages between browsers. These messages contain SDP, which among other things contains IP addresses to use for the exchange of data directly between the browsers in the future.

Why do we need P2P? Can't we just go through a server?

Sure we can go through a server. In fact, a lot of use cases will end up using a server for various needs - things like recording the session, multiparty or connecting to other networks necessitates the use of a server. But in many cases you may want to skip that server part:

• Voice and video means lots of bandwidth. Placing the burden on the server means the service will end up costing more
• Voice and video means lost of CPU power. Placing the burden on the server means the service will end up costing more
• Routing voice and video through the server means latency and more chance of packet losses, which will degrade the media quality
• Privacy concerns, as when we send media through a server, it is privy to the information or at the very least to the fact that communication took place

So there are times when we want the media or our messages to go peer-to-peer and not through a server. And for that we can use WebRTC, but we need to exchange IP addresses across browsers to make it happen. Now, this exchange may not always translate into two web browsers communicating directly - we may still end up relaying messages and media. If you want to learn more about it, then check out the introduction to NATs and Firewalls on webrtcHacks.