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Learning WebRTC peer-to-peer communication, part 1

Remember blockchain-redux? Yeah, I'm still working on it. Last time, we got it to share blocks between clients in real-time.

But that still used Firebase as the communication channel. Our blockchain isn't really distributed. โ˜น๏ธ

I have to fix that before my talk at WeAreDevelopers Congress in Vienna. Otherwise, what's the point?

WebRTC can do that for us. Get browsers to talk to each other without any servers at all.

I built a demo app that uses RTCPeerConnection to build a peer-to-peer connection with itself. You can see the code on GitHub or keep reading.

Since iOS 11, WebRTC now works in all browsers people use. You can use it for real! ๐Ÿ‘๐Ÿผ

Try my demo here. I'd embed an iframe, but browser security rules say no.

WebRTC offers 3 APIs:

  • get video and audio from devices (MediaStream)
  • establish peer-to-peer connections (RTCPeerConnection)
  • send arbitrary data (RTCDataChannel)

In this article, we'll use 2 of them. Media and peer connection.

RTCPeerConnection connects clients without servers

RTCPeerConnection process sketch

You establish a connection between 2 clients on the same page like this ๐Ÿ‘‡

  1. Instantiate two RTCPeerConnection objects
  2. Add each other as ICE candidates
  3. createOffer on the 1st object
  4. set local/remote "description" on both
  5. createAnswer on the 2nd object
  6. set remote/local "description" on both
  7. Enjoy your direct communication

Here's how that goes in code.

We start with a React component that renders 2 videos and 3 buttons. It has a bunch of default state that we'll use to drive everything.

class WebRTCPeerConnection extends React.Component {
state = {
startDisabled: false,
callDisabled: true,
hangUpDisabled: true,
servers: null,
pc1: null,
pc2: null,
localStream: null,
};
localVideoRef = React.createRef();
remoteVideoRef = React.createRef();
start = () => {
// start media devices
};
call = () => {
// initiate a call
};
hangUp = () => {
// hang up connection
};
render() {
const { startDisabled, callDisabled, hangUpDisabled } = this.state;
return (
<div>
<video
ref={this.localVideoRef}
autoplay
muted
style={{ width: "240px", height: "180px" }}
>
<video
ref={this.remoteVideoRef}
autoplay
style={{ width: "240px", height: "180px" }}
>
<div>
<button onclick={this.start} disabled>
Start
</button>
<button onclick={this.call} disabled>
Call
</button>
<button onclick={this.hangUp} disabled>
Hang Up
</button>
</div>
</video>
</video>
</div>
);
}
}

We have 3 booleans that enable/disable our buttons, null peer connections pc1 and pc2, I don't know what servers are for, and we render videos and buttons.

Step 1 โ†’ start

When you click the Start button, we ask for audio/video permissions and start a localStream.

start = () => {
this.setState({
startDisabled: true,
});
navigator.mediaDevices
.getUserMedia({
audio: true,
video: true,
})
.then(this.gotStream)
.catch((e) => alert("getUserMedia() error:" + e.name));
};
gotStream = (stream) => {
this.localVideoRef.current.srcObject = stream;
this.setState({
callDisabled: false,
localStream: stream,
});
};

Disable the start button with this.setState, get your media with navigator.getUserMedia. If you give permission, we start streaming in the localVideo element and add it to state.

Step 2 โ†’ Call

You can now press the Call button. That starts two peer connections, pc1 and pc2, and goes through the dance to get them talking to each other.

  1. call initiates the offer
  2. onCreateOfferSuccess updates both pcs and initiates the answer
  3. onCreateAnswerSuccess finishes the handshake
  4. gotRemoteStream wakes up and sets the second video
call = () => {
this.setState({
callDisabled: true,
hangUpDisabled: false,
});
let { localStream } = this.state;
let servers = null,
pc1 = new RTCPeerConnection(servers),
pc2 = new RTCPeerConnection(servers);
pc1.onicecandidate = (e) => this.onIceCandidate(pc1, e);
pc1.oniceconnectionstatechange = (e) => this.onIceStateChange(pc1, e);
pc2.onicecandidate = (e) => this.onIceCandidate(pc2, e);
pc2.oniceconnectionstatechange = (e) => this.onIceStateChange(pc2, e);
pc2.ontrack = this.gotRemoteStream;
localStream.getTracks().forEach((track) => pc1.addTrack(track, localStream));
pc1
.createOffer({
offerToReceiveAudio: 1,
offerToReceiveVideo: 1,
})
.then(this.onCreateOfferSuccess, (error) =>
console.error("Failed to create session description", error.toString())
);
this.setState({
servers,
pc1,
pc2,
localStream,
});
};

This code is mostly boilerplate.

We enable and disable the appropriate buttons, get localStream from state, and instantiate servers, pc1, and pc2.

Both pc* objects get a bunch of event listeners. onIceCandidate will connect them to each other, onIceStateChange just prints debugging info, and gotRemoteStream will add it to the right <video> element.

Then we take all tracks from localStream (audio and video) and add them to the first client. After that pc1 creates an offer to receive its video and audio.

When all that's done, we update component state.

onCreateOfferSuccess

After pc1 successfully creates an offer to be received, we update local and remote descriptions in our clients. I'm not sure what these "descriptions" are, but it's where the important stuff happens.

onCreateOfferSuccess = (desc) => {
let { pc1, pc2 } = this.state;
pc1.setLocalDescription(desc).then(
() => console.log("pc1 setLocalDescription complete createOffer"),
(error) =>
console.error(
"pc1 Failed to set session description in createOffer",
error.toString()
)
);
pc2.setRemoteDescription(desc).then(
() => {
console.log("pc2 setRemoteDescription complete createOffer");
pc2
.createAnswer()
.then(this.onCreateAnswerSuccess, (error) =>
console.error(
"pc2 Failed to set session description in createAnswer",
error.toString()
)
);
},
(error) =>
console.error(
"pc2 Failed to set session description in createOffer",
error.toString()
)
);
};

pc1 updates its local description, and pc2 updates its remote description. pc2 also creates an answer, which I think is akin to saying "Okay, I accepted your offer, let's do this".

onCreateAnswerSuccess

When pc2 successfully creates an answer, we do another round of description setting. This time in reverse order.

onCreateAnswerSuccess = (desc) => {
let { pc1, pc2 } = this.state;
pc1.setRemoteDescription(desc).then(
() => console.log("pc1 setRemoteDescription complete createAnswer"),
(error) =>
console.error(
"pc1 Failed to set session description in onCreateAnswer",
error.toString()
)
);
pc2.setLocalDescription(desc).then(
() => console.log("pc2 setLocalDescription complete createAnswer"),
(error) =>
console.error(
"pc2 Failed to set session description in onCreateAnswer",
error.toString()
)
);
};

pc1 sets its remote description and pc2 sets its local description. I think this acknowledges that, from pc1's perspective, it is local to itself and pc2 is remote, and vice-versa for pc2.

ยฏ\_(ใƒ„)_/ยฏ

At this point, we have two video streams talking to each other on the same page.

onIceCandidate

At some point during all this, both pc*s say that they've got an ICE candidate. Don't know when exactly that happens, but it gives us a chance to tell each client who they're talking to.

onIceCandidate = (pc, event) => {
let { pc1, pc2 } = this.state;
let otherPc = pc === pc1 ? pc2 : pc1;
otherPc.addIceCandidate(event.candidate).then(
() => console.log("addIceCandidate success"),
(error) => console.error("failed to add ICE Candidate", error.toString())
);
};

We guess the other client and add it as a candidate. If we had more than 2, this could get tricky.

Step 3: HangUp

Hanging up is easy. You close both clients.

hangUp = () => {
let { pc1, pc2 } = this.state;
pc1.close();
pc2.close();
this.setState({
pc1: null,
pc2: null,
hangUpDisabled: true,
callDisabled: false,
});
};

The tricky part

This works great as a tech demo. And that's when your dreams are shattered.

The first part of your connection, where two clients find each other, is called signaling. The WebRTC spec doesn't say anything about signaling.

It's easy in a single page, two client demo. Both clients are right there, in memory. And there's only two clients to boot.

But in the real world, you want those clients to run in different browsers, on different machines, far apart in the world. How do you make them find each other? What if there's thousands?

Well, you need some sort of communication channel that knows where all the clients are and can say "Yo, connect here. You, over there!". A central server of some sortโ€ฆ

That won't do for a distributed decentralized blockchain โ˜น๏ธ

Next step: Serverless WebRTC signaling. Stay tuned.

<strongThis is a Livecoding Recap โ€“ an almost-weekly post about interesting things discovered while livecoding. Usually shorter than 500 words. Often with pictures. Livecoding happens almost every Sunday at 2pm PDT on multiple channels. You should subscribe to My Youtube channel to catch me live.

Did you enjoy this article?

Published on May 7th, 2018 in blockchain, Front End, Livecoding, Technical, WebRTC

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