没有任何服务器的WebRTC甚至不可能成为信令服务器吗？

Question

I'm trying to setup an a cordova plugin for iOS which implements the webrtc functions without using any server and it will only be used on a local network . I know there is this plugin , which looks promising but i have some problems with it. My plan is not to use a TRUN, STUN or any kind of signaling server.

Maybe you think right now: " Ok this is not possible. No signaling equals no connection. " But let me explain first. As pointed out here and here it's possible to avoid using a TRUN, STUN or ICE server. I think this is a good way to start my project but there is still an open question. How shall the devices find each other if there isn't any kind signaling (in the example they use a Node.js server)? Right now i'm playing with the idea of an QR-Code which contains all the necessary information.

At the end it should look like this (black arrwos are more important):

The idea is that everyone who comes into a room has to scan a QR-Code on the RP and then the device knows the IP, port, etc. of the RP and a WebRTC connection with a DataChannel will be established.

I've been looking for an answer for days now, but due to the fact (or at least one of the reasons) that WebRTC is not even supported on iOS nativly there aren't many WebRTC examples out there which work on iOS and no one for a local network.

So my question is: Am I on the right way or is this not even possible? (I found no example for this anywhere, but if I put all the posts I read together, I think it should be possible.)

Answer 1

First of all, TURN and STUN are not signaling server. Signaling server is the term normally associated with the backend server that let's you relay the messages between two peers before the connection is established. The signaling server is thus used to establish the connection. Once the connection is established, there is no role of the signaling server in the communication, unless you intend to make any changes to the connection parameters.

TURN and STUN servers, on the other hand, are used during the connection establishment process. It helps the two peers find a direct path to each other. So when the connection is established, the peers can talk directly with each other and they don't require the signaling server to relay the messages anymore.

Now coming to your question, short answer is, no, your plan is incomplete.

Here are some changes that you'd need in order to make it work:

1. QR Code is not adequate to convey all required information. According to this answer , they can store roughly 4kb of maximum data. Thus it is not sufficient to pass all candidates.

   Not to mention that WebRTC requires both devices to share the candidates. So, you'd need a display and QR code scanner on the Raspberry PI.

   You might want to explore alternatives such as Wifi to allow for two-way data sharing between the device and Raspberry Pi. Once setup, the Wifi connection will act as the Signaling server.

   Though I am not well versed in iOS or Raspberry Pi. So I would recommend that you ask a separate question about the choice of communication channel if you are unsure about what to choose. Keep in mind that you need Raspberry Pi to be able to communicate with the device for a short period of time in order to allow WebRTC connection to be established.

2. Coming to STUN and TURN servers, you may be able to get away without using them. I have seen a few cases when my app is able to establish connection to peers within the local network without STUN and TURN servers.

   However, I would strongly recommend that you use at least a STUN server. They are often available for free of charge. Google and Firefox also provide their own STUN servers that you can use in any of your WebRTC apps. You can search on internet to get their details.

   TURN servers are required only when the two peers are behind NAT's. In such cases STUN servers are sometimes incapable of finding a direct route between them, and you need the TURN server to relay the audio/video/message stream.

3. Your plan to establish the WebRTC channel between Raspberry Pi and the phones (the black arrows) seem fine to me. It would help you establish further connections between two phones whenever required.

   However, if you eventually decide to implement something like Wifi on your Raspberry Pi, the WebRTC connection may be redundant. After all, you could use Wifi to pass the data back and forth, and don't really need an additional layer of WebRTC channel to do that.

Answer 2

Since you run your app on a local network you don't need STUN and TURN servers. But still you need a signaling server. Signaling can't be done with QR-codes, read more about WebRTC and you will understand why.

But a signaling server can be very simple. Since you have that raspberry pi in your local network, you can use this as your signaling server. Just install node, express and socket.io on it. You need only one simple javascript file, mine is only 23 lines of code. Stop wasting your time with QR-codes and you will have your signaling server up and running in no time. You can look at Google Codelab for an example. Hopes this helps you !!