Establishing WebRTC peer connection

1
votes

I have started to look into WebRTC a bit and I am using it to build a simple peer to peer chat application using the data channel. I have the following questions:

  1. Do I need to establish a RTCPeerConnection to each peer I want to talk to? So if there are three peers they each need 2 RTCPeerConnections (unless I use one of the peers as a sort of ad-hoc server).

  2. If peer A sends out a candidate and sdp when creating a offer to peer B. Can peer B connect to peer A using that info and send its answer (with candidate and its sdp) over the RTCPeerConnection, i.e. using the RTCPeerConnection (before it's been completely established) as a signaling channel? I would assume that when the offer is created by peer A it starts to listen for connections on some port.

My understanding of WebRTC is a bit limited so if I've missunderstood some concept of WebRTC in my questions above please point them out!

2
webrtc

2 Answers

3
votes
  1. Yes, as a direct P2P protocol everybody must be directly connected to everybody else if they want to communicate; unless you create some kind of mesh network in which one peer forwards messages to other peers.
  2. No, the SDP offer and answer and ICE candidates all need to be exchanged through a signalling server; the connection cannot be established until both peers have actually agreed on a specific session configuration and ICE route to use, so you cannot send the SDP answer over a connection which isn't complete yet.

Especially for a simple text-only chat, going through a server is often easier than using P2P; the processing and bandwidth requirements are so minimal that the complications of P2P connections are probably not worth it. And you need a signalling server anyway. P2P only becomes really interesting once you start sending large files or audio/video streams.

2
votes

In principle it is possible to establish a WebRTC connection without a signalling server, but that requires an out of band exchange of session tokens between the peers. I.e. the user would have to copy a token from the application, somehow send it to another user and the other user would have to paste it.

Additionally those tokens cannot be reused, so this procedure would have to be repeated every time peers want to establish a connection.

So while theoretically possible webrtc is not distributed in practical terms.

There is some noise about specifying support for incoming connections and reusable peer contacts, but the progress on that is unclear.