rendezvous-service.md

Rendezvous server for unrupt.

Unrupt uses WebRTC to connect the audio of 2 web browsers (typically safari).

To set up a WebRTC 'call' requires that browser A creates an opaque blob of data called an 'offer', which is sent to browser B browser which creates a complimentary blob called an 'answer' which is then sent to Browser A. Exchanging this 'offer/answer' is (almost) sufficient for the two browsers to be able to create a direct connection between them.

(*) - almost because there are some other (smaller) blobs exchanged (called candidates) which decribe the available network interfaces on a webRTC capable browser.

We do this exchange over websockets to a central web service that is known to both parties.

The web service needs to do the following:

1. accept websocket connections
2. associate the websocket connection with an ID as a url param
3. recieve messages (in json) over websocket(s)
4. (optionally) check that a 'from' field exists in the message and that it matches the source websocket's id
5. inspect the 'to' field in a received json message
6. determine if any current websockets have an ID that match the 'to' field
7. send the message out on the matching websocket

a nice feature would be to reject websocket connections where the id is already taken.

An implementation in scala can be found here: https://github.com/steely-glint/fingersmith

Here's how unrupt uses this service:

        var mid = getMyId();
        var socket = new WebSocket( "wss://pi.pe/websocket/?finger=" + mid);
        socket.onmessage = (event) => {
                var data = JSON.parse(event.data);
                if (data.to != mid){
                    alert("message mixup");
                }
                switch (data.type) {
                    case "offer":
                        // setOffer
                        break;
                    case "answer":
                        // setAnswer
                        break;
                    case "candidate":
                        // etc
                }
            }
        
            function sendMessage(to,from,type,data){
                var messageJ = {to:to,from:from,type:type,sdp:data};
                var message = JSON.stringify(messageJ);
                console.log("sending "+message);
                socket.send(message);
            }