WebRTC signaling

At places where WebRTC is needed (e.g. live audio playback), Angelcam uses a simple WebSocket-based signaling protocol.

Connection

Keep the WebSocket connection open for the entire WebRTC session — closing it terminates the session.

• Client SHOULD send WebSocket PING messages at regular intervals and expect a PONG within a reasonable timeout.
• Client MUST respond to PING messages sent by the server.

Message format

Both peers exchange WebSocket TEXT messages containing JSON. Every message must have a type field:

{
  "type": "MESSAGE_TYPE",
  ...
}

MESSAGE_TYPE is one of: offer, answer, candidate.

offer and answer

{
  "type": "offer",
  "sdp": "v=0\r\n..."
}

The sdp field contains the proposed or selected session description.

candidate

{
  "type": "candidate",
  "candidate": {
    "candidate": "candidate:...",
    "mid": "0",
    "ufrag": "ABCD"
  }
}

candidate can be null (end-of-candidates signal) or an ICE candidate object with:

Field	Description
candidate	ICE candidate in SDP attribute format (without the leading a=)
mid	Media ID of the stream the candidate belongs to
ufrag	Username fragment for the media stream