This page describes the protocol used by the gstreamer WebRTC signaling. The WebRTC tools for gstreamer are defined in the Rust plugin here:

net/webrtc · main · GStreamer / gst-plugins-rs · GitLab

They provides a signaling server, sink for gstreamer and a Node.js API.

Here, we will describe how their signalling protocol works so that you can implement your own signalling solution to connect it with 3rd parties software (such as Unity or aiortc in Python)

Protocol

Roles

Gstreamer signalling defines 3 roles (so the WebRTC peers are not symmetrical)

• producer (produces data, video or audio streams)
• consumer (access to a producer stream, can access all or a subset of its streams)
• listener (gets notified by the server of new producers status)

Protocol sequence diagram

sequenceDiagram
  Server-->>Producer: Welcome (*PeerId)*
	Server-->>Consumer: Welcome (PeerId)
  Producer->>Server: PeerStatusChanged  # declare yourself as producer
	Consumer->>Server: StartSession (producer PeerId)
	Server-->>Producer: StartSession (SessionId)
	Server-->>Consumer: SessionStarted (SessionId)
	Producer->>Server: Peer (offer sdp)
	Server-->>Consumer: Peer (offer sdp)
	Consumer->>Server: Peer (answer sdp)
	Server-->>Producer: Peer (answer sdp)

• The session is always at the initiative of the consumer to a remote producer. Once the session is established, the producer sends the offer, then the consumer answers.
• You can also send ICECandidate via the Peer message. Extra messages such as List, EndSession are not included in the diagram. You can find more information on those messages directly in the gstreamer Rust protocol code:

net/webrtc/protocol/src/lib.rs · main · GStreamer / gst-plugins-rs · GitLab

Messages

Peer —> Server

• NewPeer - Internal message to let know about new peers
• SetPeerStatus - Set current peer status {"type":"setPeerStatus","meta":{"name":"UnityClient"},"roles":["listener"]}
• StartSession - Start a session with a producer peer {"type":"startSession","peerId":"0cf80c2e-79fc-495e-8e45-8f25c7f6c4cf"}