Signaling in WebRTC is out of scope. It is left out to the implementer of the service to decide both on the transport to use and the signaling protocol.

For transport, there are 3 main options (assuming the use case requires interaction with a web browser):

1. XHR
2. SSE
3. WebSocket

It can also be a combination of the above.

On top of the transport protocol itself comes the signaling protocol. Usually one of these 3 options:

1. Proprietary protocol, invented for the purpose of the specific service
2. SIP (over WebSocket)
3. XMPP/Jingle
4. MQTT