Normally, ICE candidates are only gathered after RTCPeerConnection.setLocalDescription is called, because this is the point at which the PeerConnection knows how many ports it needs. The iceCandidatePoolSize member of RTCConfiguration instructs the PeerConnection to gather ICE candidates before this occurs, as a performance optimization. Thus, if setting the local description is delayed (by signaling, for example), interactions with STUN/TURN servers can occur in parallel.