RTP stats objects, of type "outbound-rtp" or "inbound-rtp" in this case, represents a WebRTC stream. The identifier of this stream is the SSRC (a number). We should collect stats for streams that "exist", but implementations and spec has disagreed on when the stream should exist: if the SSRC information is conveyed via SDP, does the stream still exist before packets are sent or received? https://crbug.com/406585888 tries to reach alignment between browsers and spec.