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The WebRTC draft and its normative references do not define how an implementation might choose to use TCP instead of UDP for media transmission. This is something that has been discussed numerous times, always with the conclusion that TCP fallback was desirable. Nothing has been captured or officially decided.
Closing as an IETF matter. The transport specification in the IETF has recently mandated that implementations support ICE TCP candidates, as well as TCP connections to TURN servers. draft-ietf-rtcweb-transport