transport-cc
Also written TWCC, transport-wide congestion control, a=rtcp-fb:transport-cc
The feedback scheme where the receiver reports the arrival time of every packet and the sender does the bandwidth estimation. Never an RFC, implemented everywhere.
Every packet leaves with a transport-wide sequence number in an RTP header extension, numbered across all streams on the transport rather than per stream. The receiver records when each one arrived and periodically sends back a compact feedback message listing those arrival times.
That is all it does. The receiver never computes an estimate. The sender compares send times to arrival times, watches the delay gradient grow, and decides for itself how much bandwidth it has.
Putting the estimate on the sender is the point. The sender is the only side that knows what it actually transmitted, and it can change its behaviour immediately instead of waiting for the next feedback round. It replaced REMB, which did the arithmetic on the receiver and shipped a single number back.
The draft expired years ago and was never published as an RFC. Every browser implements it anyway, which is worth remembering the next time someone tells you a standards document is the source of truth.