RTCP = RTP Control Protocol.
Two types of packets: RTCP Sender/Receiver Report. Typically, one report is sent per second.
- SSRC for each media
- CNAME — identifier to group medias together
- timestamps, packet count, jitter estimation (to help with QoS)
RTCP-FB = RTCP Feedback (for congestion control) — optional extensions
Google REMB (Receiver Estimated Maximum Bitrate)
- for dynamic bandwidth adaptation
- request to resend some packets
NACK PLI (Picture Loss Indication)
- request to resend a new refresh frame
CCM FIR (Full Intra Request)
- request to send a keyframe
These extensions are negotiated in SDP.