RTCP (RTP Control Protocol) is the companion control channel that runs alongside RTP, carrying not media but feedback about it. Sender reports and receiver reports periodically exchange statistics — packets lost, jitter, round-trip time — and, crucially, the sender report pairs each stream's RTP timestamp with absolute NTP time, which is what lets a receiver lip-sync separate audio and video. RTCP also carries the extended feedback messages that drive congestion control (REMB, transport-cc) and request actions like keyframes. It uses only a small fraction of session bandwidth by design. If RTP is the conversation, RTCP is the two parties constantly confirming 'I got that, here's how it's going, here's the clock.'
