What is RTCP?
The Real-time Transport Control Protocol (RTCP) accompanies RTP sessions with control information rather than media payloads. Its reports can describe reception quality, timing, packet counts, and session participants.
How RTCP works
RTCP participants periodically exchange compact binary reports alongside an RTP media session. Sender reports relate an RTP timestamp to a wall-clock reference, enabling receivers to align streams such as audio and video, while receiver reports summarize observed delivery. Source-description packets associate identifiers with participant information, and other packet types handle departure or application feedback. Monitoring and congestion logic consume this control plane without treating it as media.
Key facts
- RTCP commonly uses the same transport family as RTP but a separate port or multiplexed channel; middleboxes must permit the negotiated arrangement for reports to flow.
- Packet-loss fields are derived from RTP sequence numbers, while interarrival jitter is a statistical timing estimate and is not the same measurement as round-trip time.
- Control traffic is rate-limited relative to session bandwidth, so report intervals generally lengthen as participant count grows instead of every receiver reporting constantly.
When RTCP matters
Inspect RTCP reports to estimate packet loss, jitter, round-trip delay, and synchronization quality. Feedback can guide bitrate adaptation, but sparse or blocked reports leave the sender with an incomplete view of conditions.