1.1. RTCP (Real-time Transport Control Protocol) Overview
RTCP OV:
- The RTCP basic functionality and packet structure is defined in the RTP specification RFC 3550
- The RTP Control Protocol (RTCP) is a sister protocol of the Real-time Transport Protocol (RTP)
- RTCP provides out-of-band statistics and control information for an RTP flow.
- It partners RTP in the delivery and packaging of multimedia data, but does not transport any media streams itself.
- The primary function of RTCP is to provide feedback on the quality of service (QoS) in media distribution by periodically sending statistics information to participants in a streaming multimedia session.
- RTCP gathers statistics for a media connection and information such as transmitted octet and packet counts, lost packet counts, jitter, and round-trip delay time.
- An application may use this information to control quality of service parameters, perhaps by limiting flow, or using a different codec.
- RTCP itself does not provide any flow encryption or authentication methods.
RTCP: Control and Management - Out-of-band control information for RTP flow
- Monitors QoS for RTP in the delivery and packaging of multimedia data
- Used periodically to transmit control packets to participants in a streaming multimedia session.
- Provides feedback on the quality of service being provided by RTP.
- Gathers statistics on media connection
§ Bytes sent, packets sent, lost packets, jitter, feedback and round trip delay.
§ Application may use this information to increase the quality of service, perhaps by limiting flow or using a different codec.
- Feedback/Information channel
- Announce-Listen protocol
- Soft-state
- Tracks participants using sender and receiver reports
- Monitors quality of data delivery
- Provides for stream synchronization
RTP and RTCP Summary:
- RTP is usually used in conjunction with the RTP Control Protocol (RTCP). The RTP standard defines a pair of protocols, RTP and the Real-time Transport Control Protocol (RTCP).
- While RTP carries the media streams (e.g., audio and video) or out-of-band signaling (DTMF), RTCP is used to monitor transmission statistics and quality of service (QoS) information.
- The RTCP provides reception quality feedback, participant identification and synchronization between media streams.
- The RTCP runs alongside RTP, providing periodic reporting of this information.
- The information in RTCP may be used for synchronization (e.g. lip sync) The RTCP traffic is small when compared to the RTP traffic, typically around 5%.
- To setup an RTP session, an application defines a pair of destination ports (an IP address with a pair of ports for RTP and RTCP.
- In a multimedia session, each media stream is carried in a separate RTP session, with its own RTCP packets reporting the reception quality for that session.
- Deviations from this rule can be signaled via RTP session descriptions in other protocols (SDP).
- RTP and RTCP typically use unprivileged UDP ports (1024 to 65535).
companion control protocol of RTP - Separate data (RTP) and control (RTCP)
- RTCP allows endpoints to periodically exchange control information
- For quality-related feedback and other meta-information
- A third party can also monitor session quality and detect network problems.
- Using RTCP and IP multicast
- typically even-numbered UDP port, not fixed port;
- RTCP Port: RTCP port = RTP port (even) + 1
- RTCP uses an odd-numbered UDP port number that follows the port number selected for RTP where
- default RTP/RTCP use UDP port 5004/5005
- typical UDP size limited to few hundred bytes (OS, network, fragmentation)
- typical: one media (audio, video, . . . ) per port pair
1.2. RTCP OV: Protocol functions
RTCP Protocol functions OV:
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1.3. RTCP Protocol Message Format
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RTCP Packet Message types: RTCP distinguishes several types of packets:
RTCP Protocol Message OV:
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