2009年9月4日

IP - RTCP OV (Study Note)

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

RTP Port:

- typically even-numbered UDP port, not fixed port;

RTCP 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:

Please refer to http://tw.myblog.yahoo.com/jw!HBFkhByWERmsTjQ93kTmwdsbhyg-/article?mid=9


1.3. RTCP Protocol Message Format

Please refer to http://tw.myblog.yahoo.com/jw!HBFkhByWERmsTjQ93kTmwdsbhyg-/article?mid=9

RTCP Packet Message types: RTCP distinguishes several types of packets:

RTCP Protocol Message OV:

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