RTP Control Protocol: Difference between revisions

Revision as of 18:36, 4 January 2021

Revision as of 19:52, 14 January 2021

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{{short description|Sister protocol of the Real-time Transport Protocol that provides control information}}

{{distinguish|Real Time Streaming Protocol}}

{{short description|Sister protocol of the Real-time Transport Protocol that provides control information}}

The '''RTP Control Protocol''' ('''RTCP''') is a sister protocol of the [[Real-time Transport Protocol]] (RTP). Its basic functionality and packet structure is defined in RFC 3550. RTCP provides [[Out-of-band signaling|out-of-band]] statistics and control information for an RTP session. It partners with RTP in the delivery and packaging of multimedia data, but does not transport any media data itself.

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==Scalability in large deployments==

In large-scale applications, such as in [[Internet Protocol Television]] (IPTV), very long delays (minutes to hours) between RTCP reports may occur, because of the RTCP bandwidth control mechanism required to control congestion (see [[#Protocol functions| Protocol functions]]). Acceptable frequencies are usually less than one per minute. This affords the potential of inappropriate reporting of the relevant statistics by the receiver or causes evaluation by the media sender to be inaccurate relative to the current state of the session. Methods have been introduced to alleviate the problems:<ref>Vít Novotný, Dan Komosný, ''Large-Scale RTCP Feedback Optimization'', Journal of Networks, Vol.3 (3), March 2008</ref> RTCP filtering, RTCP biasing and [[RTCP hierarchical aggregation|hierarchical aggregation]].<ref>

[http://www.academypublisher.com/jnw/vol03/no03/jnw03030110.pdf Realtime control protocol and its improvements for Internet Protocol Television]</ref>

===Hierarchical aggregation===

The Hierarchical Aggregation (or also known as RTCP feedback hierarchy) is an optimization of the RTCP feedback model and its aim is to shift the maximum number of users limit further together with [[quality of service]] (QoS) measurement.<ref name=HA1/><ref name=HA2/> The [[RTCP]] [[Bandwidth (computing)|bandwidth]] is constant and takes just 5% of session bandwidth. Therefore, the reporting interval about QoS depends, among others, on a number of session members and for very large sessions it can become very high (minutes or even hours).<ref name=RFC3550/>. However, the acceptable interval is about 10 seconds of reporting. Bigger values would cause time-shifted and very inaccurate reported status about the current session status and any optimization made by the sender could even have a negative effect on network or QoS conditions.

The Hierarchical Aggregation is used with [[Source-Specific Multicast]] where only a single source is allowed, i.e. [[IPTV]]. Another type of multicast could be [[Any-Source Multicast]] but it is not so suitable for large-scale applications with huge number of users.

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=== Feedback Target ===

Feedback Target is a new type of member that has been firstly introduced by the Internet Draft draft-ietf-avt-rtcpssm-13.<ref name=HA4/>. The Hierarchical Aggregation method has extended its functionality. The function of this member is to receive Receiver Reports (RR) (see [[RTCP]]) and retransmit summarized RR packets, so-called Receiver Summary Information (RSI)<ref name=HA4/> to a sender (in case of single-level hierarchy).

==Standards documents==