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RTCP hierarchical aggregation

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The Hierarchical Aggregation (or also known as RTCP feedback hierarchy) is an optimization of the RTCP feedback model to shift maximum number of users limit further [1], [2]. The RTCP 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)[2]. 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 sender could even have a negative effect to network or QoS conditions.

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

Feedback Target

Feedback Target is a new type of member that has been firstly introduced by the Internet Draft draft-ietf-avt-rtcpssm-13 (see [4]). The Hierarchical Aggreagtion 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) [4] to a sender (in case of single level hierarchy).

Schema

Bacause of presence Source-Specific Multicast the unicast RTCP feedback is used and agregated with feedback targets.

Schema of hierarchicaly aggregated RTCP feedback: http://adela.utko.feec.vutbr.cz/projects/images/ssm_ha.png

References

  1. KOMOSNY D., NOVOTNY V. Tree Structure for Specific-Source Multicast with feedback Aggregation, in ICN07 - The Sixth International Conference on Networking . Martinique, 2007, ISBN 0-7695-2805-8
  2. NOVOTNY, V., KOMOSNY, D. Optimization of Large-Scale RTCP Feedback Reporting in ICWMC 2007. ICWMC 2007 - The Third International Conference on Wireless and Mobile Communications. Guadeloupe, 2007, ISBN: 0-7695-2796-5
  3. J. Chesterfield, E. M. Schooler. "An Extensible RTCP Control Framework for Large Multimedia Distributions"
  4. J. Ott, J. Chesterfield, E. Schooler. "RTCP Extensions for Single-Source Multicast Sessions with Unicast Feedback", Internet Draft draft-ietf-avt-rtcpssm-13
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