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3.3. Modes of Operation

RTCP-based feedback may operate in one of three modes (Figure 1) as described below. The mode of operation is just an indication of whether or not the receiver will, on average, be able to report all events to the sender in a timely fashion; the mode does not influence the algorithm used for scheduling the transmission of FB messages.

And, depending on the reception quality and the locally monitored state of the RTP session, individual receivers may not (and do not have to) agree on a common perception on the current mode of operation.

a) Immediate Feedback mode: In this mode, the group size is below the FB threshold, which gives each receiving party sufficient bandwidth to transmit the RTCP feedback packets for the intended purpose. This means that, for each receiver, there is enough bandwidth to report each event by means of a virtually "immediate" RTCP feedback packet.

The group size threshold is a function of a number of parameters including (but not necessarily limited to): the type of feedback used (e.g., ACK vs. NACK), bandwidth, packet rate, packet loss probability and distribution, media type, codec, and the (worst case or observed) frequency of events to report (e.g., frame received, packet lost).

As a rough estimate, let N be the average number of events to be reported per interval T by a receiver, B the RTCP bandwidth fraction for this particular receiver, and R the average RTCP packet size, then the receiver operates in Immediate Feedback mode as long as N<=B*T/R.

b) Early RTCP mode: In this mode, the group size and other parameters no longer allow each receiver to react to each event that would be worth reporting (or that needed reporting). But feedback can still be given sufficiently often so that it allows the sender to adapt the media stream transmission accordingly and thereby increase the overall media playback quality.

Using the above notation, Early RTCP mode can be roughly characterized by N > B*T/R as "lower bound". An estimate for an upper bound is more difficult. Setting N=1, we obtain for a given R and B the interval T = R/B as average interval between events to be reported. This information can be used as a hint to determine whether or not early transmission of RTCP packets is useful.

c) Regular RTCP Mode: From some group size upwards, it is no longer useful to provide feedback for individual events from receivers at all -- because of the time scale in which the feedback could be provided and/or because in large groups the sender(s) have no chance to react to individual feedback anymore.

No precise group size threshold can be specified at which this mode starts but, obviously, this boundary matches the upper bound of the Early RTCP mode as specified in item b) above.

As the feedback algorithm described in this document scales smoothly, there is no need for an agreement among the participants on the precise values of the respective FB thresholds within the group. Hence, the borders between all these modes are soft.

    ACK
  feedback
    V
    :<- - - -  NACK feedback - - - ->//
    :
    :   Immediate   ||
    : Feedback mode ||Early RTCP mode   Regular RTCP mode
    :<=============>||<=============>//<=================>
    :               ||
   -+---------------||---------------//------------------> group size
    2               ||
     Application-specific FB Threshold
        = f(data rate, packet loss, codec, ...)

                  Figure 1: Modes of operation

As stated before, the respective FB thresholds depend on a number of technical parameters (of the codec, the transport, the type of feedback used, etc.) but also on the respective application scenarios. Section 3.6 provides some useful hints (but no precise calculations) on estimating these thresholds.

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