3.2. Algorithm Outline

FB messages are part of the RTCP control streams and thus subject to the RTCP bandwidth constraints. This means, in particular, that it may not be possible to report an event observed at a receiver immediately back to the sender. However, the value of feedback given to a sender typically decreases over time -- in terms of the media quality as perceived by the user at the receiving end and/or the cost required to achieve media stream repair.

RTP [1] and the commonly used RTP profile [2] specify rules when compound RTCP packets should be sent. This document modifies those rules in order to allow applications to timely report events (e.g., loss or reception of RTP packets) and to accommodate algorithms that use FB messages.

The modified RTCP transmission algorithm can be outlined as follows: As long as no FB messages have to be conveyed, compound RTCP packets are sent following the rules of RTP [1] -- except that the five-second minimum interval between RTCP reports is not enforced. Hence, the interval between RTCP reports is only derived from the average RTCP packet size and the RTCP bandwidth share available to the RTP/RTCP entity. Optionally, a minimum interval between Regular RTCP packets may be enforced.

If a receiver detects the need to send an FB message, it may do so earlier than the next regular RTCP reporting interval (for which it would be scheduled following the above regular RTCP algorithm). Feedback suppression is used to avoid feedback implosion in multiparty sessions: The receiver waits for a (short) random dithering interval to check whether it sees a corresponding FB message from any other receiver reporting the same event. Note that for point-to-point sessions there is no such delay. If a corresponding FB message from another member is received, this receiver refrains from sending the FB message and continues to follow the Regular RTCP transmission schedule. In case the receiver has not yet seen a corresponding FB message from any other member, it checks whether it is allowed to send Early feedback. If sending Early feedback is permissible, the receiver sends the FB message as part of a minimal compound RTCP packet. The permission to send Early feedback depends on the type of the previous RTCP packet sent by this receiver and the time the previous Early feedback message was sent.

FB messages may also be sent as part of full compound RTCP packets, which are transmitted as per [1] (except for the five-second lower bound) in regular intervals.