3.2 RTCP Details
RTCP packets SHOULD be sent on every RTP session. Failure to do so can result in loss of functionality on the remote end because channel statistics such as loss rate and jitter will not be communicated. Failure to transmit RTCP packets can also cause the termination of the session by time out assuming silence suppression is enabled and there is a long period of silence. See section 3.1 or [RFC3550] section 6.3.5.
The bandwidth estimation works as follows: One host sends a pair of packets to another host, back to back. The receiver calculates the bandwidth on the link, based on the reception times and packet sizes. The receiver then combines multiple measurements to arrive at a bandwidth estimate that is communicated back to the sender through an extension to the RTCP report.
In order to accelerate bandwidth estimation, the session starts in a "normal" RTCP sending rate. Once enough RTCP packet pairs have been sent, or the receiver has successfully estimated the bandwidth, the session changes to the "fast" RTCP sending rate. A high-level overview of this behavior is illustrated in the following diagram. Detailed specifications of the states, transitions and actions are given in sections 3.2.1 to 3.2.7.
Figure 2: Bandwidth estimation
RTCP packet pairs SHOULD be sent as specified in this document. If packet pairs are not sent, the receiver MAY never send a bandwidth estimate back. Bandwidth estimates SHOULD be sent through the profile extension; failure to do so can result in reduced functionality on the remote end for features that need a bandwidth estimate. RTCP packet pairs MUST be correctly received and parsed but MAY not be used by the bandwidth calculation algorithm.