Limitation of RTCP in Rate Control

In the majority of the existing rate control schemes, RTP is used to implement the required feedback in order to establish a closed loop between the sender and the receiver. In addition, using RTP as the transport protocol, the associated control protocol RTCP can be used to give feedback the loss and delay statistics from the receiver to the sender in order to establish a decision basis for the adaptation algorithm. The shortest time between two successive RTCP messages is about 5 seconds. This limits the use of RTCP in rate control adaptation (namely the short-term reaction). In order to react quickly, adaptation schemes proposed in the literature use feedback information arriving on short time scale of only a few milliseconds or even every sent packet. Hence, the sender can react to rapid changes in network. The frequency of RTCP feedback messages indicates that an RTP sender can not react fast enough to rapid changes in the network congestion. Therefore, the goal of RTCP-based adaptation is to adjust the sender's transmission rate to the average available bandwidth and not to react to rapid changes in network conditions [124]. Also, as the RTCP messages usually acknowledge the reception of a large number of packets, the sender would be allowed to insert a large burst of packets into the network, which might lead to losses or increase them [124].