Tuesday, November 20, 2012

RTCP Timeout Interval (RFC3550 vs RFC4585)

The main advantages of using the Early Feedback Profile for RTCP-based feedback or AVPF are

  • reduced minimum interval: the 5 second minimum interval has been removed. The randomisation factor causes the interval to fall somewhere in the [2.5/1.21828, 7.5/1.21828] second interval unless the reduced minimum value is used.
  • Immediate/early feedback mode: important events can be communicated back to the sender provided that certain conditions are met.
Figure 1 illustrates the differences in the timeout intervals for unicast scenarios, where the top and bottom dotted lines represent the minimum and maximum intervals possible, the dashed line in the middle is the deterministic value of the timeout, and the solid line includes the effect of randomisation.
An average RTCP packet size of 96 bytes was used in the calculations. The initial timeout value was not considered.

Figure 1: RTCP Timeout Intervals

This shows that the mean RTCP interval for bitrates of around 100kbps and above is less than 200ms, which is a big step away from 5s.