RFC 4585 RTP/AVPF July 2006
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.
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.
Ott, et al. Standards Track [Page 9]
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