1.1. Definitions
The definitions from RTP/RTCP [1] and the "RTP Profile for Audio and Video Conferences with Minimal Control" [2] apply. In addition, the following definitions are used in this document:
Early RTCP mode:
The mode of operation in that a receiver of a media stream is often (but not always) capable of reporting events of interest back to the sender close to their occurrence. In Early RTCP mode, RTCP packets are transmitted according to the timing rules defined in this document.
Early RTCP packet:
An Early RTCP packet is a packet which is transmitted earlier than would be allowed if following the scheduling algorithm of [1], the reason being an "event" observed by a receiver. Early RTCP packets may be sent in Immediate Feedback and in Early RTCP mode. Sending an Early RTCP packet is also referred to as sending Early Feedback in this document.
Event:
An observation made by the receiver of a media stream that is (potentially) of interest to the sender -- such as a packet loss or packet reception, frame loss, etc. -- and thus useful to be reported back to the sender by means of a feedback message.
Feedback (FB) message:
An RTCP message as defined in this document is used to convey information about events observed at a receiver -- in addition to long-term receiver status information that is carried in RTCP receiver reports (RRs) -- back to the sender of the media stream. For the sake of clarity, feedback message is referred to as FB message throughout this document.
Feedback (FB) threshold:
The FB threshold indicates the transition between Immediate Feedback and Early RTCP mode. For a multiparty scenario, the FB threshold indicates the maximum group size at which, on average, each receiver is able to report each event back to the sender(s) immediately, i.e., by means of an Early RTCP packet without having to wait for its regularly scheduled RTCP interval. This threshold is highly dependent on the type of feedback to be provided, network QoS (e.g., packet loss probability and distribution), codec and packetization scheme in use, the session bandwidth, and application requirements. Note that the algorithms do not depend on all senders and receivers agreeing on the same value for this threshold. It is merely intended to provide conceptual guidance to application designers and is not used in any calculations. For the sake of clarity, the term feedback threshold is referred to as FB threshold throughout this document.
Immediate Feedback mode:
A mode of operation in which each receiver of a media stream is, statistically, capable of reporting each event of interest immediately back to the media stream sender. In Immediate Feedback mode, RTCP FB messages are transmitted according to the timing rules defined in this document.
Media packet:
A media packet is an RTP packet.
Regular RTCP mode:
Mode of operation in which no preferred transmission of FB messages is allowed. Instead, RTCP messages are sent following the rules of [1]. Nevertheless, such RTCP messages may contain feedback information as defined in this document.
Regular RTCP packet:
An RTCP packet that is not sent as an Early RTCP packet.
RTP sender:
An RTP sender is an RTP entity that transmits media packets as well as RTCP packets and receives Regular as well as Early RTCP (i.e., feedback) packets. Note that the RTP sender is a logical role and that the same RTP entity may at the same time act as an RTP receiver.
RTP receiver:
An RTP receiver is an RTP entity that receives media packets as well as RTCP packets and transmits Regular as well as Early RTCP (i.e., feedback) packets. Note that the RTP receiver is a logical role and that the same RTP entity may at the same time act as an RTP sender.