1. Introduction

Real-time media streams that use RTP are, to some degree, resilient against packet losses. RTP [1] provides all the necessary mechanisms to restore ordering and timing present at the sender to properly reproduce a media stream at a recipient. RTP also provides continuous feedback about the overall reception quality from all receivers -- thereby allowing the sender(s) in the mid-term (in the order of several seconds to minutes) to adapt their coding scheme and transmission behavior to the observed network quality of service (QoS). However, except for a few payload-specific mechanisms [6], RTP makes no provision for timely feedback that would allow a sender to repair the media stream immediately: through retransmissions, retroactive Forward Error Correction (FEC) control, or media-specific mechanisms for some video codecs, such as reference picture selection.

Current mechanisms available with RTP to improve error resilience include audio redundancy coding [13], video redundancy coding [14], RTP-level FEC [11], and general considerations on more robust media streams transmission [12]. These mechanisms may be applied proactively (thereby increasing the bandwidth of a given media stream). Alternatively, in sufficiently small groups with small round-trip times (RTTs), the senders may perform repair on-demand, using the above mechanisms and/or media-encoding-specific approaches. Note that "small group" and "sufficiently small RTT" are both highly application dependent.

This document specifies a modified RTP profile for audio and video conferences with minimal control based upon [1] and [2] by means of two modifications/additions: Firstly, to achieve timely feedback, the concept of Early RTCP messages as well as algorithms allowing for low-delay feedback in small multicast groups (and preventing feedback implosion in large ones) are introduced. Special consideration is given to point-to-point scenarios. Secondly, a small number of general-purpose feedback messages as well as a format for codec- and application-specific feedback information are defined for transmission in the RTCP payloads.