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1. Introduction

When the Audio-Visual Profile with Feedback (AVPF) [RFC4585] was developed, the main emphasis lay in the efficient support of point-to-point and small multipoint scenarios without centralized multipoint control. However, in practice, many small multipoint conferences operate utilizing devices known as Multipoint Control Units (MCUs). Long-standing experience of the conversational video conferencing industry suggests that there is a need for a few additional feedback messages, to support centralized multipoint conferencing efficiently. Some of the messages have applications beyond centralized multipoint, and this is indicated in the description of the message. This is especially true for the message intended to carry ITU-T Rec. H.271 [H.271] bit strings for Video Back Channel messages.

In Real-time Transport Protocol (RTP) [RFC3550] terminology, MCUs comprise mixers and translators. Most MCUs also include signaling support. During the development of this memo, it was noticed that there is considerable confusion in the community related to the use of terms such as mixer, translator, and MCU. In response to these concerns, a number of topologies have been identified that are of practical relevance to the industry, but are not documented in sufficient detail in [RFC3550]. These topologies are documented in [RFC5117], and understanding this memo requires previous or parallel study of [RFC5117].

Some of the messages defined here are forward only, in that they do not require an explicit notification to the message emitter that they have been received and/or indicating the message receiver's actions. Other messages require a response, leading to a two-way communication model that one could view as useful for control purposes. However, it is not the intention of this memo to open up RTP Control Protocol (RTCP) to a generalized control protocol. All mentioned messages have relatively strict real-time constraints, in the sense that their value diminishes with increased delay. This makes the use of more traditional control protocol means, such as Session Initiation Protocol (SIP) [RFC3261], undesirable when used for the same purpose. That is why this solution is recommended instead of "XML Schema for Media Control" [XML-MC], which uses SIP Info to transfer XML messages with similar semantics to what are defined in this memo. Furthermore, all messages are of a very simple format that can be easily processed by an RTP/RTCP sender/receiver. Finally, and most importantly, all messages relate only to the RTP stream with which they are associated, and not to any other property of a communication system. In particular, none of them relate to the properties of the access links traversed by the session.

2. Definitions

2.1. Glossary​

  • AIMD - Additive Increase Multiplicative Decrease
  • AVPF - The extended RTP profile for RTCP-based feedback
  • FCI - Feedback Control Information [RFC4585]
  • FEC - Forward Error Correction
  • FIR - Full Intra Request
  • MCU - Multipoint Control Unit
  • MPEG - Moving Picture Experts Group
  • PLI - Picture Loss Indication
  • PR - Packet rate
  • QP - Quantizer Parameter
  • RTT - Round trip time
  • SSRC - Synchronization Source
  • TMMBN - Temporary Maximum Media Stream Bit Rate Notification
  • TMMBR - Temporary Maximum Media Stream Bit Rate Request
  • TSTN - Temporal-Spatial Trade-off Notification
  • TSTR - Temporal-Spatial Trade-off Request
  • VBCM - Video Back Channel Message

2.2. Terminology​

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

  • Message: An RTCP feedback message [RFC4585] defined by this specification, of one of the following types:

    • Request: Message that requires acknowledgement
    • Command: Message that forces the receiver to an action
    • Indication: Message that reports a situation
    • Notification: Message that provides a notification that an event has occurred. Notifications are commonly generated in response to a Request.

Note that, with the exception of "Notification", this terminology is in alignment with ITU-T Rec. H.245 [H245].

  • Decoder Refresh Point: A bit string, packetized in one or more RTP packets, that completely resets the decoder to a known state.
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