Multimedia Congestion Control: Circuit Breakers for Unicast RTP Sessions

Abstract

The Real-time Transport Protocol (RTP) is widely used in telephony, video conferencing, and telepresence applications. Such applications are often run on best-effort UDP/IP networks. If congestion control is not implemented in these applications, then network congestion can lead to uncontrolled packet loss and a resulting deterioration of the user's multimedia experience. The congestion control algorithm acts as a safety measure by stopping RTP flows from using excessive resources and protecting the network from overload. At the time of this writing, however, while there are several proprietary solutions, there is no standard algorithm for congestion control of interactive RTP flows.This document does not propose a congestion control algorithm. It instead defines a minimal set of RTP circuit breakers: conditions under which an RTP sender needs to stop transmitting media data to protect the network from excessive congestion. It is expected that, in the absence of long-lived excessive congestion, RTP applications running on best-effort IP networks will be able to operate without triggering these circuit breakers. To avoid triggering the RTP circuit breaker, any Standards Track congestion control algorithms defined for RTP will need to operate within the envelope set by these RTP circuit breaker algorithms.

Real-time Audio-Visual Media Transport over QUIC

“…In this case, all congestion cues would also be provided via the API to the pluggable congestion controller [24,38]. The built-in QUIC congestion control could run in parallel and observe that the mid-term behavior is in line with that of a native QUIC stack, possibly provide hints of excess data transmission and, if needed, act as a circuit breaker [28,35].…”

Network Transport Circuit Breakers

“…A set of Fast-Trip CB methods have been specified for use together by a Real-time Transport Protocol (RTP) flow using the RTP/AVP Profile [RFC8083]. It is expected that, in the absence of severe congestion, all RTP applications running on best-effort IP networks will be able to run without triggering these CBs.…”

RTP Payload Format for the Mixed Excitation Linear Prediction Enhanced (MELPe) Codec

“…Since UDP does not provide congestion control, applications that use RTP over UDP SHOULD implement their own congestion control above the UDP layer [RFC8085] and MAY also implement a transport circuit breaker [RFC8083]. Work in the RMCAT working group [RMCAT] describes the interactions and conceptual interfaces necessary between the application components that relate to congestion control, including the RTP layer, the higher-level media codec control layer, and the lower-level transport interface, as well as components dedicated to congestion control functions.…”