RTCP feedback Message Timing Configuration

Introduction This document proposes controlling specific RTCP message feedback send time. This proposal help sender negotiate RTCP feedback send time, better flexibility in defining application behavior. This document defines a new Session Description Protocol (SDP) parameter to negotiate the timing configuration.

Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

SDP Definitions This section defines optional SDP parameters that are used to negotiate RTCP feedback message send time. Time defined is applicable to specific RTCP feedback message only. An OPTIONAL RTCP feedback specific parameter, "fb-min-time", indicates the minimum period T_fb_min_time in milliseconds between two same RTCP feedback or wait time before sending feedback message. The syntax is as follows: a=rtcp-fb:<rtcp-fb-pt> <rtcp-fb-param>;fb-min-time=<fb-min-time-val> where above parameters are explained in

rtcp-fb-pt: = /fmt ; as defined in SDP
rtcp-fb-param: = SP "app" [SP byte-string]
: / SP token [SP byte-string]
: / ; empty
fb-min-time-val: = feedback message minimum time value in milliseconds

“fb-min-time” may have an OPTIONAL parameter “sync-counter”, indicates synchronization counter SYNC-CONTER helps synchronize RTCP feedback with RTP timestamp change. If T0 is start of time, receiver keeps count of change in RTP timestamp as COUNT. Once COUNT is equal to parameter SYNC-CONTER or time elapsed is greater than or equal to T_fb_min_time, receiver sends the RTCP feedback. Receiver resets the counter and time, to determine when the next feedback is to be sent.

SDP description for RTCP feedback timing configuration

Payload specific RTCP feedback PLI (Picture Loss Indication) with minimum interval of 50 milliseconds. Configuration can be used by the receiver to trigger PLI when no decodable unit is available to decode for 50ms. a=rtcp-fb:96 nack pli;fb-min-time=50
RTCP feedback Generic NACK with minimum time of 1 milliseconds. Receiver to wait for 1 milliseconds before NACK RTCP feedback message is sent on packet loss. a=rtcp-fb:96 nack;fb-min-time=1
RTCP feedback transport-cc with minimum time of 50 milliseconds and synchronization counter set to 3. Receiver to send transport-cc feedback on every 3rd change in RTP timestamp change or 50 milliseconds elapsed, whichever happens earliest. a=rtcp-fb:96 transport-cc ;fb-min-time=50;sync-counter=3