The x-google-per-layer-pli FMTP parameter is a format specific parameter that can be added to a remote description in the a=fmtp: line: a=fmtp:96 x-google-per-layer-pli=1
When using simulcast with more than a single SSRC, it will change how the simulcast encoder reacts to Picture Loss Indication (PLI) and Full Intra Request (FIR) RTCP feedback.
When the parameter value is 1, a PLI requests the generation of a key frame for the spatial layer associated with the SSRC of the media source and a FIR does the same for the SSRC value of the media sender.
When the value is 0 or the parameter is missing, a keyframe is generated on all spatial layers for backward compability.
This parameter does allow for large-scale A/B testing and opt-in to the new behavior. For multiparty calls enabling it should reduce the number of keyframes sent by the client and number of key frames received by the receivers which results in a better bandwidth utilization.
This parameter is experimental and may be removed again in the future.
Since the current behavior of reacting to a PLI for a specific SSRC with key frames on all spatial layers can be considered an implementation bug this parameter is not registered with the IANA.
If experimentation shows that the current behavior is better for some codecs like VP8 which can share encoding parameters with synchronous keyframes a standardized variant of this parameter shall be registered with the IANA.