About LTR support in VideoToolbox Encoding

Hi there,

When I enable LTR by setting "kVTCompressionPropertyKey_EnableLTR" to be true, and get LTR refresh frame by setting ForceLTRRefresh to be true in VTCompressionSessionEncodeFrame() call.

Everything goes well, but I found I cannot know which encoded frame is LTR Refresh frame, I try to find out, so dumpped the attachments of the CMSampleBuffer frame, I found the normal P frame's attachment like this:

{
DependsOnOthers = 1;
EarlierDisplayTimesAllowed = 0;
EncodedFrameAvgQP = 35;
IsDependedOnByOthers = 1;
NotSync = 1;
RequireLTRAcknowledgementToken = 9177;
TemporalID = 0;
}

And LTR refresh frame may look like this:

{
DependsOnOthers = 1;
EarlierDisplayTimesAllowed = 0;
EncodedFrameAvgQP = 38;
FECGroupID = 0;
FECLastFrameInGroup = 1;
FECLevelOfProtection = 1;
IsDependedOnByOthers = 1;
NotSync = 1;
ReferenceWasRefreshed = 1;
RequireLTRAcknowledgementToken = 9180;
TemporalID = 0;
}

The laster has 4 more key-value pairs than the former:

FECGroupID = 0;
FECLastFrameInGroup = 1;
FECLevelOfProtection = 1;
ReferenceWasRefreshed = 1;

The key "ReferenceWasRefreshed" is very noticeable,

So question 1: May this key signify current frame is a LTR Refresh frame? (But It's weird I didn't find any documents about this key.)

Question 2: If this frame is LTR refresh frame, How can I know which frame this LTR frame refers to? More specifily, the corresponding 'AcknowledgementToken'.

Question 3: This question is independent. I found that if I didn't set 'kVTEncodeFrameOptionKey_AcknowledgedLTRTokens' for a while (keep the tokens be empty), I cannot get the 'AcknowledgementToken' from encoded CMSampleBuffer for a while too, that's to say this key is not contained in the attachments of CMSampleBuffer. What should I do to make the encoded frame always contains the AcknowledgementToken?

Thanks very much!