TWCC: Fix small receive delta rounding #207
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@boks1971 Thanks for the review, I made those changes. |
Prior to this change, a sequence of small receive deltas could get
erroneously encoded as a series of 0s.
If the actual receive deltas were {200μs, 200μs, ..., 200μs}, what
was appearing in the TWCC feedback packet (after marshaling) was
{0, 0, ..., 0}. (Note: the added test, without the code fixes, would
fail by returning deltas of {200, 200, ..., 200}, but marshalling the
feedback packet would truncate all of those to 0).
To fix this, receive deltas are rounded to the closest "delta tick"
period (250μs) as opposed to truncating. Additionally,
`lastTimestampUS` is moved forward by the rounded delta instead of set
to the timestamp of the last packet. This lets rounding errors
accumulate so that a series of packets will have the correct sum of
receive deltas (within the margin of error of a delta tick).
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Prior to this change, a sequence of small receive deltas could get erroneously encoded as a series of 0s.
If the actual receive deltas were {200μs, 200μs, ..., 200μs}, what was appearing in the TWCC feedback packet (after marshaling) was {0, 0, ..., 0}. (Note: the added test, without the code fixes, would fail by returning deltas of {200, 200, ..., 200}, but marshalling the feedback packet would truncate all of those to 0).
To fix this, receive deltas are rounded to the closest "delta tick" period (250μs) as opposed to truncating. Additionally,
lastTimestampUSis moved forward by the rounded delta instead of set to the timestamp of the last packet. This lets rounding errors accumulate so that a series of packets will have the correct sum of receive deltas (within the margin of error of a delta tick).