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2ec5f2bd3d
* Fixing edge cases in picture id munging.
Changes
1. Check the RTP sequence number order before VP8 temporal layer
filtering and use that ordering result while doing temporal
layer filtering.
In a sequence like below
o Packet 10 -> Picture ID 10
o Packet 11 -> missing
o Packet 12 -> Picture ID 11
it is not known if packet 11 will belong to Picture ID 10 or
Picture ID 11. The problem becomes a lot more tricky if there
is a burst loss and there is a larger hole in the picture id
space also as a result.
So, in the event of a packet loss, forward even if the current
packet belongs to a layer that can be dropped. More comments
in code.
2. Use result of sequence number ordering check while doing VP8 picture id munging.
3. When adding to missing picture id cache, have to include picture ids including
both ends. As a picture can span multiple packets and it is not known which
picture the packet belongs to, have to include both ends also in missing
picture id cache in the event of a gap.
4. As a picture can span multiple packets, it is not possible to have a simple
map of missing picture ids as an entry cannot be deleted if an out-of-order
picture id is received. There may be more missing packets belonging to that
picture id that is yet to be received.
So, have to use an ordered map and truncate the map if it grows too large.
Picked this for ordered map - https://github.com/elliotchance/orderedmap.
Has a simple API, had the highest number of stars of all the ones I checked.
And there are benchmarks.
The author also wrote a medium post at https://medium.com/swlh/an-ordered-map-in-go-436634692381
Another one which I looked at is - https://github.com/wk8/go-ordered-map.
The author of that wrote at https://morioh.com/p/990229f32171 and has a
bunch of other options at the end of that post (but does not include the
one I picked above). None of those have that many stars.
Testing:
--------
- Set max temporal layers to 0 so that temporal filtering happens and run for
an hour on sample app.
* do not let padding packets through VP8
* Correct comment
* fix comment
* Review comments from Jie
* golang naming convention