[Bug 738319] rtpsession: fix Early Feedback Transmission

[GStreamer (bugzilla.gnome.org)]

https://bugzilla.gnome.org/show_bug.cgi?id=738319
  GStreamer | gst-plugins-good | 1.4.1

--- Comment #35 from Sebastian Dröge (slomo) <slomo at coaxion.net> 2015-01-26 17:48:25 UTC ---
(In reply to comment #34)
> (In reply to comment #31)
> > If we observed a NACK with "23" and now we want to send one with "5", we will
> > send one with "5". But if we observed one for "235" and we now want to send one
> > for "456" then we will send one for "46" only. There is no merging done here,
> > only filtering of seqnums that were already requested. The merging is done in
> > rtp_source_register_nack().
> 
> I think if someone already announced that "5" was lost, and we didn't receive
> 456, then we should nack 456, otherwise the sender will think that one lost 5
> and the other lost 46 but not 5, which may be confusing. As for large groups,
> NACKs may only be used statically, not as a request for an immediate
> retransmission.

If we're talking about a group with more receivers than one, we have multicast,
right? In that case if some other receiver requested "5" again, and we want
"456" now, we can skip "5" as we would receive the retransmission for the other
one too (because it goes to the multicast group).

Or am I missing something here? There's the same reasoning for PLI and FIR btw,
we don't send our request because someone else in the multicast group did
already which would also get us the result without requesting anything.

> (In reply to comment #33)
> > What are these FIR packets? Are they this?
> > https://tools.ietf.org/html/rfc2032#section-5.2.1
> > If they are supposed to be this, I think we are creating and parsing them wrong
> > currently.
> 
> No, they're in RFC 5104 3.5.1.. Google Hangouts was using them instead of PLI
> at some point, but I understand that their correct use is really for stream
> switching.

Oh good, that's not confusing at all ;) I'll check against that RFC what would
be the correct behaviour here.

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