> A quick question about live555 behaviour with packet loss.
>
> Let's consider a (video) frame consisting of five packets [ ]. Packets have
> the following RTP sequence enumeration:
>
> [11] [12] [13] [14] [15]
>
> (all packets have the same rtp timestamp as they belong to the same frame)
>
> Now, let's imagine that a single packet is lost in the transmission and we
> get the following sorted sequence of RTP packets:
>
> [11] [12] [14] [15]
>
> Does live555 scrap frames with missing RTP sequence numbers, or does it
> consider them as valid frames, passing them on to afterGettingFrame?
It depends on whether (i) the RTP packets each contained a single frame, or
(ii) the RTP packets, taken together, made up a single frame.
In case (i) - which occurs only if the frames are small - each frame gets
passed on.
In case (ii) - which usually occurs if the frames are large (e.g., ‘key
frames’) - then all packets making up the frame must be received for the data
to be useful. If *even one* of the packets is lost, then the frame cannot be
recovered, and thus the rest of the data gets discarded.
This is why - for H.264 video - it is important that, for streaming, a large
‘key frame’ get encoded as a sequence of smaller ‘slice’ NAL units, rather than
being encoded as a single (very large) NAL unit - because very large NAL units
are very vulnerable to packet loss.
I have discussed this *many* times on this mailing list in the past; see
http://lists.live555.com/pipermail/live-devel/2011-December/014190.html
http://lists.live555.com/pipermail/live-devel/2012-August/015615.html
http://lists.live555.com/pipermail/live-devel/2013-May/016994.html
http://lists.live555.com/pipermail/live-devel/2014-June/018426.html
http://lists.live555.com/pipermail/live-devel/2014-June/018432.html
http://lists.live555.com/pipermail/live-devel/2014-June/018433.html
http://lists.live555.com/pipermail/live-devel/2014-June/018434.html
http://lists.live555.com/pipermail/live-devel/2015-March/019135.html
http://lists.live555.com/pipermail/live-devel/2015-April/019228.html
Ross Finlayson
Live Networks, Inc.
http://www.live555.com/
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