[Mp4-tech] reconstructing a 3GP file from an MP4 stream

[Slavko Glamocanin]

Hi all,
I'm trying to re-encapsulate a 3GP file, the contents of which (an mp4
video stream) have been streamed over RTP.
I get most of the information needed for the 3GP encapsulation from the
mp4 stream (how many samples, their sizes, ... ), but the problem is i
cannot find the information how the SAMPLES are grouped into CHUNKS.
Currently I'm grouping by I-frames (I-frame is a start/end of a chunk),
which works fine for a couple of seconds but not always.
I also found that i cannot detect keyframes in the stream and i have
strange visual artefacts when they occur - i set the keyframe time to
about 3 seconds. I couldnt find anything about keyframes in the
documentation.
So to summarize:
 - How are the samples grouped into chunks?
 - Is the keyframe connected to stream corruption? (how can i handle
it?)
Thank you for your help!
Cheers,
Slavko Glamoèanin
P.S.
I've attached an example of a reconstructed 3GP file (streamed from a
live feed). 
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