[asterisk-dev] why no sequence numbers with video frames over IAX ?

[Mihai Balea]

On Oct 30, 2007, at 11:52 AM, Luigi Rizzo wrote:


> I am a bit unclear on the following case:
>
> i encode a video frame (say an I-frame) and the result is a large
> chunk of data that needs to be transmitted over multiple UDP packets
> (containing in turn RTP or IAX frames).
> Now, for this video frame i have only one timestamp (in IAX they have
> 1ms resolution), and maybe 50+ packets.
> The timestamp for the next video frame is say 30 ms later.
>
> The only thing i can do here is send the individual IAX packets with
> the same timestamp, because i don't have enough different values  
> before
> the next video frame.
> But then how can i detect missing or reordered fragments ?
This is a limitation of the IAX protocol.  There was some discussion  
about improving this aspect, and I even proposed a new frame format,  
but I guess until somebody comes up with the code, nothing will be done.

> Surely if the fragmentation is done correctly i can leave it to
> the decoding code, which has more info (in the bitstream).
>
> Perhaps i should just forget about doing those checks in my code
> and leave it to the decoder.

If you're using H.264 you can leave it to the codec.  Other codecs,  
such as Theora need some extra information attached to the the  
fragments.

Mihai



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