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

[Mihai Balea]

On Oct 30, 2007, at 11:26 AM, Kevin P. Fleming wrote:

> Not really... sequence numbers (especially in the video media world)
> aren't a reliable way to order packets and check for missing packets
> anyway, as sequence numbers can be 'consumed' by non-media frames in
> some situations, and IIRC there are also video formats where multiple
> frames can be sent with the same sequence number.
>
> Using the timestamps and duration of the frames allows the receiver to
> determine if any time period is missing media.

You can also have video frames with the same timestamp.  Or better  
said, video frame fragments. If a compressed video frame is larger  
than a reasonable size (for example the standard MTU), it is usually  
split into several frame fragments that are sent via separate IAX  
packets.  These fragments should have the same timestamp, since they  
all belong to the same video frame and they should be ordered by  
their sequence number.  Unfortunately, as others mentioned before,  
IAX vide frames do not carry sequence numbers, so it becomes the job  
of the codec implementation to worry about fragment reordering.  H. 
264 has built in support for that.  For codecs like Theora, we add a  
custom header on top of the fragment.

Mihai

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