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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