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upon viewing the webrtc RTX rfc

The payload type is dynamic. If multiple payload types using retransmission are present in the original stream, then for each of these, a dynamic payload type MUST be mapped to the retransmission payload format. See Section 8.1 for the specification of how the mapping between original and retransmission payload types is done with Session Description Protocol (SDP).

but why? if retransmission use a different ssrc, using same payload type as media payload type would not affect the recovering process,right?

Just want to know the design reason behind this approch!

Leo9988
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While you could theoretically use the same payload type and demultiplex by SSRC instead of payload type, SDP can not properly express this as you would end up with two rtpmap lines with the same paylaod type but different codec names.

(one of the downsides of that requirement is that you need one RTX payload type per media type instead of just a single RTX payload type)

Philipp Hancke
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  • I would argure add a SSRC line that dedicate for RTX into sdp would not require a extra RTX payload type mapping. I just feel like a direct ssrc mapping is much simpler and easier to understand than multiple payload type mapppings. Or is there some other considerations come to play? – Leo9988 Jul 28 '22 at 06:32