Re: Issue 444: RTX APT capability?

On 08/04/2016 1:27, Robin Raymond wrote:
>
>> 2.In WebRTC 1.0, do you expect there to be multiple “rtx” entries within parameters.codecs[], one for each codec that is being retransmitted?  For example, codecs[i].sdpFmtpLine = “a=fmtp:99 apt=98;rtx-time=3000” for one rtx line and codecs[j].sdpFmtpLine = “a=fmtp:100 apt=101;rtx-time=3000” for another one.
>>
>> a. Would you support an WebRTC 1.0 PR that would remove rtx entries 
>> in codecs[] and add unsigned short rtxPayloadType to the 
>> RTCRtpCodecParameters dictionary?
>>
> [RR] I would hands down fully support this change as a setting within 
> the existing codec parameter. I think it removes the ambiguity and 
> needless cross referencing across codecs. It’s just much easier this way.
>

+1 but take into account that rtx has also the rtxtime parameter so we 
would need a rtxPayloadType  and a rtxTime parameters, I think it is 
better to move it to it's own dictionary:


|//New dictionary dictionary RTCRtpCodecRTXParameters { payloadtype 
payloadType; unsigned long rtxtime; }; dictionary RTCRtpCodecParameters 
{ DOMString name; payloadtype payloadType; ... RTCRtpCodecRTXParameters 
rtx; // NEW: rtx.payloadType };|


Take also into account that if we remove the RTX from the codecs, we 
would need to add an rtxSSRC whenever there is an ssrc attribute. 
Currently it is set only in the encodings, that would mean that include 
a new case of invalida parameters: all encodings with same ssrc MUST 
HAVE the same rtxSSRC.

>> Personally, I believe we either need to do this uniformly in both 
>> ORTC and WebRTC 1.0, or not do it.
>>
>> 3. Do you advocate keeping FEC and RED as codecs with 
>> RTCRtpCapabilities and RTCRtpCodecParameters?
>>
>
> [RR] I see those exactly like RTX codec. They are settings within the 
> same codec. The only difference RTX requires a unique PT per codec, 
> whereas the same PT value for FEC, RED could be shared across codecs 
> (respectively).
>
>

That is the main difference between RTX and FEC,RED.
TX relates 1-to-1 to a Codec payload while FEC/RED has a 1-to-1 
relationship with a RTP stream. As we don't have any object that 
represents an RTP Stream (same ssrc can be set on different encodings), 
there is not a single place where we can set the FEC/RED parameters and 
would need to be set in all encodings with same ssrc (same as the 
rtxSSRC above).

Best regards
Sergio