Re: [rtcweb] Offer/answer for heterogeneous encode/decode

Derek Pang <dcpang@highfive.com> Tue, 26 November 2013 22:07 UTC

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From: Derek Pang <dcpang@highfive.com>
Date: Tue, 26 Nov 2013 14:07:18 -0800
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To: Harald Alvestrand <harald@alvestrand.no>
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Subject: Re: [rtcweb] Offer/answer for heterogeneous encode/decode
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I am new to rtcweb community. Was there any proposal on making both Vp8 and
H264 decoders MTI in webrtc, but VP8 or H264 encoder to be optional ? Does
this offer a better option for the marketplace, while allowing
interoperability.


On Sun, Nov 24, 2013 at 8:38 AM, Harald Alvestrand <harald@alvestrand.no>wrote;wrote:

> On 11/23/2013 01:20 AM, Paul Giralt (pgiralt) wrote:
>
>> On Nov 22, 2013, at 7:06 PM, Martin Thomson <martin.thomson@gmail.com>
>> wrote:
>>
>>  On 22 November 2013 15:44, Paul Giralt (pgiralt) <pgiralt@cisco.com>
>>> wrote:
>>>
>>>> While you’re right that it would work for this scenario, my point was
>>>> really
>>>> that Offer/Answer is not really asymmetric as implied earlier. Take for
>>>> example the hypothetical case where you are only able to decode VP8 but
>>>> only
>>>> able to encode H.264. If I offer VP8 as my only codec (because it’s the
>>>> only
>>>> thing I’m able to decode therefore I never want anyone to send me
>>>> anything
>>>> other than VP8) I cannot send H.264 in the offer because that implies
>>>> I’m
>>>> able to decode it. The other side then wants to say it can only receive
>>>> H.264 so it would have to send back an answer with only H.264. I guess
>>>> there’s nothing really inherently stopping you from doing this because
>>>> as
>>>> far as I can tell, 3264 only says the answer has to be a subset of the
>>>> offer
>>>> for multicast streams, however how would the answering side know that
>>>> the
>>>> offering side is even capable to receiving H.264? Perhaps Offer/Answer
>>>> can
>>>> technically be asymmetric, but it doesn’t seem practical to use it this
>>>> way
>>>> because you cannot really indicate your send and receive capabilities
>>>> independent of each other.
>>>>
>>> Judicious application of a=sendonly or a=recvonly avoids this issue.
>>> If you want to send H.264, try a=sendonly on a line with H.264.  If
>>> you want to receive VP8, try a=recvonly on a line with VP8.
>>>
>> I gave this as an example of a possible way to do it, but that means you
>> need two separate m= lines - one for send and one for receive. Seems
>> strange to do this for something that you really want to be a single
>> bi-directional stream.
>>
>
> An application that can't talk to itself is kind of bizarre too.
>
> I think this is a corner case.
>
>
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