Re: [rtcweb] Proposal for dealing with CNAMEs and MSIDs for synchronization

[Harald Alvestrand <[e-mail]>]

On 02/09/2013 02:11 PM, Stefan Håkansson LK wrote:
> On 2013-02-08 22:37, Harald Alvestrand wrote:
>> On 02/07/2013 09:08 PM, Charles Eckel (eckelcu) wrote:
>>>> -----Original Message-----
>>>> From: [e-mail] [mailto:[e-mail]] On
>>>> Behalf Of Eric Rescorla
>>>> Sent: Thursday, February 07, 2013 3:03 PM
>>>> To: Stefan Hakansson LK
>>>> Cc: [e-mail]
>>>> Subject: Re: [rtcweb] Proposal for dealing with CNAMEs and MSIDs for
>>>> synchronization
>>>>
>>>>
>>>>
>>>> On Thu, Feb 7, 2013 at 11:59 AM, Stefan Hakansson LK
>>>> <[e-mail]> wrote:
>>>>
>>>>
>>>>     On 02/07/2013 08:43 PM, Eric Rescorla wrote:
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>             My question is basically: what if the sender creates two
>>>>             MediaStreams for which all tracks have local sources
>>>> (cam's,
>>>>             mike's), sends them to a peer, will the RTP streams for
>>>> both
>>>>             MediaStreams have the same or different CNAME?
>>>>
>>>>             I argued for that they should have the same.
>>>>
>>>>
>>>>         Yes, I think this is a separate (and fraught) question. :)
>>>>
>>>>
>>>>
>>>>     Separate question, but I think the answer should be documented
>>>> (regardless on if it is "same", "different" or "implementers choice").
>>>>
>>>>
>>>>
>>>> Agreed.
>>> I think SHOULD is the correct thing to document. I can imagine there
>>> will be cases in which the sender will be a somewhat distributed app
>>> that does not really know that the media source should be
>>> synchronized, so it will assign a different CNAME.
>>> The rest of the proposal sounded good as well.
>>>
>>>
>> SHOULD often makes me unhappy, because it can lead easily to something
>> being true so much of the time, people forget to check for whether it is
>> true or not.
>>
>> I think "all the streams have local sources" is a property that we
>> shouldn't require that one end of a PeerConnection reveals to the other
>> end.
>
> When you say "streams" above I guess you mean RTP streams (or 
> MediaStreamTracks) as it would not make sense to make this a secret 
> when they are transported in different MediaStreams but expose it if 
> they are in the same.
When they are in the same MediaStream, they are required to have the 
same CNAME (unless I lost track of the conversation), so it's only when 
they're in different MediaStreams that it matters whether the CNAME is 
the same or not.

>
> What info would reveal this ("have local sources")? Is it them having 
> the same CNAME?

Yes.