Subject: RE: [SIP] Mix or replace

From: Jonathan Rosenberg <[email-masked]>
Date: Tue, 16 Jan 2001 00:56:45 -0500

> -----Original Message-----
> From: Christer Holmberg [mailto:[email-masked]]
> Sent: Tuesday, January 09, 2001 3:46 PM
> To: [email-masked]
> Subject: [SIP] Mix or replace
> 
> 
> 
> Hi,
> 
> This issue was mentioned in, and is part of, the early media 
> replacement
> thread, but I would like to ask about here.
> 
> My question is: If we receive multiple 183 responses, with SDP message
> bodies, from different locations, should we:
> 
> 1. Mix the media streams
> 
> ...or...
> 
> 2. Replace the media stream every time we get a new 183 response.
> 
> My assumption was option 2, but since Jonathan mentioned the audio
> mixing in a previous mail I would like to raise the question.
> 
> The problem is that there be cases where one option is to be 
> used, while
> in other cases the option is to be used...

I don't think this needs to be specified. If I'm in multiple calls, the spec
does not say whether I need to play just one of the media streams at a time,
or mix them and play both. Its a local decision based on the end device.

In the case of early media, I can imagine a smart phone that creates new
call apperances as it gets multiple 183s (each will have different tags),
allowing me to select which appearance I listen to (note that there is a
problem in figuring out which media stream is associated with each call
appearance, unless we have some kind of rtcp SDES item that conveys the SIP
URL, or define an sdp attribute for ssrc or cname). Another phone might
always play just one and ignore all the others. Yet another might mix them,
although I suspect that is not what you will want in most cases.

-Jonathan R.

---
Jonathan D. Rosenberg                       72 Eagle Rock Ave.
Chief Scientist                             First Floor
dynamicsoft                                 East Hanover, NJ 07936
[email-masked]                     FAX:   (973) 952-5050
http://www.cs.columbia.edu/~jdrosen         PHONE: (973) 952-5000
http://www.dynamicsoft.com

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From: Christer Holmberg <[email-masked]>
Date: Tue, 16 Jan 2001 13:31:18 +0200

Hi,

>>used, while in other cases the option is to be used...

Well, this may work if your end device is a SIP phone, and you for
example can press some button to switch between different media streams
- or choose to listen to them all. However, that may not be possible if
you have a PSTN GW. In that case the end device is a PSTN phone, and the
user will probably have no control over the GW.

Also, I am not sure my 90 year old grandmother, or my 5 year old
brother, want to start do manual media stream switching when she/he
makes a phone call...
 
>allowing me to select which appearance I listen to 

That is never going to work. Let's assume I am listening to the first
media stream, and then I get some kind of indication that 2 other media
streams are coming. I will have to listen to both of them to figure out
what they are about in the first place, and the more streams there are
the more of each message I will never be able to hear in the first
place...

>(note that there is a problem in figuring out which media stream is associated with each call
>appearance,

Will we even be able to pick out media streams if they all go via a RTP
proxy/firewall/NAT?

>unless we have some kind of rtcp SDES item that conveys the SIP
>URL, or define an sdp attribute for ssrc or cname). 

Well, that is not a bad idea at all.

>Another phone might always play just one and ignore all the others.

True, but then we have the race condition when I may hear different
things every time I call the same number, since it will depend on which
183 message reaches me first.

>Yet another might mix them, although I suspect that is not what you will want in most cases.

I don't even think that it should be a requirement for a UA to be able
to mix media streams, since that would almost require one to implement a
simple conference bridge in the UA...

Here is an idea about having some kind of media stream indicator, BEFORE
the media itself is sent.

When an UAS receives an INVITE, and wants to send early media, he could
send some kind of REQUEST (Note: NOT provisional response!) to indicate
that. Then, if the UAC wants it, he sends back 200 OK. After that, the
UAS sends 183 and media. If the UAC sends non-200 (maybe followed by a
BYE for that specific leg if wanted), the UAS will not send media (the
UAS can also choose to terminate the call itself, even if the UAC didn't
send a BYE). No, this will not solve all problems, but it would give
more control over the media, and when it's sent.

There are probably also better ways to do this than my example, but my
basic point, and the magic word, is MEDIA CONTROL :)

Regards,

Christer Holmberg
Ericsson Finland