Subject: Re: [sipcore] About Join and Replaces Headers: //Replaces header's extension

From: [email-masked]
Date: Fri, 8 Oct 2010 00:24:36 +0800

One spelling mistake and it's correction, inlines(in blue color).

Note: Here I talk a lot about RFC3911's potential clarification points. 
IMO it is more serious problem (than the Replaces header's extension). I 
request you evaluate it.

Gao

> If user Y (using UA Z) sends an INVITE-with-Replaces to X, then the 
> original dialog (between UAs X and Y) *must* be terminated by X when
> it accepts the INVITE-with-Replaces.

I knew it. But my proposal here is to do extension for the Replaces 
header. And you may refer to the first mail of this thread for details. 
 
> What UA Z should do is send an INVITE-with-Join to X 

I MUST point out that Join header's semantics is not proper for this use 
case.

> (offering only video), 

By my experience, INVITE-with-Join(only downlink video) *could* be treated 
as a video duplication request. But how about INVITE-with-Join(only 
bidirectional video)? 
In fact, no like audio stream, there's no physical meaning for video 
streams' mixing. So, if UA Z send an INVITE-with-Join, with only 
bidirectional video, UA X may not know how to handle it.(So, I feel that 
RFC3911 need clarification!)

> and then UA Y should do a re-INVITE to remove its video 
> stream from its dialog with X (leaving the audio stream). 

Let's analyse what would happen after UA Y sending a re-INVITE to remove 
it's video stream from it's dialog. As the use case of 
"INVITE-with-Join(only bidirectional video)" is meaningless, as I showed 
in the previous section, we may analyse a similar one: 
"INVITE-with-Join(only downlink video)" .

Not like audio mixing, the session can be downgraded from three-party to 
two-party, when one leave the session. If the vedio between X and Y is 
over, I think the duplicated one between X and Z would lose it's vedio 
source.

Gao

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