Subject: [sipping] Removing media stream from all legs and from own leg in tightly coupled SIP conferencing

From: "Sedlacek, Ivo" <[email-masked]>
Date: Wed, 30 Aug 2006 12:22:07 +0200

Hello,
 
RFC4353 defines the tightly coupled SIP conferencing.
 
One of the operations defined there is 5.6. Adding and Removing Media.
 
It says:
 
------------------------------------------------------------------------
------------------------
5.6.  Adding and Removing Media
 
   Each conference is composed of a particular set of media that the
   focus is managing.  For example, a conference might contain a video
   stream and an audio stream.  The set of media streams that constitute
   the conference can be changed by participants.  When the set of media
   in the conference change, the focus will need to generate a re-INVITE
   to each participant in order to add or remove the media stream to
   each participant.  When a media stream is being added, a participant
   can reject the offered media stream, in which case it will not
   receive or contribute to that stream.  Rejection of a stream by a
   participant does not imply that the stream is no longer part of the
   conference, only that the participant is not involved in it.
 
   A SIP re-INVITE can be used by a participant to add or remove a media
   stream.  This is accomplished using the standard offer/answer
   techniques for adding media streams to a session [13].  This will
   trigger the focus to generate its own re-INVITEs.
------------------------------------------------------------------------
------------------------
 
Given the description above, it is possible that a participant decides
to use only subset of the offered media streams when the re-INVITE is
delivered to the participant.
 
Since the participant may decide to use a subset of the offered media
streams when re-INVITE is delivered to the participant, the participant
should also have the same option when the participant initiates
re-INVITE.
 
Thus when removing the media stream in re-INVITE, the participant has
two possibilities how to remove media stream:
1) either participant wants to remove the media stream from own leg
(between participant and focus)
2) or participant wants to remove the media stream from all the legs
(completely from the conference)
 
How should these two possibilities be distinguished in SIP or SDP?
 
Kind regards
 
Ivo Sedlacek
Siemens
ANF DATA MCS CD3
phone: +420 54310 6532
mailto: [email-masked]
 
Mailcode: C0gmHhFp 

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---

From: Paul Kyzivat <[email-masked]>
Date: Wed, 30 Aug 2006 08:04:56 -0400

Ivo,

The focus controls the media it offers to each participant. Using sip 
signaling a participant can only control the media it uses. Should it so 
choose, the focus could follow the lead of "special" participant about 
what to offer to the others. But that owuld be a policy of the focus. 
Otherwise you would need to use some other mechanism to control the 
behavior of the focus.

	Paul

Sedlacek, Ivo wrote:
> ------------------------------------------------------------------------------------------------
> ------------------------------------------------------------------------------------------------
>  
> should also have the same option when the participant initiates re-INVITE.
>  
> mailto: [email-masked] <mailto:[email-masked]>
>  
> Mailcode: C0gmHhFp
>  
>  
> 
>

---

From: Paul Kyzivat <[email-masked]>
Date: Wed, 30 Aug 2006 11:15:59 -0400

Sedlacek, Ivo wrote:
> Hello Paul,
> thanks for your mail.
>  > signaling a participant can only control the media it uses.
> 
> Yes. The participant directly works only with the media streams it uses. 
> But it can also cause focus to do some actions
> 
> In RFC4353 there is stated:

I think you read too much into what 4353 is saying.

Yes it is true that a participant in a conference could send a reinvite 
to offer a stream it is not currently using, and that other participants 
also are not adding. The focus could accept that or reject it. If it 
accepts it, it could offer the same to other participants and bridge the 
new media - if it wants. There is nothing to force it to do so, and no 
guarantee that it is capable of doing so.

Similarly, if all participants are using two media streams, say voice 
and video, and one participant removes the video, the focus *could* 
remove it from all participants. But that might be quite impolite to the 
other participants who might want to continue using it. It *could* 
follow the lead of a "special" participant in this regard. For instance 
if there is a moderator it might not allow any stream unless the 
moderator accepts that stream.

But in general what can be done in this way is limited, because the 
offer/answer protocol doesn't provide a suitable way to control all the 
things that a focus might need to do.

I suggest you take your question to the xcon wg. I have stopped 
following their work, but I expect they will have a solution for what 
you want to do.

	Thanks,
	Paul

>    ...
>    The set of media streams that constitute
>    each participant.
>    ...
> So there is already a method how a participant offers new media stream 
> to other participants through usage of the focus.
> 
> For media stream removal, this RFC4353 is not so clear - it is stated 
> that rejection of a media stream by a participant is valid only for the 
> participant. Other participants can continue using it.
> 
>    ...
>    Rejection of a stream by a
>  > Should it so
> Well, such policy is certainly possible.
> 
> Unfortunately, this is quite restrictive. I believe it would work only 
> in case where there is only one "special participant" and this one uses 
> all the media streams used in the conference.
> 
> Let's consider the following use case:
> - a conference is set up with 3 participants A, B, C. After set up A 
> uses media streams X, Y, Z; B uses media streams X and Y; C uses media 
> streams X and Z. B is the "special participant".
> - now B sends re-INVITE and wants to add media stream W (otherwise the 
> conference should stay as it is). So B includes in SDP offer X with 
> port!=0; Y with port!=0, Z with port=0 and W with port!=0.
> - since the focus follows the lead of B, the focus not only offers the 
> media streams X, Y, W to A and C, but it also offers removal of media 
> stream Z from A and C (which was not the intention of B).
> 
> Another use case is where there are multiple "special participants", 
> which use different (partly overlapping) sets of media streams. Which of 
> them will the focus follow?
> 
> There may be situations when the participant is allow to do both 
> following actions:
> 1. remove media stream from own leg
> 2. remove media stream from all the legs
> 
> 1. can be achieved by rejecting the media stream as specified in RFC4353.
> Is there any IETF RFC or draft which would allow participant to do 2. 
> (in situations when the participant is allowed to do both 1. or 2.)?
> 
> Kind regards
> 
> Ivo Sedlacek
> Siemens
> ANF DATA MCS CD3
> phone: +420 54310 6532

---

From: "Sedlacek, Ivo" <[email-masked]>
Date: Wed, 30 Aug 2006 16:51:23 +0200

Hello Paul,

thanks for your mail.

1. can be achieved by rejecting the media stream as specified in
RFC4353.
Is there any IETF RFC or draft which would allow participant to do 2.
(in situations when the participant is allowed to do both 1. or 2.)?

Kind regards

Ivo Sedlacek
Siemens
ANF DATA MCS CD3
phone: +420 54310 6532

Mailcode: C0gmHhFp

---

From: "Even, Roni" <[email-masked]>
Date: Thu, 31 Aug 2006 08:45:54 +0300

Hi,

RFC 4353 introduction has the following text

"This framework presents a general architectural model for
   these conferences and presents terminology used to discuss such
   conferences.  It also discusses the ways in which SIP itself is
   involved in conferencing.  The aim of the framework is to meet the
   general requirements for conferencing that are outlined in [3].  This
   specification alludes to non-SIP-specific mechanisms for achieving
   several conferencing functions.  Those mechanisms are outside the
   scope of this specification."
 
The questions you are raising have to do with conference functions that
are discussed in XCON working group and are addressed in the drafts
prepared there.
 
Having said that I would agree with Paul that you can define a specific
policy to your conference server that will define its behavior based on
users input including having only the common media, adding media if one
of the participants adds it. Having more complicated policies will
require some sort of protocol and authorization to make changes on the
conference media which again in the scope of XCON group.
 
Thanks
Roni Even 

________________________________

From: Sedlacek, Ivo [mailto:[email-masked]] 
Sent: Wednesday, August 30, 2006 5:51 PM
To: Paul Kyzivat
Cc: [email-masked]
Subject: RE: [sipping] Removing media stream from all legs and from own
legin tightly coupled SIP conferencing

Hello Paul,

thanks for your mail.

Kind regards

Ivo Sedlacek
Siemens
ANF DATA MCS CD3
phone: +420 54310 6532

Mailcode: C0gmHhFp