Subject: [MMUSIC] Question about bundling SDP m=lines

From: [email-masked]
Date: Tue, 03 Oct 2006 19:46:12 +0900

Folks,

I hope I would like to ask an question on SDP offer/answer model.

Imagine the case where the offerer initiates a video call.
Is there any parameter which indicates offerer doesn't want
answerer to answer an SDP with video medium only,
without an audio medium?

An SDP offer for video call includes two m=lines, m=audio and m=video. 
According to RFC4566/3264, the answerer may read
each m=line independently, and may return a SDP answer
with port 0 in either of the two m=lines.
Session may be established with video only, without audio,
which is not the caller's intention.

  offerer                             answerer
     |                                  |
     | INVITE (m=video & m=audio)       |
     |--------------------------------->|
     | 200 OK (m=video & m=audio port 0)|
     |<---------------------------------|
     |     RTP session (video only)     |
     |<================================>|
     |                                  |

To avoid this, offerer have to tell the answerer that
m=audio and m=video are bundled and that it doesn't
want to establish with either of the media.

I hope I would like to ask you how to express the offerer's intention.

I guess this should be achived by m=line group attribute
defined by RFC3388, because it says in Chapter 1 that "expression of 
how different media streams within a session description relate to 
each other." I guess that this would make it possible to express
that m=audio and m=video are bundled.

What the "semantics" field should be set, 
if I use a group attribute in this way? 
Four values, "LS", "FID", "SRF" and "ANAT" are registered on IANA,
but none of them seems to be intended to influence UA's offer/answer.
Is a new semantics parameter needed to meed the requirement?

Please let me know your advice.

Does this topic match to another mailing list more?
I apologize if this subject is off-topic for this mailing list.

Thank you,
Jun Koshiko

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From: Gonzalo Camarillo <[email-masked]>
Date: Mon, 30 Oct 2006 08:40:28 +0200

Hi,

yes, you could use the grouping framework to group a few streams 
together so that they should be accepted or rejected as a group. As you 
point out, none of the current 'semantics' does this. Therefore, you 
would need to define new 'semantics' for the framework.

Cheers,

Gonzalo

---

From: Paul Kyzivat <[email-masked]>
Date: Mon, 30 Oct 2006 09:43:03 -0500

This seems to be one more example of a whole family of cases where SDP 
and offer/answer aren't sufficient to state the criteria desired for a 
call. Other examples are:
- several codecs can be supported but only one at a time, so any
   answer listing more than one codec is unacceptable. (Unless of
   course there are two and one of them is telephone-events.)

- I'd like an audio call or a fax call, but not both

One solution to this is to come up with a new notation to express each 
new kind of constraint, and add it to SDP.

Another possible solution might be to use an attribute to characterize 
media streams as "required" or "optional", with the expectation that a 
call should not be completed unless the "required" streams can be 
established.

Yet another possible solution is to use preconditions. I haven't thought 
out the details, but it seems it would be possible to define a 
precondition type related to media sufficiency. This would delay 
alerting while the offerer gets to see the initial answer. At that point 
the offerer can call things off if the answer isn't sufficient. This 
would avoid the need to express all possible types of constraints, 
reducing it to a case of "I'll know if its good when I see it".

Unfortunately, it seems that use of preconditions has not caught on. So 
this kind of solution isn't likely to be very popular.

	Paul

Gonzalo Camarillo wrote:
>> What the "semantics" field should be set, if I use a group attribute 
>> in this way? Four values, "LS", "FID", "SRF" and "ANAT" are registered 
>> on IANA,

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From: Jun KOSHIKO <[email-masked]>
Date: Tue, 31 Oct 2006 17:03:57 +0900

Hi,

Thank you for the valuable comments.

I understand that your comments are the followings.

- Protocol Extension is needed to match the requirement
- There are three possible solutions as followings
  1. Grouping m=lines of SDP (+ new "semantics")
  2. New attributes (required/optional) in media description
  3. Use precondition (+ new "precondition-type")

Is my understanding correct?

I hope I would like to compare and consider the advantages
and disadvantages of the three solutions.

Thank you,
Jun Koshiko

> >>      | INVITE (m=video & m=audio)       |

---------------------------------------------------------
Jun KOSHIKO
Next Generation Strategy
Research and Development Center
NTT-east Corporation

Telephone  +81 3 5359 5104
Facsimile  +81 3 5359 1236
E-mail: [email-masked]
19-2 Nishi-shinjuku 3-chome Shinjuku-ku Tokyo 163-8019 Japan
---------------------------------------------------------

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From: "Schmidt, Christian" <[email-masked]>
Date: Tue, 31 Oct 2006 09:36:02 +0100

We are working on a new draft about dependencies between media streams (see attachment).
We propose to use an additional SDP attribute, called dependency.
With this new attribute, you could define mandatory and optional dependencies between media streams.
The draft could be extended to cover other types of dependencies between media streams as well.

Could this new dependency attribute solve your problem? Comments are welcome.

Christian Schmidt

-----Ursprüngliche Nachricht-----
Von: Paul Kyzivat [mailto:[email-masked]] 
Gesendet: Montag, 30. Oktober 2006 15:43
An: Gonzalo Camarillo
Cc: [email-masked]
Betreff: Re: [MMUSIC] Question about bundling SDP m=lines

	Paul