Subject: [MMUSIC] Answerers alternative selection of m lines(one IPv4 and the other IPv6)

From: "Hiroshi Waki" <[email-masked]>
Date: Fri, 15 Feb 2002 18:29:31 +0900

Hi all,

Suppose the offerer is dual stack and does not know which version of 
IP is supported by the answerer.  The answerer may support both or 
only one IP version, either v4 or v6.

In this case, the offerer may use the offer that includes two m lines 
(one IPv4 and the other IPv6) grouped using FID.  Both lines contains 
the same codec(s).  What the offerer expects is that the answerer 
selects only one of the two m lines, i.e. the answerer accepts 
one m line with an appropriate port, and explicitly rejects 
the other m line with port=0 in the corresponding m lines in the answer.

However, the answerer may accept both of them, and if so, it means that 
two RTP sessions at the same time, as the draft-ietf-mmusic-fid-05 says, 
'Remember that if several "m" lines grouped together using FID semantics 
contain the same codec the media agent MUST send media over several 
RTP sessions at the same time' in 7.4.1.

How can the offerer prevent it?
Do we need a new mechanism to show "alternative" m lines to let 
the answer know that it must select only one of them?

Regards,
Hiroshi Waki

NTT Network Service Systems Laboratories
tel +81 422 59 3404  fax +81 422 59 3494
e-mail  [email-masked]

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From: Jonathan Rosenberg <[email-masked]>
Date: Fri, 15 Feb 2002 10:17:45 -0500

> the answer know that it must select only one of them?

You can always send an updated offer with the v4 stream disabled. In
fact, you can avoid fid altogether, and send the offer with both v4 and
v6, and once you get the answer, send an updated offer with just one of
them immediately (if needed).

-Jonathan R.

-- 
Jonathan D. Rosenberg, Ph.D.            72 Eagle Rock Avenue
Chief Scientist                         First Floor
dynamicsoft                             East Hanover, NJ 07936
[email-masked]                 FAX: (973) 952-5050
http://www.jdrosen.net                  PH:  (973) 952-5000
http://www.dynamicsoft.com

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From: Takumi Ohba <[email-masked]>
Date: Sat, 16 Feb 2002 10:20:18 +0900

Hi,

> them immediately (if needed).

Establishing two RTP sessions is not the offerer's initial intention.
Sending an updated offer will cause increasing setup delay of the call that 
matches to the offerer's intention.
In addition, I'm not sure that all implementations of dual stack can handle 
both V4 and V6 at one time.

Is there a mechanism that has been already discussed in this WG and be able 
to show the offerer's intention for the answerer to select only one of multi
ple (or at least two) choices?

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From: Jonathan Rosenberg <[email-masked]>
Date: Fri, 15 Feb 2002 22:24:14 -0500

> matches to the offerer's intention.

No; the call can proceed while the second pass occurs. I agree its not
ideal, but a lot of problems can be solved by multi-pass offer-answers
if needed.

> both V4 and V6 at one time.

There is no need to actually support both at the same time, only two
advertise two of them, and then pick one once the answer comes.

> ple (or at least two) choices?

Not that I am aware of. Doesn't mean its not a good idea; we've
discussed this kind of thing in the context of nat, where a caller can
use stun to obtain a public address. THen, it sends its invite with both
the public and private addresses, and wants the callee to pick just one.

-Jonathan R.