Subject: [Sip] Comments on "unify" draft - Part II

From: Christer Holmberg <[email-masked]>
Date: Mon, 21 Jan 2002 08:51:44 +0200

Hi,

Here are some comments on unify. To make all issue easier to follow, I
will continue the numbering from where Rohan ended.

Chapter 5.1 describes how the One-of-N codec negotiation takes place.

My questions related to this:

7) If the same method, using two INVITEs, is used also for re-INVITEs in
the middle of the call (I assume it must be used also if the re-INVITE
contains a set of codecs and we need to pick one), how does it affect
the rule that "there must be no media cut while the codec
re-negoatiation takes place"? We do set the mode to "inactive" in the
first INVITE, so it would affect the media...

Chapter 5.4 describes how the early media takes place.

My question(s) related to this:

8) How does the UAC know there will be early media, so it sets the mode
to "sendrecv", when 5.1 says it shall set the mode to "inactive" for the
first INVITE?

9) Since 5.1 describes how the media negoatiation is done using two
INVITEs, how does the UAS suddenly know which codec to use (for the
early media) after receving only one INVITE? I think the One-of-N codec
problem affects also the early media, doesn't it?

10) IF the COMET (Rohan had some issues about that) is used to refuse
early media, why can't it also be used to accept early media - and
include the "final" codec for the early media?

Regards,

Christer Holmberg
Ericsson Finland

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From: Jonathan Rosenberg <[email-masked]>
Date: Thu, 24 Jan 2002 23:05:15 -0500

> first INVITE, so it would affect the media...

Good question. You wouldn't use a=inactive here. You've already done an
exchange, so presumably you know the codecs supported by the other side
(because they included a bunch in the prior m line, or because of
simcap, or because you sent OPTIONS in the dialog), so you would do the
re-invite with the specific codecs you wanted.

> to "sendrecv", when 5.1 says it shall set the mode to "inactive" for the

It can do either; there is no requirement. Setting it to senrecv means
that it might get early media. It needs to be prepared to receive media
upon sending INVITE anyway, so there is no real change here.

> problem affects also the early media, doesn't it?

Not really. The UAS would send its list in the answer in a 183 (as
opposed toa  200 OK for regular media), and then the UAC would update
its offer in a COMET. Same SDP exchange as regular media, just a
different set of sip messages. Thats the idea here, session exchanges
through SDP are decoupled from dialog state.

> include the "final" codec for the early media?

I am trying to define one general mechanism that solves all the
problems. COMET isn't used to refuse or accept early media, its used to
update session state, and that can be used to do anything.

-Jonathan R.

-- 
Jonathan D. Rosenberg, Ph.D.            72 Eagle Rock Avenue
Chief Scientist                         First Floor
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From: Christer Holmberg <[email-masked]>
Date: Fri, 25 Jan 2002 08:07:25 +0200

Hi,

>re-invite with the specific codecs you wanted.

Would it be worth putting a few words in the text about this, saying
that a client SHOULD use one of the codecs it received by the other side
when the call was established, because then only a single re-INVITE is
needed? And, IF one wants to include a NEW set of codecs in the
re-INVITE, one should be aware that TWO re-INVITEs are needed, since a
re-negoatiation must be done.

Regards,

Christer Holmberg
Ericsson Finland

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From: Jonathan Rosenberg <[email-masked]>
Date: Fri, 25 Jan 2002 01:25:35 -0500

> re-negoatiation must be done.

I think it is premature to discuss adding wording to this document to
clarify things. This document is not meant to proceed as a work item.
Rather, it is meant to solicit opinions on whether the proposed
direction is correct. Much needs to be done to distill concise semantics
from the proposal.

-Jonathan R.