Subject: [Sipping] RE: Meaning of status codes

From: "Peterson, Jon" <[email-masked]>
Date: Thu, 5 Dec 2002 17:41:04 -0500

Clipping avoidance is a separable issue from the matter at hand. What we're
discussing now is whether or not vanilla SIP UAs should use an SDP answer in
a 180 to establish an early media session that is disjointed from the media
of the call. If we can agree that this is bad, and that there are more
attractive alternatives, we will have made significant progress, I think, in
so far as we will have chosen a more narrow and practical scope for the
problem.
 
That much said, I think that sending an SDP answer in a 180 for continuous
media (media that will transition to the call requested by the initial
INVITE) is also problematic. I don't think the "just play it" philosophy is
actually a panacea for clipping. Paradoxically, the cases in which early
media is used in this fashion are actually the hardest to solve, I think,
not the easiest. There is a design trade-off built in to SIP that makes the
use of early media for this purpose inherently difficult for SIP (due to
forking and related issues about how status codes are used). This is a
subject for a separate thread, but suffice it to say that I'm not sure I
find the strategy you detail below practicable.
 
Jon Peterson
NeuStar, Inc.

-----Original Message-----
From: Francois Audet [mailto:[email-masked]]
Sent: Thursday, December 05, 2002 2:21 PM
To: 'Peterson, Jon'; 'Gonzalo Camarillo'
Cc: 'William Marshall'; [email-masked]';
[email-masked]'; [email-masked]'; 'Alan Johnston';
'sipping'; 'Adam Roach'
Subject: RE: Meaning of status codes

[snip]

While I personally don't really have a problem with this, it seems to me
that other people have (in particular for clipping avoidance).

Perhaps the way to deal with this is the following (from the receiver of the
INVITE's point-of-view):

*	

	It is OK to send an SDP in a 180 in order to avoid clipping (and
even perhaps recommended).
*	

	It is not a good idea to send in-band tones/announcements after
sending a 180/SDP (although striclty speaking, legal as the calling party
has no way of distinguishing this from clipping avoidance). Other mechanism
are preferable (as described in Jon's email above): 183/SDP (with or without
a 180 sent later), the URL method, text, etc.

From the originator of the INVITE's point of view:

*	

	SDP in 180/183 means listen and play if/when media is available
(overriding any optional locally-generated tone).
*	

	180 means "far-end user is being alerted", and CAN result in a local
indication (GUI display, locally-generated tone, etc.).

This seems fairly simple and implemenatable.

---

From: Rohan Mahy <[email-masked]>
Date: Fri, 6 Dec 2002 14:51:09 -0800

On Thursday, December 5, 2002, at 02:41 PM, Peterson, Jon wrote:

> Clipping avoidance is a separable issue from the matter at hand. What 
> we're discussing now is whether or not vanilla SIP UAs should use an 
> SDP answer in a 180 to establish an early media session that is 
> disjointed from the media of the call. If we can agree that this is 
> bad, and that there are more attractive alternatives, we will have 
> made significant progress, I think, in so far as we will have chosen a 
> more narrow and practical scope for the problem.
>  
> That much said, I think that sending an SDP answer in a 180 for 
> continuous media (media that will transition to the call requested by 
> the initial INVITE) is also problematic.

Why is this problematic?  What breaks in the "just play it" model that 
is any harder to deal with than receiving the same answer in a 183?

If I get a 180 with an answer, I will probably play local ringback 
until I receive media from that answer.

thanks,
-rohan

> I don't think the "just play it" philosophy is actually a panacea for 
> clipping. Paradoxically, the cases in which early media is used in 
> this fashion are actually the hardest to solve, I think, not the 
> easiest. There is a design trade-off built in to SIP that makes the 
> use of early media for this purpose inherently difficult for SIP (due 
> to forking and related issues about how status codes are used). This 
> is a subject for a separate thread, but suffice it to say that I'm not 
> sure I find the strategy you detail below practicable.
>  
> Jon Peterson
> NeuStar, Inc.
>
> [email-masked]'; [email-masked]'; 'Alan 
> Johnston'; 'sipping'; 'Adam Roach'
> While I personally don't really have a problem with this, it seems to 
> me that other people have (in particular for clipping avoidance).
>
> Perhaps the way to deal with this is the following (from the receiver 
> of the INVITE's point-of-view):
>
> • 	It is OK to send an SDP in a 180 in order to avoid clipping (and 
> • 	It is not a good idea to send in-band tones/announcements after 
> sending a 180/SDP (although striclty speaking, legal as the calling 
> party has no way of distinguishing this from clipping avoidance). 
> Other mechanism are preferable (as described in Jon's email above): 
> 183/SDP (with or without a 180 sent later), the URL method, text, etc.
>
> • 	SDP in 180/183 means listen and play if/when media is available 
> • 	180 means "far-end user is being alerted", and CAN result in a 
> local indication (GUI display, locally-generated tone, etc.).
>