Subject: [Sipping] Comments on draft-stucker-sipping-early-media-coping-03.txt

From: Gonzalo Camarillo <[email-masked]>
Date: Tue, 31 Oct 2006 13:31:13 +0200

Hi,

a few comments on draft-stucker-sipping-early-media-coping-03.txt

Regarding the usefulness of such a mechanism, it will depend on how many
legacy systems we have out there. In an environment where almost all UAs
do not understand the new mechanisms, there will be no obvious gains.
Also, the mechanism would not be very useful in environments with many
gateways that marked everything as "unknown".

Regarding the early media classification proposed by the draft, what is
the incentive for UAs not to cheat? That is, I will send advertisements
labeled as "Critical" announcements. This happens continuously with high
priority emails, for instance.

Regarding the mechanism itself, the proposed SDP parameter bundles
together two things that are actually orthogonal:

1) the direction attribute for early media exchanges
2) the correlation between the description of a stream in an SDP offer
or answer with its associated media packets

To do 1), I would suggest to use the same tokens as we typically use for
direction attributes. That is, we would use "inactive" instead of "none".

Being able to do 2) could be useful for regular media as well.
Therefore, it should be done in a different SDP parameter.

As a result, we would have:

m=audio 49920 RTP/AVP 0
a=em: sendrecv
a=ssrc: 23a73c01

Cheers,

Gonzalo

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---

From: "Brian Stucker" <[email-masked]>
Date: Tue, 31 Oct 2006 08:05:47 -0600

Thanks for the comments Gonzalo,

Responses below.

Regards,
Brian

> -----Original Message-----
> From: Gonzalo Camarillo [mailto:[email-masked]] 
> Sent: Tuesday, October 31, 2006 5:31 AM
> To: Stucker, Brian (RICH1:AR00)
> Cc: sipping
> Subject: Comments on draft-stucker-sipping-early-media-coping-03.txt
> 
> Hi,
> 
> Regarding the usefulness of such a mechanism, it will depend 
> on how many legacy systems we have out there. In an 
> environment where almost all UAs do not understand the new 
> mechanisms, there will be no obvious gains.

This is true of many new protocol mechanisms. If you can't get the
legacy boxes to signal anything, there's nothing you can do about it.
Gains may be slow initially until these boxes can be upgraded.

> Also, the mechanism would not be very useful in environments 
> with many gateways that marked everything as "unknown".

Which is essentially what we have today, hence the draft to try to fix
that.

> 
> Regarding the early media classification proposed by the 
> draft, what is the incentive for UAs not to cheat? That is, I 
> will send advertisements labeled as "Critical" announcements. 
> This happens continuously with high priority emails, for instance.

I think that with the way SIP works, the UA can always cheat to some
extent. They could simply send back a 200 OK to the INVITE w/ SDP to
start up early media and then use re-INVITE or UPDATE to make any
further SDP changes necessary for final media. 

I don't see how cheating makes the situation worse than what we have
today which is chaos, although I hadn't specifically thought about
preventing it. Should probably add it as a consideration into the draft.
Hopefully one can base decisions around the "believe-ability" of the
information you're receiving in the SDP answer off of other information.

> 
> Regarding the mechanism itself, the proposed SDP parameter 
> bundles together two things that are actually orthogonal:
> 
> 2) the correlation between the description of a stream in an 
> SDP offer or answer with its associated media packets
> 
> To do 1), I would suggest to use the same tokens as we 
> typically use for direction attributes. That is, we would use 
> "inactive" instead of "none".

Fair enough.