Subject: RE: [Sipping] Some thoughts on fixing early media

From: "GARCIN Sebastien RD-CORE-ISS" <[email-masked]>
Date: Mon, 17 Jul 2006 08:28:11 +0200

Although you said that the SDP answer would be typically sent in a 200 OK, early media by definition takes place before the 200 OK. So the SDP answer is typically sent in the first reliable 18x. 

sébastien

-----Message d'origine-----
De : Dean Willis [mailto:[email-masked]] 
Envoyé : jeudi 13 juillet 2006 23:19
À : GARCIN Sebastien RD-CORE-ISS
Cc : sipping
Objet : Re: [Sipping] Some thoughts on fixing early media

On Jul 13, 2006, at 11:29 AM, GARCIN Sebastien RD-CORE-ISS wrote:

> Dean,
>
> With regards to your proposal below, having a SIP indication is more 
> effecient than having to do a entire new offer/answer exchange via 
> "re-INVITE". Another comment is that early media can sometime be 
> bidirectional (e.g. for collecting DTMF in the foward direction) so 
> "sendonly" is not appropriate. The only way would be to indicate 
> "inactive" in the reliable SDP answer for "non early-media" cases and 
> then send re-INVITEs for setting up the media session...

I'd argue that if you're engaging in "early media" using bidirectional media flow, you really need to have delivered an SDP answer so that the UAC knows what kind of media it can send.

--
Dean

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---

From: Dean Willis <[email-masked]>
Date: Mon, 17 Jul 2006 17:38:13 -0500

Early media, as currently used in unidirectional UAS->UAC  cases,  
doesn't require that an answer be sent yet, or in fact, every sent at  
all.

Want to "steal calls"?

Get two user agents that corroborate by sending initial INVITE  
messages with "offers", subsequent INVITE messages in the reverse  
direction with "answers", and use early media in both directions to  
simulate a bi-directional call.

If done carefully, you can make a call with no 200 OK, no ACK, no  
BYE, and without even any 100-class provisional messages (although  
using provisionals as acknowledgements cuts down the error rate).

Will this fork and interwork with the PSTN correctly? No, but neither  
does regular SIP.

--
Dean

On Jul 17, 2006, at 1:28 AM, GARCIN Sebastien RD-CORE-ISS wrote:

> Although you said that the SDP answer would be typically sent in a  
> 200 OK, early media by definition takes place before the 200 OK. So  
> the SDP answer is typically sent in the first reliable 18x.
>
> sébastien
>
> I'd argue that if you're engaging in "early media" using  
> bidirectional media flow, you really need to have delivered an SDP  
> answer so that the UAC knows what kind of media it can send.
>
> --
> Dean
>

---

From: "Hadriel Kaplan" <[email-masked]>
Date: Tue, 18 Jul 2006 10:18:26 -0400

> -----Original Message-----
> From: Dean Willis [mailto:[email-masked]]
> Want to "steal calls"?

No you can't; at least not if the provider is blocking early media.  They
block it when a call is terminating to a subscriber UA.  So in your example,
while they're both calls originating from a sub UA, they're also both
terminating at a sub UA and the media will be blocked. (depending on the
provider equipment, of course)

But then the real question is: given you have 2 user agents under your
control that you wish to call between, why would you bother making the call
through a provider??  Why not just call direct?  ;)

-hadriel