Subject: [Sipping] A question about early session

From: [email-masked]
Date: Tue, 24 Jul 2007 19:41:52 +0800

Dear Sipping Group,

In order to solve the media clipping problem in case of forking, RFC 3960 
recommends application server model and RFC 3959 defines a new 
early-session disposition type for the Content-Disposition header field. 

"Sending early media using a different offer/answer exchange than the one 
used for sending regular media helps avoid media clipping in cases of 
forking. The UAC can reject or mute new offers for early media without 
muting the sessions that will carry media when the original INVITE is 
accepted. The UAC can give priority to media received over the latter 
sessions. This way, the application server model transitions from early to 
regular media at the right moment."(RFC 3960 Section 4.The Early Session 
Disposition Type Page 4) 

So I have a question for clarification. If a UAC has received session 
media, can it switch to early session media in the subsequent? Let's 
discuss a CFNA-like (call forward no answer) service for example. User X 
calls user Y but Y is not answering, the AS of user Y forwards the call to 
another user Z. If user Z is not answering, the AS of user Y forwards the 
call to another user W. 
So if user Y has a color ring back tone service and the AS use early 
session to play color ring back tone media, user X hears user Y's color 
ring back tone at first. Because user Y is not answering, the AS of user Y 
forwards the call to another user Z .If user Z is a PSTN user behind a 
PSTN gateway, as we all known, PSTN gateway would not use early session, 
it uses session media to play ring back tone and now user X hear a ring 
back tone. And if user Z is not answering either, the AS of user Y 
forwards the call to another user W and user W has a color-ring service. 
So the AS of user W users early session to play user W's color ring back 
tone. But user X has received session media from PSTN gateway and now can 
it switch to hear the early session media from the user W's AS?

Regards,

Shen

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