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RE: [Sip] Open Issue #58: Rejected mid-call SDP in 2xx

"Brett Tate" <brett@broadsoft.com> Fri, 05 October 2001 18:47 UTC

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Reply-To: brett@broadsoft.com
From: Brett Tate <brett@broadsoft.com>
To: 'Victor Paulsamy' <victor@zapex.com>
Cc: sip@ietf.org
Subject: RE: [Sip] Open Issue #58: Rejected mid-call SDP in 2xx
Date: Fri, 05 Oct 2001 14:24:08 -0400
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> > > If the UAS were to return the "previous SDP", 
> > > what purpose does re-INVITE without SDP serve?
> > 
> > It helps to avoid an SDP change loop when 
> > a 3PCC/B2BUA unholds the session or switches
> > between lines on behalf of the controlled party.
> > 
> 
> Could you please elaborate it to help understand?

An example of using reinvite-without-sdp is depicted 
within Figure 9: Mid-Call Announcement of 
http://www.softarmor.com/sipwg/drafts/draft-rosenberg-sip-3pcc-02.txt

Using basic SIP without reinvite-without-sdp, 
the SDP change loop could have been introduced.
The controller would have to send "previous" 
SDP in a reinvite on behalf of one party.
However once that party receives a reinvite, 
it can change its SDP; thus triggering another 
invite and potentially a new SDP in response,
thus triggering ... sdp change loop.

A similar sdp change loop is discussed 
within section 2.2 of the same draft.


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