For several years, we have been advocating that UAs should implement the
Join header for the INVITE request. (See RFC 3911. We have a test for
this in the interop server.) The original intent of INVITE-with-Join is
to implement ad-hoc conferencing, with the media mixing done by a UA.
But it turns out that a more important usage is to allow SIP agents to
insert themselves into existing SIP dialogs. (Call the flow detailed
below.)
For this purpose, the period when more than one dialog' media streams
are mixed is quite short, and so there is no requirement that the UA do
the media mixing at all well.
Currently, none of the sipXecs UAs implements INVITE-with-Join at all.
But I was thinking that we could start with sipXbridge, for which I
expect the implementation effort will be less than for the C++ stack.
Comments?
Dale
----------
Call flow:
Initially, there is a dialog D between UA 1 and UA 2:
UA 1 <----------------> UA 2
UA 3 wants to insert itself into the dialog as a B2BUA. It knows the
dialog identifiers of dialog D. The first step is for one "side" of UA
3 to send an INVITE with "Join: D" to UA 1, thus connecting itself to
the UA 1 end of dialog D:
UA 1 <----------------> UA 2
<------+
|
|
UA 3
The second step is for the "other side" of UA 3 to send an INVITE with
"Replaces: D" to UA 2. This routes the media from UA 2 to UA 3:
UA 1 <----------------> UA 2
<------+ +------>
| |
| |
UA 3
UA 2 sends a BYE on dialog D to UA 1, causing that leg of the conference
at UA 1 to end, leaving UA 1 talking solely to UA 3:
UA 1 UA 2
<------+ +------>
| |
| |
UA 3
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