Hi List!

I am running into an „issue” that I cannot really explain.
I have a call from Station A to Station B with both legs connected to Asterisk 
via Kamailio.
Asterisk used is latest 18.1.0 with chan_pjsip.

Station A -> Kamailio -> Asterisk -> Kamailio -> Station B

Now when Station B gets the INVITE it answers with 183 Session Progress to 
initiate Early Media.
However Asterisk not only passes this 183 Session Progress on to Station A, it 
creates a second 183 Session Progress with basically the same
Content (only some Header lines might have switched position but their content 
is identical) to Station A.

The first question that comes to mind is:
Why does Asterisk send the second 183 Session Progress in the first place (it 
did not get anything from Station A or Station B that would explain this)?


And in turn this leads to an issue if I replace station A with another Asterisk 
and put station A behind this Asterisk.

Station A -> Asterisk2 -> Kamailio -> Asterisk -> Kamailio -> Station B

In that case whatever was in the SDP of the first 183 Session Progress (e.g. 
audio=recvonly video=inactive) will cause the second Asterisk to put both media 
streams to sendrecv, and starts sending RTP packets from Station A for both 
audio and video which clearly is a bug in Asterisk. By the way we tested with 
different versions from the latest 18 back to an early 16 version. They all 
show this behaviour.
To recap both 183 Session Progress contain the same SDP, the first one leads to 
wanted behaviour, the second one causes Asterisk to send both audio and video 
even though it shouldn’t.
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