[pjsip] 3PCC transfer implementation

Benny Prijono bennylp at pjsip.org
Wed Nov 21 09:46:29 EST 2007

Anshuman S. Rawat wrote:
> Hi Benny,
> Joegen mentions -
> "In addition to the dangers of 3PCC call transfer is that some user agents
> ignore RTP packets whose ssrc has changed.  I have seen implementation
> that throws away rtp packets with different ssrc than what it was previous 
> receiving."
> Is this an issue with pjsip? We are testing 3PCC transfer and have noticed 
> that the transferee cannot hear anything but can be heard by the party it 
> was transferred to. Putting more clearly -
> - A calls B
> - B transfers A to C
> C can hear A but A cannot hear C. For traces, we can see that signalling is 
> fine and bi-directional RTP flows between A & C.

I think so. pjsua_lib didn't maintain SSRC for subsequent media 
update. In fact I've had this on my to do list for loong time: 
http://www.pjsip.org/trac/ticket/14 (notice the ticket number!).

But the good news is, finally it's been fixed. :)

And coming back to the loss audio problem. If pjmedia (the RTP 
session) receives RTP packet with different SSRC, it will mark the 
packet as having "bad ssrc", but it shouldn't drop the packet. So I 
think the loss of audio shouldn't be caused by this.

If you have any doubts on whether pjmedia may drop RTP packets for 
any reason, you'll find useful information at log level 4 or 5.


> Regards,
> Anshuman

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