[Kamailio-Users] How to solve early media during serial forking

[Iñaki Baz Castillo]

Hi, AFAIK there is an issue in typical SIP/PSTN enviroments in the case of 
failover using different gateways serially.

The main issue is that SIP doesn't define how a UAC should handle the case in 
which it receives various "183 Session Progress" from different 
early-dialogs, each of them containing early-media.

It occurs to me that a SIP/PSTN gateway sometimes replies 183 with early media 
for 1 or 2 seconds, and inmediatelly it replies a 502. Well, this 502 could 
be caused by other node in the PSTN network, it's not necessarialy a problem 
fo the gateway [1].
In this case I do serial forking forwarding the request to other gateway.

But the UAC already received early media from the first gateway, and most of 
the SIP phones just render the first early media they receive in case of 
multiple 183, and drop others. So basically the problem is that the UAC user 
doesn't listen the ringing coming from PSTN after the request has been 
forwarded to the second gateway.

Is there something I could do at proxy level to avoid this issue? Sincerely I 
don't think so, since it makes no sense to disallow the first 183 (I cannot 
know that the gateway will later reply a 502).

Thanks.


[1] Anyway, *who* is creating the early media? Why the PSTN generates it if 
the request doesn't arrive to the called? "imagine there is no PSTN..."


-- 
Iñaki Baz Castillo