[SR-Users] Manage multiple RTP streams with different TO-tags (forking)

[Yuriy Gorlichenko]

Hello.
The first thing: to tag can't be changed due session once it passed.

If provider uses forking mechanism the it has to be hidden from your
system.

Regarding SDP according
https://datatracker.ietf.org/doc/html/rfc3261#section-13.2.1
They can't change SDP answer description on the fly. Only first SDP answer
used as proper, all othe answers will be ignored.

On Fri, 27 Aug 2021, 11:27 B. Tietz, <benjamin.tietz at berlin.de> wrote:

> Hello,
>
> following situation.
> I have a Kamailio (5.4) using rtpengine to loadbalance calls.
>
> If a call from Alice comes in, Kamailio decides to send the call to
> Carrier B from Bob.
> Bobs Phone is ringing and the carrier B send a 183 Session Progress with
> SDP and To-tag=abcd. The SDP has G722 as codec and port 1234.
> A few moments later carrier B send a second 183 Session Progress with SDP
> and TO-tag=fghi. The SDP has G711 as codec and port 5678. This is done, to
> play some funky music as ringtone -.-
> If Bob answers the call, carrier B sends a 200 OK WITHOUT SDP and
> TO-tag=abcd. So this should instruct our Kamailio to switch to the first
> G722 and port 1234.
>
> But sadly, this is just not working as expected.
>
> We tried to set the flags media-handover and port-latching for the
> rtpengine options and additionally set a to-tag when using rtpenging_manage.
> But this doesn't solve the codec change, so we have only audio when Bob
> answers the call, but no ringtone-music. If we allow G711 only in the
> outgoing INVITE to Bob, we have also tha ringtone-muisic, because there is
> no codec-change.
>
> Carrier B tells us, they are using a fork-mechanism.
>
> Is there something we can do, to support the codec change in 183? Or
> enforce carrier B to send SDP in 200 OK? Or anything else?
>
> Carrier B can not change anything in the ringtone-music-backend. They are
> stuck on G711.
>
> Thanks!
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