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

Yuriy Gorlichenko ovoshlook at gmail.com
Fri Aug 27 11:45:29 CEST 2021

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

Regarding SDP according
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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