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

[Arsen Semenov]

it's legal to get responses with different to-tags, since the request is
forked. this is what's happening in this scenario.
I think this is an issue of UAC/rtpengin how it is reacting to the second
SDP in 183

are the SDPs in 183 responses carried reliably?

On Fri, Aug 27, 2021 at 2:51 PM Yuriy Gorlichenko <ovoshlook at gmail.com>
wrote:

> 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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-- 
Arsen Semenov
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