[asterisk-users] Weird 'hairpin' call rtp audio problem

[Benoit Panizzon]

Hello List

Asterisk 13.14.1 in use with pjsip stack.

On the remote side is a SBC which performs some 'nat' detection. I
suppose this means the SBC listens from where it is getting RTP data
and then replies to that ip.

As long as the asterisk is initiating the call this is fine, the
asterisk start sending RTP to the media IP of the SBC and the SBC is
sending media back.

Now I want to do a hairpin call, simulating call forward on no answer
(yes this is the situation I observed the problem first)

So incoming AND outgoing calls are via SBC.

exten => destination,1,Progress()
exten => same,n,Playtones(ring)
exten => same,n,Wait(5)
exten => same,n,Dial(PJSIP/sip:external at sbc)

What I now observe when I dissect this call via Wireshark (and set rtp
debug on etc).

Call to destination is established, up to the Wait(5) we have two way
RTP audio between the SBC and the Asterisk.

The external destination picks up the call. From what I see the media
ip addresses and ports are correct, no direct media is attempted. So
asterisk should 'simple bridge' oder 'native bridge' the call localy.

But for some reason, the asterisk server is NOT forwarding any rtp, nor
is the SBC forwarding any rtp it is getting from it's remote side which
is definitely sending rtp data. (yes I have access to the SBC and did
sniff both sides).

I fear, that both, the asterisk side and the sbc side are attempting
the same kind of nat detection and do not forward rtp until they
receive any packets.

I did probably try all possible permutations of:

direct_media=no
rtp_symmetric=yes
force_rport=yes

But still no audio.

Any hints on how to force asterisk to send the first rtp packet?

Mit freundlichen Grüssen

-Benoît Panizzon-
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