Yes, what you are encountering is RTPEngine's "learning" behaviour, where it 
waits for a single RTP frame from a (presumably NAT'd) endpoint and latches 
onto its source IP:port in order to send RTP (symmetrically) back to it.

Check out the "endpoint learning" section here:

https://rtpengine.readthedocs.io/en/latest/rtpengine.html

-- Alex

> On Jul 26, 2023, at 11:53 AM, Duarte Rocha <[email protected]> wrote:
> 
> Hello all,
> 
> I'm currently doing some experiments with RTPENGINE and I need some help 
> configuring it.
> 
> First scenario i'm under NAT and Kamailio receives an INVITE with a private 
> address, rewrites (c) line on SDP with it's own IP address and relays the 
> message to the destination endpoint. After that the originator receives the 
> 183 Session Progress with kamailio's IP as (c) parameter. After receiving the 
> first RTP packet from the originator the media streams flow normally and I 
> have sound on both sides including ringing.
> 
> Second scenario I use different equipment that is not under NAT, but the 
> remaining scenario is the same, except the originator doesn't send an RTP 
> packet after the 183 Session Progress.  However, in this case, no RTP stream 
> is relayed to the originator and as a consequence it doesn't hear a ring or 
> voice when the call is answered.
> 
> As i see it,  it looks like Kamailio is on passive mode to the A-Leg of the 
> call and waits for a RTP packet to establish its stream. Is this correct? If 
> so, how do i change this behaviour?
> 
> Cheers
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-- 
Alex Balashov
Principal Consultant
Evariste Systems LLC
Web: https://evaristesys.com
Tel: +1-706-510-6800

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