It is possible with TCP. Moreover Kamailio is trying to reuse the tcp connection whenever the destination address is matching an active one.
The standard way is good, but in the real world you could face some devices which do not support "outbound" properly, so you can leverage functions exposed by nathelper module to ensure correct routing.

On Thu, Nov 11, 2021 at 4:50 PM Ross McKillop <ross@rsmck.co.uk> wrote:


On 11 Nov 2021, at 07:22, Olle E. Johansson <oej@edvina.net> wrote:



10 nov. 2021 kl. 18:21 skrev Ross McKillop <ross@rsmck.co.uk>:

Hi,

Thanks, I was almost certain that is set but it seems it may not be so will double check that, thank you :) 

Now to solve the other issue…. 
Just a nit-picking note: All these are non-standard fixes. The standard based way is to use the outbound module, it’s the way to allow the sip server to use an inbound TCP connection for outbound requests. This applies to client2server connections.

For server2server connections there is a requirement of mutual TLS auth in order to be able to reuse the connection in both directions

I thought this would be possible with TCP too.

I looked at the outbound module but this is a 'drop in' replacement for something that already exists, so I'm trying to avoid rewriting headers as would be required for flow IDs etc, however it's a definite preferred method for improvement.

As an aside, and it may be the same issue in https://www.mail-archive.com/sr-users@lists.kamailio.org/msg15589.html, when you force an outbound socket on a specific port and a reply is received on that port, both $Rp and $Rut show the port on the first listen directive, not the port it was actually received on, which makes it a bit harder to do what I was trying to as well :/

Ross

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Arsen Semenov