[Serusers] Proxy-to-Proxy authentication

[Victor Huertas Garcia]

Hi all!

I'm newie in this mailing list and I am working with SER at this momen in a 
project.

However I have a doubt I would like to clarify.

Does anyone know if there is a way to perform SIP Proxy to SIP Proxy 
authentication (I mean in SIP protocol in general)?

If a SIP proxy receives an INVITE from another SIP Proxy, how does the proxy 
which receives the INVITE that the originating proxy can be trusted?

I have read something about TLS but I took it from an article of 2003... 
Which is the most used method nowadays?

Thank you very much for your attentio

Regards

Victor