[Serusers] STUN and symmetric nat again

[Istvan De]

Hi!

I've read about the above topic in the arcives and now I've got the same
problem.

User A is behind a symmetric NAT -> a new port is bound for every
different
(external ip, external port) pair.
User B has a connection to the net without any NAT or firewall.

User A wants to have a SIP+RTP session with user B. How?

I guess basically what is needed is forming of a SIP
message like "send the your reply to the same ip and
same port from where you have received this" and user
A can send this to user B.

For the RTP session, a pretty similar thing would be
needed like, A says to B: "I send you the RTP stream
on your x port, send your rtp stream back to the same
ip and port from where you receive mine".

How to setup such a SIP packet?
Or maybe there is a standard solution for this already?
(STUN wont help in this case I guess, and TURN is not
an option for me.)

I've read about "symmetric SIP + symmetric RTP" maybe
that is the same as I've described above?

Thanks
Istvan