When client
use STUN, it can detect the NAT type. When client
register, it contains x-NAT (0:unknown, 1: full cone, ..., 6:
symmetric), which will be helpful for the server to detect NAT type.
Again, where are you finding these nonsymmetric clients? The amount of
non-symmetrical SIP and RTP implementations out there in the wild at
this point is negligible.
You're confusing "symmetric RTP" (as defined in RFC 4961) with
"symmetric NAT" (as defined in RFC 3489).
Simon