[Serusers] Can SER determine NAT type?

[Chris St Denis]

I want to use STUN as a primary nat solution, with the nathelper or
mediaproxy module doing rewrites as a secondary/backup solution.

However, this will not work for symmetric NATs. I want to proxt the rtp data
from these (and only these) kind of nat. Can SER determine that?

My client adds a 
Warning: 399 spa "Restricted Cone NAT Dectected".
Header to the register message (determined by stun), but that looks
proprietary and not something I can expect to be the same from other clients
so I'm not sure I want to header match on that.