Have you ever had to deal with SIP and NAT issues? How many times you or your customers were frustrated by one way audio? How much time have you spent debugging such cases?
It is not a simple problem, and there were many attempts to deal with NAT in SIP and other media signalling protocols that are using offer/answer model. Some of them are server centric, others are implemented on the client side and a few of them use mix of server and client implementation. Lets review some of them.