[Serusers] Theory behind a media proxy and NATed UA

[Andreas Granig]

Felipe Louback wrote:
> I read that a media proxy put his address at the sdp body and both UA
> talks with the media proxy instead of each other. At the time a NAT
> user sends an INVITE, there is no corresponding port on the NAT. So
> How does a media proxy contact the NATed UA?
>  Isn't this the same problem the called part will have to contact the
> NATed party?

After the first UDP packet received on the mediaproxy socket it knows 
where to send the packets to. So both parties have to send at least one 
packet to the mediaproxy before the streams can be bridged together.

This is why some protocols like T.38, where one party waits for data 
from the other party before replying, don't work with NAT.

Andy