1.
[ S N O M 4 S N A T F I L T E R ]
•The first exception is a REGISTER request. When a user agent tries to register and needs the support of the filter, the filter will set up a local data structure representing the user agents. It will make sure that the connection to the user agents stays alive. It will also make sure that requests destined to the user agents will be forwarded properly.
•The second exception is an SDP attachment. The filter checks if the user agent needs support (or must be recorded) and will in that case add a local contact to the SDP that can be used for media relay.
•The third exception occurs when the filter queries a web server for routing information. In this case, it will send a provisional response to stop the UAC from repeating messages.
These three exceptions make sure that all user agents will work behind NAT, no matter what
