UDP behind NAT


There are challenges when communicating through NAT.
1. client is behind NAT. It doesn’t know it’s own public IP, which is needed by, e.g. RTSP
The public IP can be learned from STUN server.
2. server is behind NAT. No private-to-public port map in NAT initially.
server talks to STUN to establish the map, and STUN tells client server’s public IP-port.
With STUN, for both restricted and unrestricted NAT, two hosts can talk to each other directly.
TCP is not discussed here, which is more complicated.

A few words on NAT type:
Cone: one private IP-port maps to one public IP-port, when talking to multiple destinations
Symmetric: one private IP-port maps to different public IP-port pairs for different destinations

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