Here is an example use case; I am curious to see how this is suppose to work and why:
Someone has a NAT router from their ISP.
Behind the router are 2 PCs running Linux and using ntpd. ntpd sends UDP messages to servers with source and destination port both set to 123.
When the packets come out of the router (WAN port) and go to the NTP server, what should their src port be?
Is the router suppose to map the src on the LAN to a different src on the WAN so it knows how to map the responses?
If not, and they carry src 123 out to the internet, how does the router know which PC to send the reply to?