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> Ziggy then sends a message to both Alice and Bob. The message to Alice contains Bob’s public IP and port, and the message to Bob contains Alice’s.

Is this done by modifying the source address on the outgoing UDP packet? If so, wouldn't modern filtering prevent the spoofed packets from getting to their destination?

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no, ziggy just acts as a directory of public ip addresses. Once alice and bob know each other's public ip they can start communicating each other. Suppose alice is at A.A.A.A:1234 and bob is at B.B.B.B:5678. Alice sends

A.A.A.A:1234 -> B.B.B.B:5678

and bob simulataneously sends

B.B.B.B:5678 -> A.A.A.A:1234

When the packets arrive to the routers the NAT already has an opposite rule because of the sent packet. As this is UDP there is no SYN-ACK handshake or other state tracking information in the packets and they traverse the NAT.

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