I'm STUNed

My router (Asus RT-AC68R) has a nice traffic monitor screen. I check it regularly. One day I noticed that an iPhone had a large amount of traffic attributed to STUN.

Off I went to figure out what STUN was.

It turns out that STUN stands for Session Traversal Utilities for NAT. Interestingly, STUN messages are sent in the lower overhead User Datagram Protocol (UDP) packets, not Transmission Control Protocol (TCP).

Still, what on the iPhone was using STUN?

Facetime.

Here's Apple's chart of port usage that calls out STUN.

Check off that we've learned something today.