Mobile Networking · How Apps Talk · Lesson 5 of 5

Challenge: Jo's News App

Follow one app's traffic from a name lookup to a shared public address.

The Case Brief

Your cousin Jo asks you to explain what her phone is doing when she opens her favorite news app. You agree to follow one session from start to finish.

Here's what you know so far:

• Jo is at home on Wi-Fi. Her phone's settings show the address 192.168.1.40.
• The app gets its stories from a server called news.example.
• Jo has never opened the app on this network before, so nothing about it is stored yet.

She taps the app icon. Before a single story can load, the phone has a problem to solve. The app only knows the server by its name, but packets need something else in their header to find their way across the internet.

Think about what the phone needs before it can send the first packet to news.example, and who it asks for help.

The Answer Expires

The lookup works. The answer for news.example comes back with a TTL of 300. The resolver stores it, and Jo's stories load.

Jo closes the app, makes a cup of tea, and opens it again 10 minutes later. Her phone asks the resolver for news.example one more time.

You notice this second lookup takes a little longer than you expected. The resolver had stored the answer earlier, so why would it go back out and ask again?

A Newer Protocol

You open the app's settings with Jo and find a line that says it fetches stories using HTTP/3.

Jo has heard that web traffic uses TCP, because every byte of a page has to arrive correctly. You tell her that's true for older versions of HTTP, but HTTP/3 works differently underneath.

The stories still arrive complete and encrypted. Something sitting between HTTP/3 and the transport protocol adds the reliability and the encryption, so the transport protocol itself doesn't have to.

Leaving the House

Jo heads out and her phone switches from home Wi-Fi to mobile data. You both visit a website that shows your public IP address. It shows a different address than it did at home.

Then you look in her phone's settings. The address listed there is 100.72.14.9, which matches neither the old home address nor the one the website just showed.

Jo wonders whether a website log with her public address could point straight to her and nobody else.