Layer 3 · Go deeper · 9 min
Go deeper
Size it, build it, then make the calls an interviewer will push you on.
Back of the envelope
Size it up
Tap through each step of the arithmetic.
A 1 MB image in a Flipkart-style sale: how many bytes cross the wire?
How much room does a tunnel leave inside a 1,500-byte link?
Build-up · 6 steps
Wrap one HTTPS request for the wire
A UPI-style app asks for a payment’s status over HTTPS. Going down the stack, each layer adds its header without reading what’s inside.
Step 1 of 6
200 bytes of HTTP
The app asks for one payment’s status. These 200 bytes are all the server cares about; every layer below exists to carry them.
4 interview moments
You’re the candidate
The interviewer pushes on the hard parts. Pick an approach and see how it holds up.
Deep dive 1 of 4
Interviewer
“A new VPN to the office is up.
sshand the login API work, but every file upload hangs. What do you do?”The tunnel’s inner MTU is 1,446 bytes. A firewall in front of the server drops all ICMP.
Your move. Pick an approach:
Deep dive 2 of 4
Interviewer
“During an IRCTC tatkal rush, a botnet hammers search and login with valid HTTPS requests. Where do you stop it?”
Your move. Pick an approach:
Deep dive 3 of 4
Interviewer
““The API is down.”
pingreaches the host, butncto port 443 times out. What do you check first?”Your move. Pick an approach:
Deep dive 4 of 4
Interviewer
“Two services in one AWS cluster placement group copy bulk data all day. What MTU do you set?”
Some of their traffic also leaves the VPC over a VPN.
Your move. Pick an approach:
Build-up · 7 steps
Follow the request hop by hop
The same request travels from a phone to a service. Each box unwraps only as far as its job needs, decides, and wraps it up again.
Step 1 of 7
Leave the phone
The Wi-Fi access point reads only MAC addresses and swaps the Wi-Fi header for an Ethernet one. That swap is layer 2’s point; link headers are local.