Networking — The Wire Under Every Request

Every bug report that says “the API is slow” is a networking question first. Was it DNS? A connection that never opened? A TLS handshake repeated a thousand times? A pool that ran dry? Most backend engineers can’t answer — they restart the service and hope. Roughly 80% of “the code is broken” incidents turn out to be wire incidents: timeouts, stale DNS, refused connections, half-closed sockets. The code was fine. The wire wasn’t. This track makes you the person in the room who can tell the difference in five minutes, from a keyboard, with tools you already have.

Where This Track Begins And The Other One Ends

The Web Infrastructure track taught you how the internet is owned and configured — registrars, zone files, hosting plans, certificates as things you buy and point. That’s the ops view: you standing in a dashboard, editing records.

This track is the other side of the same wire: what your code experiences at runtime. When splitease-api calls out — or someone calls in — what actually crosses the network, in what order, and where it dies when it dies. Where the two overlap (DNS, TLS), we link the ops doc and go deep only on the client, runtime, and debugging side. Nothing is re-taught.

Web Infrastructure asked	This track asks
Where do I edit the DNS record?	Why did my app cache the old IP for an hour?
Which certificate do I install?	Why did the handshake fail at 2am, and how do I see it?
Which hosting plan?	Why is every request 300ms slower than the code accounts for?

The Journey

flowchart LR
    A["01 How a request travels"] --> B["02 TCP reliability"]
    B --> C["03 DNS at runtime"]
    C --> D["04 HTTP on the wire"]
    D --> E["05 TLS trust"]
    E --> F["06 Timeouts retries pools"]
    F --> G["07 Capstone debug the wire"]

First the stack itself — layers, IPs, ports, sockets. Then each layer a request climbs through, in the order a real request climbs through them. Then the production patterns that keep fintech APIs alive at 2am. Then a capstone where things break and you diagnose them cold.

The Toolbelt

Every tool in this track is already on your Windows 11 machine. No installs, no admin rights, no Wireshark (yet).

Tool	The question it answers
`curl`	What does the raw HTTP exchange actually look like — headers, status, timing?
`nslookup`	What IP does this name resolve to, right now, from this machine?
`tracert`	What path do my packets take, and which hop is eating the latency?
`netstat`	What connections and listening ports does this machine have open, and which process owns them?
`Test-NetConnection`	Can I reach that host on that port at all — and if not, refused or timeout?
DevTools Network tab	What did the browser send, what came back, and where did the milliseconds go?

The Modules

#	Module	What it gives you
01	How A Request Travels	The four-layer stack, IPs, ports, sockets — the map everything else pins to
02	TCP: The Reliability Machine	Handshakes, ACKs, retransmission — why “connection refused” and “timeout” mean opposite things
03	DNS: Finding The Server	Resolution from your code’s point of view — caches, TTLs, and the bugs they cause at runtime
04	HTTP: The Protocol You Speak Daily	The raw text under every `@GetMapping` — methods, status codes, headers that matter
05	TLS: Trust On The Wire	What the handshake costs, what encryption hides and doesn’t, certificate failures decoded
06	Timeouts, Retries & Connection Pools	The production patterns — idempotency for payments, pool sizing, the settings that prevent 2am
07	Capstone: Debug The Wire	Staged failures, real errors, a diagnosis playbook you run start to finish

Before You Start

splitease-api runnable. The hands-on parts point real tools at your own API on localhost:8080. If it doesn’t start, go back to the Spring Boot capstone and get it running — this track inspects it from the outside.
The sprint schedule already points here. Week 3 is the calendar view of this material; this track is its textbook. Same content, full depth.
Public targets: example.com and httpbin.org — real, free, and built for exactly this kind of poking.

Before Photo — No Shame In Zero

Take this now, before module 01. Scoring zero is the honest baseline — the whole track exists to make these feel insultingly easy.

Wherever you landed: same next step. Start at Networking 01 — How A Request Travels.