DNS

TCP gets heavier the more you talk about it, while UDP is often reduced to “connectionless, unreliable”. That is not wrong, but it is too thin. What really matters in engineering is not which features UDP lacks, but why it deliberately does not do them, and what gets pushed back onto the application or a higher-layer protocol when it does not.

You change an A record in production, but browsers keep reaching the old address. The authoritative server already returns the new value, yet users in different regions still see different answers. Packet captures clearly ask for www.example.com, but the first few hops do not return an IP directly.

The hard part of DNS is not record types. It is the model behind them: DNS is never a real-time global lookup table. It is a resolution system held together by hierarchical delegation, recursive resolution, and TTL-based caching.