The End-to-End Argument
Functions belong at the endpoints; lower layers can only optimize, never guarantee. The single most useful design principle in networking.
Networking
Token Buckets, Arrival Curves, and Network Calculus
Rate limiting has a theory: bound the arrival curve and you get a provable bound on delay and queue depth.
The Noisy-Channel Coding Theorem
You can communicate at capacity with arbitrarily low error. Shannon's most surprising result: reliability is free below the limit.
The Two Generals Problem
Agreement over a lossy channel is impossible. Why exactly-once delivery is a lie and idempotency is the real answer.
Hyrum's Law and Postel's Principle
Every observable behavior of your API is depended upon by someone; and the robustness principle that made that worse, plus the modern case against it.