Protocols
Intro
Protocols are the agreed-upon rules that let machines communicate: what to send, in what order, and what to do when things go wrong. Every layer of the stack (link, network, transport, application) has its own protocol set, and production debugging often means knowing which layer broke the contract. Example: an HTTP 502 can mean the upstream is down, or it can mean a TLS version mismatch between proxy and origin that never shows up in application logs.