Recap: The OSI Model and Network Protocols

Every request climbs the same stack, and every failure lives on one layer of it.

A request crosses every layer in order, and a failure stops it at the first one that does not complete.

Opening a page runs ARP, DNS, TCP, TLS and HTTP, each needing the one before.
Troubleshooting by layer starts where the symptom points and proves one layer at a time, bottom up or top down.
Each tool proves one layer: ping and traceroute the network, dig the name, nc the port, openssl the certificate, curl the response.
Cloud components work at the layer of the header they read; an application load balancer reports 502, 503 and 504 for faults behind it.
A new HTTPS connection costs three round trips before the first byte with TLS 1.3, one when the connection is reused.

The stack as a debugging map
Layer	Protocol	Tool	Common failure
7, application	HTTP, DNS	curl -v, dig	4xx and 5xx responses, NXDOMAIN
Security, between 4 and 7	TLS	openssl s_client	Expired or mismatched certificate, missing intermediate
4, transport	TCP, UDP	nc -vz	Connection refused, idle timeouts
3, network	IP, ICMP	ping, traceroute	No route, timeouts, a TTL that runs out
2, data link	Ethernet, Wi-Fi, ARP	ip neigh	No gateway MAC address, a duplicate IP
1, physical	Cables and radio	Link lights, Wi-Fi signal	No link at all

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