Part I · How it works
6.1 Network layers: OSI and TCP/IP
alsoosiosi modelnetwork layerstcp/ip modelnetwork stacklayers
TL;DR
- A network is built in layers, each using the one below: link (Ethernet, Wi-Fi), network (IP), transport (TCP, UDP), application (HTTP, DNS, SSH).
- The OSI model names seven layers, the Internet uses four, and people still say "layer 2" for Ethernet and "layer 7" for HTTP.
- When something fails, check from the bottom up:
ip -br link # 2: is the link up?
ip -br addr; ip route # 3: an address, and a way out?
ping -c 3 1.1.1.1 # 3: does IP reach the Internet?
dig example.com # 7: do names resolve?
curl -I https://example.com # 4 to 7: TCP, TLS, HTTP
On your machine
$ atfq 6.1 L2 link wlp0s20f3 up · neighbour 192.168.1.1 is f4:92:bf:0e:21:7a on wlp0s20f3 L3 IP docker0 172.17.0.1/16, wlp0s20f3 192.168.1.23/24, wlp0s20f3 fe80::8ec6:81ff:fe2a:107f/64 · gateway 192.168.1.1 · forwards packets for others L4 TCP 12 connections open now; 30,120 started and 410 accepted since boot · 1,840 of 3.1 million segments resent L4 UDP 95,000 datagrams in, 61,000 out, since boot L7 DNS 127.0.0.53
An example: Alice's laptop, one of the test machines. In your terminal, this block shows your own machine, read live.
Pitfalls
ping 1.1.1.1works butping example.comdoesn't: it's DNS, not the network.pinguses ICMP, which some networks block: a failed ping doesn't prove a host is down.- OSI layers 5 and 6 (session, presentation) have no clear place of their own on the Internet; TLS is usually put between 4 and 7.
Deep dive 2 questions
Where does the OSI model come from?
From an ISO standard published in 1984, meant to define a complete set of network protocols. Those protocols lost to TCP/IP, which already ran on the ARPANET and in BSD Unix. The seven-layer vocabulary stayed, as the common way to say where a problem sits.
What does each layer add?
On the way out, each layer wraps what comes from above in its own header: TCP adds ports and sequence numbers, IP adds the source and destination addresses, Ethernet adds MAC addresses. The receiver unwraps them in reverse order. A packet carrying a few bytes is mostly headers.