The OSI model: the 60-second story

The OSI model

You courier Diwali sweets to Kochi. No hub opens the box.

Hubs read the PIN code and pass it on. Every network hop reads only as deep as its job needs.

Seven layers, each with one job

Bits on a wire, frames to a neighbour by MAC, packets across networks by IP, segments to a process by port, messages by URL.

Does the internet run seven layers?

Bad: Seven, one protocol each

OSI is a reference model, not the internet’s blueprint.

Good: Four, and TLS straddles

Link, internet, transport, application. TLS spans layers 5 and 6, riding on TCP.

One 200-byte request, wrapped four times

TLS, TCP, IP and Ethernet each wrap it in their own header, never reading inside. 280 bytes cross the wire; 80 are headers.

Each box reads only as deep as its job

MAC addresses change at every router. IP survives unless NAT rewrites it. TLS survives only to the first box holding the key.

Which layer stops a login botnet?

Bad: Packet filters

Every request completes TLS from a real address, like a user.

Good: A WAF at L7

It reads each decrypted request’s path and rate. You pay to end TLS.

“The API is down”: walk up the stack

L3: ping and traceroute reach the host
L4: nc connects to port 443
TLS: openssl finishes the handshake
L7: curl -v shows what the server says

The office VPN is up. ssh works, the login API works. Every file upload hangs.

The cause

The tunnel cut the MTU to 1,446. The ICMP “too big” reply was blocked, so full-size packets silently vanish.

Remember

OSI’s seven layers are shared vocabulary; the internet runs four. Every box reads up to some layer: a router reads IPs, an L4 balancer the 5-tuple, an L7 proxy or WAF the decrypted request.