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OSI & TCP/IP models
The OSI seven-layer model beside the TCP/IP model, and what each layer does.
OSI ↔ TCP/IP layers
OSITCP/IP
L4 · TransportPDU: Segment
End-to-end delivery. TCP is reliable and ordered; UDP is fast and best-effort. Ports live here.
Protocols: TCP, UDP
A layered model splits networking into levels, each serving the layer above it and using the layer below. The OSI reference model has seven layers; the practical TCP/IP model collapses them into four.
Reading the stack
- Data travels down the sender's stack — each layer wraps it in a header (*encapsulation*) — and back up the receiver's stack.
- The data unit is named per layer: segment (transport), packet (network), frame (data link), bit (physical).
Tip:Mnemonic for OSI layers 7→1: *All People Seem To Need Data Processing.*
Where common protocols live
| Layer | Example protocols |
|---|---|
| Application | HTTP, FTP, SMTP, DNS |
| Transport | TCP, UDP |
| Network | IP, ICMP |
| Data link | Ethernet, ARP |