OSI and TCP/IP Layers

One HTTP request gains a header per layer going down, crosses the wire as bits, and is unwrapped going up

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steps

Act 1 of 8·Two Models

step 1

OSI: seven layers to talk about · TCP/IP: four layers that actually run

L7ApplicationHTTP, DNS, SMTP

L6Presentationencoding, TLS*

L5Sessionsessions, RPC

L4TransportTCP, UDP, QUIC*

L3NetworkIP, ICMP

L2Data LinkEthernet, Wi-Fi, ARP

L1Physicalcopper, fibre, radio

Application

Transport

Internet

Link

Engineers still say "layer 2 switch" and "layer 7 load balancer" — OSI numbers are the shared vocabulary. But the internet runs the TCP/IP suite, which folds OSI's top three layers into the application and its bottom two into the link. The starred entries show why: TLS and QUIC do not sit cleanly in any one OSI box.

Header inspector — fills in while the request moves through a stack (acts 2, 3 and 5).

Speednormal

Getting bytes from a browser to a server means solving several unrelated problems at once: encoding the request, splitting it into pieces and recovering losses, finding a route across many networks, and putting signals on a cable. Layering gives each problem its own owner. The OSI model names seven layers, numbered from the cable up.

The TCP/IP stack

Applicationmessage

addressed by URL / host47 B here

Transportsegment

addressed by port20 B header

Internetpacket

addressed by IP address20 B header

Linkframe

addressed by MAC address14 B + 4 B

Active Recall

Question 1 / 3

A switch receives a frame. Which header field does it use to decide where the frame goes?

DSAverse

OSI and TCP/IP Layers

See also

OSI and TCP/IP Layers

See also