The Runtime Theory

OSI Request Journey: Seven Layers from Application Bits to Wire and Back

Watch one HTTP request walk the OSI model — from application to TLS, TCP, IP, Ethernet frames, and physical bits — and get reassembled in reverse at the receiver.

The Runtime Theory Team09 stages

trace / request.md

APPLICATIONBUILDS REQUESTTLS ENCRYPTSPAYLOADSESSIONSTATE GOVERNSTCP SEGMENTSAND PORTSIP PACKETSAND ROUTINGETHERNET FRAMESBITS ONTHE MEDIUMLAYERS STRIPPEDIN REVERSETHE JOURNEYREPEATS

readyThe application writes an HTTP request into a socket. The bytes are opaque here — the app never sees headers, segments, or frames that follow.

The OSI model is not a law — it is a map of what the machine actually does, in order. On the way down, each layer wraps the previous layer's data in its own header: TCP adds ports and sequence numbers to the HTTP bytes, IP adds addresses to the segments, Ethernet adds MACs to the packets. On the way up, each layer strips its own header and passes the rest along.

Two details make the ordering concrete. First, addressing changes meaning at each layer: TCP ports address processes, IP addresses address hosts across the internet, MAC addresses address the very next hop on the wire — nothing more. Second, not every header survives the trip: frames die at each hop, while packets and segments persist end to end. That is why the journey is drawn as a V: seven layers down, seven layers back up, with the physical medium as the bottom — and why diagnosing a network problem means asking which layer's contract is being violated.