Internet Model (Layers, Protocols, Encapsulation)
Context: FIT1047_MOC · THE framework of the networks half — every later protocol slots into one of these five layers · abstraction, same trick as the OS
Quick Revision
- 🎯 Objective: five layers, each with a job, an interface to the layer above/below, and a protocol to its peer on the other machine.
- 📦 Core Components: Application → Transport → Network → Data Link → Physical, each wrapping the message in its own PDU.
- ⚡ Key Constraint: protocols run HORIZONTALLY (peer layer ↔ peer layer), interfaces run VERTICALLY (layer ↔ adjacent layer) — mixing these up breaks every layer question.
📝 Core
The Five Layers (Internet model)
| Layer | Job (lecture phrasing) | PDU | Example protocol |
|---|---|---|---|
| Application | manage user data | Message | HTTP (World Wide Web (HTTP and HTML)) |
| Transport | break into packets, make sure they arrive | Segment | TCP |
| Network | where should packets go? (routing) | Packet | IP |
| Data Link | computer-to-computer (one hop) | Frame | Ethernet |
| Physical | cables, Wi-Fi, fibre | Bit | — |
- Why layers ➔ wildly diverse hardware/software must interoperate ⟹ hierarchical abstraction with well-defined tasks + interfaces — “typical software engineering”.
- Encapsulation ➔ going down the stack, each layer wraps the upper layer’s PDU and adds its own header: Message → Segment → Packet → Frame → Bits; unwrapped in reverse on arrival.
- Who climbs how high ➔ end hosts run all five layers; a switch operates up to Data Link; a router up to Network — that’s why routers can join different link technologies (Ethernet, fibre, ADSL, Wi-Fi).
⚠️ Common Mistakes
- 💡 Protocol vs interface ➔ protocol = peer-to-peer language across machines; interface = the service boundary between adjacent layers on ONE machine.
- 💡 PDU names are per-layer ➔ segment/packet/frame aren’t synonyms; the exam asks exactly which layer calls it what.
- 💡 Routers don’t read Transport headers ➔ a device only unwraps as far as its top layer.
🧠 Active Recall
Trace a YouTube request from the browser down the stack and across a router — naming each PDU and where unwrapping stops.
Answer
- Short answer: HTTP Message → TCP Segment → IP Packet → Ethernet Frame → bits; the router unwraps to the Packet (Network layer), re-wraps in a NEW frame for the next link technology.
- Why: Per-hop re-framing ➔ Data Link is hop-scoped, Network is end-to-end — precisely the layer split’s purpose.
Why does the layered model let Wi-Fi, ADSL, Ethernet and fibre coexist in one connection?
Answer
- Short answer: Layers above Data Link never see link technology; each hop swaps the frame type while Packet and above pass through unchanged.
- Why: Abstraction boundary ➔ interfaces isolate change — swap a layer’s implementation without touching the others.