Layering and architecture
Introduit
Vocabulaire
| English | Sens |
|---|---|
| computer networks | computer networks |
| layering/ˈleɪərɪŋ/ | layering |
| service/ˈsɜːvɪs/ | Activities performed to meet people's needs or wants rather than supplying a physical good. |
| physical/ˈfɪzɪkl/ | Describing security measures involving tangible barriers, facilities or protection of people and equipment. |
| link/lɪŋk/ | link |
| network/ˈnetwɜːk/ | A set of connected devices or nodes that can exchange information. |
| transport/ˈtrænspɔːt/ | transport |
| application | application |
| encapsulated | encapsulated |
English
The stack
- Computer networks are built by layering: each layer offers a service to the one above and consumes the one below. The five-course stack: physical, link, network, transport, application.
- Data is encapsulated downward — each layer adds its header — and decapsulated upward, each layer seeing only its own promise.
{exercise:uni-cs0901-stack}
Pratique
A 100-byte payload leaves with how many Ethernet+IP+TCP header bytes?
Difficulté 1/5 · 100 pièces + un petit bonus lors de la première réussite aujourd'hui
14 + 20 + 20 = 54: one header per layer crossed.
English
The independence
- A change confined to one layer needs no change elsewhere IF the service interface holds. Ethernet went from 10 Mb/s to 100 Gb/s under identical IP because the link service (best-effort frames) never changed.
- A leaky interface destroys it: a browser that reads the TTL below is written once and breaks forever.
{exercise:uni-cs0901-interface}
Pratique
Ethernet could evolve under identical IP because —
Difficulté 1/5 · 100 pièces + un petit bonus lors de la première réussite aujourd'hui
The service (best-effort frames) never changed, so the layer above never noticed.
English
One message, twice wrapped
- A 100-byte payload leaves the application as $14 + 20 + 20 = 54$ bytes of headers around it: the NIC strips 14, IP strips 20, TCP strips its 20.
- The internet's hourglass: narrow waist (IP), wide top (applications), wide bottom (links) — one protocol at the waist lets everything above talk to everything below. OSI's 7 layers split our link in two and add presentation and session on top.
{exercise:uni-cs0901-hourglass}
Pratique
The waist of the internet hourglass is —
Difficulté 1/5 · 100 pièces + un petit bonus lors de la première réussite aujourd'hui
One narrow protocol (IP) lets every application talk over every link.