Principles of layering

A network stack splits one big job into layers. Peers on different machines follow a protocol; adjacent layers meet at an interface; each layer offers a service upward. OSI has 7 layers (Physical, Data link, Network, Transport, Session, Presentation, Application); TCP/IP has 5 (top three merged). Layers 1–3 are hop by hop and run on routers; 4–7 are end to end. Going down, each layer encapsulates: message → segment → packet/datagram → frame (+ trailer) → bits. Hub = 1, switch = 2, router = 3.

From 1.1.1 Why layer: protocol, service, interface, peers

• Layer: one job in the stack; serves the layer above, uses the layer below.
• Protocol: rules between peers (same layer, different machines): format, meaning, behaviour.
• Service: what a layer does for the layer above. Interface: how the layer above asks for it.
• Peer talk is logical; the real path is down → link → up.
• Layering = modularity + reuse − some overhead.

From 1.1.2 The OSI 7 layers and the TCP/IP model

• OSI bottom-up: Physical, Data link, Network, Transport, Session, Presentation, Application.
• Scope: link → hop → host-to-host → process-to-process.
• Routers: layers 1–3; hosts: 1–7.
• TCP/IP (5): application (= OSI 5–7), transport, network, data link, physical.
• Encryption/compression = presentation; dialogue control = session.

From 1.1.3 Encapsulation, headers, PDU names, addresses and devices per layer

• Encapsulation: payload + header (+ trailer at data link); decapsulation in reverse.
• PDUs: message (app) → segment (transport) → packet/datagram (network) → frame (data link) → bits.
• Addresses: link (per hop, changes) · network (end to end, fixed) · which-program id · names.
• Devices: hub/repeater 1 · bridge/switch 2 · router 3 · gateway/host up to 7.
• Efficiency = data / (data + all headers + trailer).

1. MCQ · 1 mark

   Which pair is correctly matched?

   1. Transport — frame
   2. Network — segment
   3. Data link — frame
   4. Application — packet
2. MSQ · 1 mark

   Which are end-to-end layers (run only on hosts) in OSI?

   1. Transport
   2. Session
   3. Network
   4. Presentation
3. NAT · 1 mark

   A 2000-byte message gets headers of 20, 20 and 14 bytes and a 4-byte trailer. Total frame size in bytes?

   Write your answer before opening the key.

4. MCQ · 1 mark

   Two layer-4 entities on different hosts exchange units following agreed rules. These rules form a:

   1. service
   2. interface
   3. protocol
   4. topology
5. MCQ · 1 mark

   A bridge operates up to which OSI layer?

   1. 1
   2. 2
   3. 3
   4. 4
6. NAT · 2 marks

   A message of 400 bytes descends 6 layers, each adding a 10-byte header. What percentage of the transmitted bytes is the message (2 decimals)?

   Write your answer before opening the key.

7. MCQ · 1 mark

   Compression belongs to which OSI layer?

   1. Application
   2. Presentation
   3. Session
   4. Transport
8. MSQ · 1 mark

   Which statements about layering are true?

   1. Only the physical layer actually transmits signals.
   2. A change in the physical medium forces changes in application programs.
   3. Each layer adds overhead.
   4. Peer communication is logical.
9. MCQ · 2 marks

   Which address in a frame is rewritten at every router hop?

   1. Network-layer destination
   2. Link-layer source and destination
   3. Which-program identifier
   4. None