Network Layer l

Question 1

What are the two main functions of the network layer?

Answer 1

• Forwarding: Router-local movement of packets from input to output links (data plan)
• Routing: Network-wide path determination from source to destination (control plane)

Question 2

How does the network layer handle transport layer segments?

Answer 2

Encapsulates segments into datagrams on sending side; extracts and delivers segments to transport layer on receiving side

Question 3

What is the data plane?

Answer 3

Per-router functions (forwarding) that handle packet transfer between input/output links. Implemented in hardware for speed

Question 4

What is the control plane?

Answer 4

Network-wide logic (routing) to determine paths. Traditionally in-route but separated in SDN (remote controller)

Question 5

How does a router use its forwarding table?

Answer 5

Matches header fields (destination IP) to table entries to determine the output link

Question 6

What’s the difference between forwarding and routing?

Answer 6

• Forwarding: Local, hardware-based, nanoseconds
• Routing: Global, software-based, seconds

Question 7

How are forwarding tables configured traditionally?

Answer 7

Each route runs a routing algorithm and exchanges route messages with neighbours to populate tables

Question 8

How does SDN differ from traditional approach?

Answer 8

A remote controller computes and distributes forwarding tables to router via messages

Question 9

What are the four key components of a router?

Answer 9

1. Input ports: Physical/link-layer termination + lookup (forwarding tables)
2. Switching fabric: Connects input to output ports
3. Output ports: Queueing + link/physical-layer transmission
4. Routing processor: Runs control plane (routing protocols/SDN controller communication)

Question 10

What’s the goal of a routing algorithm?

Answer 10

Find the least-cost path from sender to receiver

Question 11

Centralised (LS) vs Decentralised (DV) Algorithms

Answer 11

• LS: All routers know full topology
• DV: Routers know only neighbours

Question 12

Static vs Dynamic Algorithms

Answer 12

• Static: Manual routes (slow changes)
• Dynamic: Adapts to traffic/topology

Question 13

Load-sensitive vs Load-insensitive Algorithms

Answer 13

• Sensitive: Link costs reflect congestion
• Insensitive: Fixed costs

Question 14

How is a network modelled as a graph?

Answer 14

• Nodes (N): Routers
• Edges (E): Links
• Cost (c): Metric like delay, bandwidth

Question 15

How is path cost calculated?

Answer 15

Sum of link costs

Question 16

Forwarding vs Routing

Answer 16

Forwarding is where (local), Routing is how (global)