What are the two main functions of the network layer?
- Forwarding: Router-local movement of packets from input to output links (data plan)
- Routing: Network-wide path determination from source to destination (control plane)
How does the network layer handle transport layer segments?
Encapsulates segments into datagrams on sending side; extracts and delivers segments to transport layer on receiving side
What is the data plane?
Per-router functions (forwarding) that handle packet transfer between input/output links. Implemented in hardware for speed
What is the control plane?
Network-wide logic (routing) to determine paths. Traditionally in-route but separated in SDN (remote controller)
How does a router use its forwarding table?
Matches header fields (destination IP) to table entries to determine the output link
What’s the difference between forwarding and routing?
- Forwarding: Local, hardware-based, nanoseconds
- Routing: Global, software-based, seconds
How are forwarding tables configured traditionally?
Each route runs a routing algorithm and exchanges route messages with neighbours to populate tables
How does SDN differ from traditional approach?
A remote controller computes and distributes forwarding tables to router via messages
What are the four key components of a router?
- Input ports: Physical/link-layer termination + lookup (forwarding tables)
- Switching fabric: Connects input to output ports
- Output ports: Queueing + link/physical-layer transmission
- Routing processor: Runs control plane (routing protocols/SDN controller communication)
What’s the goal of a routing algorithm?
Find the least-cost path from sender to receiver
Centralised (LS) vs Decentralised (DV) Algorithms
- LS: All routers know full topology
- DV: Routers know only neighbours
Static vs Dynamic Algorithms
- Static: Manual routes (slow changes)
- Dynamic: Adapts to traffic/topology
Load-sensitive vs Load-insensitive Algorithms
- Sensitive: Link costs reflect congestion
- Insensitive: Fixed costs
How is a network modelled as a graph?
- Nodes (N): Routers
- Edges (E): Links
- Cost (c): Metric like delay, bandwidth
How is path cost calculated?
Sum of link costs
Forwarding vs Routing
Forwarding is where (local), Routing is how (global)
