WEEK 7 CS2005-NetworkLayerI-2024_25

Understand the following key areas:
- The Network Layer’s key functions
- Forwarding/Routing
- Routing algorithms

Logical Communication: Provides logical communication between hosts.
Components of Network Layer:
- Data Plane: Deals with per-router functions that forward datagrams arriving at input links to appropriate output links.
- Control Plane: Contains network-wide logic controlling the routing of datagrams from source to destination using routing algorithms.
Integration in Traditional Routers: Traditionally, forwarding (data plane) and routing (control plane) functions are implemented together within routers.
Software-Defined Networking (SDN): Separates data plane and control plane by implementing control functions as a remote service.

Sending Side Process:
- Encapsulates segments from the transport layer into datagrams.
- Sends datagrams to nearby routers.
Receiving Side Process:
- Receives datagrams from routers.
- Extracts transport layer segments to deliver to the transport layer.

Definition: Moving a packet from an input link to the appropriate output link when it arrives at a router.
Importance: Most common and critical function in the data plane.
Additional Tasks:
- Blocking packets from exiting a router (e.g., known malicious hosts).
- Duplicating packets for transfer over multiple links.

Definition: Determines the path packets take from sender to receiver.
Routing Algorithms: Algorithms that calculate paths, part of the control plane.

Forwarding:
- Local action at the router level.
- Fast process (often in nanoseconds, usually hardware-implemented).
Routing:
- Network-wide, longer timescale process (often seconds, typically software-implemented).
Forwarding Table: Key element in routers; forward packets based on header fields indexing the forwarding table.

Functionality: Each router contains algorithms for computing forwarding tables.
Communication: Routers exchange routing information through routing messages based on predefined protocols.

Centralized Control: A remote controller computes and distributes forwarding tables to routers.
Router Function: Routers focus solely on forwarding, managed by the remote controller which may ensure reliability and redundancy.
Communication: Involves exchanging messages carrying forwarding tables and other routing information.

Input Ports:
- Terminate incoming physical links and perform link layer functions.
- Consult forwarding tables to determine output port.
- Forward control packets to the routing processor.
Switching Fabric:
- Connects input ports to output ports within the router.
Output Ports:
- Store and transmit packets on outgoing links.
Routing Processor:
- Executes routing protocols and manages routing tables.
- In SDN, communicates with the remote controller for forwarding table entries.
- Handles network management functions.

Objective: Determine the best paths from senders to receivers.
Path Definition: Sequence of routers packets pass through to reach destination.
Path Evaluation Criteria: Cost-effectiveness (fastest, least congested).
Graph Representation:
- Graph G = (N, E):
  - N (Nodes): Set of routers (e.g., {u, v, w, x, y, z})
  - E (Edges): Set of links (e.g., {(u,v), (u,x), (v,x), etc.})
- Cost Calculation: Summation of link costs for paths.
Routing Algorithm Classifications:
- Centralized: Routers have complete network topology.
- Decentralized: Routers know only their immediate neighbors.
- Static vs. Dynamic: Static routes change slowly; dynamic responds to traffic/load changes.
- Load-sensitive vs. Load-insensitive: Load-sensitive routes adjust costs based on congestion conditions.

Understand the following:
- Key functions of the network layer including forwarding and routing.
- The distinct roles of data and control planes.
- Internal router architecture and processing of input ports for destination-based forwarding.
- An overview of routing algorithms.