Network Layer Functions, Routing, and Congestion Control

Posted on May 6, 2026 in Computer Engineering

Network Layer: Functions and Protocols

The Network Layer is the third layer of the OSI Model. It is responsible for routing, addressing, and forwarding data packets from the source to the destination across multiple networks. (10 Marks Answer)

Main Functions

  • Logical Addressing: Assigns IP addresses to devices and ensures each device is uniquely identified.
  • Routing: Determines the best path for data transmission using routing algorithms and tables.
  • Packet Forwarding: Moves packets from one network to another using routers to deliver data.
  • Fragmentation: Breaks large packets into smaller ones for transmission and reassembles them at the destination.

Key Devices and Protocols

Router: The main device used in the Network Layer; it connects multiple networks and directs traffic.

  • IP (Internet Protocol): Handles addressing and routing.
  • ICMP: Used for error reporting.
  • Routing Protocols: RIP and OSPF.

Features and Performance

  • Provides end-to-end delivery across networks.
  • Works with packets (data units).
  • Handles congestion control and traffic management.

Advantages

  • Enables communication between different networks.
  • Efficient routing of data.
  • Supports large-scale networks like the Internet.

Disadvantages

  • Complex routing mechanisms.
  • Possible delays due to congestion.
  • Packet loss can occur.

Routing Algorithms: Classification and Goals

Routing algorithms are methods used by routers to determine the best path for forwarding data packets from source to destination. They operate at the Network Layer of the OSI Model. (10 Marks Answer)

Goals of Routing Algorithms

  • Find the shortest or best path.
  • Avoid congestion.
  • Ensure reliable and efficient delivery.
  • Adapt to network changes.

Classification of Routing Algorithms

1. Non-Adaptive (Static) Routing

Routes are fixed and predefined. They do not change based on network conditions.

  • Example: Fixed routing tables.
  • Advantages: Simple, low overhead.
  • Disadvantages: Not suitable for dynamic networks.

2. Adaptive (Dynamic) Routing

Routes change based on network conditions. Routers exchange information to update paths.

a. Distance Vector Routing
  • Each router maintains a table of distances to all nodes.
  • Uses algorithms like Bellman-Ford.
  • Example protocol: RIP.
  • Features: Simple implementation, slower convergence.
b. Link State Routing
  • Each router has complete knowledge of the network topology.
  • Uses Dijkstra’s Algorithm.
  • Features: Faster convergence, more complex.
c. Path Vector Routing
  • Maintains the entire path information.
  • Used in large networks.
  • Example: BGP (Border Gateway Protocol).

Summary of Routing Performance

  • Advantages: Efficient data delivery, supports large networks, adapts to failures and congestion.
  • Disadvantages: Complex algorithms, overhead due to updates, convergence time issues.

Congestion Control Algorithms and Techniques

Congestion control algorithms are techniques used to prevent and manage network congestion, which occurs when too many packets are sent into the network, causing delays, packet loss, and reduced performance. These algorithms mainly operate at the Network Layer and Transport Layer of the OSI Model. (10 Marks Answer)

Causes of Congestion

  • High traffic load.
  • Limited bandwidth.
  • Slow routers or processing.
  • Excessive retransmissions.

Types of Congestion Control Algorithms

1. Open Loop Congestion Control (Prevention)

Prevents congestion before it occurs without feedback from the network.

  • Traffic shaping: Controls the rate of data transmission.
  • Admission control: Limits entry of new traffic.
  • Resource reservation: Allocates resources in advance.
  • Advantage: Avoids congestion proactively.
  • Disadvantage: Cannot react to unexpected conditions.

2. Closed Loop Congestion Control (Feedback-Based)

Detects and reacts to congestion using feedback.

  • Backpressure: Congested node asks upstream nodes to slow down.
  • Choke Packets: Router sends a message to the sender to reduce the rate.
  • Implicit Signaling: Sender detects congestion through delays or packet loss.
  • Explicit Signaling: Network explicitly informs the sender about congestion.

Transport Layer Algorithms (TCP Based)

  • Slow Start: Starts with a small transmission rate and increases it exponentially.
  • Congestion Avoidance: Increases transmission rate slowly after a threshold.
  • Fast Retransmit: Retransmits lost packets quickly without waiting for a timeout.
  • Fast Recovery: Reduces the congestion window and continues transmission.

Pros and Cons

  • Advantages: Improves network performance, reduces packet loss, ensures fair bandwidth usage.
  • Disadvantages: Adds complexity, may reduce transmission speed temporarily, requires continuous monitoring.

Quality of Service (QoS) in Networking

Quality of Service (QoS) refers to the set of techniques used in networking to manage and prioritize traffic so that important data (like voice or video) gets better service than less critical data. It ensures reliable and efficient communication across a network.

Need for QoS

  • Different applications have different requirements.
  • Real-time services (video calls, streaming) need low delay and high reliability.
  • Prevents network congestion and ensures smooth performance.

QoS Parameters

  • Bandwidth: Amount of data that can be transmitted per second.
  • Delay (Latency): Time taken for data to travel from source to destination.
  • Jitter: Variation in delay between packets.
  • Packet Loss: Number of packets lost during transmission.

QoS Techniques

  • Traffic Shaping: Controls the rate of data transmission (e.g., Leaky Bucket, Token Bucket).
  • Scheduling: Determines the order in which packets are sent (e.g., FIFO, Priority Scheduling).
  • Resource Reservation: Reserves bandwidth for important applications.
  • Admission Control: Limits new traffic when the network is congested.

QoS Models

  • Integrated Services (IntServ): Provides guaranteed QoS using resource reservation.
  • Differentiated Services (DiffServ): Classifies traffic into different priority levels; more scalable than IntServ.

Pros and Cons

  • Advantages: Improves performance of real-time applications, ensures better user experience, efficient use of network resources.
  • Disadvantages: Complex to implement, requires additional configuration, may increase cost.

Internetworking Concepts and Devices

Internetworking is the process of connecting multiple independent networks to form a larger network, enabling communication between devices across different networks. The best example of internetworking is the Internet.

Need for Internetworking

  • Different networks use different technologies and protocols.
  • Allows communication between geographically distant systems.
  • Enables resource sharing across networks.
  • Supports global connectivity.

Devices Used in Internetworking

  • Router: Connects different networks and uses IP addresses to forward packets.
  • Gateway: Connects networks with different protocols and performs protocol conversion.
  • Bridge: Connects similar networks at the Data Link Layer.
  • Switch: Connects devices within a LAN.

Key Concepts and Types

Key concepts include Addressing (unique IP addresses), Routing (best path determination), and Packet Switching (independent data packets).

Types of Internetworking

  • LAN-to-LAN: Connecting two local networks.
  • LAN-to-WAN: Connecting a local network to a wide area network.
  • WAN-to-WAN: Connecting large networks across regions.

Pros and Cons

  • Advantages: Enables global communication, resource sharing, scalability, and flexibility.
  • Disadvantages: Complex network management, security issues, higher cost of setup and maintenance.

The Network Layer in the Global Internet

The Network Layer in the Internet is responsible for delivering packets from source to destination across multiple networks. It corresponds to the Internet layer in the TCP/IP model and the Network Layer of the OSI Model.

Main Functions

  • Logical Addressing: Uses IP (Internet Protocol) addresses to uniquely identify devices.
  • Routing: Routers use routing tables and algorithms to determine the best path.
  • Packet Forwarding: Moves packets from one router to another across interconnected networks.
  • Fragmentation: Breaks large packets into smaller ones to match network limits, reassembled at the destination.

Key Protocols

  • IP (Internet Protocol): Core protocol providing connectionless, best-effort delivery.
  • ICMP (Internet Control Message Protocol): Used for error reporting and diagnostics (e.g., Ping).
  • ARP (Address Resolution Protocol): Maps IP addresses to MAC addresses.

Characteristics and Role of Routers

  • Connectionless: No dedicated path is established.
  • Best-effort delivery: No guarantee of delivery.
  • Packet-based communication: Data is sent in discrete units.
  • Routers: The main devices that examine destination IPs and forward packets accordingly.

Pros and Cons

  • Advantages: Enables global communication, scalable, flexible, and supports heterogeneous networks.
  • Disadvantages: No guarantee of delivery (packet loss possible), congestion may cause delays, requires complex routing mechanisms.