Develop understanding of Network layer (design issues, routing algorithms, Congestion control algorithms, Internetworking, the network layer in the internet (IPv4 and IPv6), Quality of Service)

Routing is the process of forwarding packets from one network to another. It relies on routing algorithms to determine the optimal path.

Types of Routing Algorithms

• Adaptive Routing Algorithms: These are "intelligent" algorithms that dynamically adjust to changing network conditions (like congestion or link failure).

  • Centralised: A central control center calculates the best path for everyone.
  • Isolated: Each router makes decisions based only on information from its immediate neighbors.
  • Distributed: Routers communicate with each other to build a global picture of the network (e.g., OSPF, EIGRP).

• Non-Adaptive Routing Algorithms: These use static routing tables that do not change.

  • Flooding: Sends packets to all neighbors (except the sender). Ensures delivery but creates high traffic.
  • Random Walks: Sends packets to a random neighbor. Unreliable for finding the best path.

Routing Protocols

These are the software programs that implement routing algorithms.

• Interior Gateway Protocols (IGP): Used inside a single organization (e.g., RIP, OSPF).
• Exterior Gateway Protocol (EGP): Used between different organizations or to the Internet (e.g., BGP).

Supporting Protocols

• Address Resolution Protocol (ARP): Translates a known IP address into a MAC (hardware) address so devices on the same local network can communicate.
• Internet Control Message Protocol (ICMP): Used for error reporting and diagnostics (e.g., the "Ping" command).

Network congestion occurs when too much data floods the network, causing delays and packet loss. The network layer uses algorithms to manage this flow.

• Leaky Bucket Algorithm (LAB): Controls the rate of packets entering the network. It acts like a bucket with a hole; water (data) leaks out at a constant rate. If the bucket is full, incoming data is discarded.
• Token Bucket Algorithm (TBA): Allows for bursty traffic. Data can only be sent if there are "tokens" in the bucket. The bucket refills at a fixed rate. This permits short bursts of high-speed transmission without overwhelming the network.

QoS mechanisms prioritize certain types of traffic (like voice or video) over others (like email) to ensure performance for critical applications.

• Importance: Essential for real-time applications (video conferencing, VoIP) where delay is unacceptable.
• Key Components:
  • Traffic Prioritization: Classifying data (DiffServ).
  • Bandwidth Management: Allocating specific amounts of bandwidth to different types of traffic.
  • Packet Scheduling: Algorithms like Weighted Fair Queuing (WFQ) determine the order in which packets are transmitted.

The Network layer uses IP addresses to identify devices and locate them on the network.

IPv4 Addressing

An IPv4 address is a 32-bit number represented in dotted decimal notation (e.g., 192.168.3.24). It consists of a Network ID (identifying the specific network) and a Host ID (identifying the specific device within that network).

• Classes:

  • Class A: Large networks (1.0.0.0 to 126.255.255.255). Mask: 255.0.0.0 (/8).
  • Class B: Medium networks (128.0.0.0 to 191.255.255.255). Mask: 255.255.0.0 (/16).
  • Class C: Small networks (192.0.0.0 to 223.255.255.255). Mask: 255.255.255.0 (/24).

• Private IP Addresses: Reserved for internal networks (not routed on the Internet):

  • 10.0.0.0/8
  • 172.16.0.0/12
  • 192.168.0.0/16

Subnetting

Subnetting divides a large network into smaller, manageable sub-networks. This improves performance and security.

Worked Example: Creating Subnets A company has the IP network 192.168.1.0/24 and needs to create 4 subnets.

1. Determine the subnet mask:

   • Original default mask: 255.255.255.0 (/24).
   • To get 4 subnets, we need to borrow 2 bits ($2^2 = 4$).
   • New bits in the host portion become network bits (1s).
   • New Mask: 255.255.255.192 (/26) or 11111111.11111111.11111111.11000000.

2. Calculate subnet addresses:

   • The "block size" is 256 - 192 = 64.
   • Subnet 1: 192.168.1.0 (Range: 192.168.1.1 – 192.168.1.62)
   • Subnet 2: 192.168.1.64 (Range: 192.168.1.65 – 192.168.1.126)
   • Subnet 3: 192.168.1.128 (Range: 192.168.1.129 – 192.168.1.190)
   • Subnet 4: 192.168.1.192 (Range: 192.168.1.193 – 192.168.1.254)

Note: The first address is the Network address and the last is the Broadcast address, so they cannot be assigned to hosts.

IPv6 Addressing

IPv6 was developed to address the shortage of IPv4 addresses. It uses a 128-bit address (e.g., 2001:0db8:3c4d:0015:0000:0000:abcd:ef12).

• Address Types: Unicast (one-to-one), Multicast (one-to-many), Anycast (one-to-nearest).
• Header: Simpler and more efficient than IPv4, with built-in support for security and QoS.

In Tanzania, understanding the Network layer is crucial for a network administrator at a school or a small business in Dar es Salaam. For instance, if a school wants to set up a computer lab with 30 computers and separate the staff Wi-Fi network from the student network, they must apply subnetting. Using a private IP block like 192.168.1.0/24, they can create subnets to isolate traffic, improve security, and manage bandwidth allocation for different departments using Quality of Service (QoS) settings on their router.