Key Functions of the Network Layer

• Routing: The network layer is responsible for determining the path that a packet should take from the source to the destination. When a packet arrives at a router's input link, the router makes decisions on how to forward it to the appropriate output link. For instance, a packet from source S1 to destination R1 needs to be routed through the next router on the path to S2.
• Logical Addressing: While the data link layer deals with physical addressing, the network layer handles logical addressing. Logical addresses help differentiate between source and destination systems. When forwarding a packet, the network layer adds a header containing logical addresses for both the sender and the receiver.
• Internetworking: The primary role of the network layer is to establish logical connections between different types of networks. It enables communication across diverse network architectures and technologies.
• Fragmentation: Fragmentation involves breaking packets into smaller individual data units to facilitate transmission through various networks. This process ensures that data can traverse networks with varying maximum packet sizes.

Services Offered by the Network Layer

• Guaranteed Delivery: The network layer provides a service ensuring that packets will reach their destination reliably.
• Guaranteed Delivery with Bounded Delay: This service guarantees that packets will be delivered within a specified host-to-host delay bound, ensuring timely communication.
• In-Order Packets: The network layer ensures that packets arrive at the destination in the same order in which they were sent, maintaining the sequence of transmitted data.
• Guaranteed Max Jitter: This service ensures that the time between two successive transmissions at the sender is equal to the time between their receipt at the destination, minimizing variation in transmission timing.
• Security Services: The network layer enhances security through the use of a session key between the source and destination hosts. Datagram payloads sent from the source are encrypted, and the destination host decrypts the payload, maintaining data integrity and source authentication.

Types of Network Layer Services

Within the network layer, services are categorized based on connections, presenting two distinct types:

• Connectionless Service

  Connectionless services involve the routing and individual insertion of packets into the subnet without requiring any additional setup. This approach facilitates independent packet handling.

• Connection-Oriented Service

  For connection-oriented services, the subnet must provide a reliable service, ensuring that all packets are transmitted over a single route. The implementation of this service involves:

  • Implementation of Connectionless Service: Packets, termed as "datagrams," are transmitted independently to a router via a few protocols. When the message size to be transmitted is four times the size of a packet, the network layer divides it into four packets, each with a destination address, and routes them independently.
  • Implementation of Connection-Oriented Service: This service operates by first establishing a connection, utilizing it for data transmission, and then releasing it. Data packets in connection-oriented services are delivered to the receiver in the same order as sent by the sender. There are two ways to implement this service:
  • Circuit Switched Connection: Involves establishing a dedicated physical path or circuit between communicating nodes before transferring the data stream.
  • Virtual Circuit Switched Connection: The data stream is transferred over a packet-switched network, creating the illusion of a dedicated path from the sender to the receiver. A virtual path is established, with the possibility of other connections also utilizing the same path.

Virtual Circuit switching vs. Datagram Networks

The Network Layer plays a crucial role in managing communication between devices. As we navigate through this layer's functionalities, we encounter a significant choice that influences data transmission strategies—Virtual Circuit Switching and Datagram Network. These two approaches offer distinct methods for forwarding data packets across networks, each bringing its own set of characteristics and implications. To gain a comprehensive understanding of this decision-making process, it's imperative to explore the core concepts of Virtual Circuit Switching and Datagram Network and appreciate how they contribute to the efficiency and reliability of data transfer within the Network Layer.

IPv4 in the Network Layer

It stands for Internet Protocol version 4. IPv4 stands as a cornerstone protocol, serving as the underlying communication mechanism that facilitates data transfer across interconnected devices. As we delve into the intricacies of the Network Layer, understanding IPv4 becomes imperative, as it forms the backbone of the internet and enables the seamless exchange of data packets. IPv4, along with its associated protocols, plays a pivotal role in addressing, routing, and ensuring the reliable delivery of information between devices within a network. Let's explore the key protocols associated with IPv4 and unravel the mechanisms that underpin its crucial role in modern computer networking.

• IPv4 serves as a foundational protocol within the Network Layer, specifically designed for packet-switched networks. Operating on a best effort delivery model, IPv4 does not guarantee delivery, proper sequencing, or avoidance of duplicate delivery. Widely recognized as the fourth revision of the Internet Protocol, IPv4 plays a pivotal role in data communication across diverse networks.
• Connectionless in nature, IPv4 establishes logical connections between network devices by assigning unique identifications to each device. The protocol is extensively employed in packet-switched layer networks like Ethernet, providing essential addressing and routing functionalities. IPv4 configuration methods vary, including manual and automatic configurations, depending on the network type.
• IPv4 is formally defined and specified in the IETF publication RFC 791. It utilizes 32-bit addresses, divided into five classes—A, B, C, D, and E—for Ethernet communication. Classes A, B, and C differ in bit length for addressing the network host, while Class D addresses are reserved for military purposes, and Class E addresses are reserved for future use.
• With 32-bit (4-byte) addressing, IPv4 accommodates 2^32 addresses. These addresses are expressed in dot-decimal notation, comprising four octets separated by periods. For instance, an IPv4 address may appear as 192.168.1.5.

Network Layer Design Issues

The network layer introduces several design issues, each playing a crucial role in shaping the functionality of this layer. These issues are outlined as follows:

• Store and Forward Packet Switching

  In the context of network layer design, the first notable issue is "Store and Forward packet switching." Here, the host sends a packet to the nearest router, where it is stored until fully arrived. Once the link is fully processed, including checksum verification, the packet is forwarded to the next router in a sequential manner until it reaches its destination.

• Services provided to Transport Layer

  Through the network/transport layer interface, the network layer extends its services to the transport layer. These services are provided with specific goals in mind:

  • Offering services that do not depend on router technology.
  • Protecting the transport layer from details such as the type, number, and topology of available routers.
  • Ensuring uniform numbering patterns for network addresses used by the transport layer, both at LAN and WAN connections.

Repeater

• A repeater operates at the Physical layer of the networking model.
• Its primary function is to receive incoming signals, and subsequently, re-transmit these signals to extend their reach over longer distances. Essentially, repeaters help overcome the limitations of signal attenuation during transmission.
• Contrary to common belief, repeaters do not amplify the signal. Instead, when the signal weakens, repeaters replicate the signal's data bit by bit and then regenerate it back to its original strength. This process ensures that the signal remains intelligible and usable despite the distance it travels.
• Structurally, repeaters are designed with two ports: one for signal input and another for signal output. This allows them to intercept incoming signals, reinvigorate them, and then forward them to the next segment of the network.
• Limitations: While repeaters are effective at combating signal degradation, they are not a universal solution. They cannot address issues like latency, noise, or errors introduced during transmission. For these challenges, other networking components like routers, switches, and error correction mechanisms come into play.
• Chain of Repeaters: In scenarios where very long distances need to be covered, a chain of repeaters can be used. Each repeater in the chain restores and retransmits the signal, further extending its reach.
• Application: Repeaters find use in various communication mediums, including wired and wireless networks. For example, they are employed in Ethernet networks to counteract signal degradation over lengthy cable runs.

Hub

• A hub functions as a multi-port networking device, similar to a multiport repeater, due to its ability to regenerate and distribute signals to multiple connected devices.
• Operating primarily at the Physical layer of networking, a hub's primary role is to facilitate connections among computers within a Local Area Network (LAN), allowing them to communicate and share resources.

Hubs are of three types:

1. Active hub: Equipped with its own power supply, the active hub not only rejuvenates weak signals but also amplifies and reinforces them across the network. Essentially functioning as both a repeater and a central point for connecting cables, the active hub enhances signal quality and serves as a distribution hub.
2. Passive hub: This hub serves as a collection point for network nodes' wiring while drawing its power supply from an active hub. It does not possess signal amplification capabilities but rather facilitates the physical interconnection of devices within the network.
3. Intelligent hub: Operating in a manner similar to an active hub, the intelligent hub offers the additional feature of remote management capabilities. This enables network administrators to monitor and manage the hub's performance, connections, and potentially connected devices from a central location.

Bridge

• Operating at the Data Link layer of the networking model, a bridge is a device that combines the functionalities of a repeater with the ability to intelligently filter content based on the source and destination MAC addresses.
• Bridges serve as a means of interconnecting two separate Local Area Networks (LANs) that operate on the same network protocol, extending network connectivity.
• Unlike hubs, which indiscriminately broadcast data to all connected devices, bridges implement a level of intelligence by selectively transmitting data only to the devices that require it. This filtering is achieved by examining the MAC addresses of both the source and destination devices in the data frame.

• Filtering Mechanism: A bridge maintains a table known as a MAC address table or bridge table. This table records the MAC addresses of devices connected to each of its ports. When a data frame arrives, the bridge checks the destination MAC address against the entries in its table and forwards the frame only to the port associated with the destination device, reducing unnecessary network traffic and improving efficiency.

Switch

• Functioning at the Data Link layer of the networking model, a switch is a multiport networking device that builds upon the foundation of a bridge. It combines intelligent content filtering with enhanced efficiency and performance features.
• Similar to a multiport bridge, a switch includes additional capabilities, such as buffering and specialized design considerations, that contribute to its improved performance compared to traditional bridges.
• One of the key differentiators between a switch and a bridge lies in the number of available ports. While bridges typically have only two ports, switches are designed with multiple ports, enabling numerous devices to simultaneously establish connections and communicate.
• Unlike older networking setups where bridges could transmit data only between two devices at a time, switches can concurrently process data from multiple devices, reducing latency and enhancing overall network throughput.
• Similar to bridges, switches also employ MAC address tables to learn the association between MAC addresses and the corresponding switch ports. As devices transmit data, the switch populates its table to optimize data forwarding.

Router

• A router is a device similar to a switch, but it operates by directing data packets based on their IP addresses.
• Its primary role falls within the network layer of networking.
• While a switch excels at connecting devices within a network, a router has the unique ability to connect different networks together, including LANs and WANs.
• One distinctive feature of routers is their dynamically updated routing table. This table acts as a map that guides routers in deciding where to send data. It holds vital information, including various IP addresses.
• Routers are like postal workers for the internet. They look at IP addresses on data packets and decide how to best deliver them, ensuring data reaches its intended destination.
• Routers play a critical role in managing internet traffic. They determine the fastest and most efficient paths for data to travel across the complex web of networks that make up the internet.

Gateway

• A gateway is a network node that links two separate networks using different protocols.
• It functions as a "gate" that connects and enables communication between these distinct networks.
• A gateway can take various forms, such as a router, firewall, server, or other devices. Its role is to allow traffic to smoothly enter and exit a network.
• One of the key roles of a gateway is to translate data between different network protocols. This is crucial when connecting networks that use different rules for communication.
• A gateway acts as a mediator, helping networks with different communication methods understand and exchange information.
• In the conNetwork Layer of the internet, a gateway often refers to a device that connects a local network to the broader internet, managing data flow and security.