Network Layer and Internetworking

Table of Contents

• Network Layer
• Network Layer Functionalities
• Network Layer Services
• Internetworking

At the physical and data link layers, the TCP/IP reference architecture does not prescribe any specific protocol. Instead, it accommodates a broad spectrum of standard and proprietary protocols within the underlying network. In TCP/IP internetwork, the network can take the form of either a local-area network or a wide-area network.

The network layer, often referred to as the TCP/IP Internet Layer, serves as the linchpin holding the entire architecture together. Its primary function is to enable hosts to inject packets into any network and allow them to traverse independently to their destination, potentially spanning different networks. These packets might even reach the destination in a different order than they were originally sent. In such cases, it becomes the responsibility of higher layers to rearrange them if achieving in-order delivery is a desired outcome.

 

Network Layer

The Network Layer, positioned as the third layer in the OSI model, manages service requests initiated by the transport layer and subsequently directs these service requests to the data link layer. This layer is tasked with facilitating source-to-destination delivery of a packet across diverse networks. Its role ensures that every packet successfully traverses from its origination point to its intended destination. Notably, the network layer does not establish any inherent relationship between these packets; instead, it treats each packet independently, as if they all pertain to distinct messages.

 

Network Layer Functionalities

The primary function of the network layer is to facilitate the movement of packets from a sending host to a receiving host. It accomplishes this by translating logical addresses into physical addresses, determining the route from the source to the destination, and effectively managing traffic issues such as switching routing, and congestion control of data packets.

Key functionalities of the network layer include:

1. Internetworking: This involves logically connecting heterogeneous physical networks to appear as a unified network to the upper layers.
2. Logical Addressing: When a packet crosses network boundaries, the header contains information about the logical addresses of both the sender and the receiver.
3. Routing: In scenarios where independent networks or links are interconnected to form an internetwork or a larger network, the connective devices route the packet to its final destination.
4. Packetizing: The network layer encapsulates packets received from upper-layer protocols, with the Internet Protocol commonly employed for packetizing.
5. Fragmenting: Routers process incoming frames, encapsulating them based on the protocol used by the target physical network to which the frame is destined.

 

Network Layer Services

As the third layer in the OSI model architecture, the Network layer is the uppermost interface with the transmission channel. Its primary services include:

1. Guaranteed Delivery: This service ensures the packet's reliable arrival at its intended destination.
2. Minimum Bounded Delay: Guaranteeing a specified host-to-host delay bound, this service ensures the timely delivery of packets.
3. In-Order Packets: This service guarantees that packets reach the destination in the exact order they were sent.
4. Guaranteed Max Jitter: Ensuring consistent time intervals between successive transmissions at the sender and their reception at the receiver, this service minimizes variation in timing gaps.  

Additionally, the Network layer provides security services such as:

• Data Integrity: Utilizing a session key between the source and destination hosts, the Network layer encrypts the payloads of datagrams at the source host, ensuring the secure transmission of data.
• Source Authentication: With the use of a session key, the Network layer authenticates the source, decrypting the payload at the destination host to ensure data integrity and source verification.

 

Internetworking

Internetworking refers to the establishment and management of internetwork, which is a collective arrangement of individual networks interconnected by intermediate networking devices. This interconnected system operates cohesively as a unified and extensive network. An internetwork, often termed a network of networks, consists of numerous smaller networks linked by nodes known as routers, sometimes referred to as gateways.

Essentially, internetworking involves connecting two or more computer networks through network devices like routers or gateways, employing a common routing technology. The outcome of this process is referred to as internetwork, commonly abbreviated to the term " internet ."

Internetwork

The illustration above showcases various network technologies interconnected by routers and other networking devices, forming an internetwork. The Internet Protocol (IP) or Internetworking Protocol serves as the fundamental tool in constructing scalable and diverse Internetworks.