Internet/Network Layer TCP/IP

Introduction of TCP/IP Network Layer

• The TCP/IP Network Layer is often referred to as the Internet Layer.
• The TCP/IP Network Layer provides the essential functions of addressing, routing, and packet forwarding to enable seamless data exchange across vast and interconnected networks, making the global internet possible.

Definition of TCP/IP Network Layer

• The TCP/IP Network Layer is a critical part of the TCP/IP protocol suite that is responsible for addressing, routing, and forwarding data across networks. 
• The Network Layer forms the backbone of the internet by ensuring that packets are addressed and routed efficiently from one device to another, regardless of the physical network or geographical location. It sits between the Link Layer (responsible for local network communications) and the Transport Layer (which ensures reliable data transmission).

Features of TCP/IP Network Layer

• In the TCP/IP model, this layer corresponds closely to the Network Layer (Layer 3) in the OSI model.
• The Network Layer is connectionless, meaning that each packet is treated independently, with no established connection between the source and destination. This is why IP is often described as a “best-effort” service, with no guarantees that packets will be delivered in order or even delivered at all. Each packet has a TTL(Time-to-Live) value, which is decremented by routers as it passes through them. When TTL reaches zero, the packet is discarded to prevent infinite looping.
• The network layer examines network or logical addresses in the data instead of physical addresses seen in the Data Link layer.
• The network layer establishes, maintains, and terminates logical and/or physical connections.
• The network layer is responsible for translating logical addresses or names into physical addresses.

Functions/Roles of TCP/IP Network Layer

There are the following common functions performed by the TCP/IP network layer-

• Logical Addressing (IP Addressing):
  • The Network Layer is responsible for assigning logical addresses (IP addresses) to devices.
  • This layer ensures that every device connected to the network has a unique IP address to identify both the source and destination of the data packets.
  • IPv4 and IPv6 are the two main versions of IP addresses used in the Network Layer.
• Routing:
  • The Network Layer determines the best path for data packets to travel from the source to the destination across multiple interconnected networks (internet).
  • In this layer, Routers operate at this layer and forward packets based on the destination IP address.
  • In this layer, Routing protocols like OSPF (Open Shortest Path First), BGP (Border Gateway Protocol), and RIP (Routing Information Protocol) are used to make routing decisions.
• Packet Forwarding:
  • In this layer, once the best path is determined, packets are forwarded from one router to the next until they reach their final destination.
  • The layer encapsulates the data received from the upper layers into IP packets and handles fragmentation, if necessary, for sending over different types of networks.
• Fragmentation and Reassembly:
  • The Network Layer divides large data units into smaller fragments, which can fit within the maximum transmission unit (MTU) of the network being used.
  • On reaching the destination, these fragments are reassembled into the original data by the receiving device.

Protocols of TCP/IP Network Layer

• Network layer protocols are considered as management protocols that can be categorized into :-
  • Routing protocols
  • Multicast Group Management Protocols
  • Network-Layer Address Assignment Protocols
• There are the following TCP/IP network layer protocols –