Network-Layer Protocols in the TCP/IP Protocol Suite

 

Network-Layer Protocols in the TCP/IP Protocol Suite

In the TCP/IP protocol suite, the network layer is responsible mainly for delivering packets from a source host to a destination host across one or more interconnected networks.

The IPv4 network layer can be viewed as consisting of one main protocol and three auxiliary protocols.

1. Protocols at the IPv4 Network Layer

The four important protocols are:

ProtocolFull FormMain Function
IPv4    Internet Protocol version 4Packetizing, forwarding, and delivery of packets
ICMPv4    Internet Control Message Protocol              version 4Handles/report certain errors related to network-layer delivery
IGMP    Internet Group Management ProtocolSupports multicasting
ARP    Address Resolution ProtocolMaps network-layer addresses to link-layer addresses

The relationship can be represented as:

              TCP/IP Protocol Suite

Application Layer
        │
        │
Transport Layer
     TCP / UDP
        │
        │
────────────────────────────────
       Network Layer
────────────────────────────────
        │
        ├── IPv4      ← Main protocol
        │
        ├── ICMPv4    ← Error handling
        │
        ├── IGMP      ← Multicasting
        │
        └── ARP       ← Address mapping
        │
────────────────────────────────
      Data-Link Layer
────────────────────────────────
        │
────────────────────────────────
       Physical Layer
────────────────────────────────

2. IPv4 – Main Network-Layer Protocol

IPv4 (Internet Protocol version 4) is the main protocol at the network layer.

Its major responsibilities are:

a. Packetizing

Data received from the upper layer is encapsulated into an IPv4 packet (datagram).

Transport-layer data
        ↓
    IPv4 Header
        +
       Data
        ↓
    IPv4 Datagram

b. Forwarding

When a packet reaches a router, IPv4 information is used to determine where the packet should be forwarded next.

Source
  ↓
Router 1
  ↓
Router 2
  ↓
Router 3
  ↓
Destination

c. Delivery

IPv4 provides the mechanism for delivering the packet from the source host toward its destination across interconnected networks.

3. ICMPv4

ICMPv4 (Internet Control Message Protocol version 4) is an auxiliary protocol that helps IPv4 deal with certain errors that can occur during network-layer delivery.

For example, when a packet cannot be delivered, ICMP can be used to communicate information about the problem.

Therefore:

ICMPv4 → Error reporting and network-layer control information

It works alongside IPv4 rather than replacing it.

4. IGMP

IGMP (Internet Group Management Protocol) is another auxiliary protocol associated with IPv4.

Its purpose is to support multicasting.

Multicasting

In multicasting, one sender can send information to a group of receivers.

             Receiver 1
                 ↑
                 │
Sender ─────── Network ────→ Receiver 2
                 │
                 ↓
             Receiver 3

Thus:

IGMP → Helps IPv4 support multicasting.


5. ARP

ARP (Address Resolution Protocol) acts as a link between the network layer and data-link layer.

Its main purpose is to map a network-layer address to a link-layer address.

For example:

Network Layer
IP Address
    ↓
   ARP
    ↓
Data-Link Layer
Link-layer / MAC Address

Thus:

ARP → Maps network-layer addresses to link-layer addresses.


6. Position of the Four Protocols

A useful way to understand their positions is:

                    TCP/IP Protocol Suite
                           
             ┌─────────────────────────────┐
             │      Application Layer      │
             └─────────────────────────────┘
                         │
                         ↓
             ┌─────────────────────────────┐
             │       Transport Layer       │
             │          TCP / UDP          │
             └─────────────────────────────┘
                         │
                         ↓
       ┌───────────────────────────────────────────┐
       │             NETWORK LAYER                 │
       │                                           │
       │  IPv4  ─── Main network-layer protocol   │
       │                                           │
       │  ICMPv4 ─ Error reporting/support        │
       │                                           │
       │  IGMP  ── Multicast support              │
       │                                           │
       │  ARP   ── Address mapping to link layer  │
       └───────────────────────────────────────────┘
                         │
                         ↓
             ┌─────────────────────────────┐
             │       Data-Link Layer       │
             └─────────────────────────────┘
                         │
                         ↓
             ┌─────────────────────────────┐
             │        Physical Layer       │
             └─────────────────────────────┘

Important point about ARP

ARP is particularly important because it connects the network-layer addressing scheme with the data-link layer. Therefore, although it is associated with the network layer in this discussion, it interacts closely with the data-link layer.

7. Nature of IPv4

We characterizes IPv4 using three important terms:

1. Unreliable

IPv4 does not guarantee successful delivery.

A packet may:

  • be lost,
  • be corrupted,
  • arrive out of order, or
  • be delayed.

IPv4 itself does not provide mechanisms such as retransmission to guarantee reliability.

2. Connectionless

IPv4 does not establish a connection before sending packets.

Each packet is treated as an independent datagram.

For example:

Packet 1 ──→ Route A ──→ Destination
Packet 2 ──→ Route B ──→ Destination
Packet 3 ──→ Route A ──→ Destination

Packets belonging to the same message may follow different routes.

3. Best-Effort Delivery

IPv4 provides a best-effort delivery service.

This means that IPv4 makes an effort to deliver packets, but it does not guarantee that every packet will reach the destination correctly.

We can  compares this with ordinary postal mail: the postal system makes an effort to deliver a letter, but it does not guarantee delivery of every ordinary letter.

8. Relationship Between IPv4 and TCP

An important concept is that IPv4 does not provide reliability.

If an application requires reliable communication, IPv4 can work together with a reliable transport-layer protocol such as TCP.

Application
     │
     ↓
    TCP
     │
     ↓
   IPv4
     │
     ↓
Data-Link
     │
     ↓
Physical

Here:

  • TCP provides reliability, sequencing, retransmission, etc.
  • IPv4 provides packetizing, forwarding, and delivery.
  • Data-link layer handles delivery across an individual link.

So, reliability can be provided by a higher layer, while IPv4 itself remains a best-effort, connectionless protocol.

9.Summary

              IPv4 NETWORK LAYER

                 ┌─────────┐
                 │  IPv4   │
                 │  MAIN   │
                 └────┬────┘
                      │
        ┌─────────────┼──────────────┐
        ↓             ↓              ↓
     ICMPv4          IGMP            ARP
   Error handling   Multicast    Address mapping
                                   to link layer


IPv4 Characteristics
─────────────────────
• Unreliable
• Connectionless
• Datagram-based
• Best-effort delivery
• Packets handled independently
• Packets may follow different routes
• Reliability, when required, is provided by higher layers


The network layer in the TCP/IP protocol suite provides packet delivery across interconnected networks. In IPv4, the network layer consists of IPv4 as the main protocol, supported by ICMPv4 for error handling, IGMP for multicasting, and ARP for mapping network-layer addresses to link-layer addresses. IPv4 provides an unreliable, connectionless, best-effort datagram delivery service.

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