Network Layer – Design Issues

 Network Layer – Design Issues

• The Network Layer is the third layer (Layer 3) of the OSI model. 
• Its main responsibility is to deliver packets from the source network to the destination network, even when multiple networks and routers exist between them.

Basic Network Layer Diagram

┌──────────────┐
│ Source Host  │
│ 192.168.1.10 │
└──────┬───────┘
       │
       ▼
  ┌─────────┐       ┌─────────┐       ┌──────────────┐
  │ Router 1│ ────► │ Router 2│ ────► │Destination   │
  └─────────┘       └─────────┘       │Host          │
                                      │192.168.3.10   │
                                      └──────────────┘

             NETWORK LAYER
       Provides packet delivery
       across multiple networks

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What are Network Layer Design Issues?

• Network Layer design issues are the important problems and decisions that must be considered while designing the Network Layer so that packets can be delivered efficiently, reliably, and correctly.

The major design issues are:

1. Store-and-Forward Packet Switching
2. Services Provided to the Transport Layer
3. Implementation of Connectionless Service
4. Implementation of Connection-Oriented Service
5. Routing
6. Congestion Control
7. Quality of Service (QoS)
8. Internetworking
9. Fragmentation and Reassembly

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1. Store-and-Forward Packet Switching

• A router generally receives a packet, stores it temporarily, examines its header, and then forwards it to the next router.
• This process is called Store-and-Forward Packet Switching.

Diagram

       Packet
         │
         ▼
┌────────────────┐
│    Router 1    │
│    Receive     │
│    Store       │
│    Forward     │
└───────┬────────┘
        │
        ▼
┌────────────────┐
│    Router 2    │
│    Receive     │
│    Store       │
│    Forward     │
└───────┬────────┘
        │
        ▼
   Destination

• The router normally does not forward the packet until it has received enough information to process it.

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2. Services Provided to the Transport Layer

• The Network Layer provides services to the Transport Layer (Layer 4).
• The service should define how packets are delivered from source to destination.

Two important approaches are:

Connectionless Service

• Each packet is treated independently.

Packet 1 ──► Router ──► Destination
Packet 2 ──► Router ──► Destination
Packet 3 ──► Router ──► Destination

• This is commonly associated with the datagram approach.

Connection-Oriented Service

• A logical connection is established before data transfer.

Connection Setup
       ↓
Data Transfer
       ↓
Connection Release

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3. Routing

• Routing is one of the most important functions of the Network Layer.
• Routing determines the path that packets should take from source to destination.

Example

             ┌─────────┐
        ┌───►│ Router 1│───┐
        │    └─────────┘   │
        │                  ▼
Source ─┤              ┌─────────┐
        │              │ Router 3│ ───► Destination
        │              └─────────┘
        │                  ▲
        │    ┌─────────┐   │
        └───►│ Router 2│───┘
             └─────────┘

• The routing algorithm selects an appropriate path.
• Examples of routing protocols include:

• RIP
• OSPF
• BGP

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4. Congestion Control

• Congestion occurs when too many packets are present in a network and the network cannot handle the traffic efficiently.

Diagram

PC1 ──┐
PC2 ──┤
PC3 ──┼────► Router ─────► Destination
PC4 ──┤       ▲
PC5 ──┘       │
          Too much traffic
               ↓
            Congestion

Congestion can result in:

• Packet loss
• Increased delay
• Reduced network performance

Therefore, the Network Layer needs mechanisms to manage congestion.

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5. Quality of Service (QoS)

• Quality of Service (QoS) refers to techniques used to provide appropriate network performance for different types of traffic.

Important QoS parameters include:

• Bandwidth
• Delay
• Jitter
• Packet loss

For example, voice and video applications can be sensitive to delay and jitter.

Voice/Video ──► Network ──► Destination
                  │
                  ▼
             QoS Management

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6. Internetworking

• Different networks may use different technologies and characteristics.
• The Network Layer helps connect these different networks into an internetwork.

 LAN 1              WAN              LAN 2

┌─────────┐      ┌─────────┐      ┌─────────┐
│ PCs     │─────►│ Routers │─────►│ Servers │
│ Network │      │         │      │ Network │
└─────────┘      └─────────┘      └─────────┘

• IP is the major protocol used to provide internetworking across heterogeneous networks.

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7. Fragmentation and Reassembly

• Sometimes a packet may be larger than the Maximum Transmission Unit (MTU) supported by a network.
• The packet may need to be fragmented according to the protocol and network conditions.

IPv4

• In IPv4, fragmentation can be performed by the source or routers, depending on the packet and configuration.

             Large Packet
                  │
                  ▼
              ┌───────┐
              │Router │
              └───┬───┘
                  │
           ┌──────┼──────┐
           ▼      ▼      ▼
        Fragment Fragment Fragment
           │      │      │
           └──────┼──────┘
                  ▼
             Destination

IPv6

• IPv6 routers do not fragment packets. If fragmentation is required, it is performed by the source using the appropriate IPv6 fragmentation mechanism.

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8. Connectionless vs Connection-Oriented Service

Feature	Connectionless	Connection-Oriented
Connection setup	Not required	Required
Packet treatment	Independent	Part of a logical connection
Example concept	Datagram	Virtual circuit
Routing	Packets may be routed independently	Typically associated with an established path/state
Overhead	Generally lower	Generally higher

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9. Important Network Layer Design Issues

• For examination, remember the following:

              NETWORK LAYER
                    │
       ┌────────────┼────────────┐
       ▼            ▼            ▼
    Routing     Congestion    QoS
                  Control
       │            │            │
       └────────────┼────────────┘
                    ▼
             Internetworking
                    │
                    ▼
             Packet Delivery
                    │
              ┌─────┴─────┐
              ▼           ▼
       Connectionless  Connection-
          Service       Oriented

Short Exam Definition

• Network Layer design issues are the major design considerations involved in providing efficient packet delivery across interconnected networks. 

• They include routing, congestion control, quality of service, internetworking, packet forwarding, service models, and fragmentation.

Key Point

The main goal of the Network Layer is to provide efficient and correct delivery of packets from the source network to the destination network through one or more routers.