UNIT 3: Network Layer

The network layer is the third layer in the OSI model (after the data link layer and before the transport layer). It is responsible for the logical addressing, routing, and forwarding of data packets across networks. The network layer ensures that data can travel between different networks.

Let's break down each topic and sub-topic in detail with diagrams and flowcharts.

3.1. Network Layer

The network layer is responsible for determining the best path for data to travel from source to destination. It handles logical addressing, routing, and forwarding.

3.1.1. Design Issues in Network Layer

The network layer is responsible for several crucial design issues that affect how efficiently data is transmitted across networks:

Routing: One of the most critical design issues. The network layer determines the most efficient path for data packets to travel across the network from the source to the destination.
- Static Routing: Involves manually configuring the routing tables, which is not flexible but is simpler.
- Dynamic Routing: Involves using algorithms and protocols to automatically adjust to network changes.
Addressing: The network layer uses logical addressing (IP addresses in IPv4 or IPv6) to uniquely identify devices on different networks. The main issue is to ensure that every device in the network can be uniquely identified.
Packet Forwarding: The network layer needs to decide how to forward data packets from one device to another. It uses forwarding tables to help decide the next hop based on the destination address.
Congestion Control: The network layer needs to manage congestion in the network, ensuring that the network does not become overloaded with too many packets.
Fragmentation and Reassembly: Large packets may need to be broken down into smaller fragments to fit the size limits of the underlying network layer. Once the packets reach their destination, they are reassembled.

Diagram for Network Layer Design Issues:

+-----------------+
|  Routing        |
|  Addressing     |
|  Packet         |
|  Forwarding     |
|  Congestion     |
|  Control        |
|  Fragmentation  |
+-----------------+

3.1.2. Example Protocols (IPv4)

One of the most widely used protocols in the network layer is IPv4 (Internet Protocol version 4). Let's explore it in detail.

IPv4 (Internet Protocol version 4):
- IPv4 is used for routing and addressing packets across networks.
- An IPv4 address is a 32-bit address, which allows for approximately 4.3 billion unique addresses (e.g., 192.168.0.1).
Structure of an IPv4 Address:
- An IPv4 address is divided into four octets (8 bits each), written as decimal numbers, separated by periods.
- Example: 192.168.1.1 – Here, 192, 168, 1, and 1 are each an octet.
IPv4 Packet Structure:
- The IPv4 packet consists of a header and data section.
IPv4 Header contains:
- Version: 4 bits indicating the protocol version (IPv4).
- IHL (Internet Header Length): 4 bits indicating the length of the header.
- Type of Service (ToS): 8 bits specifying the quality of service.
- Total Length: 16 bits for the total length of the packet (header + data).
- Identification, Flags, Fragment Offset: These help in fragmenting and reassembling data if necessary.
- Time to Live (TTL): 8 bits specifying how many hops the packet can take before being discarded.
- Protocol: 8 bits specifying the protocol used in the data section (TCP, UDP, etc.).
- Header Checksum: 16 bits for error-checking the header.
- Source and Destination IP Addresses: 32 bits each, indicating the origin and destination of the packet.
IPv4 Packet Example:
```
+--------------------+-------------------------+--------------------+
| Version | IHL | ToS | Total Length | Identification | Flags |
+---------------------------------------------------------------+
| Fragment Offset | TTL | Protocol | Header Checksum |
+---------------------------------------------------------------+
| Source IP Address (32 bits) | Destination IP Address (32 bits) |
+---------------------------------------------------------------+
```

3.2. Routing

Routing is the process of selecting paths for traffic in a network. The network layer uses various routing algorithms and protocols to ensure that data packets are sent efficiently from source to destination.

3.2.1. Routing Principles and Issues

Routing Principles: The primary goal of routing is to find the most efficient path for packets. Efficient routing is essential for ensuring minimal delay, high bandwidth, and low cost.

Routing involves two main concepts:
1. Routing Algorithms: These are algorithms that determine the best path based on various factors (e.g., distance, cost, or load).
2. Routing Protocols: These protocols are used to exchange routing information between routers.
Routing Issues:
- Dynamic Routing: Dynamic routing uses routing protocols that can automatically adapt to network changes (e.g., link failure or congestion).
- Static Routing: Static routing requires manual configuration of the routing table, and it doesn’t adapt to network changes.
- Scalability: As networks grow larger, routing needs to scale accordingly.
- Convergence Time: The time taken for routers to agree on the best path after a network change.

Diagram: Routing Process:

 +---------------------+             +-----------------------+
 | Source Device       |             | Destination Device    |
 | (Host or Router)    |----> Route --->(Host or Router)     |
 +---------------------+             +-----------------------+

3.2.2. Routing Algorithms and Protocols

Routing algorithms are used to determine the best path for data to travel. There are two main types of routing algorithms:

Distance-Vector Routing Algorithm:
- Each router shares information about its routing table with its neighbors.
- Routers use this information to calculate the shortest path to each destination.
- Example Protocol: RIP (Routing Information Protocol).
Working of Distance-Vector Algorithm:
- Every router maintains a table that contains the cost to reach every destination.
- The router sends its table to its neighbors periodically, and updates its table based on the received information.
- Example:
```
Router A -----> Router B -----> Router C
(Calculates distance to various destinations)
```
Example Protocol - RIP:
- RIP is a simple routing protocol based on the distance-vector algorithm.
- RIP uses hop count as a metric, where the maximum number of hops allowed is 15.
- RIP operates using a broadcasting method where routers send their entire routing table to their neighbors periodically.
Link-State Routing Algorithm:
- In this algorithm, each router shares information about the state of its links (directly connected neighbors) with all other routers.
- Routers then calculate the shortest path to every destination using the information from all routers.
- Example Protocol: OSPF (Open Shortest Path First).
Working of Link-State Algorithm:
- Each router sends link-state advertisements (LSAs) to all other routers in the network.
- Routers use the Dijkstra algorithm to calculate the shortest path tree based on these LSAs.
Example Protocol - OSPF:
- OSPF is a link-state routing protocol.
- OSPF operates within an Autonomous System (AS) and ensures that all routers within the AS share the same routing information.
- OSPF divides the network into areas for scalability.
Diagram for Link-State Algorithm (OSPF):
```
+------------+      +------------+
| Router A   |----->| Router B   |
+------------+      +------------+
      |                  |
      v                  v
+------------+      +------------+
| Router C   |----->| Router D   |
+------------+      +------------+
```

Comparison of RIP and OSPF:

RIP: Distance-vector, uses hop count, less complex but slower convergence.
OSPF: Link-state, uses cost based on bandwidth, faster convergence, more complex.

Summary of Key Concepts

Network Layer is responsible for routing, logical addressing, and packet forwarding across networks.
Design Issues include routing, addressing, packet forwarding, congestion control, and fragmentation.
IPv4 is a widely used protocol at the network layer, and its structure includes logical addressing and packet forwarding.
Routing is the process of determining the best path for packets, and routing protocols like RIP (distance-vector) and OSPF (link-state) are used to achieve this.

Flowchart for Routing Process:

+---------------------+
| Source Device       |
+---------------------+
        |
        v
+---------------------+
| Routing Algorithm   |
| (RIP or OSPF)       |
+---------------------+
        |
        v
+---------------------+
| Routing Table Update|
+---------------------+
        |
        v
+---------------------+
| Forward Data Packet |
+---------------------+
        |
        v
+---------------------+
| Destination Device  |
+---------------------+

This explains the core concepts related to the network layer, routing principles, algorithms, and protocols.

UNIT 3: Network Layer