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Unit - 4Network Layer 6L 4.1 Network Layer ServicesThe Network Layer is the third layer of the OSI model. It handles the service requests from the transport layer and further forwards the service request to the data link layer. The network layer translates the logical addresses into physical addresses It determines the route from the source to the destination and also manages the traffic problems such as switching, routing and controls the congestion of data packets. The main role of the network layer is to move the packets from sending host to the receiving host. The main functions performed by the network layer are:Routing: When a packet reaches the router's input link, the router will move the packets to the router's output link. For example, a packet from S1 to R1 must be forwarded to the next router on the path to S2. Logical Addressing: The data link layer implements the physical addressing and network layer implements the logical addressing. Logical addressing is also used to distinguish between source and destination system. The network layer adds a header to the packet which includes the logical addresses of both the sender and the receiver. Internetworking: This is the main role of the network layer that it provides the logical connection between different types of networks. Fragmentation: The fragmentation is a process of breaking the packets into the smallest individual data units that travel through different networks. Services Provided by the Network LayerGuaranteed delivery: This layer provides the service which guarantees that the packet will arrive at its destination. Guaranteed delivery with bounded delay: This service guarantees that the packet will be delivered within a specified host-to-host delay bound. In-Order packets: This service ensures that the packet arrives at the destination in the order in which they are sent. Guaranteed max jitter: This service ensures that the amount of time taken between two successive transmissions at the sender is equal to the time between their receipt at the destination. Security services: The network layer provides security by using a session key between the source and destination host. The network layer in the source host encrypts the payloads of datagrams being sent to the destination host. The network layer in the destination host would then decrypt the payload. In such a way, the network layer maintains the data integrity and source authentication services. 4.2 Packet SwitchingThe packet switching is a switching technique in which the message is sent in one go, but it is divided into smaller pieces, and they are sent individually. The message splits into smaller pieces known as packets and packets are given a unique number to identify their order at the receiving end. Every packet contains some information in its headers such as source address, destination address and sequence number. Packets will travel across the network, taking the shortest path as possible. All the packets are reassembled at the receiving end in correct order. If any packet is missing or corrupted, then the message will be sent to resend the message. If the correct order of the packets is reached, then the acknowledgment message will be sent. 
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