The network layer is responsible for two primary functions. One is to divide segments into network packets and then reassemble the packets on the receiving end. The second function is to route packets by identifying the best path over a physical network. To route packets to a target node, the network layer employs network addresses, usually Internet Protocol(IP) addresses.
Features of Network Layer
- Transport data packets from source to destination without changing or using them.
- Fragment the packets if the packed are too large for delivery.
- Decide the route for transmission of packets from available multiple routes.
- Source and destination addresses are added to the data packets inside the network layer.
Services of Network Layer
Packetizing
Packetizing is the process of encapsulating the data received from the upper layers of the network, which is the payload at the source via the network layer, and decapsulating the payload from the network layer packet at the destination.
While encapsulating the data at the source upper-layer protocol adds a header that has the sender address and the address of the receiver including information that is needed by the network layer and transmits the packet to the data-link layer.
The routers in the route are not permitted to change the source and destination addresses and are also not permitted to decapsulate the packets until it is received by the destination unless they are allowed to be fragmented.
When the receiver receives the packets from the data-link layer, decapsulates the packet, and passes the packet data to the upper-layer protocol.
Routing and Forwarding
The process of finding the best possible route from several available routes in the network is called routing. For this process, routing protocols are used to assist the routers to coordinate with each other and create communication via the network.
The actions taken by each router when a packet is received at one of its interfaces is known as forwarding. When a router receives a packet from one of its associated networks, it must transfer the packet to another attached network (unicast routing) or to several attached networks (in the case of multicast routing).
Issues of Services related to the Network Layer
Error Control
Error checking is inefficient since the packets can be fragmented at each router although it can be implemented.
Flow Control
Flow control limits how much data a source may deliver without overloading the receiver. To manage the flow of data, the receiver should make feedback to the sender indicating that it is overloaded. Because of insufficient flow control and not giving direct flow control in the network layer, the source sends packets of data when they are ready, without considering the receiver’s readiness.
Congestion Control
When the volume of packets of data delivered by the source exceeds the capacity of the network or routers, congestion arises. If the congestion continues, the system may eventually fail and no data packets will be transmitted. There is insufficient congestion control at the network layer although congestion management is performed indirectly.
Advantages and Disadvantages of Network Layer Services
| Advantages | Disadvantages |
| Packetizing provides ease of transportation of data packets. | Flow control is lower in the design of the network layer. |
| Packetizing removes individual points of failure in data communication systems. | Too many datagrams in the network lead to congestion. |
| Reduce network traffic by routers creating collision and broadcast domains. | Lack of proper error control mechanisms due to fragmented data packets. |
| Forwarding helps to transfer data packets from one place to another in the network. | |
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