Tuesday, July 16, 2013
- The network layer comes at number three in the
model of networking.
- The duty of this layer is to forward and route the packets via the intermediate routers.
- It comes with functional as well as procedural means for the transfer of data sequences with variable length from a source host to a destination host and across one or more networks.
- During the transfer it also takes the responsibility for the maintenance of the services functions’ quality.
There are many other functions of this layer such as:
Ø Connection-less communication: In IP, a datagram can be transmitted from one host to another without any need for the receiving host to send an acknowledgement. Protocols that are connection oriented are used on the higher levels of the OSI model.
Ø Host addressing: Every host in the network is assigned a unique address that determines its location. A hierarchical system is what that assigns this address. These are the addresses that are known as the IP (internet protocol) addresses.
Ø Message forwarding: The networks are sometimes divided in to a number of sub – networks which are then connected to other networks for facilitating wide – area communication. Here specialized hosts called routers or gateways are used for forwarding the packets from one host to another.
Characteristics of Network Layer
- One of the characteristics of the network layer is encapsulation.
- Network layer ought to provide encapsulation facilities.
- It is necessary that the devices must be identified with the addresses.
- Not only the devices but the network layer PDUs must be assigned such addresses.
- The layer 4 PDU is supplied to the layer 3 during the process of encapsulation.
- For creating the layer 3 PDU, a layer 3 label or header is added to it.
- In reference to the network layer, this PDU thus created is referred to as a packet.
- On creation of a packet, the address of the receiving host is included in the header.
- This address is commonly known as the destination address.
- Apart from this address the address of the source or the sender host is also stored in the header.
- This address is termed as the source address.
- Once the encapsulation process is complete, the layer 3 sends this packet to the data link layer for preparing it to be transmitted over the communication media.
- The services provided by the network layer for directing the packets to the destination addresses define this characteristic.
- It is not necessary that the destination and the source hosts must always be connected to the same network.
- In actual, the packet might have to go through a number of networks before reaching the destination.
- During this journey the packet has to be guided to reach the proper address. - This is where the routers come in to action.
- They help in selecting the paths for guiding the packets to the destination.
- This is called routing.
- During the course of routing of the packet, it may need to traverse a number of devices.
- We call the route taken by the packet to reach one intermediate device as “hop”.
- The contents of the packet remain intact until the destination host has been reached.
- On the arrival of the packet at the destination address, it is sent for processing at the third layer.
- The destination address is examined by the host system for verifying whether the packet is meant for itself or not.
- If the address is found to be correct, the decapsulation process is carried out at the network layer.
- This layer passes the layer 4 PDU to the transport layer for appropriate servicing.