Computer Networking

Transport Layer

Network Layer

Network layer is the 5th Layer from sender and 3rd layer of receiving side in the OSI model.

At Sender Side, Network layer receives segments from transport layer and convert these segments into packets/datagrams and transmit these packets/datagrams to data link layer.

At Receiver Side, Network layer receives frames from data link layer and convert these frames to packets/datagrams and then transmit to transport layer.

Functionality of Network Layer

Network layer has various function let explain all functions one by one,

1. Source to Destination Delivery

Network layer provides Source to destination Delivery which also called HOST to HOST delivery. Following figure shows host to host delivery

2. Routing 

Networking layer uses the Router device to determine the best optimal path out of the multiple paths from source to the destination. Router uses routing protocols (i.e. RIP, OSPF etc.)

3. Addressing

 As we know, Network layer is used when source to destination delivery is required in different networks or over the internet. Network layer use the Logical (IP) Address to communicate over the internet. IP address contains the network ID and Host ID of destination machine. Network layer use IPV4 or IPV6 for addressing purpose.

4. Fragmentation

 Sometimes when a sender sends a packet to router then router may not have enough space to accommodate entire packet. So, it is required to break these packets into fragmentations (parts).

So, fragmentation of packets/datagram is also responsibility of network layer.

5. Congestion Control

The case, when maximum nodes send the data at a time to same router even with fragmentations, then buffer of router may be out of capacity. In this case, traffic must be control. Controlling of traffic is called congestion control.  So, in some cases congestion control is required which is also the responsibility of Network layer.

6. Flooding

When a data packet arrives at a router, then router sent this data to all the outgoing links from router except the link from which the data arrived.

Explain with example

Suppose there are 5 routers (A, B, C, D and E) which are connected though transmission lines as given below

By using flooding technique

  • An incoming data packet from sender will send to Router A.
  • Router A will forward the data packet to other routers B, C and D.
  • B will send the packet to C.
  • C will send the packet to B,D and E.
  • D will send the packet to C and E.
  • E will send the packet to D.

Note: Main advantage of flooding is that, the shortest path is always chosen by flooding because in flooding each router holds the information’s of their neighbors.

In the upcoming lectures we will see the details of network Layer functionalities.

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