The network layer is responsible for delivering packets across multiple networks from source to destination by adding a header containing logical addresses. It provides routing to direct packets through routers to their final destination when independent networks are connected. The transport layer ensures whole messages are delivered intact and in order from specific processes on one computer to specific processes on another by using port addresses and sequencing packets. It can provide connection-oriented or connectionless services and performs error control through retransmissions to ensure reliable end-to-end delivery of data.

2. NETWORK LAYER
IT IS RESPONSIBLE FOR THE SOURCE-TO-DESTINATION DELIVERY OF A PACKET
ACROSS MULTIPLE NETWORKS.
WHEREAS THE DATA LINK OVERSEES THE DELIVERY OF THE FRAME BETWEEN
TWO SYSTEMS ON THE SAME NETWORK.

3. . . . .
IF TWO SYSTEMS ARE CONNECTED TO THE SAME NETWORK, THERE IS NO NEED
OF THE NETWORK LAYER.
HOWEVER IF THE TWO SYSTEMS ARE CONNECTED TO TWO DIFFERENT
NETWORKS, THERE IS A NEED OF NETWORK LAYER TO ACCOMPLISH SOURCE TO
DESTINATION DELIVERY.

4. Relationship b/w network ,transport and data link layers.

6. LOGICAL ADDRESSING
PHYSICAL ADDRESSING IMPLEMENTED BY THE DATA LINK LAYER HANDLES THE
ADDRESSING PROBLEM LOCALLY.
BUT IF A PACKET PASSES THE NETWORK BOUNDARY, WE NEED ANOTHER
ADDRESSING SYSTEM TO HELP DISTINGUISH THE SOURCE AND DESTINATION
SYSTEMS.
NETWORK LAYER ADDS A HEADER TO THE PACKET COMING FROM THE UPPER LAYER
THAT INCLUDES THE LOGICAL ADDRESSES OF THE SENDER AND RECEIVER.

7. ROUTING
WHEN INDEPENDENT NETWORKS OR LINKS ARE CONNECTED TO CREATE A
LARGE NETWORK, THE CONNECTING DEVICES (ROUTERS) ROUTE THE PACKETS
TO THEIR FINAL DESTINATION.
NETWORK LAYER PROVIDES THIS MECHANISM.

8. Source-to-destination delivery

9. TRANSPORT LAYER
TRANSPORT LAYER IS RESPONSIBLE FOR SOURCE-TO DESTINATION (END-TO-
END) DELIVERY OF THE ENTIRE MESSAGE..
WHEREAS THE NETWORK LAYER OVERSEES END-TO-END DELIVERY OF
INDIVIDUAL PACKET AND IT DOESN’T RECOGNIZE ANY RELATIONSHIP BETWEEN
THE PACKETS.IT TREATS THEM INDEPENDENTLY (EVEN IF THEY ARE NOT).
THE TRANSPORT LAYER ENSURES THAT THE WHOLE MESSAGE ARRIVES INTACT
AND IN ORDER.

10. Relationship b/w session, transport and network layer.

12. SERVICE-POINT ADDRESSING
COMPUTERS RUN SEVERAL PROGRAMS AT SAME TIME. THIS MEANS DELIVERY
NO ONLY FROM ONE COMPUTER TO OTHER BUT ALSO FROM SPECIFIC PROCESS
ON ONE COMPUTER TO SPECIFIC PROCESS ON OTHER.
THE TRANSPORT LAYER THEREFORE INCLUDE A TYPE OF ADDRESS CALLED
SERVICE-POINT ADDRESS(PORT ADDRESS) .NETWORK LAYER GETS EACH
PACKET TO THE CORRECT COMPUTER AND THE TRANSPORT LAYER GETS THE
ENTIRE MESSAGE TO THE CORRECT PROCESS ON THE COMPUTER.

13. SEGMENTATION AND REASSEMBLY
A MESSAGE IS DIVIDED INTO TRANSMITTABLE SEGMENTS WITH EACH SEGMENT
CONTAINING A SEQUENCE NUMBER.
THESE NUMBERS ENABLE THE TRANSPORT LAYER TO REASSEMBLE THE
MESSAGE CORRECTLY AND IDENTIFY AN REPLACE PACKETS THAT WERE LOST IN
TRANSMISSION.

14. CONNECTION CONTROL
THE TRANSPORT LAYER CAN BE CONNECTIONLESS OR CONNECTION ORIENTED.
A CONNECTIONLESS TRANSPORT LAYER TREATS EACH SEGMENTS AS
INDEPENDENT PACKETS AND DELIVER IT TO TRANSPORT LAYER AT THE
DESTINATION MACHINE.
A CONNECTION-ORIENTED TRANSPORT LAYER MAKES A CONNECTION WITH
TRANSPORT LAYER AT THE DESTINATION MACHINE FIRST BEFORE DELIVERING
THE PACKETS. AFTER ALL DATA IS DELIVERED THE CONNECTION IS TERMINATED.

15. ERROR CONTROL
TRANSPORT LAYER CONTROLS ERROR BY PROVIDING A MECHANISM TO DETECT
DAMAGED, LOST AND DUPLICATE SEGMENTS.
ERROR CORRECTION IS ACHIEVED BY RETRANSMISSION OF THE SEGMENTS.
ERROR CONTROL IS PERFORMED PROCESS-TO-PROCESS RATHER THAN OVER A
LINK.

16. FLOW CONTROL
IF THE RATE AT WHICH THE DATA ABSORBED BY THE RECEIVER IS LESS THAN
THAT PRODUCED BY THE SENDER, THAN THE TRANSPORT LAYER IMPOSES A
MECHANISM TO AVOID OVERWHELMING THE RECEIVER.
THIS IS PERFORMED END-TO-END RATHER THAN ACROSS THE LINK.