The use of transport layer is it manages the transport of data between two hosts over a network.
It slices the data into small chunks that can be easily sent over the network.
And finally reassembles that data segments into the original message at the receiver end. This layer is responsible for reassembling the data on the receiver end.
Network Layer chooses the best route and send the packets over the network between hosts. And it assigns the ip addresses to all the devices to identify the source and destination hosts in the network.
Network layer receives each data segment from the transport layer from the sending host and wraps it in a data packet along with routing data.
On the receiving host, the network layer unwraps the packet received extract the segment and sends it up to transport layer.
PDU means Protocol Data Units. Basically it is a data container used by each protocol to exchange data between the hosts in a network. Each and every PDU contains data payload and control information that helps the protocol to find out what to do with the data payload.
Usually the control information is stored in the header of the PDU and also sometimes stores in the trailer.
RARP means Reverse address resolution Protocol that provides the reverse method of finding the IP from a host's hardware address
Open shortest Path first is a dynamic routing protocol used by the routers and network devices to exchange routing information over the network.
This protocol finds the shortest path, detect the data route changes,and link failures. Also it reroutes the traffice to build a loop-free environment.