Transport Layer

The transport layer shares information with the SESSION layer above it and the NETWORK layer beneath it:

A good analogy for the transport layer is the railway system.

Imagine that the information ‘package’ which the session layer wants to move across the network - is a train.

The session layer will tell the transport layer “This session wants to shift some information - Arrange a method for me to get it to its destination”. The session layer does not care how the transport layer is going to do it – all it cares about is that it is going to take place.

Now, there could also be another session also wanting to move its information across the network at the same time. This complicates things, because there are only a certain number of railway lines (connections) available. Therefore the transport manager has to allocate who gets to use the routes without collisions taking place. (Collisions are seriously bad news even in the data network world!).

Another part of the transport task is to split up the information package into convenient sized packets – the equivalent of the train carriage. It then sends the packets on their way.

Sometimes, there are two or three connections going to the same place, and so it makes sense to split up the stream of packets into two streams and send them all at the same time – time saved, money saved.

So one of the problems for the transport layer has at the other end is to re-arrange the information package back into the right order.

If some of the information is missing – the transport manager either spots the error and corrects it or it asks for the correct information packet to be re-sent.

All in all, the transport layer has a lot of work to do.

To summarise its tasks:

	Talks to the SESSION layer above it and the NETWORK layer below it.
	Manages the connections across the network
	Splits outgoing information into convenient chunks (packets).
	Re-arranges incoming information into the correct order.
	Checks that the incoming information is error-free
	Corrects the errors if it can.
	If it cannot fix the errors, then it requests that the missing information is re-sent.
	Informs the transport manager at the other end that the information arrived OK