Transport layer is the Layer 4 of the OSI model in a computer network. The basic function of the transport layer is to service the session layer above it. Segmentation is done at the sending end and reassembly at the receiving end. What the transport layer does is, takes the data from the session layer, splits it into smaller units if the data to be transfered is greater than the maximum size allowed and then sends it. At the receiving end, the transport layer re assembles these packets and delivers it to the session layer above it.
A session layer is responsible for maintaining sessions. Whenever a session is established, the transport layer creates a new connection for transferring the data of that particular session. Suppose that the data transfer is of high priority and the efficiency must be very high. In that case, what the transport layer of the OSI model does is, it creates several network connections for the same session. It shares the load between these connections for achieving higher throughput.
In some cases it would not be feasible to have multiple connections due to the high costs involved. In such cases, what the transport layer will do is that, it will multiplex all the signals into a single connection. It makes sure that the session layer is unaware of such issues.
The session layer may have various requirements. Sometimes it may require a error free point to point connection and at other times it may require just a isolated message transfer. Whatever the session layer requires, the network layer decides what kind of service it must provide. This choice will also indirectly affect the end user who uses the host machine.
As technology progresses, so does the ability of computers to process requests and other complicated calculations. This means that there will be multiple connections for a single host the transport layer deals with. There must be some means of identifying the messages associated with each connection and the transport layer makes this possible with the help of transport headers.
Network connection establishment and release is also a function of network layer. If you remember, the network layer is associated with flow control. In a similar manner, the transport layer is also concerned with flow control, wherein the traffic from the sender is controlled to match the receiving hosts ability to receive the data.
