Layer 3: Network layer

           The network layer is responsible for packet forwarding including routing through intermediate routers, where the data link layer is responsible for media access control  flow control and error checking.

          A network is a medium to which many nodes can be connected, on which every node has an address and which  permits nodes connected to it to transfer messages to other nodes connected to it by merely providing the content of a message and the address of the destination node and letting the network find the way to deliver  ("route") the message to the destination node.

In this layer switching, Addressing ,Route definition, Route discovery operation is perform.

Layer 4: Transport layer

            This layer is the heart of the whole protocol hierarcy. Its task is to provide reliable cost-effective data transport from the source machine to destination machine, Independently of the physical network or network currently in use . It is upper layer of OSI model.

                 IN this layer flowing operation is perform :-

  Packet & sequencing , flow control, error control, Multiplexing, End to End delivery Addressing , Loss control.

Layer 5: session layer

     IN session layer it establishes, manage and terminate connection b/w application. The session layer sets up, co-ordinates and terminate conversations, exchange and dialogues b/w the application at each end.

  It deals with session and connection co-ordination.

 Session layer enables connection in different mode.

1.   Simplex

2.   Duplex

1.   Half duplex     2. Full duplex.