Transport Layer (Computer Network)

 Transport Layer -

The transport layer is the core of the internet model. The application layer programs interact with each other using the services of the transport layer. Transport layer provides services to the application layer and takes services from the network layer. figure shows the position of the transport layer in the 5 layer internet model. The transport layer in fourth layer in this model.

 

Duties of  Transport Layer -

                               Responsibilities of the Transport Layer :

Responsibilities of the transport layer include 

• Service-point Addressing

  Computers often run several programs at the same time. For this reason, source-to-destination delivery means delivery not only from one computer to the next but also from a specific process (running program ) on one computer to a specific process (running program) on the other. The transport the layer header must therefore include a type of address called a service-point address (or port address). The network layer gets each the entire message to the correct process on the computer.

• Segmentation and reassembly 

A message divided into transmittable segments,with each segment containing a sequence number. These numbers enable the transport layer to reassemble the message correctly upon arriving at the destination and to identify and replace packets that were lost in transmission.

• Connection control  

The transport layer can be either connectionless or connection-oriented. A connectionless transport layer treats each segment as an independent packet and delivers it to the transport layer at the destinations machine. A connection-oriented transport layer makes a connection with the transport layer the destination machine first before delivering the packets. After all the data are transferred, the connection is terminated.

• Flew control 

Like the data link layer, the transport layer is responsible for flow control. However,flow control at this layer is performed end to end rather than across a single link.

• Error control 

Like the data link layer, the transport layer is responsible for error control. However error control at this layer is performed process-to-process rather than across a single link. The sending transport layer makes a sure that the entire message arrives at the receiving transport layer without error (damage, loss or duplication). Error correction is usually archived through retrans-mission.