Transport Layer – OSI Model

The main aim of transport layer is to be delivered the entire message from source to destination. Transport layer ensures whole message arrives intact and in order, ensuring both error control and flow control at the source to destination level. It decides if data transmission should be on parallel path or single path

Transport layer breaks the message (data) into small units so that they are handled more efficiently by the network layer and ensures that message arrives in order by checking error and flow control.

FUNCTIONS OF TRANSPORT LAYER:

1. Service Point Addressing : Transport Layer header includes service point address which is port address. This layer gets the message to the correct process on the computer unlike Network Layer, which gets each packet to the correct computer.
2. Segmentation and Reassembling : A message is divided into segments; each segment contains sequence number, which enables this layer in reassembling the message. Message is reassembled correctly upon arrival at the destination and replaces packets which were lost in transmission.
3. Connection Control : It includes 2 types :
   • Connectionless Transport Layer : Each segment is considered as an independent packet and delivered to the transport layer at the destination machine.
   • Connection Oriented Transport Layer : Before delivering packets, connection is made with transport layer at the destination machine.
4. Flow Control : In this layer, flow control is performed end to end.
5. Error Control : Error Control is performed end to end in this layer to ensure that the complete message arrives at the receiving transport layer without any error. Error Correction is done through retransmission.