The transport layer is a fourth layer from the top. The transport layer's central role is to support the broadcasting services precisely to the application procedures running on multiple hosts.
It includes the transport service of the session layer entities quality, where quality is determined in the condition of residual error rate, throughput interruption and other quality checking frameworks.
It can do splitting, multiplexing, blocking or segmenting on the requirement. It provides end to end connectivity. It can carry out the messages sequencing and exercise end-to-end error control to provide the transport service quality.
The transport layer provides its services to session layer entities in a cost-effective manner. The transport layer can offer either or both types of services, i.e., connection-oriented mode services or connectionless-mode services.
The connection-oriented mode transport service definition is described in ISO 8072, transport service definition. In such services, the establishment of the transport connection, release and data transfer, and expedited data transfer are provided.
On the other hand, the only service offered by connectionless-mode transport service is unit data.
There are various elements of the transport layer, which are as follows −
The transport layer also specifies service supported to the session layer users. An error-free point-to-point communication to deliver communication in the order transmitted is one of the transport layer's essential functions.
Error detection and error recovery are an integral part of reliable service, and therefore they are essential to perform error control mechanisms on an end-to-end basis. It is used to control errors from lost or duplicate segments. The transport layer allows specific segment sequence numbers to the message's multiple packets, creating virtual circuits, allowing only one virtual circuit per session.
The basic structure of flow control is to support synergy between a quick process and a slow process. It also allows a fast method to hold pace with a moderate one. Acknowledgements are transmitted back to manage end-to-end flow control. Go back, N algorithms can request retransmission of packets starting with packet number N. Selective Repeat is used to request specific packets to be retransmitted.
This layer can create and launch the connection across the network. This contains a naming structure so that a process on one device can denote with whom it wants to connect.
It can establish a separate network link for each transport link needed through the session layer.
When the transport layer accepts a vast message from the session layer, it divides the message into lower units based upon the specification. This procedure is known as fragmentation. After that, it is developed into the network layer. Conversely, when the transport layer acts as the receiving process, it reorders the message pieces before reassembling them into a message.