The fourth layer of the OSI Model-Transport layer
The Transport layer is the fourth layer of the OSI Model. It provides the transparent transfer of the data between the source and the destination and the data communicated by the transport layer is encapsulated in a transport layer and send to the network layer. It is the responsibility of the transport layer to choking the correct delivery of the stream of data. The transport layer manages the end to end control like-it determines whether the entire packet has arrived at the destination point and error checking. It ensures the complete data transfer from sending host to the receiving host.
The lower Data link layer is only responsible for the delivery of the packet from one host to another and if the packet gets lost in a router somewhere in the network the Transport layer detect that. And also if the data is send incorrectly this is the transport layer responsibility to ask for the retransmission of the data. The Transport layer act as an interface between the bottom and the Top three layers.
The Transport layer provides two types of services-the first is connection oriented and the second one is connectionless oriented .With the connection oriented services first a virtual circuit is established between the two station and after the connection is established segment do not have to be routed individually to search the destination .This services is very useful for the large data transfer. And with the connectionless services there is no need to establish a virtual link between the two stations, in this service each segment is routed individually.
Responsibilities of the transport layer-
1. The transport layer is responsible for delivery of the entire message from the source to destination where the network layer is responsible to end to end delivery of the individual packet. The network layer does not recognize the any relationship of the entire message it deal every packet independently.
2. The transport layer ensures that the whole message arrives in the order from the source to destination point.
3. In the transport layer the message is divided into transmittable segment and each segment contain the segment number. These numbers enable the transport layer to reassemble the message correctly at the destination point.
4. Transport layer responsible for the flow control and performed end to end.
5. Like Data link layer the transport layer is responsible for the error control and the sending transport layer make sure that the receiving transport layer receives the entire message without any error.
6. The Transport layer uses the protocols – TCP and UDP.TCP is the connection oriented protocol and the UDP is the connectionless protocol.
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