OSI Model – Transport Layer (Layer 4)

In data communication process, after Session Layer has created sessions by creating dialogue control between devices and nodes, the data is handed to Transport layer. 

Transport Layer, well the name gives us some hint that it has something to do with the transportation of the data.  This layer decides what type of transportation will be done, viz. reliable (TCP) or unreliable (UDP)

Based on the category of the Transportation type, when this layer receives the data stream, it segments them, i.e. it will chop the data stream, into many segments and label them like 1 of 1, 2 of 1, 3 of 1, etc..and provides end to end data transport services and establish a logical connection between the sender and the receiver.

If the category of transportation decided was TCP, this layer will wait for acknowledgements from the receiver before sending further segments; it is also concerned with Flow control.

And if it is UDP which is an unreliable transportation, it doesn’t care about acknowledgements from the receiver, and neither is it concerned about flow control and sequencing.

To Summarize, Transport Layer is connectionless which means unreliable or UDP(User Datagram Protocol), or Connection-oriented which means reliable or TCP(Transmission Control Protocol).

User	Service
Application Layer	Web Browser, Email Clients
Presentation Layer	Formats data, compression, encryption, etc
Session Layer	Maintains dialog control, creating session and controlling session and ending session.
Transport Layer	End-to-end communication

Now the data segments are ready to be handed to the next layer which is the Network Layer.

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