The fourth layer in the Open Systems Interconnection (OSI) Model, is actually called the host-to-host Transport Layer

The fourth layer in the Open Systems Interconnection (OSI) Model, is actually called the host-to-host Transport Layer but the name has been shortened to spare the inconveniences of pronouncing and writing such a long name. It is referred to as the Transport Layer and you can find it just above the internet Layer and below Network Layer, the two-layer it shares its' functioning with. It is defined as a conceptual division of the methods that are in the layered architecture of protocols.


The major role of this layer is to provide direct logical communication between applications running on different hosts and end-to-end communication on a network.

1. For connection-oriented transmissions of data, Transport Layer uses Transmission Control Protocol (TCP) which not only sends the data but also verifies that it has been delivered without compromise in accuracy and sequence. For doing this, TCP uses a mechanism called Positive Acknowledgement with Re-transmission (PAR), which makes it capable of sending data again and again until it gets the confirmation of the delivery. Since it has the responsibility of making sure the packets are delivered in a strict sequence, it can also fix any error that may occur that threatens the disorganization of this sequence. The only fault in this method is that it increases the network load and can cause congestion and slow down because it requires confirmation of each and every delivery.


2. By using the User Datagram Protocol (UDP), Transport Layer can provide a simple message carrier service which may allow the transmission of the messages only. Some lesser-known and additional protocols used in the transport layer are the Datagram Congestion Control Protocol (DCCP) and Stream Control Transmission Protocol (SCTP).


3. Transport layer uses the checksums mechanism to detect any error in the data transmission. The role of the checksums is to make sure that the data received by the receiver is the same as the data sent by the sender. This mechanism is not only used in this particular layer but in all the layers through the model.


4. The only medium of interaction between the two devices are the layers and these devices are therefore not capable of detecting the capacity of each other in terms of data transmission. In case the sender sends data faster than the receiver can receive, it can cause a breakdown in the process. To control the flow of data and prevent breakdown by overwhelming is also the role that is played by the Transport Layer.

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