What is Transport Layer in the OSI Model and Its Elements

The Open Systems Interconnection (OSI) was developed by the ISO (International Organization for Standardization) in the year 1984. At present, it is considered as an architectural model of inter-computer communications. The OSI model is a theoretical model of a telecommunication otherwise computing system that differentiates the functions of communication. In one computer to another, this model mainly explains how the information can be transmitted from a software application of one computer to the software application of another computer. This OSI model includes seven layers where each layer is used for a specific network function. So this article discusses one of the OSI model layer namely the transport layer.

Transport Layer in the OSI Model

The OSI model divides the entire task into seven layers where every layer can be used for a particular task because they are self-contained. So, the task assigned to every layer in the OSI model can be executed independently. The layers in the OSI model mainly separated into two types namely upper layers & lower layers.

Transport-Layer-in-the-OSI -Model
transport-layer-in-the-OSI -model

The upper layer is mainly used to deal with the issues based on the application. These are executed within the software only. The application layer in the model is nearest to the end customer. Both the application layer & customer interact with the applications of software. The lower layer in the OSI model deals with the issues of data transport.

What is the Transport Layer?

Definition: The fourth layer in the OSI model is known as the transport layer from the top. This layer provides the communication services directly to the running application processes on different hosts. Even though these processes on various hosts are not linked physically, and uses logical communication to transmit the messages to each other. Here, this layer provides logical communication.

The protocols of this layer are implemented within the end systems, not in the routers of the network. A transport layer in the computer network gives above one protocol to the applications of the network. For instance, the two transport layer protocols like TCP &UDP provides different services to the network layer. All the protocols in this layer provide different services like multiplexing, de-multiplexing, reliable data transfer, bandwidth & delay guarantees.

Elements of Transport Layer

The elements of this layer include the following.


Service-point Addressing

Computers frequently run several programs at a time, because of this reason, the delivery of source to destination in two computers is not possible. So this layer is used for an exact kind of address to its header which is known as a port address or service point address. Through this address, every packet arrives at the correct computer & also transport layer attains the total message to the exact method on that computer.

Segmentation & Reassembly

In a segmentation process, a message can be separated into communicable segments where each segment includes a sequence number that enables the layer to rebuild the message. Once the message arrives from the source system to the destination system then the message can be reassembled properly, packets can be identified and replaced because of the lost data in transmission.

Connection Control

These are two types namely connectionless and connection-based. In connectionless, the transport layer treats every packet like an individual & transmits it to the destination computer. In this kind of transmission, the receiver doesn’t transmit an acceptance to the sender regarding a packet receipt. This is an earlier communication method.

Connection based Transport Layer

This layer makes a connection with the layer at the destination of the computer before sending the packets toward the target. The connection can be created by using the following steps:

  • Establishment of Connection
  • Transfer of Data
  • Termination Connection

So connection-based service is used for the entire data transmission, whereas connectionless service is less consistent

Multiplexing & De-multiplexing

Several packets are transmitted across a system from various applications by using control systems and these are found within the transport layer. This layer allows packets using different methods, and these packets are distinguished through their port numbers & transmit them to the layer once adding correct headers.

In the De-multiplexing process, the data can be obtained which is transmitted from different processes. It gets the data segments from this layer to deliver it to the suitable method running on the machine at the receiver end.

Flow Control

The transport layer is also accountable for the flow control between the contiguous layers of the TCP/IP model. It doesn’t execute across a single link even it executes a continuous node. By impressive flow control methods, loss of data can be stopped from the sender & slow receiver.

For example, it uses the sliding window protocol technique. In this technique, the receiver transmits a window back toward the sender to update the received data size.

Error Control

This is also achieved back-to-back similar to the data link layer. In this layer, the total message can appear at the receiving end of the layer without any fault. The improvement of error can be attained through the packet re-transmission. The integrity of data can be checked once the data has arrived or not by using the services like the ACK & NACK to update the sender.


1). What is the transport layer in the OSI model?

The fourth layer in the OSI model is the transport layer.

2). What is the transport layer segment?

When the unit of data transmits from TCP protocol to the network layer is known as Segment

3). What is the main function of the transport layer?

This layer is responsible for back-to-back communication above a network

4). What is the application of TCP?

This communication protocol is used for interconnecting network devices using the internet

5). What is the full form of FCP?

FCP is a fiber channel protocol

Thus, this is all about an overview of the transport layer. The main function of this layer is to allow data from the above layer & divide it into slighter units and transmits to the network layer & ensures that all the small pieces arrive properly at the other end. Here is a question for you, what are transport layer devices?