Top 6 major functions of transport layer in network of the OSI model

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Do you know about the Transport layer? How many functions of transport layer of the OSI model? If you don’t know then you have come to the right place. In this post, we will talk about what is the transport layer, and functions of transport layer of the OSI model. So without wasting any time let’s start.

What is the transport layer of the OSI model?

The transport layer is the fourth layer of the OSI model. It receives data from the Application layer and sends it to the network layer. In the transport layer, data is delivered from one port to another. It is also called process to process or end to end delivery.

A question must have come to your mind why do we call the transport layer as process-to-process delivery? Because so many processes keep running inside each host. That process communicates with other processes in the other network to send or receive data. That’s why this is called a process-to-process delivery.

TCP and UDP protocols are used in the transport layer. Because of this, the problem in data transmission is very less. In the network layer, the data is given an independent source and a destination base. There is no reliability of data in it, but due to the TCP protocol in the transport layer, the data packet stays away from problems like Congestion, irregular delivery, error control, and flow control.

Functions of Transport Layer

The transport layer has a variety of functions. So let’s discuss about functions of transport layer.

End-to-End Delivery

In the data link layer, the data packet reaches the destination by node-to-node delivery. In the network layer, data packets are delivered from host to host. But in the transport layer, the data packet is carried through the port-to-port delivery method. Which is also called end-to-end delivery. This three-layer delivery system is given in the image below.

functions of transport layer

The main functions of transport layer is to, deliver the data packet from one application end to another application end. The method of sending data from one computer application to another computer application is called end-to-end delivery. In this, both the systems are in different networks.

We run some applications on our computers through the Internet. Like sending mail from a browser, downloading a file, uploading a file, browsing a website, etc. So all these tasks are done through some protocol.

For example, sending mail is happen due to SMTP (Simple Mail Transfer Protocol).

All protocols have a port number. Transport layer transmits data from one end to another through this protocol. That’s why it is called port-to-port delivery. below I have given some important protocol and their port numbers.

FTP(File Transfer Protocol)20/21
SSH(Secure Shell)       22
SMTP(Simple Mail Transfer Protocol)25
DNS(Domain Name System)53
HTTP(Hypertext Transfer Protocol)80
POP (Post office Protocol)110


In the network layer, Internet Protocol is used to send data. In which the data is not secure or reliable. But in the transport layer, with the help of TCP (Transfer Control Protocol) and UDP (User Datagram Protocol), data transfer or receive takes place. Whose data is secure and reliable.

When some data is sent through the transport layer to other applications, it is received at the receiver side in the same sequence. This means that data is sent or received in sequence order.

For an Example:

If you send some files to your friend in a sequence. In the same manner, your friend will receive the same file in that order. It means you send files like A1, A2, A3, and A4. The receiver side will also receive A1, then A2, then A3, and A4.

The chances of data loss in the transport layer are very less. Because TCP is used in it. If there is any data loss at the time of transportation, then the receiver gets to know about it.

In the Transport Control Protocol, a connection is made from the source to the destination before sending the data. After that, the data packet is sent. Therefore the transport layer is reliable for sending data.

Connectionless Vs Connection Oriented

In connectionless service, when the data packet is sent from the sender to the receiver before no connection is made between them. There is no number of data packets in this service. Due to this reason, there is a delay in the data transfer time to reach the data destination or there is a possibility of a loss of data. The sequence in which the data is transferred gets spoiled. The transmission in which the UDP protocol is used is called a connectionless service.

In a connection-oriented service, a connection is established from the source to the destination before the data is sent. After that, the data is sent from one point to another. When the data reaches the destination safely, then the connection is closed. Connection-oriented service is established with the help of Transmission Control Protocol.

Error control

In the transport layer, the source process sends the data to the destination process. After the data is reached at the destination, the second process checks the data by the error control method. This error control method is called checksum.

In this method, the second process tries to match the checksum of the data coming from the first process to seven of its own checksums. If the checksums of both processes are the same, then the second process receives it, otherwise, it sends the acknowledgement to the sender process.

Congestion Control

When all the nodes in a network send data simultaneously, then the network becomes full. To solve this problem, the Congestion control method is used.

Different types of protocols are used in the Congestion control method. If data is being transferred with speed from any port or if any error data is sent then it is controlled through this Congestion control method. thereby reducing network collisions. TCP uses the AIMD method (Additive Growth Multiplier Reduction) to control Congestion.

Flow Control

By using flow control technology in the transport layer, it controls the traffic that happens in the network. If the sender is sending the data faster or the receiver is receiving the data in a slower mode, then it controls the data loss occurring in it. It also uses Stop-and-Wait, Go-back-N, and Selective Repeat methods.

The advertising window method is used to control the flow control in the transport layer. In this, the receiver sends a window to the sender before the data is sent. This window means capacity. According to the capacity of that window, the sender fills the data in it and then sends it to the receiver. In this way, the flow control method is used in the transport layer of the OSI model.

Multiplexing and Demultiplexing

This Multiplexing and demultiplexing methods are also used in the transport layer to send and receive data.

functions of transport layer


When many processes send data to the transport layer. For this reason, the multiplexing technique is used. In this, all the processes are assigned according to their port number and united to all. The data is sent to the network layer by adding the header in that packet. Due to multiplexing, data of many processes can be sent simultaneously.


When data from the network layer is received by the transport layer, first it checks the error. After that, it divides all the processes through demultiplexing technique and sends that process to the correct port number.


In this article, we have explained about functions of transport layer of the OSI model. We always try our best to give complete information about what we have written. So that you do not have to go to any other site and you can get complete information on this site only.

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