04. Transport Layer of OSI Model (Layer4)

Transport Layer is the fourth layer in 7Layer OSI Model after Network Layer. Similar to L2 & L3, this layer also performs addressing & multiplexing … but at Transport Layer. OSI Model divides the network communication processes into seven layers in order to simplify it. Each layer performs specific functions to support the layers above it. The 7x Layer model starts from Physical till Application Layer. The core concept behind Transport is “support of Multitasking” … it allows same computer, browser & internet connection to work on multiple applications simultaneously … & this is achieved through Port Numbers, Transport Layer Addressing & Multiplexing.

Transport layer is in the middle of the OSI model as in below figure. It is a part of both the lower and upper of layer groups.

Lower layers, because it is involves the transport of data,

Upper Layers, because its functions are also somewhat high-level.

PDU at Transport Layer is called Segment.

OSI Model Layers Waqas Karim


Functions/Duties of Transport Layer

Each Layer in OSI Model Performs some important duties. Important functions performed by Transport Layer are listed here:

  • Sequencing: Sequencing is a connection-oriented service that takes TCP segments that are received out of order and place them in the right order
  • Error Control: Is uses error control mechanisms to ensure reliable delivery of data. Due to this acknowledgement mechanisms, the receiver can detect how many bits have been corrupted during transmission. Receiver then requests the sender to send those bits again to ensure that no data is lost during transmission.
  • Other Functions of Transport Layer include: E2E Connection Mgmt, Transmission, Segmentation, Flow Control
  • Transport Layer is responsible for Layer4 Addressing which is also called Process Level Addressing. It allows a computer to use multiple network layer protocols simultaneously.

Transport Layer also performs Multiplexing and De-multiplexing of data to allow multiple programs to run on same computer using different Port numbers. In modern multi-tasking environments, many network applications need to run on a computer simultaneously. So, there should be some mechanism to identify which application should receive the incoming data.

To make this work correctly, incoming data from different applications is multiplexed at the Transport layer and sent to the Media layers. On the other side of the communication, the data received from the Media layers are de-multiplexed at the Transport layer and delivered to the correct application. This is achieved by using Layer4 “Port Numbers”.

The range of Transport Layer Port numbers is from 0-65,535.

65000, because Port number is a 16-bit number & maximum range through which it can span is 65,535.

The port numbers are divided into three ranges:

OSI Model Transport Layer Waqas Karim Pic1

These are some Famous & Well Known Port Numbers:

OSI Model Transport Layer Waqas Karim Pic2

Transport Layer Protocols

The OSI Model provides a conceptual framework for communication between computers, but the model itself is not a method of communication. Actual communication is made possible by using communication protocols. Each layer on the OSI Model has some protocols associated with it. Some important protocols on Transport layer are listed in below:

OSI Model Transport Layer Waqas Karim Pic4

Network Equipment/Components at Transport Layer

Similar to protocols, each layer has associated equipment with it. Some important Equipment that operate at Transport Layer of OSI Model are listed in below:

OSI Model Transport Layer Waqas Karim Pic5



  • It is the 4th Layer in OSI 7 Layer Model
  • It performs important functions like L4 Addressing, Multiplexing/De-multiplexing, Sequencing, Error Control, E2E Connection Mgmt, Transmission,  Segmentation, Flow Control …
  • Important Protocols at Transport Layer include TCP, UDP, SPX, BGP, ESP, Fiber Channel Protocol, iSCSI, SCTP, …
  • Equipment operating at Transport Layer include Firewalls, Gateways, Load Balancers.
  • PDU is called Segment


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