Transport Layer: Functions of layer 4 in networking

len Alfred Ajibola - 17th October, 2021 @ 01:03 PM

Topics in ICT

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Functions of the Transport Layer:

  1. The transport layer appropriately addresses the respective port on both communicating device via its headers port addressing. For this reason, data segments are correctly directed to the listening ports on both communicating devices. This is termed port addressing of service point addressing. It is shown in the image below:

    Transport layer of osi model - Lenacad

    Please read an introduction to the transport layer of the OSI model here.

  2. If TCP is utilized at this layer, data will be transfered in a reliable manner, thus guaranteeing bandwidth usage. In order to achieve this, the transport layer of the sending device uses the segmentation of data. These segmented data are called segments. The segments are reassembled into the appropriate data by the receiving device before passing them to its upper layers.

  3. The transport layer prevents the sender from overwhelming the receiver with excessive data. The process through which it does this is termed flow control. Flow control involves a sliding window process (called windowing) in which data segments are sent in bytes. During windowing, the sending device begins by sending few segments at a time. If the network conditions are perfect, it gradually increases the number of segments sent, as far as the receiving device approves of them through an acknowledgement (ACK) segment. If the sender becomes overwhelmed with segments such that it starts dropping them, it sends a negative acknowledgement called NACK (or NAK). With this message, the sending device reduces the bytes of segment sent. This is the concept of flow control at the transport layer. It is shown in the image below:

    Transport layer windowing - Lenacad

  4. The transport layer prevents the sending and reception of duplicate data segments by both communicating device. The process through which it achieves this is termed error control. Error control at layer 4 is a function of the Transmission Control Protocol's (TCP's) reliable connection.

  5. The transport layer carries out multiplexing and demultiplexing functions. In this regard, the transport layer accept packets (from layer 3 below) and data (from layer 5 above). It differentiates the packets and data into segments (segmentation). It eventually send multiple segments simultaneously across the network depending on its available bandwidth. Multiplexing increase the rate of data transfer across the network.

  6. Error detection and correction is a function of the transport layer. This is true because it computes a checksum and checks it against incoming data. If the data remained intact during transit, it sends an acknowledgement (ACK). Conversely, if the data was found to be corrupt, it sends a NACK to the sending device. The image below shows TCP's error detection and correction:

    Error control at transport layer - Lenacad

    Understand that the lightweight User Datagram Protocol (UDP) does not carry out error detection and correction.

  7. The transport layer brings about real-time communication when dealing with Voice over Internet Protocol (VoIP). This is a function of the User Datagram Protocol (UDP) in a process termed connectionless or stateless data communication. This is actually a good feature since it prevents uncessary echoes or hearing of oneself in voice conversations. In this regard, dropped segments during transmission aren't resent; and the words of the caller aren't clearly heard at that specific point in time.


Meanwhile, recall that the transport layer in networking is famously termed as the layer 4 of the OSI model. The above listed functions are performed by two major protocols present at these layers. They are:

  1. Transmission Control Protocol (TCP)

  2. User Datagram Protocol (UDP)

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Alfred Ajibola is a Medical Biochemist, a passionate Academician with over 7 years of experience, a Versatile Writer, a Web Developer, a Cisco Certified Network Associate and a Cisco CyberOps Associate.

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