Transport Layer in Computer Networks

In this tutorial, we will be covering the concept of the Transport layer in the OSI reference model in detail.

The Transport layer is the layer-4 of the OSI reference model. The transport layer is mainly responsible for the process-to-process delivery of the entire message. A process is basically an application program that is running on the host.

The basic function of the Transport layer is to accept data from the layer above, split it up into smaller units, pass these data units to the Network layer, and ensure that all the pieces arrive correctly at the other end.

Furthermore, all this must be done efficiently and in a way that isolates the upper layers from the inevitable changes in the hardware technology.

The Transport layer also determines what type of service to provide to the Session layer, and, ultimately, to the users of the network. The most popular type of transport connection is an error-free point-to-point channel that delivers messages or bytes in the order in which they were sent.

The Transport layer is a true end-to-end layer, all the way from the source to the destination. In other words, a program on the source machine carries on a conversation with a similar program on the destination machine, using the message headers and control messages.

• The transport layer also identifies errors like damaged packets, lost packets, and duplication of packets, and provides sufficient techniques for error correction.

• The protocols of the network layer are only implemented in the end systems but not in the network routers.

• There are many services that are provided by the protocols of the Transport layer such as multiplexing, demultiplexing, reliable data transfer, the guarantee of bandwidth.

This layer mainly provides the transparent transfer of data between end-users, also provides the reliable transfer of data to the upper layers.

Figure: Transport Layer

Functions of Transport Layer

1. Service Point Addressing: Transport Layer header includes service point address which is the port address. This layer gets the message to the correct process on the computer unlike Network Layer, which gets each packet to the correct computer.

2. Segmentation and Reassembling: A message is divided into segments(that are transmittable); each segment contains a sequence number, which enables this layer in reassembling the message. The message is reassembled correctly upon arrival at the destination and replaces packets that were lost in transmission.

3. Connection Control: It includes 2 types:

   • Connectionless Transport Layer: Each segment is considered as an independent packet and delivered to the transport layer at the destination machine.

   • Connection-Oriented Transport Layer: Before delivering packets, the connection is made with the transport layer at the destination machine.

4. Flow Control: In this layer, flow control is performed end to end rather than across a single link.

5. Error Control: Error Control is performed end to end in this layer to ensure that the complete message arrives at the receiving transport layer without any error. Error Correction is done through retransmission.

Let us now understand the process-to-process delivery by the transport layer.

Illustration of Process-to-Process Delivery

Thus the Transport layer is mainly responsible for the delivery of messages from one process to another.

Design Issues with Transport Layer

• Accepting data from the Session layer, split it into segments, and send it to the network layer.

• Ensure the correct delivery of data with efficiency.

• Isolate upper layers from the technological changes.

• Error control and flow control.