In the previous tutorial, we have studied about the various aspects of network layer, we explored the functions that occur at this layer. In this tutorial, we will look at the transport layer protocol and its responsibilities.

We have studied that at the data link layer, delivery of frames take place between two nodes connected by a point-to-point link or a LAN, by using the data-link layers address, say MAC address. At the network layer, delivery of datagram can take place between two hosts by using IP address. At the transport layer, communication can take place between processes or application programs by using port addresses. Let us look at the transport layer in some greater detail in this tutorial.

Getting Started

The transport layer is the 4th layer in the open system interconnection model responsible for end to end communication over a network. It provides a logical communication between two processes located on two different hosts.

Responsibilities of the Transport Layer Protocol –

• End to End Connectivity : It means a transport layer works at the process level and includes the usage of port numbers.
• Flow Control: Transport layer uses selective repeat protocol to manage the flow control at its level.
• Error Control: Checksum is implemented to control and manage the errors at transport layer. Thus, transport layer ensures quality and reliability to the end user.
• Transport layer is also responsible for segmentation i.e. transport layer divides the large sized data coming from application layer into smaller chunks called segments and forwards them to the network layer. We will learn about segmentation in greater detail in the upcoming tutorials.
• Multiplexing and de-multiplexing also occurs at the transport layer.
• One of the major responsibilities of transport layer is the congestion control during communication.
• Transport layer ensures a reliable message delivery. The byte stream received at the transport layer is broken into smaller chunks called segments. The receiver sends the acknowledgements for the segments. Transport Control Protocol (TCP) maintains a timer. If ACK is not received in time then it is retransmitted.

Now that we know about the transport layer and the network layer responsibilities, here arises a question.

Why do we need a transport layer when network layer is already present?

Answer: In TCP/IP (transmission control protocol/ internet protocol), IP uses datagram service which does not guarantee reliability. Thus, IP is unreliable at the network layer and to ensure reliability we use TCP at the transport layer.

Exercises:

Let us now answer a few basic questions on the transport layer.

Transport layer aggregates data from different applications into a single stream before passing it to
a) network layer
b) data link layer
c) application layer
d) physical layer

A _____ is a TCP name for a transport service access point.
a) port
b) pipe
c) node
d) none of the mentioned

Transport layer protocols deals with
a) application to application communication
b) process to process communication
c) node to node communication
d) none of the mentioned

That was all about the introduction to the transport layer protocol in the upcoming set of tutorials. We will look at the services like congestion control provided at transport layer in greater detail. In the next tutorial, we will learn a little about session layer and presentation layer protocol.