What Is the Transport Layer?

Key Responsibilities of the Transport Layer #

• Extends network to the applications: the transport layer takes messages from the network to applications. In other words, while the network layer (directly below the transport layer) transports messages from one end-system to another, the transport layer delivers the message to and from the relevant application on an end-system.

The Post Analogy #

Imagine you post a package across the world. Presumably, a ship or an airplane would carry the message to the relevant country. However, the post system of that country would take it to the relevant address. The plane/ship is the network layer and the post system is the transport layer.

Have a look at the slides for a clearer explanation.

Here are some other responsibilities of the transport layer.

• Logical application-to-application delivery, the transport layer makes it so that applications can address other applications on other end-systems directly. This is true even if it exists halfway across the world. So it provides a layer of abstraction.

• Segments data. The transport layer also divides the data into manageable pieces called ‘segments’ or ‘datagrams.’

• Can allow multiple conversations. Tracks each application to application connection or ‘conversation’ separately, which can allow multiple conversations to occur at once.

• Multiplexes & demultiplexes data. It ensures that the data reaches the relevant application within an end-system. So if multiple packets get sent to one host, each will end up at the correct application.

Where It Exists #

• The transport layer does not have anything to do with the core of the network. Its only responsibility is to take messages from an application on a machine and hand them off to the network layer. The network layer transfers messages from one host to another.

• The transport layer also receives messages from the network layer and transports them to the correct application.

Therefore, the transport layer and its protocols reside on end-systems! It is also the first layer in the OSI reference model (from the bottom) that distinguishes between applications.

Transport Layer Protocols #

The transport layer has two prominent protocols: the transmission control protocol and the user datagram protocol. In general, an application developer will have to choose between the two. We’ll discuss the intricacies of each in detail in upcoming chapters, but here is a quick overview.

Quick Quiz! #

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In the next lesson, we’ll have a more in-depth look at multiplexing and demultiplexing!