The Transport Layer

A transport layer is the interface between the application layer and the complex hardware of the network. It is designed to allow peer entities on the source and destination hosts to carry on conversations.

Data may be user data or control data. Two modes are available, full-duplex and half duplex. In full-duplex operation, both sides can transmit and receive data simultaneously, whereas in half duplex, a side can only send or receive at one time.

Interaction between the transport layer and the layers immediately above and below are shown in figure 2.2. Each of the applications in the application layer has the ability to send a message using TCP or UDP. The application communicates with the TCP or the UDP service, which ever it requires. Both the TCP and UDP communicate with the Internet Protocol in the internet layer. In all cases communication is a two way process. The applications can read and write to the transport layer. The diagram only shows two protocols in the transport layer. T/TCP will also reside in this layer alongside the other two protocols and function in the same manner.

 

Figure 2.2: Interaction with Application, Transport and Internet Layers

A message that is to be sent originates in the application layer. This is then passed down onto the appropriate protocol in the transport layer. These protocols add a header to the message for the corresponding transport layer in the destination machine for purposes of reassembling the message. The segment is then passed onto the internet layer where the Internet Protocol adds a further header. Finally the segment is passed onto the physical layer, a header and a trailer are added at this stage. Figure 2.3 shows the structure of the final segment being sent.

 

Figure 2.3: Transmitted Segment from TCP/IP Network

In the following chapters we will look at three protocols that are used in the transport layer, UDP, TCP and T/TCP.