Principles Digital Communication System & Computer Networks (Charles River Media Computer Engineering)

As we saw in the previous chapter, IP does not provide a reliable service— it does not guarantee successful delivery of packets. However, from an end user's point of view, reliable service is a must—when you transfer a file, you want the file to be delivered intact at the destination. The TCP layer provides the mechanism for ensuring reliable service between two end systems. In this chapter, we will study the details of TCP. We will also discuss UDP, another transport layer protocol that provides less overhead than TCP.

22.1 SERVICES OF THE TRANSPORT LAYER

When we use a network for a service (such as a file transfer), we expect reliable service so that the file reaches the destination intact. However, IP does not provide reliable service. The packets may be lost, they may arrive out of order, they might arrive with variable delay, and so on. The transport layer protocols provide end-to-end transmission by taking care of all these problems; it provides a reliable data transfer mechanism to the higher layers.

Suppose you want to transfer a file from one system to another system. You invoke a file transfer application on your system. The networking software first has to establish a connection with the other end system, transfer the file, and then remove the connection. The transport layer provides the necessary functionality to establish the connection, transfer the packets to the other system reliably even if there are packet losses because of the IP's limitations, and then remove the connection. The transport layer protocol does this job by providing the following services:

• Type of service

• Quality of service

• Data transfer

• Connection management

• Expedited delivery

• Status reporting

Type of service: The service can be connection-oriented or connectionless. In connection-oriented service, flow control and error control are incorporated so that the service is more reliable and sequence of packets is maintained. However, there is an overhead to connection establishment. In connectionless service, there are no overheads for connection establishment and so it is efficient, but reliable data transfer is not guaranteed. Connectionless service is used for applications such as telemetry, real-time applications such as audio/video communication, and so on. On the Internet, TCP provides connection-oriented service, and UDP provides connectionless service.

Quality of service: Depending on the application, the quality of service parameters have to be defined, such as acceptable error levels, desired average and maximum delay, desired average and minimum throughput, and priority levels. Note that IP also provides the services of priority, delay, and such. However, in the TCP/IP networks, the required quality of service is not guaranteed. The quality of service parameters are very important when transmitting voice or video data because delay, packet loss, and so on will have an impact on the quality of the speech or video.

Data transfer: This service defines whether the data transfer is full duplex, half duplex, or simplex. Transfer of data and control information is the function of this service.

The communication can be full duplex, half duplex, or simplex. In some communication systems, the communication can be only one way (simplex). In such a case, it is not possible to transmit acknowledgements at all. UDP can be used in such systems. An example of such a system is a satellite communication system in which the VSATs can receive data but cannot transmit.

Connection management: In connection-oriented service, the transport layer is responsible for establishment and termination of the connection. In case of abrupt termination of a connection, data in transit may be lost. In case of graceful termination, termination is prevented until data has been completely delivered.

The services provided by the transport layer are connection management and data transfer. In addition, this layer handles issues related to quality of service, status reporting, and delivery of urgent data.

Expedited delivery: When some urgent data is to be sent, the interrupt mechanism is used to transfer the urgent data.

Status reports: Status reporting of performance characteristics is also done by this layer. The performance characteristics are delay, throughput, addresses (network address or port address), current timer values, and degradation in quality of service.