16.5 TRANSPORT LAYER

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The transport layer can provide two types of services—connection-oriented and connectionless. In connection-oriented service, a connection is established between the two end systems before the transfer of data. The transport layer functionality is to ensure that data is received error-free, packets are received in sequence, and that there is no duplication of packets. The transport layer also has to ensure that the required quality of service is maintained. Quality of service can be specified in terms of bit error rate or delay. In connectionless service, the packets are transported without any guarantee of their receipt at the other end.

The transport layer provides end-to-end reliable data transfer between two end systems. This layer ensures that all packets are received without error and the packets are received in sequence.

The transport layer can provide either connection-oriented service or connectionless service. TCP provides connection-oriented service and UDP provides connectionless service. Note that only TCP provides reliable data transfer.

In TCP/IP networks, TCP is an example of a connection-oriented transport layer, and UDP is an example of a connectionless transport layer.

Note

Quality of service parameters such as delay and throughput are important for some applications. For example, in voice communication, there should not be variation in delay in receipt of packets. The transport layer provides the facility to specify quality of service parameters.

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