How to support a reliable communication in transport layer ?

At the Transport layer, each particular set of pieces flowing between a source application and a destination application is known as a conversation.To identify each segment of data, the Transport layer adds to the piece a header containing binary data. This header contains fields of bits. It is the values in these fields that enable different Transport layer protocols to perform different functions.

Reliability means ensuring that each piece of data that the source sends arrives at the destination. At the Transport layer the three basic operations of reliability are:
- tracking transmitted data.
- acknowledging received data.
- retransmitting any unacknowledged data.

This requires the processes of Transport layer of the source to keep track of all the data pieces of each conversation and the retransmit any of data that did were not acknowledged by the destination. The Transport layer of the receiving host must also track the data as it is received and acknowledge the receipt of the data. These reliability processes place additional overhead on the network resources due to the acknowledgement, tracking, and retransmission. To support these reliability operations, more control data is exchanged between the sending and receiving hosts. This control information is contained in the Layer 4 header.

Determining the Need for Reliability
Applications, such as databases, web pages, and e-mail, require that all of the sent data arrive at the destination in its original condition, in order for the data to be useful. Any missing data could cause a corrupt communication that is either incomplete or unreadable. Therefore, these applications are designed to use a Transport layer protocol that implements reliability.