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.

Open System Interconnection Reference Model - OSI Model

The OSI Reference Model is founded on a suggestion developed by the International Organization for Standardization (ISO). The model is known as ISO OSI (Open Systems Interconnection) Reference Model because it relates with connecting open systems – that is, systems that are open for communication with other systems. It was developed in 1984. It defines a networking framework for implementing protocols in seven layers.
Layers in the OSI model are ordered from lowest level to highest.The stack contains seven layers in two groups:The layers are stacked this way:
* Application
* Presentation
* Session
* Transport
* Network
* Data Link
* Physical
The upper layers of OSI model are application, presentation and session layer. The software in these layers performs application-specific functions like data formatting, encryption, and connection management.
The lower layers are transport, network, data link and physical layer. These layers provide more primitive network-specific functions like routing, addressing,and flow control.

Benefits of OSI model

- Helps users understand how hardware and software elements function together.
- OSI is independent of country.
- OSI is independent of the operating system.
- Makes troubleshooting easier by separating networks into manageable pieces.
- Helps users understand new technologies as they are developed.