As it is clear from the name of this protocol, the idea behind Transport layer is in delivering data. It is not the only function of this layer but delivering is essential. Transport layer ensures successful delivery in correct order, without errors. Data is being verified by the recipients Transport layer. Its integrity should be exactly the same as on senders end. One of the most commonly used Transport layer protocols is the TCP (Transmission Control Protocol) that delivers data between server and client in both directions. Protocols for the Transport layer can be divided to connection oriented and connectionless.
Connection oriented protocols are establishing two way connection between the nodes before data will be transmitted. We can explain on the TCP example. When the browser is trying to open the webpage, Transport layer would send synchronization packet to the web server, expecting synchronization-acknowledgment response. When it receives response, it sends its own response to the server.
After this process is done (called three-way handshake) the data can be transmitted. After every time data is being sent, Transport layer expects expects acknowledgement response. If there is no response, data would be resend.
Figure 1: Transport Layer in OSI Networking System
Another purpose of Transport layer protocol is to ensure data integrity by evaluating checksum. It is the value added to the data on the source, defining data. If the checksum at the recipients end does not match the initial checksum, the data would be retransmitted.
The protocols that does not require reliability and are not using the checksum and three way handshake are called connectionless. Those are usually used for the purpose of video and audio live streaming when speed is more important than reliability.
Another essential functions of Transport layer protocols are:
Segmentation – dividing data in small fragments that can be transmitted over network. Since different networks have different Maximum Transmission Unit (MTU) that defines maximum size of one data piece which can be carried through the network, data is being divided and transmitted to recipient. The MTU size is being acknowledged by Transport layer during discovering routine while establishing connection with the recipient .
Reassembly – after data received, Transport layer protocols on the receiving end are reconstructing data to its initial form. Though, data can be received in different order then it was send, Transport layer rearranges pieces of data in the correct order,
Sequencing – often the time has been synchronized between two nodes, and time-stamps been added to the data, it can be organised in groups of subdivided data segments. It identifies the order of groups of data issue and interpretation.
