Transport Layer

Transport layers work transparently within the layers above to deliver and receive data without errors. The send side breaks application messages into segments and passes them on to the network layer. The receiving side then reassembles segments into messages and passes them to the application layer.

The transport layer can provide some or all of the following services:

• Connection-Oriented Communication: Devices at the end-points of a network communication establish a handshake protocol to ensure a connection is robust before data is exchanged. The weakness of this method is that for each delivered message, there is a requirement for an acknowledgment, adding considerable network load compared to self-error-correcting packets. The repeated requests cause significant slowdown of network speed when defective byte streams or datagrams are sent.
• Same Order Delivery: Ensures that packets are always delivered in strict sequence. Although the network layer is responsible, the transport layer can fix any discrepancies in sequence caused by packet drops or device interruption.
• Data Integrity: Using checksums, the data integrity across all the delivery layers can be ensured. These checksums guarantee that the data transmitted is the same as the data received through repeated attempts made by other layers to have missing data resent.
• Flow Control: Devices at each end of a network connection often have no way of knowing each other's capabilities in terms of data throughput and can therefore send data faster than the receiving device is able to buffer or process it. In these cases, buffer overruns can cause complete communication breakdowns. Conversely, if the receiving device is not receiving data fast enough, this causes a buffer underrun, which may well cause an unnecessary reduction in network performance.
• Traffic Control: Digital communications networks are subject to bandwidth and processing speed restrictions, which can mean a huge amount of potential for data congestion on the network. This network congestion can affect almost every part of a network. The transport layer can identify the symptoms of overloaded nodes and reduced flow rates.
• Multiplexing: The transmission of multiple packet streams from unrelated applications or other sources (multiplexing) across a network requires some very dedicated control mechanisms, which are found in the transport layer. This multiplexing allows the use of simultaneous applications over a network such as when different internet browsers are opened on the same computer. In the OSI model, multiplexing is handled in the service layer.
• Byte orientation: Some applications prefer to receive byte streams instead of packets; the transport layer allows for the transmission of byte-oriented data streams if required.