The Transport Layer Explained

The transport layer is the fourth layer in the OSI and TCP/IP layers models. It manages the transport of data between two hosts over a network. Since the data can be transmitted into small chunks called data segments, it breaks the data stream into segments and attaches the necessary information to reassemble them back into the data stream on the sending device. On the receiving device, it reads the information attached to each segment and reassembles all segments back into the original data stream.

The transport layer provides end-to-end transport services. It establishes a logical connection between the source and destination host on a network. The connection can be unreliable or reliable.

For reliable connections, it is responsible for error detection and correction. When it detects an error, it resends the data. For unreliable connections, it provides only error detection. It leaves the error correction for the application layer.

Unreliable connections provide faster data transfer speed than reliable connections. But they do not give any guarantee of data delivery. It could be delivered or lost in transportation. It uses the UDP (User Datagram Protocol) for unreliable connections.

For reliable connections, it uses the TCP (Transmission Control Protocol). Reliable connections provide slower data transfer speed than unreliable connections. But they offer guaranteed data delivery. To provide guaranteed data delivery, TCP uses the three-way handshake, and flow control techniques.

Through the three-way handshake process, TCP establishes a reliable connection before transmitting data. It uses flow control techniques during the data transmission. It labels a sequence number to each segment on the sending device. On the receiving devices, it uses that sequence number to identify missing segments. It sends acknowledgments for all received segments. If the sending device does not receive an acknowledgment for a segment, it resends that segment. After transmitting all segments, it terminates the connection.

Functions of the transport layer

• It establishes, maintains, and terminates connections between the sending and receiving devices.
• On the sending device, it breaks the data stream into smaller segments and attaches a sequence number to each segment.
• On the receiving device, it joins all segments and produces the original data stream back.
• To join segments, it uses sequence numbers.
• It provides reliable and unreliable data delivery.
• For reliable data delivery, it uses TCP protocol.
• For unreliable data delivery, it uses the UDP protocol.
• It uses reliable data delivery when accuracy is more important than speed such as transmitting software and user documents.
• To provide reliable data delivery, it sends/receives an acknowledgment for each transmitted data segment.
• It uses unreliable data delivery when speed is more important than accuracy such as network broadcast and video call.
• It provides connection multiplexing.
• Connection multiplex allows multiple applications to send and receive data simultaneously on the same networking device.

The Transport layer terminology

The following table lists the transport layer's terms and their meanings.