The Transport Layer Explained

The transport layer is the fourth layer in the OSI and TCP/IP layers models. It manages the exchange of data between two devices over the network. On the sending device, it breaks the data stream into smaller pieces, known as segments, and on the receiving device, it reassembles the original data stream from the received segments. This tutorial introduces the transport layer and explains its core features and functions.

The transport layer provides end-to-end transport services. It establishes a logical connection between the source and destination hosts 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 TCP (Transmission Control Protocol). Reliable connections provide slower data transfer speeds than unreliable ones. However, they offer guaranteed data delivery. To ensure data delivery, they use the three-way handshake and flow control.

In the three-way handshake, TCP establishes a reliable connection before transmitting data. It uses flow control techniques during 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 an acknowledgment for each received segment. If the sending device does not receive an acknowledgment for a segment, it resends that segment. After transmitting all segments, it terminates the connection.

Basic functions

• 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 combines all segments to produce the original data stream.
• To join segments, it uses sequence numbers.
• It provides reliable and unreliable data delivery.
• For reliable data delivery, it uses the 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 calls.
• It provides connection multiplexing.
• Connection multiplex allows multiple applications to send and receive data simultaneously on the same networking device.

Terminology

Conclusion

The transport layer initiates and manages connections between communicating devices. It controls the data flow and provides error corrections. This tutorial described basic features and explained the terms used in various functions of this layer.