Transport Layers

The transport layer in the standard Internet ISO/OSI stack is between the Media and Application layers. Sometimes, it is a part of the so-called host layer protocols, which are composed of the top 4 layers: transport, session, presentation, and application.

In IoT protocols, it is commonly a part of the other layers, commonly networking. Besides pure IoT protocols, this book also contains regular internet ones (such as WiFi), so we briefly introduce the transport layer focused on the IP.

The transport layer is responsible for end-to-end communication and provides segmentation and aggregation mechanisms, error detection and correction, flow control, port-based addressing and quality of services. IP-based protocols use two kinds of connections:

• TCP: connection-oriented mode, where the connection with endpoints is kept for the whole transmission time open. One can imagine it as making a phone call: while connected, you can hear the other side's speech, breathing, and background noises so that you can tell the connection is not dropped. Examples of services using this connection on the regular Internet are Web (HTTP), FTP, and SSH. TCP is more complex than UDP and is stateful.
• UDP: connectionless mode, where the sender sends the data and “hopes” to let it be delivered to the receiver. There is no warranty for the message to have been delivered. Moreover, because of the routed nature of the connections, consecutively sent messages may be delivered partially, unordered or not delivered at all. For this reason, error checking and correction and quality of service in UDP are implemented in higher layers (if any). UDP is lightweight and, therefore, is typical for video streaming, e.g. video conferencing. One can imagine UDP protocol as sending an SMS message: you “send and forget”, and your friend may reply to it, to acknowledge, or may not. Delivery time is not precisely known and can only be estimated. This kind of connection is simpler than TCP and is stateless.

A choice between UDP and TCP is driven by other network layers, e.g. some IoT network stacks do not provide TCP-like connections. Higher-level protocols also drive the selection of the transport layer connection standards, e.g. MQTT generally requires TCP while CoAP uses UDP; see the following chapters for details.