The fourth layer is transport, which ensures that packets go from sender to receiver. This is achieved with acknowledgement (ACK) and negative-acknowledgement (NACK) messages from the destination, which can trigger a repetition of the messages, until they are received correctly. There are also other mechanisms in play, such as splitting a message into chunks for transmission (segmentation), reassembling the parts in a single message (desegmentation), and checking if the data was sent and received successfully (error control).

The OSI model specifies five different transport protocols—TP0, TP1, TP2, TP3, and TP4. TP0 is the simplest one, which executes only the segmentation and reassembly of messages. The other classes add other functionalities on top of it—for instance, retransmission or timeouts.