What is the name of the protocol data unit (PDU) at Layer 4?

At Layer 4 of the OSI model, which is the Transport Layer, the Protocol Data Unit (PDU) is called a segment. This layer is responsible for end-to-end communication, ensuring that data is transferred reliably and in order between devices. The primary functions of the Transport Layer include establishing a connection, flow control, and error detection and recovery.

Segments are created by transport protocols such as TCP (Transmission Control Protocol) and UDP (User Datagram Protocol). TCP segments include header information necessary for establishing a connection and managing data flow, while UDP segments have a simpler header since they are used for simpler, connectionless communication.

Understanding the correct terminology at each layer of the OSI model is essential for networking professionals, as it provides clarity on the functions and mechanisms employed at each level of network communication. The other terms refer to PDUs at different layers; for instance, packets are associated with Layer 3 (the Network Layer), frames are related to Layer 2 (the Data Link Layer), and messages may refer to application-layer data but are not specific to Layer 4.