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Concept
Protocol data unit
Summary
In telecommunications, a protocol data unit (PDU) is a single unit of information transmitted among peer entities of a computer network. It is composed of protocol-specific control information and user data. In the layered architectures of communication protocol stacks, each layer implements protocols tailored to the specific type or mode of data exchange.
For example, the Transmission Control Protocol (TCP) implements a connection-oriented transfer mode, and the PDU of this protocol is called a segment, while the User Datagram Protocol (UDP) uses datagrams as protocol data units for connectionless communication. A layer lower in the Internet protocol suite, at the Internet layer, the PDU is called a packet, irrespective of its payload type.
In the context of packet switching data networks, a protocol data unit (PDU) is best understood in relation to a service data unit (SDU).
The features or services of the network are implemented in distinct layers. The physical layer sends ones and zeros across a wire or fiber. The data link layer then organizes these ones and zeros into chunks of data and gets them safely to the right place on the wire. The network layer transmits the organized data over multiple connected networks, and the transport layer delivers the data to the right software application at the destination.
Between the layers (and between the application and the top-most layer), the layers pass service data units (SDUs) across interfaces. The higher layer understands the structure of the data in the SDU, but the lower layer at the interface does not; moreover, the lower layer treats the SDU as the payload, undertaking to get it to the same interface at the destination. In order to do this, the protocol (lower) layer will add to the SDU certain data it needs to perform its function; which is called encapsulation. For example, it might add a port number to identify the application, a network address to help with routing, a code to identify the type of data in the packet and error-checking information.
Official source
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