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 Forphysical example,layer sendingsends ones and zeros across a wire,fiber,or etcfiber. isThe donedata bylink thelayer physicalthen layer,organizes organizing thethese ones and zeros into chunks of data and gettinggets them safely to the right place on the wire. isThe donenetwork bylayer transmits the data link layer, passingorganized data chunks over multiple connected networks, is done byand the networktransport layer and delivery ofdelivers the data to the right software application at the destination is done by the transport layer.
Between the layers (and between the application and the top-most layer), the layers pass service data units (SDUs) across the 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 (computing)|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 (networking)|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. All this additional information, plus the original service data unit from the higher layer, constitutes the ''protocol data unit'' at this layer.
The SDU and metadata added by the lower layer can be larger than the maximum size of that layer's PDU (known as the [[maximum transmission unit]]; MTU). When this is the case, the PDU must be split into multiple payloads of a size suitable for transmission or processing by the lower layer; a process known as [[IP fragmentation]].
