Editing Protocol data unit (section)

==Packet-switched data networks==
{{Unreferenced section|date=June 2023}}
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 (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]].

The significance of this is that the PDU is the structured information that is passed to a matching protocol layer further along on the data's journey that allows the layer to deliver its intended function or service.  The matching layer, or "peer", decodes the data to extract the original service data unit, decide if it is error-free and where to send it next, etc.  Unless we have already arrived at the lowest (physical) layer, the PDU is passed to the peer using services of the next lower layer in the protocol "stack".  When the PDU passes over the interface from the layer that constructed it to the layer that merely delivers it (and therefore does not understand its internal structure), it becomes a service data unit to that layer.  The addition of addressing and control information (encapsulation) to an SDU to form a PDU and the passing of that PDU to the next lower layer as an SDU repeats until the lowest layer is reached and the data passes over some medium as a physical signal.

The above process can be likened to the mail system in which a letter (SDU) is placed in an envelope on which an address (addressing and control information) is written, making it a PDU.  The sending post office might look only at the postcode and place the letter in a mailbag so that the address on the envelope can no longer be seen, making it now an SDU.  The mailbag is labeled with the destination postcode and so becomes a PDU until it is combined with other bags in a crate when it is now an SDU, and the crate is labeled with the region to which all the bags are to be sent, making the crate a PDU.  When the crate reaches the destination matching its label, it is opened and the bags (SDUs) removed only to become PDUs when someone reads the code of the destination post office.  The letters themselves are SDUs when the bags are opened but become PDUs when the address is read for final delivery.  When the addressee finally opens the envelope, the top-level SDU, the letter itself, emerges.