Protocol data unit: Difference between revisions

Revision as of 23:50, 27 August 2020

Revision as of 06:33, 28 August 2020

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Between the layers (and between the application and the top-most layer), the layers pass service data units across the interfaces. The higher layer understands the structure of the data in the SDU, but the lower layer at the interface does not; the lower layer treats the SDU as [[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 SDUPDU 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.