Service Data Unit (SDU)

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In computer networking, particularly protocol layered models like OSI and/or TCP/IP, a Service Data Unit (SDU) is an unencapsulated unit of data received from an upper level.

This unit of data is readable by the receiving layer as a raw segment but not yet ready for transmission through lower network layers.

As an example; the network layer receives an encapsulated SDU from the transport layer directly above. This SDU is actually a transport layer PDU—meaning that the transport layer has encapsulated that segment.

When the SDU reaches the network layer below, it hasn’t yet been encapsulated for transmission to the link-layer below. In a sense, this means it is still “raw” SDU data from the perspective of the current layer.

Zack West
Entrepreneur, programmer, designer, and lifelong learner. Can be found taking notes from Mother Nature when not hammering away at the keyboard.