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.

Zαck West
Full-Stack Software Engineer with 10+ years of experience. Expertise in developing distributed systems, implementing object-oriented models with a focus on semantic clarity, driving development with TDD, enhancing interfaces through thoughtful visual design, and developing deep learning agents.